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TH E C URRENT LITERATURE PUBLIS HING C OMPANY NEW Y ORK

I 909 I N T R O DU CT IO N S BY

P F E D A M S c . RO SSOR E . E . BARNARD, .

Yerkes Astronomical Observatory .

F h E E K E E P D . . . . PRO SSOR CHARL S BAS RVILL , . , F C S

o f o f o f . Professor Chemistry, College the City New York

E D T S c . DIR C OR WILLIAM T . HORNADAY, . , o f ZoOlo ical President New York g Society.

Ph . F E M . D E E E K T S . S . . PRO SSOR FR D RIC S ARR , , ,

Professor of Anthropology, Chicago University.

h D F E M P . . R R A I . EY ER B . S . A . . , P O SSO C SS US J K S , , , A dr ain o f a Professor Mathematics, Columbi University

i E WAR . EE ER A . M . L tt . D . D D J WH L , , , ‘ ’ Editor of Current Literature .

Ph LL.D. F E B . D . TE E A . . . . PRO SSOR HUGO MUNS RB RG, , M D , , ,

o o f . Pr fessor Psychology, Harvard University EDIT O R IA L B O A R D

0L I— E KA EM PF F ER T .V . WALD MAR , ‘ ’ Scientiﬁc American . — L II E . O . C E. V . HAROLD E SLAD , — M L III E E TT EW A . O . V G ORG MA H , — M . E . V OL . III F E . RE PRO SSOR WILLIAM J MOO , ,

Assistant Professor of Mechanical Engineering, Brooklyn

Polytechnic Institute .

V OL I — E . V WILLIAM ALL N HAMOR, e of the Research Chemist, Chemistry Department, Colleg

City of New York .

L — E K A M V O . . T E . V CAROLIN E S AC POL , ’ Tutor in Biology, Teachers College , Columbia University. — Ph D . L I M B h B A M . V O . V W . TT EW A . . P . . D . MA H , , , n Assistant Curator, Vertebrate Paleontology, America

Museum of Natural History .

L I— I V O . V T A . M MAR ON E . LA HAM , . , or Tut in Botany, Barnard College, Columbia University.

OL II— - V . V E T E E . FRANCIS ROL WH L R, S . T D . — V OL. V II E M M . D D E E A . TH O OR H . ALL N, . — V OL . L E B A M KE A . . . VIII . L LAND LOC , , for Brooklyn Training School Teachers . — V OL . Z I h E E A M . P . D VIII FRAN B LL NG R, . , . — V OL . S . F IX J . WOOL .

— - V OL . IX I RA T EE ER . . . F NC S ROL WH L , S T D

VOL X— . F E h E D . W P . PRO SSOR CHARL S GRAY S HA , , f o . Professor Ethics and Philosophy, N ew York University

E TT L ONARD ABBO , ‘ ’ r Associate Editor Cur ent Literature .

T H E E N C E H I S U DR T H E U N I V E R S

V O LU ME I

ASTRO NOMY

By W ALD EMAR KAEMPF F ERT

INTRODU CTION

P . A By ROFESSO R E E. B RNARD C i 1 0 b opyr ght, 9 9, y CU RRE N T LITE RAT U RE P U B LI S H I N G CO MPANY C ON T EN T S

T T F E E D IN RODUC ION BY PRO SSOR E . . BARNAR CHAPTER PACE

T H E T F T E I EVOLU ION O AS RONOMICAL ID AS . II T H E EVOLUTION OF OBSERVATI ONAL M ET HODS I I III T H E RI SE OF ASTROPHYSICS IV CELESTIAL PHOTOGRAPHY V T H E LA W OF GRAVITATION VI PLAN ETARY DISTANCES VII PLAN ETARY MOTIONS VIII T H E SOLAR SYSTE M IX T H E S UN X SOLAR EN ERGY XI M ERCURY XII VE NUS XIII T H E EART H XIV T H E MOON XV XVI JUPITER XVII SATURN XVII I U R A N U s AND NEPTUNE XIX T H E PLA N ETOI Ds

CoM ETs ETE ETE TE XX , M ORS AND M ORI S XXI T H E CONSTELLATIONS XXII T H E MOTIONS OF T H E STARS XXIII VARIABLE AND BINARY STARS XXIV N EB U LzE AND STAR CLUSTERS XXV COSMOGONY AND S TELLAR EVOLUTION The Editor desires to express his gratitude to the uni

v e r si ti e s , the learned societies and the libraries which have placed their facilities at his disposal in connection

with this work . Especial thanks are due to the Columbia

a f University libraries , not only for the Opportunities

but forded , also for the interest shown in forwarding

research work from their collections . Recognition of courtesy is due to the many publishers who have granted permission for certain quotations from their copyrighted volumes , among them being Messrs . D .

Co . o M Clur e . C . . . c Appleton , the Macmillan , the S S Co

r and The Mc G aW Publishi n g Co . To acknowledge the personal indebtedness to the mem bers of the Editorial Board , the Contributors , and all who have assisted with suggestion and advice would make too

li st but . long a ; mention should be made of Mr . Edward J

Wheeler, Litt . D Editor of Current Literature , to whose scholarly j udgment and discrimination is largely due wh at

- . R W merit may be found herein F . . . IN T R O D U C T I ON

I N the present volume there have been covered in a comprehensive and popular manner the various depart

deﬁ ments of Astronomy . Owing to its treatment in a n it el y historical and descriptive manner, however, it may be possible to supplement the general review by a few brief statements of some of the results and problems that confront us in the actual work of the observational as tr n om - o y of to day . There is frequently brought before the astronomer the fact that certain subj ects that were apparently exhausted have proved through the more advanced methods of to

or day, perhaps by chance , to be veritable mines of dis cover y, richer by far than had been anticipated in all the previous investigations . A remarkable illustration of this fact is the splendid work of Professor Hale at the Solar

Observatory of the Carnegie Institution at Mount Wilson ,

California . The Sun had almost been relegated to that limbo from which nothing new can ever come . With the

’ exception of Hale s development of the spectroheliograph , whi ch made possible the continuous photographic study of the surface of the Sun and of the solar prominences , but little advance had been made in solar research for a very long period of time . Even with the new instrument the ix INTRODUCTION work seemed to be conﬁned to the photography of the prominences and a few other features of the Sun that were already observable visually with the spectroscope . Before this the Sun was somewhat of a curiosity and but little new information was had concerning it . It only became really interesting when a total eclipse was immi nent, at which time the corona could be seen and studied . The spectroheliograph was the ﬁrst great step in the study of the Sun . Even though this made possible a continuous photographic record of the prominences and kindred fea tures it could not record the more attenuated and delicate

- corona . Indeed , we seem to day as far as ever from any sight of this mysterious obj ect without the aid of the friendly Moon , which for a few minutes at long intervals hides the Sun and gives us our only view of the corona . But the great work done by Professor Hale and his associates at Mount Wilson (which was foreshadowed by his work at the Yerkes Observatory) in the discovery o f the solar vortices and magnetic ﬁelds of sun -spots has revolutionized the study of that body and opened up new ﬁelds of investigation in this direction that are almost unlimited .

Mr . Abbot, of the Smithsonian Institution , has also established a permanent station at Mount Wilson for the investigation o f the solar constant and a general study o f the heat of the Sun . The solar investigations , therefore , that are going On at Mount Wilson are among the most

n ot important that have ever been un dertaken . They are only of the highest interest, but may ultimately lead to important results bearing upon the commercial life o f the world by revealing to us some possible means o f forecast INTRODUCTION xi

ing conditions upon the Earth . Any vagaries in the Sun must have more or less direct influence on the conditions of the Earth which owes its every throb of life to the mighty influence o f the Sun . Much of the ordinary spectroscopic Work may be said to be in its infancy because o f the vast fields of research that are open to it . It is already laying the foundation for a very accurate determination of the distance o f the visible binary stars where both stars can be observed with — the spectroscope an accuracy that can never be attained f by the ordinary methods o parallax work . Already this has given results of precision in the case of Alpha Cen tauri , whose distance has been determined by Professor

o f ob Wright, the Lick Observatory, from spectroscopic s r v ti n f e a o s combined with the known orbit o the star .

Time , however, is an element in this work, and after a sufﬁciently long interval a valuable harvest of knowledge of star distances will result . The spectroscopic material for such investigations is being specially obtained by Pr o fessor Frost and his associates at the Yerkes Observatory

of ( as well as by others elsewhere) , where spectrograms the various visual binaries that are bright enough to give a measurable spectrum are being carefully and accurately accumulated . A possible improvement of the spectro scope, whereby a larger percentage of the light can be

o f o f utilized , will make possible the extension this class

0 work, for at least 9 per cent . o f the available light cannot

. ﬁ at present be utilized If this can be done , the ef ciency o f the spectroscope will be vastly increased and a great number of obj ects at present beyond the reach of accurate spectro scopic study will be investigated and their nature xii INTRODUCTION

and physical conditions become known . A step in this direction is the intended erection on Mount Wilson of a reﬂecting telescope one hundred inches in diameter . The great light- grasping power of this instrument will enable much fainter obj ects to be studied than can be observed with the present means . Only a few years ago our knowledge of comets seemed to be satisfactory . What we could see with the naked eye or with the telescope apparently readily agreed with certain theories that were formulated to explain them . The tails of various comets were sorted out and assigned to di fferent classes . This one was a hydrocarbon tail and that a hydrogen tail , etc . The spectroscope had shown that comets in general consisted of some form of hydro carbon gas ( such as cyanogen ) . Such gas or gases are evidently mixed up with minutely divided matter which is disrupted and expelled from the comet ’ s head and thrown out backward from the comet away from the Sun . This

L ebedew was shown later by the experiments of , Nichols and Hull to be due to the pressure of the Sun ’ s light upon the smaller particles of the comet , which drove them away into space with increasing velocity to form the tail . The simple phenomena thus seen by the eye were rather easy of explanation . Photography, however, has revealed such a mass of strange phenomena in these bodies that the theories which seemed so satisfactory before are now

r seriously questioned, and some of them appea to be entire ly inadequate to explain some of the phen omena shown by the photographic plates . But little indication of many of the most extraordinary changes and peculiari

’ ties o f comets tails is seen by the eye . In part this is

xiv INTRODUCTION

the tail was twisted or corkscrew shaped , as if it had gone

con out in a more or less Spiral form . Areas of material n ected with the tail would become visible at some distance

from the head , where apparently no supply had reached

it from the nucleus . Several times the matter of the tail was accelerated perpendicularly to its length . At one time the entire tail was thrown forward and violently curved perpendicularly to the radius vector in the general direction of the sweep of the tail through space . This peculiarity is opposed to the laws of gravitation . There is no known cause for this freak of the tail . Evidently we have here, and in many other of the phenomena of this body, some unknown inﬂuence at work in the planetary spaces . What this is , is one of the great problems for the

o f future to solve . It has been suggested that many the unaccountable phenomena of this comet are electrical and can be attributed to the same inﬂuence that produces our magnetic storms and auroras on the Earth , and these are believed to be due to abnormal disturbances on the Sun . It is to be hoped that the present return of Halley’ s comet will add much to a solution of this problem . The study of the dark or apparently vacant regions of the sky, especially in the Milky Way, is of paramount importance . The photographic plate has shown that the “ ” dark regions ( the so- called coal sacks ) are generally connected with masses of nebulosity or gaseous matter . These are especially remarkable in the regions of the

hi uchi hi uchi stars Theta O p and Rho O p . In the latter case we ﬁnd a magniﬁcent nebula in a rich region of the Milky Way occupying a hole that is apparently devoid of stars . Some astronomers have attributed the general INTRODUCTION xv — absence of stars here to absorbing matter to an opacity and partial dying out of the nebula that cuts off the light of the stars which are beyond it . What these apparent vacant regions really are is , therefore , an unsolved prob

f to lem at present . Some o them are evidently due the thinning out and actual absence of stars in those parts of

sk the y. But the others , which are connected with nebu

losi ties . , seemingly must have some other explanation One fact that appears to be brought out by the great nebula of Rho O phi uchi is that the groundwork o f the Milky Way i n this region , and by inference elsewhere , may be made up of stars actually much smaller than the average of

so those seen in the general sky . I f this were it would materially change our ideas of the Milky Way . This supposition comes from the fact that the great nebula is

s connected with some of the brighter stars in thi region , while at the same time there is apparently evidence that it is connected with the faint stars that form the ground

o f work the Milky Way here . If, however, the dark regions about and near the nebula are due to the absor p

o f tion light by an opacity of the nebula , the supposition

to for u as the relative sizes would not hold , the neb la in that case might be very much nearer to us than the Milky

Way . It will be evident that an understanding o f the nature of the dark regions o f the Milky Way is o f the utmost im portance to a proper knowledge o f our stellar universe . The great nebulous regions of the sky that photography has revealed to us are intimately connected with the

Milky Way . They cover very large regions of the heavens and must be almost inconceivably great . In no case has it been possible to determine the exact dimensions o f these xvi INTRODUCTION

wonderful obj ects , because we do not know their distances .

It is possible, however, by assumptions that are j ustiﬁed by facts to arrive at some idea of their minimum extent .

I f they are no further away than the nearest ﬁxed stars , and from their evident connection with certain stars we know that they must be much further away, we can form some idea of their vastness . Our own Sun if removed to the distance of the nearest ﬁxed stars would present an apparent diameter of about the hundredth part of a second of arc . Its known diameter is something like a million miles ( accurately miles ) . Some of these '

" n nebulous regions a r e ma y degrees in diameter . The one connected with the Pleiades is ten degrees in diameter . It is certainly connected with the cluster whose distance is much beyond the nearest fixed stars . From this it will be readily seen that this great nebulous region must be at least some four million times greater i n diameter than our Sun , or over one hundred thousand times greater than the entire diameter of our known solar system . These are figures that appear to be appallingly great . But they are only relatively so and only shock us because the facts are new and we are not yet used to them . What i s the ultimate function of these enormous masses of gaseous matter that we find lying in space ? Are we sure that they are the primitive matter from which worlds and systems are finally to be evolved ?

These , very brieﬂy, are a few of the problems that we encounter in astronomy as developed by the subtle means of research in use at the present time .

R . E . E . BARNA D A S T R ON O M Y

CHAPTER I

TH E EVOLUTION O F ASTRONOMICAL IDEAS

HERBERT SPENCER has stated that evolution is a change to from the indeﬁnite to the deﬁnite , from the incoherent r e the coherent . I f any proof of that doctrine were o f quired , it would assuredly be found in the development astronomical conceptions . In this chapter an attempt will be made to outline in a general way the manner in which the present theories were evolved from the mysticism o f

- folk lore and religion . Some of the matter herein pre ’ “ sented is drawn from Arrhenius Die Vorstellung vom Welt ba i ge ude m Wandel der Zeiten . The astronomical beliefs of prehistoric man were n o doubt Similar to those entertained by the Eskimo of the Arctic regions and the untutored tribes of Argentine

Republic, South Africa and Australia , tribes who , living to only for the day , concern themselves but little with

morrow and yesterday and care nothing about the universe . Somewhat more cultured than these Eskimo an d South American and South African tribes are primitive nations who have endeavored to account for the origin o f

th e Earth and the heavens by anthropomorphic theories . The universe must have been created by some Personal

Being who had at his disposal something to mold . The idea that the universe was made out of nothing is a philo Sophical assumption which was introduced by the highly 2 ASTRONOMY

Th o f cultured philosophers of the East . e something out which th e universe was created is usually regarded as water, an element which to the eye at least is perfectly homogeneous , shapeless , and chaotic . That the fertilizing mud was deposited by ﬂoods must have attracted the at tention of ancient primitive races , for which reason they m ay have assumed that all the Earth was slowly and gradually deposited from water . Thus we ﬁnd that Thales

0 . ( 5 5 B C . ) argued that all things were created from water

Yet other substances were assumed as primordial matter, A n aximi n es and later of Miletus , who also flourished in the sixth century , called the generative principle of things air or breath , while Heraclitus , who flourished at Ephesus near the end of the sixth century , believed that all bodies were transformations of on e and the same element, which he called fire . The belief that primordial water is the origin o f all things was deeply rooted in Asiatic races , for it occurs over and over again in many creation myths , among others in the Chaldean and in the Hebrew . Instead of water we sometimes find that an egg may be taken as the primal u nit, no doubt because every organism springs from an a pparently lifeless seed . Thus we find that the egg plays a most important part in the creation myths o f the Japa n ese as well as in narratives from India , China, Polynesia,

Finland , Egypt and Phenicia . h o f . as In many of these creation myths , which I Riem c - ollected no fewer than sixty eight , more or less inde

o f . pendent one another, deluges are prominent features In nearly all of them it is supposed that after the water s to ubsided the land was exposed, fertilized and made bring forth . All of these creation myths are interwoven and inter T connected with religious belief . o the savage mind every thing that moves is endowed with a Spirit . Accordingly primitive man endeavors to propitiate the Spirit by magic, knowledge of which art is given only to the medicine man EVOLUTION O F IDEAS 3

or to the priest . In a certain sense , therefore , magic is the l n precursor of natural science , and the myths and ore upo which the practice o f magic i s based are remotely ante cedent to our scientiﬁc theories . According to Andrew as Lang, myths are based much upon primitive science , a s con resting upon superstition , upon primitive religious ti n cep o s. ’ “ ’ In Maspero s Histoire Ancienne des Peuples de l O r i en t Classique we ﬁnd an account Of the Chaldean conception o f the universe . Surrounded on all sides by the ocean , the Earth rises in the middle like a high mountain whose summit i s covered with snow from which the Euphrate s

Springs . The Earth is encircled by a high wall , and the a byss between the Earth and the wall i s filled by the ocean . i s o f su Beyond it the abode the immortals . The wall p of fir mamen t the ports the vault the , shaped by Marduk, od out o f h Sun g , a hard metal , w ich shines in the daytime i but which at night s like a blue bell set with stars . In the morning the Sun enters the vault by an eastern en t rance and at night makes its exit by a western outlet . Marduk arran ged the year according to the course of the of Sun and divided it into twelve months , each which counted three periods of ten days . The year, therefore , r numbered three hundred and sixty days . Every sixth yea a so r on an special year was intercalated , that the yea had -fiv average three hundred and sixty e days . A s the lives of the Chaldeans were t o a high degree a s influenced by a change in the seasons , they l id great stres o f upon division time . In the beginning they probably based their chronology upon the movements of the Moon , like many another race . Soon they recognised that the S ufi in exerted a stronger influence , and accordingly they t r oduced a solar year whose divisions they ascribed to s Marduk . The stars were observed because their position c determined the seasons . Since the seasons govern organi n o f life , a pernicious belief in the i fluence the stars took r a for oot, belief which prevailed twenty centuries and 4 ASTRONOMY which crippled the advance of science up to the time o f

. Di odor us S i culus Galileo , a contemporary of Julius a C esar, describes this astrology in the following words , as given in a translation by Philemon Holland ( 1 700 ) “ Therefore from a long observation of the Stars , and a n exact Knowledge of the motions and inﬂuences of every one of them , wherein they excel all others , they (the Chaldean astrologers ) foretell many things that are to come to pass . “ They say that the Five Stars which some call Planets , but they Interpreters , are most worthy of Consideration , both for their motions and their remarkable inﬂuences , especially that which the Grecians call Saturn . The o f brightest them all, and which often portends many and great Events , they call Sol , the other Four they name our Mars , Venus , Mercury, and Jupiter, with own Country

Astrologers . They give the name of Interpreters to these

Stars , because these only by a peculiar Motion do portend of things to come , and instead Jupiters , do declare to Men beforehand the good -will of the gods ; whereas the other Stars ( not being of the number o f the Planets ) have a constant ordinary motion . Future Events ( they say) are pointed at sometimes by their Rising, and sometimes by their Setting, and at other times by their Colour, as may be experienced by those that will diligently observe it ; sometimes foreshewing Hurricanes , at other times Tem

estuous . p Rains , and then again exceeding Droughts By of these , they say, are often portended the appearance o f o Comets , Eclipses the Sun and M on , Earthquakes and all other the various Changes and remarkable effects in n ot the Air, boding good and bad , only to Nations in gen

to . eral , but Kings and Private Persons in particular o f Under the course these Planets , they say are Thirty o f Stars , which they call Counselling Gods , half whom observe what is done under the Earth , and the other half take notice of the actions of Men upon the Earth , and what is transacted in the Heavens . Once every Ten Days

6 ASTRONOMY

of chaos once existed and that out of it Gaa , the mother son of e all things , and her , Uranos, the god heaven , wer created . The Greek cosmogony was adopted by the Romans with it out noteworthy development . Hence is that Ovid wrote on the origin of the universe much as Hesiod had done seven hundred years before . In that long interval of seven centuries the study of nature had advanced but little . Indeed it was not until the invention of the telescope that astronomy was lifted entirely out of the hands of the B e priesthood and placed upon a sure scientific footing . o f fore the invention the telescope , therefore , astronomy o f appears merely in the garb a myth . At its best it was metaphysical . The rudiments of astronomical science are to be found ff to in the e orts of the Chaldeans , Egyptians and Greeks f n eces devise calendars and to mark time . That ef ort si tated a study of the motions of the celestial bodies . o f n Moreover, exigencies husbandry rendered ecessary some method of keeping track of the seasons so that seed i t me and harvest could be ascertained . The regular occur rence of such events as the Nile flood made requisite suit a ble preparations . Hence the early Egyptians so built their temples that they might know the time o f the summer solstice and hence the time when the flood might be ex e t c ed . p This was a matter of practical importance, but not merely connected with religion or priestcraft , on which the lives and the happiness of the people o f n Egypt depended , and might be compared with the moder time observations made at the great national observatories . The observation of the stars was carried on with at least of this obj ect in View, and gradually with the development civilization time reckoning from the stars became an im portant consideration closely connected with the lives of the people . With the study of the stars for such a purpose naturally an amount of information as to their positions r acti and motions was accumulated, and for centuries the p EVOLUTION O F IDEAS 7 c al side o f astronomy was the study o f the position of the s tars and the motion of the planets . The astrology of the Chaldeans spreading westward increased rather than di minished the interest in the stars , for not only was the connection of the planets with natural phenomena and the mere reckoning of time studied, but the mystical element involving prophecy of future events attracted attention .

- In other words , astrology was a pseudo science , for which r eason it is difficult to estimate its benefits or to exaggerate i t s evils . In its scientific aspect it involved the observa t ion and record o f the position of the heavenly bodies with all the exactness that the mathematical and observational methods of the time could achieve . It enabled the motions of the planets to be stud ied as well as the positions of the fixed stars and the course of the Sun as it passed through on them . But, the other hand , when the interpretation of th e o f an d appearance the skies was involved , superstition poetic fancy had full sway, in which no doubt cer t ain elements of self-interest and deception on the part o f the priests or astrologers were not lacking . Hence these men did not study the sky to interpret phenomena on a s o f cientific basis . Confined in the narrow limits super s ti ti on n ot , they only made no progress but actually held b ack astronomy as they did other sciences . That the work o f the astrologer was mysterious there can n o n o be doubt, and as reason was assigned for the m o f o r w as ovement the planets the position of the stars , it a natural assumption on the part of the people that some s upernatural agency was at work, which also was con n ecte d with their lives and their future . With the begin n ing o f the development of scientific astronomical theory proper the power and position of the astrologers began to — i n wane slowly, it is true, for when Tycho Brahe was vited to deliver lectures on astronomy at the University o f Copenhagen , the first dealt very largely with astrol a str on o ogy. Cardan and Kepler among the distinguished of S i r mers the Middle Ages , Roger Bacon , Burton and 8 ASTRONOMY

Thomas Brown were among the men of mind who were interested , at least in part , in the teachings of the underly ing basis of the cult . As explanations of the motions of the heavenly bodies on a rational basis were forthcoming, the sc i en doom of the astrologer, so far as participation in the t ifi c . creed of the day was concerned , was sealed I f there was a natural explanation that could be accepted , how could supernatural influences condition the movements of the planets or the positions of the stars ? If then these movements were natural and made in obedience to natural ff laws , how could they a ect the future course of life and future occurrences that O bviously had no connection with ? natural phenomena The law of gravitation , which ex plained the solar system and the movement of the planets , corroborated this View and left only the comets as striking n atural phenomena which could not be explained in a way that the popular mind could grasp . With the rise of learning and the growth of observation , the explanations of natural phenomena by astronomers secured acceptance ’ by the people . Finally, when Halley s prediction of the 1 0 1 8 return of his comet, first made in 7 5 , was verified in 75 , the reign of natural law in the world of the heavens be or came an accepted fact, from which only the ignorant superstitious could dissent . Distinctly di fferent and apart from astrological influence was the work of Copernicus, whose researches mark the beginning of the new and philosophical science of astron o m y, in which the element of mysticism was gradually dis placed and observation and reasoning were depended on .

Copernicus , as will be seen when the development of the o r i e s of the solar system is considered in an early chapter, returned to many of the fundamental ideas of Pythagora s and the early Greek philosophers , especially that the Sun was the center of the universe . He was a thoughtful student not only of Greek philosophy but of the work of so such later astronomers as Ptolemy and his successors , that when he announced a theory of the solar system i n EVOLUTION OF IDEAS which the Earth and other planets revolved around the Sun o f as a center, it was based upon the fullest knowledge e previous reasoning and theory . Neverthel ss he was cast ing to on e side the tradition and the science Of the day as it was then understood and presenting what was a co n cep tion o f the heavenly world no less daring than original . H i s theory was a natural outcome of the revival of learn o f u ing in the Renaissance , foreshadowed by the work s ch scien men as Leonardo da Vinci and others , in whom the tific spark , had been awakened . With Copernicus . the evolution o f his heliocentric theory was a matter Of sci en tifi c reasoning rather than o f direct observation . But it marked the beginning of a series of epoch -making dis c ove r i e s so presented in a clear and positive form , that the theory of the revolution O f the planets around the Sun on became e of the fundamental canons of astronomy . the Thus , as will appear in the course of our history, Copernican theory in which the revolution of the planets ’ t around the Sun is made clear, Kepler s theory of plane ary a r e stated ti motion in which laws to account for this mo on , l ’ u and finally, Newton s announcement of the great niver sa l on law of gravitation , are the foundation stones which modern astronomical science firmly rests . The invention o f the telescope established the similari ty in the bodies of the solar system and revealed facts fhat f previously had been hidden from observers o the heavens .

Indeed, with the invention of the telescope and the growth d e of mathematical science, there began an era of sc r ipti on al astronomy in which exact observation was com bi n ed t with careful computa ion and mathematical analysis, an era which continued into the nineteenth century with undiminished vigor . Brilliant discoveries were made po s o sible by improved and powerful instruments , acc mpanied b f y theoretical work of even greater value . In the middl e o the nineteenth century new instruments were put ‘ at the

’ command o f the scientist which had a remarkable e ﬂect i n extending the boundaries of the science . The telescope I O ASTRONOMY h ad facilitated merely the Observation of the stars . The b to spectroscope , on the other hand , ena led the astronomer ascertain their composition . With the application of the spectroscope to astronomy began the welding of physics and chemistry with as t r o n omy and the birth of that modern science o f astro ff o f physics , which has a orded data for the study the seri ous problems connected with the evolution of the universe . From the soothsaying star- gazer of Chaldean times to the modern astrophysicist , who works in a laboratory as well r es on as in an observatory , we have a development that is p sible for the aggregation of knowledge which we now pos o f sess the vast universe with its suns , planets, stars and nebulae. The spectra o f distant celestial bodies recorded on the photographic plate by the spectrographs of large telescopes ar e now studied in comparison with the spectra of ter r e st r i al substances produced in the physical laboratory . ‘ Not only the nature an d composition of the stars can be be ascertained , but also their motion in space which are yond the range of any telescope . The New Astronomy has become on its astrophysical side almost an experimental science with the methods and accuracy of the chemical or physical laboratory . It is from this modern astronomy, with its breadth and resourcefulness , that modern science looks not only for advances in its own particular ﬁeld , but in the broader and ever interesting problems of cosmogony as concerned in the evolution of the stars and other bodies making up the universe . CHAPTER II

T H E EVOLUTION OF ASTRONOMICAL M ET HODS OF OBSERVATION

T H E history of astronomi cal Observation is the history ’ f H i s o man s attempt to bring the stars nearer to him . own senses are so feeble and so very subj ect to error that he has been constrained to devise subtle artiﬁcial senses f which take the place o eyes and hands . Thus early he -ﬁn d er s invented position , which enabled him to determine ’ with more or less precision a star s direction or position at a given time and not merely to guess at that position ; great eyes , called telescopes , that see what his eyes can never see and also determine positions with greater a o ’ curacy ; wonderful spectroscopes that analyze a star s com position as nicely as if it were a stone picked up in the road ; and photographic devices that reveal secrets Of star structure that otherwise would never be disclosed by hi s unaided senses . F or determining the position of the heavenly bodies the instruments used have always been comparatively simple .

All are based on certain rudimentary geometric principles . A s geometry was a science fairly well developed among f 4 the ancients , it is not di ﬁcult to realize that they had vari ous means of measuring angles , both vertical and horizon tal . In most ancient cases , however, the observers have m th e failed to hand down their methods , erely recording results without indicating the circumstances in which they

obtai n ed so . were , , that it is impossible to discuss the values o f the Observations and correct them in the light of recent

1 1 1 2 A STRONOMY

discoveries . It is evident that the instruments of the c an ients were simple, but their precise nature is altogether uncertain . The earliest astronomical observations o f which there is record were made by the Chinese . The Shu King, the oldest known scientiﬁc work, states that two thousand years before the present era the Chinese determined the — seasons that is to say , the positions of the Sun at the — equinoxes and solstices by means of four stars which have since been identiﬁed and found to be so suitable that a modern astronomer could not have made a better Choice .

The Chinese also determined , eleven hundred years before the present era , the obliquity of the ecliptic , which they

2 . found equal to 3 deg 54 min . The obliquity, which

2 . varies , is now 3 deg 37 min . , and calculation shows that at the epoch of the Chinese observations it must have been 2 1 3 deg. 5 min . Hence the error of the Chinese determina tion was only three minutes of arc . Among the few astronomical values which have r e mained constant during the history of man are the times o f revolution of the planets . The Hindus determined the revolution of Mercury with an error of 7 1 0000 of a day .

For Venus the error was of a day , for Mars

7 1 00, of a day . In the case o f Jupiter the error amounts

- to one quarter of a day, but it is to be remembered that 1 1 the period of revolution of this planet exceeds years , so that the same observer could not Observe many returns of the planet to the same point of its orbit . This comment applies with still greater force to Saturn , the revolution of 2 which occupies 9 years . Hence it is not astonishing that in this case the Hindus were six days in error . Among the ancient Greeks is a measurement of a ter r est r i al meridian made about 2 00 B C . by Eratosthenes

2 6 B C . 1 1 6 B C ( 7 to 95 or 9 ) , who found the circumference o f the Earth equal to stadia by measuring the a n gular distance of the Sun from the zenith at the summer solstice both at Alexandria and at Syene in Upper Egypt

1 4 A STRONOMY

1 example , was of 9 feet radius and had its circumference graduated to single minutes . Various forms of armillary spheres were constructed in which the stars were placed in the i r relative positions on great circles of the celestial sphere . Such devices served for much of the early astro n omi cal work , taking the place of modern star charts .

Tycho B rahe , like his predecessors , employed wooden ’ instruments . One of these was a large Ptolemy s ring , surmounted by a post carrying horizontal arms , by which it was turned in bearings like a capstan , so that the ring could be brought into any vertical plane . Tycho B rahe l a so constructed a mural circle , by means of which vertical angles could be measured . Hence it was by using the naked eye and rudimentary instruments that he a ccumu lated observations of such precision that they served Kepler as the basis of the researches which led to the dis ov e r c y of the laws of planetary movement . The eye can distinguish an obj ect whose diameter is equal to about of its distance , which corresponds with an angular diameter of about one minute of arc . This was the measure of the precision of early observa tions . Its value may be appreciated by stating that it cor responds with the diameter of a lead - pencil seen at a dis

0 . tance of 7 feet The telescope , by increasing the distance at which obj ects can be distinguished , therefore has been and is now the chief reliance of the astronomer in deter

- mining position . While the naked eye to day may be said to have been very largely supplanted by spectroscopic and photographic observation , yet the telescope has constantly met the demands of astronomers as its power has increased and its scope widened . By chance or otherwise it was found by a Dutch Spec 1 608 tacle maker , Lippershey , about , that two lenses when placed at some distance apart would act to magnify distant obj ects , j ust as a single lens would enlarge the image of a - b near y obj ect . This action of the lens can be explained by considering the effect on a prism of transparent material METHODS OF OB SERVATION 1 5

of placed in the path of a beam o f light . When a beam light falls on on e o f the angular faces of the prism at a direction other than perpendicular to the face it is forced to on change its direction account of refraction , due to the O b change in medium . That is , a ray of light passing li ue l i s q y through air into a denser medium , such as glass , out a bent toward the perpendicular, and in passing from denser to a rarer medium is bent away from the pe r pen icul r o f d a . A lens may be considered as a collection of f t prisms constantly changing angles , so that the e fec would be to bend parallel rays coming from a point at inﬁnite distance in such a way that they would all be on brought to a single point known as the focus . C se

- quently a telescope may be regarded as a light gatherer . ’ The importance to astron omy of Lippershey s invention can be appreciated from the fact that as soon as Galileo heard Of it he constructed such an instrument which, e hardly the size of a small toy spyglass , magni ﬁed thre times , or brought the heavenly bodies three times as near 1 0 He applied it to celestial observation in 6 9 . The value of the telescope as an astronomical instrument s hi s became apparent immediately . It wa from the use of ” the Optik tube , as he called it, that Galileo arrived at —conclusion that Ptolemy was wrong and Copernicus right how will become apparent from a consideration of the how-1 discoveries made by Galileo . He did more than this , ever ; for by the application of the telescope to the Observa tion o f the stars he became in truth the founder o f our f modern science o astrophysics . Galileo saw hosts o f stars never before revealed to th e unaided eye . The six stars in the Pleiades now appeared as 6 a s 3 , and various nebulous obj ects of light , such the , o f ﬁn e Milky Way, were found to consist of multitudes t stars clustered together . But his crowning achievemen on 1 61 0 occurred January 7 , , when in turning his telescope toward Jupiter he discovered four satellites o f that planet an d determined that their periods o f revolution ar oun di 1 6 ASTRONOMY

Jupiter ranged from about forty -two hours to sev e n tee n days . Here was a miniature system simi lar to that conceived by Copernicus . Was it any won der that Galileo abandoned the Ptolemaic teaching ? Thus Galileo was able to strike a serious blow at the infallibility o f Aristotle and Ptolemy, by whom no mention had been made of the existence of such extra bodies . At this time, however, others besides Galileo were working with the fele sco e 1 60 - 1 62 1 p , among them Thomas Harriott ( 5 ) in 1 0 1 2 England , Simon Marius ( 57 6 4) and Christopher 1 1 6 0 Scheiner ( 575 5 ) in Germany . Thenceforth observa tion a l astronomy with the telescope was anchored on a ﬁrm basis .

As was quite natural , telescopes eventually formed an important part of the equipment o f the observatory o f Tycho B rahe and of John Kepler ( 1 57 1 In on e of Kepler ’ s works on Optics is contained a suggestion for the use of a convex lens for an eye - piece in the c on str uc ’ tion of the telescope . Galileo s instrument consisted of a

- lead tube containing a large double convex lens , which

- served as an Obj ective , and a small double concave lens at — the eye - end in order to give an erect image an arrange ment which finds its counterpart in the modern opera glass . ’ Kepler s suggested improvement provided a more effi n cie t and fairly modern astronomical telescope . The actual construction of an instrument of this type , however, n ot is credited to Scheiner rather than Kepler, who was ably deficient in mechanical skill . After considerable ex p er i me n ti n g by various astronomers and instrument mak f ers , it was ound that a comparatively small obj ective with a considerable focal length was most useful and effective . 1 6 2 In 7 Capani , of Bologna , constructed an instrument of 6 this kind I 3 feet long, while Auzout actually made a tele ‘ o 600 . sc pe feet in length , which , however, failed to work

These , of course , were skeleton structures not mounted in s tubes . Perhaps the best of them were those of Huygen METHODS OF OB SERVATION 1 7

( 1 62 9 whose skill in grinding lenses stood him in such good stead that he was able to construct a telescope ’ with which he determined the ring of Saturn . Huygens telescope had considerable focal length . He placed the O bj ect glass on a tall vertical pole or staff so balanced that

i — ’ F . I H A I E E P g UYGE NS E R AL T L SCO E .

“ it could be moved in any direction by means of a cord . The observer on the surface of the Earth was supplied w ith an eye - pi ece which he maintained in a straight line w ith the star he was observing by means of a cord . “ ” sub All these telescopes were refractors . They were 1 8 ASTRONOMY

ect j to certain inherent defects , chief among which was the difﬁculty o f bringing to a single focus all the rays of di ffer - d ent colors . The seventeenth century philosophers believe it impossible to overcome the unequal refrangibility O f the “ di fferent colored rays of light which produced chromati c aberration ” and resulted in an image indistinct for the blurring Of various colors . Accordingly they gave up the

" R ays o f li g h t 5 m t h e S ta r

—R F T F i . 2 E RA I N g C C TELE SCO PE . R EF LE CTI N C T E LE SCO PE . idea o f perfecting the refracting telescope and directed their attention to constructing an instrument on a di ff erent principle , using a concave mirror to form the image of the O 1 6 obj ect bserved . Mersenne , in 39, suggested the employ ment of a spherical mirror, but the idea appears to have

ed . been dropp Quite independently, James Gregory, in 1 66 3 , proposed a similar arrangement , using, however, a O f parabolic in place a spherical mirror . At that time he could not ﬁnd a workman able to construct such a mirror . METHOD S O F OB SERVATION 1 9

In the Gregorian instruments the parabolic reflector is placed at the lower end of the tube , while on its axis and a short distance beyond its focus is placed a small concave reflector . The light from the distant obj ect falls upon the large mirror, from which it is reflected back to the small one, which throws it back through a hole in the cen

‘ ter of the large reflector . It then passes into the eye ’ i n piece , which , indeed , Gregory s time had been much i mproved by Huygens . Gregory ’ s efforts turned Newton ’ s attention to reflecting 1 66 telescopes . In 9 he cast his first disk and began to grind

1 6 2 . it, but it was not until 7 that he had real success Then he made two small instruments , one of which was only about an inch in diameter, with a magnifying power of ’ ff a bout 38 . The principle of Newton s telescope di ered from that of Gregory ’ s in that it had a small plane mirror placed in the cone of light from the reflector at an angle f o . 45 degrees Being placed inside the focus , this mirror brought the light cone at right angles to its original di r e c t ion , thus forming the image outside the tube and obviating the necessity of a hole in the parabolic reflector . About the same time that Newton completed his i n str u ment the Cassegrain construction was proposed , which was similar in construction to the Gregorian telescope , except for the small mirror . In the Gregorian this mirror was concave . Cassegrain ( 1 672 ) proposed the use of a con cave mirror inside the focus . It brought the light from the obj ect to a focus through a hole in the center of the large parabolic mirror . O wing to the difficulty in obtaining a suitable mirror alloy, little progress was made for some time in construct i n x eflecti n 1 1 8 g g telescopes . In 7 , however, Hadley , the o f inventor the sextant , constructed one on the Newtonian principle , 5 feet in length . The instrument magnified over, 2 00 times and revealed as much as the old refracting tele s copes . Perfect as this Newtonian telescope seemed to be , the Gregorian type held the field until 1 774. 2 0 ASTRONOMY

By using a small Gregorian telescope Herschel had hi s

i n attention directed to the wonders of astronomy . His come being too limited to purchase an instrument , he set about making one for himself . During his life he is said 00 to have made upward of 4 telescopes , mostly of the New t on i an ff type . Among his earliest e orts was the construe

- tion of a 5 foot reﬂector, which was a wonderful success . i n st r u Then came one 7 feet in length . The largest of his

1 8 . ments was completed under George III . in 7 9 This f telescope surpassed all his previous e forts , for it was actually 40 feet long and had a reflecting mirror 4 feet in ’ diameter . The story of Herschel s work with this great telescope would fill a volume . The largest telescope of the reflecting type was con » structed by Lord Rosse , an Irish peer, and used at Par son stown O f . It had mirror 54 feet focus and a diameter 6 of feet, but it could be used only for observations on or near the meridian . While out of use for many years , it n ow long held the record for size , which , however , is taken by the I oo- inch reflector recently completed for the

Mount Wilson Solar Observatory , California . e The case of the refracting telescope , which as we hav seen had been all but abandoned on account of its ch r o matic aberration by the seventeenth - century astronomers and physicists , was not as hopeless as they believed . Not withstanding Newton ’ s dictum that it was useless to try r e to improve it , owing to the impossibility of producing th e fraction without dispersion , Euler read a paper before Berlin Academy in 1 747 proving mathematically the pos sibility of correcting both the spherical and chromati c aberration of an Obj ect glass . Upon reading Klingen ’ ’ sti e r n a s paper corroborating Euler s views , John Dollond made a series of most valuable experiments which led him to the solution of the problem of the achromatic obj ect — glass namely, that by properly combining two kinds of glass , ﬂint and crown , he could unite the colored rays fairly

22 ASTRONOMY

o f ﬁ direction the telescope was practically de ned , and the micrometer was invented by William Gascoigne ( 1 61 2 ﬁlar which was the forerunner of the micrometer, so essential to modern astronomy , where an image at the focus of a telescope can be measured . The micrometer is indeed an important adj unct to the u telescope , for , unless ang lar distances can be measured , the mere bringing nearer of the celestial bod ies would mi r me have but a limited amount of usefulness . In the c o ter of William Gascoign e two pointers carried by a single screw were placed at the focus of a telescope . When these r evolv pointers were parallel they pointed to zero ; but, by i n n ; the screw, they could be separated and the number of revolutions or parts of a revolution could be read from a t o divided head . Consequently all that it was necessary know was the distance between two successive threads of the screw in order to obtain an exact value for any dis N ow e tance which the pointers might separate . if it wer desirable to determine the angular distance between two stars , each pointer was set on a star and the distance between them was thus gradually measured , so that by sim ple mathematics the corresponding angular distance could be computed . Micrometers soon became an important part of exact

Observation with a telescope . Auzout and Picard made subsequent improvements , so that finally a micrometer resulted in which a spider filament was placed on a frame i n moved by a screw with graduated head , thus enabling creased precision of observation to be obtained . This is the fundamental device now used with various improve t o ments and refinements . Roemer, who was the first determine the velocity of light, improved the micrometer in 1 672 by adding springs to take up the lost motion . He also constructed the first meridian telescope in 1 68 9. By the middle of the seventeenth century the use of telescopic sights for determining the position of the stars had become

established . The precision of the observations of that METHOD S OF OB SERVATIO N 2 3 e 1 0 o f poch may be estimated at seconds arc, which corre sp on ds to the diameter o f a lead pencil seen at a distance

Of about 5 50 feet .

Methods and instruments continued to improve . The observations of Lalande attained a limit o f precision of one second of arc , corresponding to a pencil at feet . At the beginning o f the nineteenth century great improve 1 ments were made . In 875 the limit of precision had been - reduced to one half a second , which removes the lead pen 2 cil to feet or more than miles . For minute measurements one of the most useful devices O has been the heliometer or divided bj ect glass micrometer, the ﬁrst really available type of which was constructed by n i s r Fraunhofer for the observatory at KO g be g . In this instrument an obj ect glass or lens is used which is divided along its diameter . The two parts of the glass are mounted so that they can be moved laterally with respect to each other . Consequently each half supplies a distinct image of the same obj ect, but separated by a strictly measurable amount . Thus , if a double star is under examination , the ‘ two half lenses into which the obj ect glass is divided can be moved until the upper star in one image is brought into i am e coincidence with the other star in the lower g , so that the distance apart becomes known by the amount of motion employed . By using screws with heads of considerable size to move the halves of the obj ect glass , the heliometer can be read to the thousandth part of a revolution , and KOn i sbe r in the case of the g g instrument such a division , equivalent to of a second of arc , could be measured with accuracy . This new instrument, which was not 1 8 2 F r aun mounted until 9, three years after the death of hofer, was at once employed by Bessel to solve the problem o f the star distances . His measurement of the parallax of “ ” 6 1 star known as Cygni , corresponding with a distance 600 about times that of the Earth from the Sun , not only n was co sidered ascertained beyond question , but is spoken “ of by Miss Clerke as memorable as the ﬁrst published 2 4 ASTRONOMY instance o f the fathom line so industriously thrown into celestial space having really and indubitably touched bot ” 1 8 tom . In 74 th e heliometer was applied to the observa 1 8 tion of the transit o f Venus, and again in 77 , when Mars came into Opposition with the Sun , Sir David Gill , using the heliometer, made a valuable determination of the solar parallax , Obtaining a value of seconds , corresponding with a distance of miles . By this time the heli ome ter had become an accepted method for improving as ’ f ’ tronomers knowledge o the Sun s distance . A number of heliometers were employed in coOper a ti on at diff erent ’ points of the Earth s surface, the work of Professor Elkins at Yale in connection with Sir David Gill at the Cape of

Good Hope Observatory being especially notable . Another modern development of t elesc0 pi c astronomy has been the direct measurement of the magnitude and th e brightness of a star, thus superseding to a great degree j udgment of the eye upon which the older astronomers had depended from the days of Hipparchus . The photometers e ( light measurers ) used with .telescopes for this purpos consist either of those designed to cut down the amount of light furnished by a measurable amount and thus cause so the star to disappear, or those in which conditions are arranged that the light of the star appears j ust equal to so- d some standard light . Under the ﬁrst head is the calle “ ’ ” s cat s eye , in which a wedge of dark neutral tinted glas is placed close to the eye , either at the eyeball of the eye piece O r at the principal focus where the micrometer wire s are usually placed . As the wedge is introduced until the star j ust disappears the graduation is read , which gradu ation can be reduced to a scale of magnitudes . In the other class of photometers the light O f the star is compared with an arbitrary artiﬁcial star formed by light from a n oil lamp shining through a small aperture . To Huygens is due the application in 1 655 o f the pen dulum to the practical measurement of time , thus giving us a clock so regulated that it was possible to make ac METHODS O F O B SERVATION 2 5

curate time observations . The invention of the pendulum 1 6 clock , patented in 57 , therefore marks a distinct epoch in astronomy . The most usual and most useful form of mounting for a i s telescope is the equatorial , the principal axis of which inclined at an angle equal to the latitude of the Observatory and is directed toward the North Pole in the Northern Hemisphere and toward the South Pole in the Southern

Hemisphere . The axis of the instrument is thus parallel to the Earth ’ s axis of rotation and is there fore called the polar axis . It carries a graduated circle which is parallel m to the celestial equator, known as the hour circle , fro which circle may be read the hour angle of the body U pon which the telescope happens to be pointed . The polar axis also carries the bearings of the declination axis , which is perpendicular to the polar axis and carries the telescope itself and the declination Circle . When the equatorial tele scope is directed toward a star or a planet it is necessary only to use clock—work machinery to cause the polar axis of the instrument to turn with a uniform motion in order to follow any star or planet which otherwise would soon be carried out of the ﬁeld of view by the rotation of the Earth . The equatorial also enables the Observer to look at once to a particular part of the heavens where a given body i s expected to be at a given time . The mounting for a modern equatorial telescope requires large and heavy moving parts . Where a solar or stellar image is desired it does not seem desirable to employ such 1 868 a heavy mechanism . To Léon Foucault, about , the idea occurred to construct a ﬁxed telescope with a mirror,

- moving with one half the angular velocity of the Sun , deﬂecting a beam in a ﬁxed direction . Such an instrument was constructed and was employed with good results , altho its operation was marred by imperfections in its driving mechanism . However, the device did not attract much attention until plans were made in the eclipse expedition of 1 890 for extensive photographs o f the phenomenon . It 2 6 ASTRONOMY was proposed to use the instrument in connection with a second mirror to produce an image which would not move . “ ” This device , now called a coelostat, was found admirable for eclipse photography . Experiments were made at the Yerkes Observatory to construct such an instrument for solar work . The work was subsequently transferred to the

Carnegie Institution Observatory on Mount Wilson . An e xtension of the same principle may be found in the tower t o f o elescope that institution , where a c elostat is mounted on the top of a skeleton tower and a beam of light is r e flected — to a laboratory beneath . To day the most modern a n d e fli ci en t reflecting telescope of large size is the 1 00 inch instrument designed for the same observatory by

Prof . G . W . Ritchey . At the present time both refracting and reﬂecting tele s copes are in use and have been brought to a great degree o f perfection . Just which is the better it would be hard to l say . The O d speculum metal reﬂector has been almost dis c arded and glass , coated with silver, has been substituted .

The glass is much superior to the metal , as it can be ﬁgured m ore accurately , and i f tarnished the silver can be removed without changing the ﬁgure of the mirror .

Again , much study has been given in France to a form of telescope known as the equatorial coudé , in which the o ptical axis of the telescope is parallel to the axis of the Earth and the light of the star is reﬂected into it by two mi rrors . Such an instrument , constructed for the Pari s

Observatory , has been very convenient for the astronomer, who can sit in his chair and observe the stars as easily as defin i h e can use his microscope . But the loss of light and o f tion by the double reflection , as well as the deflection the mirrors and the varying temperatures to which the ff di erent parts of the instrument are subj ected, render this construction far from perfect . CHAPTER III

T H E EVOLUTION OF ASTRONOM I CAL INSTRUM ENTS A N D — — M ET HODS T H E RISE OF ASTROPHYSICS T H E SPECTROSCOPE A N D I TS MODI F ICAT I ONS

To GA I N a knowledge o f the composition and nature O f th e celestial bodies i s the fundamental problem of astron o m m y. Unable to bring a celestial body or a specimen fro l it , except in the rare case o f a meteorite , to the chemica laboratory for study , the astronomer is dependent entirely on a study of the energy that it emits in the form of light h and heat rays . Strange as it may seem , these rays furnis as true a record of their birth a n d li fe history as if a sam ple from the distant star had been tested in the assay fur e nace or with the reagents of the chemist . The simpl instrument called a spectroscope gives an accurate an d permanent record which affords complete data for th e studies o f the astronomer . White light i s composed o f various forms o f vibration s which , taken by themselves , will supply light of variou colors from red to violet . It was found by Sir Isaac New ton in passing a beam of white light through a prism that not all of the rays are bent equally toward and away from t he perpendicular , but that the amount of bending de o r pended upon their color , as it is now termed , their wav e length and position in the solar spectrum . Thus , when h e permitted a beam of white light , emerging from a hole i n a shutter , to fall upon a prism in a dark room , he found that there was produced after its emergence a brightly col 3 7 2 8 ASTRONOMY o red band with the red at one end , where the waves were w refracted the least , and shading through yello and green to violet, where the waves were bent or refracted most .

Consequently , i f there were a source of light capable o f furnishing one color and that only , it would be obvious , w herever that color appears in the bright band produced by the prism , that it radiated from a particular source . 1 M elvi ll Before 753 a young Scotchman , Thomas , no ticed that when various compounds of sodium were intro d uced into an alcohol ﬂame and viewed through a prism

Th e P r i s m

— D I PE I O LI T T H E I S RS N GH BY PR SM .

there appeared a particular shade of yellow light , which was always bent or refracted to a ﬁxed and deﬁnite de

. y gree Others repeated these experiments , and ﬁnall 1 8 r e Fraunhofer ( 7 7 a great Optici an of Munich , d iscovered this deep yellow ray and found its place in the e sp ctrum . The same phenomenon was noticed by many

o . h ther experimenters Indeed , the omnipresence of t e y ellow light was often an embarrassment in spectral r e s earch . That this yellow line was due to sodium was pointed out by William Swan . Finally , it was noted that the distribution O f sodium was so general and the prism test o f its presence so delicate that its absolute exclu s ion was well nigh impossible .

3 0 ASTRONOMY

ui sh ed g German physicist , Professor Gustav Robert Kir c h o ff ( 1 8 2 4 He it was who sent a beam of bright s unshine through sodium vapor and discovered that the “ ” ff D line o f Fraunhofer, instead of being e aced by the

ﬂame , was strengthened . The same held true with iron . T he inference was of course drawn that sodium and iron w ere constituents of the glowing atmosphere of the Sun a n d th at light of the particular wave length in passing t hrough such an atmosphere was absorbed . This principle has been formulated by Miss Clerke as “ follows : Substances of every kind are opaque to the pre — c ise rays which they emit at the same temperature that i s to say , they stop the kinds of light or heat which they a r e then actually in a condition to radiate . But it does not

follow that co ol ~ bo di es absorb the rays which they would f g ive out i f su ﬁciently heated . Hydrogen at ordinary tem e r a tur e p , for instance , i s almost perfectly transparent , but i f raised to the glowing point—as by the passage of elec t r i ci t —i t y then becomes capable of arresting, and at the

same time of displaying in its own spectra , light of four d istinct colors . In these few words we have the essence of s ectr osc0 i c p p chemistry and astrophysics . Materials of t h e Earth when heated to incandescence give a bright line s pectrum characteristic of the individual element , but the same materials in the Sun show a spectrum marked by d ark lines . While spectrum analysis was applied to chemistry and f terrestrial materials by Bunsen , Kircho f worked industri o usly and made a map of the solar spectrum some eight

feet in length , in which the various lines of the ele m entary bodies were represented . The spectroscope , a s ff constructed by Kircho , consisted of a slit placed a t the principal focus of the convex lens to make

the rays parallel for thei r passage through the prism .

In order to secure greater dispersion , several prisms were added and the emerging beam was passed through

t h e telescope to form an image of the spectrum . The RI SE O F ASTROPHYS ICS 3 1 n ew i nstrument at once presented an enormous num s ber o f lines for study , not only o f the Sun , but of variou d ob ser v other celestial bo ies . It was soon applied to the ing end of an astronomical telescope , so that the celestial e image was formed directly at the slit of the sp ctroscope . By increasing the number o f prisms the dispersion o f the spectroscope can be increased , and a longer spectral band produced , in which otherwise closely adj acent lines a are increasingly separated . But in passing through number of prisms there is considerable loss o f light by

— i f i i n e i e o f . e d t o a d o ce i . S t o c o at t e co d ct e w F g 5 , sl ll m g l s p r r s ur li ht e n o f c o i a ti n t e e c o e t o e n d e a a e a g , l s ll m g l s p r r p r ll l r ys i i i n s h ch n d L . e f o S . P . B . b a e on w t a r m ; , pr sm ; , s pr sm s s ; , l .

o f O b e i n t e e co e E . e e e c e o f ob e i n t e e co e . s rv g l s p ; , y pi s rv g l s p

s et th e reflection and absorption , so that a limit is soon to number of prisms employed . Another form of spectro w scope, hich makes use of a grating or a number of fine lines ruled very closely together on a transparent or a reflecting surface , has been found to possess greater dis

solving power without any accompanying loss of light . s In fact, the resolving power of a perfect grating depend so simply upon the total number o f lines it contains , that the light eﬂi ci en cy per unit area may be as great for a

large grating as for a small one . The principle o f the grating depends upon the i n te r fer ence of th e various minute light waves caused by a series

o f . lines , amounting to from ten to twenty thousand to the i Th e nch , ruled on a transparent or a reflecting surface . 3 2 ASTRONOMY mathematical discussion of the formation of the spectra by the interference of the light waves in passing through or being reflected from such a grating can hardly find a place here . The result , however , is essentially the same as in the case o f the prism . As soon as the dispersion of light was obtained by this means it was found that it could be stud i ed quantitatively, and that the grating could be used for

’ i — T F g . 6 KI RCH O FF S SP E C RO SCO P E . a stronomical measurements with as great facility as the

prism spectroscope .

Lewis M . Rutherfurd of New York was able to make ex c ellen t 1 86 gratings about 4, but it remained for Professor

- Henry A . Rowland ( 1 848 1 90 1 ) at the Johns Hopkins

University to construct a dividing engine with a screw ,

practically free from error, which would move a small plate of polished speculum metal by regular intervals of 7 1 5 000 of an inch under a diamond point which traced sharp

and regular lines . This machine not only was remarkably RI SE OF A STROPHYSICS 33

sensitive in its action , but automatically compensated for any minute irregularities in the screw . It was made to work at a constant temperature . It automatically pro c eeded with its ruling night and day until a grating o f the desired length was completed . Professor Rowland for the first time ruled gratings on concave surfaces and used them in place of the prism o f the ordinary spectroscope . The Spectra obtained with these diffraction gratings in conj unction with special lenses were many feet in length and could be photographed in sections on photographi c 2 0 plates , each o f about inches in length . The grating spectroscope has been modified by Pro fess r o . . A A Michelson o f the University of Chicago , who has devised a new form of grating in which a series o f glass plates precisely equal in thickness are placed one on another like a flight of steps . A parallel beam o f light when transmitted through them i s resolved into spectra o f ’ a very high order , exceeding even those of Rowland s largest gratings , so that compound lines in the spectrum can be studied with facility . The application of the spectroscope has made of as t r on om i n str u y an experimental science, with methods and ments for research and future progress fully as promising as may be found in any of the physical or natural sci m on o e ees . The spectroscope has not only amplified a str n ew my , but it has developed the science o f astrophysics , in which astronomy is combined with physics . New meth ods an d instruments for research already have brought to light striking discoveries which have compelled the modi fi c ati on o f older astronomical and cosmical theories . In connection with the spectroscope it i s possible to measure the temperature of the radiations sent out from th e Sun and the stars with a high degree o f accuracy by means o f the bolometer , a sensitive thermometer , invented fin . . e by Professor S P . Langley It consists o f two very threads of platin um wire about o f an inch in thick n e ess , mounted side by side within a constant temperatur 34 ASTRONOMY

chamber . On one of these wires the radiation is permit ted to fall , while the other is carefully shielded . Any change in the temperature of the wire on which the light or heat waves fall produces a di fference in its electrical r e si sta n c e that can be measured with a high degree of pre c i s i on ff - , so that a di erence of less than one millionth of a degree in the temperature can be clearly indicated . The spectrum formed by the spectroscope is caused to move slowly across the exposed platinum wire of the bolometer and a galvanometer in the circuit reflects from so its mirror a Spot of light upon a photographic plate , that the deflections o f the magnetic needle are photo graphed and registered , thus indicating the intensity and f i n energy at the di ferent parts of the spectrum . This strument was first used by Langley to determine the amount o f heat received from the Sun on the top of Mount 1 88 1 Whitney in , and since that time it has been employed by him and his successors at the Astrophysical Laboratory m at Washington and also at Mount Wilson . The proble that the bolometer seems capable o f solving is to deter mine the atmospheric absorption of light and heat in the ’ n passage of the Sun s rays to the Earth . It has also bee used to measure the heat spectrum of the Moon and som e of the brighter stars ; in the case of the former showing that the Moon is very cold , as there is a considerable quan tity of heat radiated having a wave length greater than that of the heat radiated from a block o f ice . After the fundamental work o f Kirchoff in identi fying the Spectral lines of the Sun and the stars with variou s terrestrial materials it was but natural that the compo s iti on of stars as shown in their spectra should be th o r oly wa s attacked by astronomers . Among the first of these a Sir William Huggins , who devoted the greater part of use ful scientific li fe to research of the heavenly bodies especially as revealed by the spectroscope .

1 862 h . In Huggins , Secc i and Lewis M Rutherfurd began their researches in stellar spectra that enabled them RISE O F ASTROPHYSICS 35 to classify and compare the spectral bands furnished by ff the di erent stars . It was the spectroscope in the hands of Sir William Huggins that made possible the solution of the ae riddle of the nebul , the nature of which for long years had been a vital point of discussion among astronomers . On 2 1 86 August 9, 4, directing his spectroscope to the planetary ae nebul in Draco , Huggins saw, instead of the bright band r he anticipated, a single line which subsequently was e solved into three lines . Thus he proved that the nebula was not an aggregation of stars or incandescent solid ma t e r i a ls t m , which would have afforded a continuous spec ru crossed by dark bands and a luminous gas . The effect of the spectroscope on astronomical research “ is thus summarized by Professor Hale : In astronomy the introduction of physical methods has revolutionized t he oh s er v ato r y, transforming it from a simple observing station into a laboratory . The interest of the student of a st r ophys i cs is no longer confined simply to celestial phenomena . a l For astrophysics has become, in its most modern aspect , th e d most an experimental science . in which some of fun a mental problems o f physics and chemistry may find thei r solution . The stars may be regarded as enormous crucibles , in some o f which terrestrial elements are subj ected to temperatures and pressures far transcending those obtain us able by artificial means . In the Sun , which appears to not merely as a point of light like the stars , but as a vast globe whose every detail can be studied in its relation -1 O f n ship to the general problem the solar constitutio , the immense scale of the phenomena always Open to observa t h e tion , rapidity of the changes and the enormous masses o f material involved provide the means for researches wh ich could never be undertaken in terrestrial labora tories . Hence it is that astrophysics may equally well be regarded as a branch of physics or as a branch of a str on o ~ ” my . The great advantage of the spectroscope over the eye or t he direct image from the photographic plate is its ability 36 ASTRONOMY

to analyz e the action of light . While the intensity of light ff su ers in its j ourney through Space , yet the nature or character o f the light undergoes practically no change , so that the light from a distant star , separated from the

Earth by an interval that seems to us almost inﬁnite , can be received in our spectroscope and be resolved into a spectral band with diﬂi culty but slightly greater than that which would be found in the employment o f a luminous source of light at the opposite end of the laboratory table .

The spectroscope , therefore, can be used in astronomy to determine the composition of a distant body according to the principles of spectrum analysis . But this is not all . It also enables the determination whether the light from a luminous body in the heavens i s approaching or reced ing, and whether the light emitted from such a body i s the

- same to day as it was yesterday or a half century ago , and whether it comes from one or more bodies which the eye and perhaps the telescope cannot separate , but which to o are distinctly separate . Hence the astronomer is only glad to remove the eyepiece from his telescope and put in its place some Sp ect r O S CO pi c device which will analyz e the light into separate colors and give him much valuable information as to the constitution and motions of even the most distant star . The value of the spectroscope is greatly increased by the application of photography . The general nature of a spectroscopic investigation can best be indicated by ab s tr a cti n g from Professor Hale his description o f solar “ spectrum analysis : Sunlight must be reflected from a mirror to a heliostat ( driven by clockwork , to maintain w the beam in a fixed direction ) to the slit . Bet een the sli t and the heliostat a lens i s introduced , for the purpose o f n formi g an image of the Sun upon the slit . When the illu t mination is secured in his way , the whole grating is filled with light from the diverging rays . The grating then pro duces an image of the solar Spectrum upon the photo

38 ASTRONOMY

drawing deﬁnite conclusions . The matter is usually de t er mi n ed by ascertaining whether certain well - known groups o f lines , which for various reasons are considered to be especially characteristic of an element , are actually

. co represented I f these groups are absent , an apparent incidence with certain less characteristic lines belonging to the same element should be regarded with suspicion . In f the case o f gases , the comparison is e fected by the aid ' o f vacuum tubes , in which the gas , usually at low pressure , s i s illuminated by an electric discharge . Thus the line given by a hydrogen tube in the laboratory have been shown to coincide in position with lines ascribed to hy

‘ drogen in the Sun . “After many years of study of the solar spectrum by these methods Rowland reached the conclusion that th e chemical composition of the Sun closely resembles that of the Earth . Certain elements , such as gold and radium , iodine, sulphur and phosphorus , chlorine and nitrogen , have not been detected in the Sun . But this does not th e prove that they are certainly absent, as their level in ! solar atmosphere , or the low degree of their absorptive ff O n e ects , might prevent them from being represented . th e the other hand, various substances not yet found on Earth are shown by many unidentiﬁed lines o f the solar spectrum to be present in the Sun . Some i f not all o f a s these will probably be discovered by chemists , j ust ite t e helium was found by Ramsay in clev . Rowland marked that i f the Earth were heated to a suﬂi ci en tly high temperature it would give a spectrum closely resembling ” that of the Sun . CHAPTER IV

T H E E TI OF A TR I A I TR E TS A ND VOLU ON — S ONOM C L NS UM N M ET HODS C ELESTI AL PHOTOGR APHY

F TE t A R the spectroscope , photography has been the mos u seful tool of the astronomer, and to its aid must be cred i ted some of the most important work of the latter half o f l the nineteenth century . For the development of celestia photography as outlined in the present chapter an interest ing paper by Professor E . E . Barnard supplies in large a part the material . According to Professor Barnard , the p plication of photography to astronomy may be said to date ’ from the very ﬁrst announcement O f Daguerre s wonderful discovery of the production of a permanent image by the f “ e fect of light upon silver salts . The celebrated French astronomer, Arago , quickly foresaw its great possibilities, especially in the faithful delineation O f the surface features o f the Sun and the Moon , for these two obj ects at least were bright enough to register themselves with the slug ” gish materials then in use . It was of course obvious to as t r on omer s and physicists that the formation of an image on a sensitized plate was in no way different from that produced at its focus by the telescope lens and that the image of a celestial body could be produced as well as any other .

It was from America that the ﬁrst practical work came, and “within less than one year from the announcement of ’ 1 8 0 Daguerre s discovery , in March , 4 , D r . John W . Dra p er O f New Yo r k city succeeded in getting pictures o f the 39 40 ASTRONOMY

‘ Moon which , though not very good, foreshadowed the possibilities of lunar photography . Five years later , at the

Harvard College Observatory , Bond, with the aid o f

. h Messrs Whipple and Black , of Boston , succeeded in O taining still better pictures of the Moon with the 1 5 - inch refractor . These pictures on daguerreotype plates aroused ifﬁ . d great interest, especially in England However, the culti e s encountered led to failures generally, except in the o f case De la Rue , Dancer and one or two others . In 1 8 8 1 - r eﬂect 5 De la Rue , using a 3 inch metal Speculum ing telescope , without clockwork , and guiding it by follow ing a lunar crater seen through a plate, made the most ” f H i s important of the early e forts at lunar photography . photographs were the best until those made in America,

1 860 . in , by Dr Henry Draper, son of the illustrious John

W . Draper . He secured excellent photographs of the c Moon , superior to any previously made , and apable of considerable enlargement . These pictures were the best e taken until Lewis M . Rutherfurd began his remarkabl work about 1 865 . His admirable photographs of the Moon 1 1 - were made with a refractor of inch aperture, which , t constructed under his immediate supervision , was the ﬁrs telescope corrected especially for the photographic rays . “ The completion of the Lick Observatory in 1 888 marked another decided advance in astronomical photography , es p eci ally of the Moon . The great focal length of this mag n i ﬁc en t instrument gave an unenlarged I mage o f the Moon In about six inches in diameter , which itself was a great ” advantage . Good results were also secured with the

Yerkes refractor . a Admirable lun r photographs have been made by MM .

Loewy and Puiseux , with the equatorial coudé , at Paris, and have shown the usefulness of this singular instrument r fo such work . “ The ﬁrst picture o f the Sun seems to have been made on a daguerreotype plate by Fizeau and Foucault in “ o f says Professor Barnard . During the total eclipse T H E G E T N E B I N I O R A ULA OR N .

CELESTIAL PHOTOGRAPHY 41

2 8 1 8 1 de ue r r eot e the Sun on July , 5 , a g yp was secured with the Konigsberg heliometer inches in diameter 2 and feet focus ) by Dr . Busch , which appears to have; been the ﬁrst photographic representation of the corona . ” It showed considerable detail quite close to the Moon . But in the early eclipses photographic work seems to have been devoted mainly to representations of the so a s lar prominences , which at that time were as rarely seen “ 1 86 the corona itself . During the eclipse of 9, however, Professor Himes secured a photograph which showed the f brighter structure O the corona . Similar pictures were i also Obtained dur ng the same eclipse by Mr . Whipple, o f

Boston . The corona was also slightly shown on picture s 60 1 8 . . made as early as by M Serrat None of them , how ever, showed more than slight traces of the corona , ex ’ s tending only for a few minutes of arc from the Moon , limb . Nearly all the pictures seem to have been taken h wit an enlarging lens , which was doubtless used to get ” the prominences on a larger scale . “ The ﬁrst really successful photographs of the corona 2 2 1 8 0 i t were obtained at the eclipse of December , 7 , when was shown on the plate to a distance o f about half a de ’

o . gree from the Mo n s limb This picture , made by Mr . b e B rothers , at Syracuse , Sicily , showed a considera l amount of rich det ail in the coronal structure ; the sam e can also be said O f the photographs of this eclipse taken by i ’ m Colonel Tennant and Lord L ndsay s party . These see to have been the first pictures that really showed the great e value o f photography for coronal delineation . The eclips 1 8 1 o f 7 was still more successfully photographed, and an t o ? excellen representation of the cor na , full o f beauti ful ” detail , was secured . “ In 1 878 extensive preparations were made to observe 2 the eclipse of July 9 of that year . Photography was to play u s v er v an important part , tho gh a tronomers did not rely strongly upon it ; for it appears that all were prepared to U n for tu make the customary drawings of the corona . n ate l y each person faithfully carried out that purpose . A most suggestive illustration of the uncertainty of such w ork is found in the large collection of drawings pub li sh ed in a volume issued by the United States Govern 1 8 8 ment relating to the eclipse of 7 . An examination o f t hese forty or fifty pictures shows that scarcely any o f them would be supposed to represent the same obj ect , and none o f them at all closely resembled the photographs . The method of free - hand drawing of the corona made un d er the attending conditions of a total eclipse received i ts death blow at that time , for it showed the utter inability of the average astronomer to sketch or draw what he really ” saw under such circumstances . 1 882 1 886 1 88 h oto r a h t In the eclipses of , and 9 p g p y played a part O f increasing importance in the Observa tions . In the latter year there were a large number of a mateur photographers who took advantage of the eclipse t o make many photographs , which , in a number of cases , were taken in a systematic and scienti fic manner . At the L ick Observatory a beginning was made in eclipse photog r aphy with an extemporized apparatus and successful ex o r e 1 8 6 p su s were made . During the eclipse of 9 important work was done in photographing the flash spectrum or the momentary reversal o f the F r a un ho fer lines which occurs when the edge of the . Sun disappears behind the Moon or reappears from it and for an instant exposes the revers ing layer , which was first seen by Professor Young at the 1 8 0 y eclipse o f 7 . This photograph was made by a oung

a. Englishman , William Shackleton , who , on exposing plate t at the critical instant of the reversal of the lines , caugh for the ﬁrst time the fugitive bright lines which are vis ible for only about a minute . This gave a permanent vis ible record of the phenomenon which removes it from the class of hasty visual observations , whose results depend u pon the memory of the observer . The photographing of such a minute point of light as a s tar is quite di fferent from that of a luminous or brilliant

44 ASTRO N OMY would be made to make use of them in a quantitative a n d systematic way . Previously , for the production of star m aps and catalogues , elaborate series of observations were m ade at the various O bservatories and the positions o f the s tars computed and incorporated in large volumes . At the R oyal Observatory at the Cape o f Good Hope Sir David 1 88 2 Gill , in , a fter making some pictures with a larget m c a era of the comet of that year, found that not only d i d but the plate Show the stars visible to the naked eye, m m A c a nu ber as s all as the ninth or tenth magnitude . c o r din gly it occurred to him that such photographs fur n i sh e d a novel and excellent method of cataloguing the s tars and mapping the heavens , as it was necessary only to m easure on the glass negatives the positions of the various s tars and refer them to certain well - known points of ref 1 88 1 8 1 e rence . From 7 to 9 the entire southern heavens ° from 1 8 south declination to the celestial pole were d uly photographed . The half million stars found on the n egatives were then measured and the magnitude of each d Ka te n etermined by Professor J . C . p y at the Univer s i t 1 8 y of Groningen , Holland . Thus in 99 was ﬁnished the ” Cape photographic Durchmusterung, which is published i n three quarto volumes and contains the magnitude and a pproximate position of every star photographed , the mag n i tud e o f the stars on each plate being reduced to a visual s cale . At the time when Si r David Gill began his photographic “ w . ork, Dr Barnard states , the Henry brothers of Paris were making a chart o f the stars along the ecliptic in their s earch for planetoids . They had at this time reached the region o f the Milky Way , and the marvelous wealth of stars they encountered on entering the b oun da r I e s of that vast zone completely discouraged them from carrying thei r charts through the rich region traversed by the ecliptic . While hesitating as to the advisability of continuing their work , the photographs of the great comet came to their n otice . They were struck with the great number of stars CELESTIAL PHOTOGRAPHY 45 shown on these pictures together with the image of th e n comet . The idea at o ce occurred to them that they could use this wonderful process to make their charts . They a began at once the construction , with their own hands , of suitable photographic telescope of 1 3% inches diameter for the photography of the stars . This instrument pro u d ced exquisite star pictures , which were marvels of a deﬁnition , as well as photographs of the nebul e, of Saturn ” and Jupiter, the Moon , etc . It was the success of the Henry brothers ’ work that

- led to the International Astro Photographic Congress , 1 which met at Paris in 886. This Congress undertook th e organization o f an International Commission engaged i n the preparation o f a photographic chart and catalogue o f the heavens , and the work since that time has been actively in progress . Uni form instruments o f the same apertur e and focal length are used at the eighteen observatorie s participating in this work and two sets of plates are being b e made , one to include all the stars that are capable of ing photographed and the other one those of the eleventh magnitude . With this photographic map astronomers s anywhere can compile their own catalogues , and portion of such catalogues by various national observatories have t already been issued . The method of preparing the char consists in photographing the whole sky upon glass plate s 8 about inches square . Each observatory has had assigned o f to it definitely its part of the sky , and about plates the size specified will be required to complete the task . Each plate of course carries one or more well determined so catalogue stars , whose position is known with accuracy , that from such points of reference it is possible to deter mine exactly the position of any other star on the plate . “ The photographic plate not only did away with the n e ce ssit y of making the star charts by eye and hand , so essen did tial to facilitate the discovery of planetoids , but it also away with the necessity of the charts themselves for that th e purpose . The little planet, which is moving among 46 ASTRONOMY s tars , now registers its own discovery by leaving a shor t — — trail its path during the exposure on the photographic plate . The first of these photographic discoveries of

. 1 8 2 ob planetoids was made by Dr Max Wolf in 9 , and his s er vato r y at Heidelberg subsequently became a h eadquar ters for discoveries of this kind . Planetoids are now found w m ” holesale in this anner by photography . In the early days of photography nebulae were considered t h e most unpromising subj ect for the photographic plate to deal with . Most of these obj ects appeared so faint that but little encouragement in that direction was offered the celestial photographer . “ One of the brightest and most promising of nebulae is o f that in the sword of Orion , and this was naturally one the ﬁrst of these obj ects to receive photographic attention . I n 1 88 0 September , , D r . Henry Draper began photo a 1 graphing nebul e with this obj ect , and succeeded , with 5 m inutes exposure , in getting a good picture of the brighter p ortions on dry plates . This was the ﬁrst nebular photo g raph . It was followed by other photographs , one of which showed stars down to the magnitude which w ere visually beyond the reach of the same telescope . T hese pictures marked a new era in the study of n a ebul e. When the results were communicated to the

F . rench Academy by Dr Draper , Janssen took up the w ork with a reflecting telescope having a silver- on - glass m 1 8 0 irror of very short focus , constructed in 7 for the 1 1 total solar eclipse of 87 . With this Janssen found it easy to photograph the brightest parts of a nebula with comparatively short exposures . Un fortunately for 1 88 2 s . cience, the death of Dr Draper , in , put a stop in A' merica to the work he had inaugurated, but it was at o - nce taken up in England by Common , who , with a 3 foot r eflector , attained rapid and immediate success . His pho r a h tog p s of the great nebula of Orion are still classic . T hey were a great advance over the work o f Draper, for t h e reflector was not only a larger telescope, but was CELESTIAL PHOTOGRAPHY 47

also better adapted for photographic purposes, and espe i ll n c a ae. see y for photographi g nebul In fact, as we shall a in a later chapter on nebul e, much of the progress in their ” study has been due to photography . The photography of nebulae was carried on with remarkable success at Lick Observatory during the i n n c umbe cy of Professor James E . Keeler as Director . th Using the Crossley reﬂecting telescope , presented to e

Observatory by Dr . Common , he made a photographi c ae study o f nebul , and reached the conclusion that there are at least of the spiral type within the range o f s this instrument . Professor Perrine , who succeeded to thi work on the death of Professor Keeler, believes that hal f a million is nearer the ﬁgure , and that with more sensitive photogr aphic plates and longer exposures the number of spirals would exceed a million . b Not only stellar motion , but stellar distances , can e

. O x measured by photography Professor Pritchard , at ford, has used the sensitive plate to sound the celes h tial depths . His ﬁrst experiments were undertaken wit 6 1 2 00 the star Cygni , and by measuring negatives which had been made in 1 886 he derived for that star a parallax ’ o f which was in satisfactory agreement with Ball s value o f This work was subj ected to detailed at scrutiny , and the Astronomer Royal was convinced th ’ ob it was more accurate than that of Bessel s results , tain ed o f with the heliometer . This was the beginning the method o f measuring a parallax from photographi c Ka 1 h plates . Professor pt eyn showed in 889 that from suc be plates , exposed at desired intervals , parallaxes could 1 00 o a derived wholesale . He applied his system in 9 t 2 8 i . t group of 4 stars with encouraging success In fact , was suggested that a photographic parallax Dur chmus ” terung should be undertaken after the completion of th e a a str ogr aphi c l chart of the heavens . When used in connection with the sp ect r o sc op e th e photographic plate has a ﬁeld singularly suited to display 48 ASTRONOMY

a n i ts possibilities . Here it deals not alone with what c be seen , but it enters into regions where the eye takes no ’ cognizance of things . For tho it is partly b ind to the light f which a fects the eye , it can readily penetrate the regions where man , in turn , is blind . By special treatment of the plate photography registers those rays invisible to the eye a n d permits their accurate measurement . s ec to r a h a The p g p , or combination of photographic p paratus with spectroscope , must be so arranged as to show with distinctness the greatest number of lines , the individual lines being separated ; consequently there are various types of spectrograph , depending upon the pur pose for which they are to be employed . One of the combinations of the spectroscope with the photographic apparatus is found in Professor Hale ’ s spec t r oh eli o r a h g p , which consists of a spectroscope across whose slit the solar image moves at a uniform speed . In s tead o f the eyepiece there is a second slit which permits light from only a single line to pass and fall on the moving p hotographic plate, so that an image of the Sun , or a s un spot in light of a single wave length , can be made to fall upon the plate and thus be recorded . The general eff ect of photography in astronomy may b e summarized in the brief statement that it has removed the astronomer from the eyepiece of the telescope and h a s substituted the more sensitive photographic plate with “ - i t s permanent record . Hence it is that the present day s tudent of astrophysics does not correspond with the the traditional idea of astronomer , says Professor Hale . “ His work at the telescope is largely conﬁned to such t asks as keeping a star at the precise intersection of two - cross hairs , or on the narrow slit of a spectrograph , in o a be rder that stars and nebul e, or their spectra , may sharply recorded upon the photographic plate . His most o a r interesting work is d ne , and most of his discoveries e made , when the plates have been developed and are sub ” ected j to long study under the microscope . CHAPTER V

T H E LA W OF GRAVITATION

WH EN the astronomers of old tried to account for the apparent motions of the heavenly bodies by complete sys s tems of epicycles , they must surely have asked themselve , Why do the planets move so regularly ? What makes them ? e move thus I f they did, they troubled themselves but littl to answer the inquiries in anything but a perfunctory way. For the most part they were content to regard the stars as the playthings of divinity , and the cause of their motions , therefore , as a mystery forever veiled to human eyes . of Still one astronomer, Anaxagoras , did have some idea a force which holds the planets in their orbits and which might be the same as that which Operates upon substances f at the surface o the Earth . - 2 B n ot After his day (499 4 7 C . ) the idea seems to have been expressed by any one until the awakening of science in the seventeenth century . Then Kepler darkly of hinted at some attractive force , because his discovery the mathematical curve described by the planets seemed to demand the existence of some constantly exerted con ’ ‘ trolling force and also because he had read Gilbert s De ’ Magnete, in which he was made acquainted with the phe n om n e a of electrical attraction . Such a force as he had in mind would act to maintain the motion of the planets and to drive them along in their S o f orbits . But this was hardly the olution the problem , f f t since as Galileo found, the motion o a body o itself mus 49 5 0 ASTRONOMY

continue indefinitely , unless there is some cause at work to alter or stop it . This formed the first and most impor tant of the laws of motion which , if not independently dis t covered by Newton , were subsequently o be stated by him with greater force and conciseness . The laws were of primary importance , because they afforded a new and correct way of considering not only the underlying reasons for the motions of the planets but of all mechanical prob lems involving matter and motion . Aside from the three great laws o f planetary motion established by Kepler as the result of many observations , n was the most important lesson taught by him , and o e that of th e readily learned by Newton , was that the motions planets were not to be attributed to the influence of mere old geometrical points , such as the centers of the epicycles , but to the actual presence of other bodies . Kepler sug gested , in particular, that the planets might be considered as connected with the Sun and therefore as sharing to ’ some extent the Sun s motion of revolution . From the Sun emanated that special kind of influence which he as sumed . Yet, while Kepler considered the Sun as the source of this hypothetical force , he believed in a more general gravity or attraction between bodies . to it He was unfortunate enough , however, conceive of diminishing simply in proportion to the distance between two as the bodies , a mathematical impossibility, was dem tr on s ated by Newton . This is the more surprising as he had demonstrated that the intensity o f light was r ecipr o cally proportional to the surface over which it was Spread and that it varied inversely as the square of the distance

from the luminous body. It was also unfortunate that , ’ while Kepler s ideas o f the nature of gravity were sound and accurate in many respects , they bore no particular logi cal connection either on e with another o r with his theory of of t planetary motion . They are , however, worthy commen as indicating the situation before Newton took these and other spe ci ulative ideas and the three isolated laws of

5 2 A STRONOMY

never brought forward convincing proof o f his claims. S o f far as Newton is concerned , the great merit o his work lay n ot so much in conceiving the law of gravitation as i n hi s brilliant demonstration of its truth . ’ h e Starting with Kepler s laws of planetary motion , n ot Showed only that they were true , which was hardly a task of merit after Kepler had considered the observation s o f Tycho Brahe and all other astronomers whose recorded but observations would throw any light on the subj ect , d why these laws were true , and why no other laws coul have accounted for the conditions actually observed in the motion of the planets . And , furthermore , underlying these famous planetary laws he discovered must be the attraction f o gravitation . By a mathematical analysis unrivaled in the history of astronomy he proved his theorem com letel p y. Not only did he suggest, as did Kepler, that the o f power attraction resided in the Sun , but he proved mathematically that as a necessary consequence of that attraction every planet must revolve in an elliptical orbit on e around the Sun , having the Sun as focus ; that the ’ radius of the planet s orbit must sweep over equal areas in equal times and that in comparing the movements o f two planets it is necessary that the squares of the periodic times be proportional to the cubes of the mean distances . These facts were discovered by Kepler ; they were ex o f plained by Newton , with the aid the powerful and cele Th b r ated mathematical reasoning which he had created . e explanation was the law of gravitation . It occurred to Newton that if a diagram of the path of the Moon for any given period , say one minute , be made , it would be found that the Moon departs from a straight line during that period by a measurable distance . In other words , the Moon has been virtually pulled toward the Earth by an amount that is represented by the difference between its actual position at the end o f the minute and the

‘ position it would occupy had it moved in a straight line, ’ fol which according to Galileo s law of motion , it should TH E LA W O F GRAVITATION 53 l o w unless some external force deﬂected it . By measuring

o f . d eflec ti on m the amount , he had a basis for deter ining t h e amount of the deflecting force . This deflection New t ton found by his calculation to be thir een feet . Obviously the force that acted on the Moon made it fall toward the Earth a distance of thirteen feet during the first minute o f its fall . ’ Galileo had shown that the rapidity of a body s fall to the Earth increased at a uniform rate— what is now termed the acceleration of gravity . In other words , the higher the starting point of the fall , the greater will be the final v t elocity . Hence the amoun of the attracting force is in s ome way related to the distance between the two bodies , a relatio n which Newton expressed by stating that the falling body is attracted to the Earth by a force which varies i h r l v e se y as the square of the distance between them . I f the attracting force then varies inversely as the square o f the distance , would the Moon drop toward the Earth at t h e calculated rate of thirteen feet in the first minute ?

That was the problem which presented itself to Newton .

The mathematical solution was simple , based as it was on ’ a comparison of the Moon s distance with the length of the ’ Earth s radius . Unfortunately there were no accurate di m en si on s of the Earth available when Newton made his s 1 f 666. o ﬁr t calculation in Hence he found , on the basis to the erroneous data at his disposal , that the Moon fell ward the Earth ﬁfteen instead o f thirteen feet during the

first minute , a discrepancy so great that he dismissed the m atter from his mind . When in 1 68 2 his attention was called to a new and ap ’ p a r en tly accurate measurement of a degree of the Earth s at meridian made by the French astronomer Picard , he tacked the problem anew . As he proceeded with his com putation it became more and more certain that this time t h com e result harmonized with the Observed facts . So p letely was he overwhelmed that he was forced to ask a friend to complete the simple calculation . When the com 54 ASTRONOMY putation was ended it was known that the force which causes bodies to fall to the Earth extends outward to the s Moon , and that by reason of this force the Moon circle around the Earth . It required but a slight stretch o f the imagination to as sume that a force which can span the distance between the Earth and the Moon may also span the distance from the Sun to the Earth and the other members of the solar sys tem . That such is really the case , Newton proved by a ’ mathematical calculation of the orbital motions of Jupiter s satellites and of the various planets . These discoveries and fundamental principles enunciated by Newton were elaborated with great exactness in hi s ‘ ’ motIon S Principia , and the section which discusses the f of the Moon , confessedly one of the most di ficult problems in celestial mechanics , has been termed by Sir George Airy the greatest chapter on physical science of ever written . That it has stood the test time is demon ’ str ated by the fact that Newton s results have scarcely been extended in the centuries which have elapsed since thei r publication . The entire work is a marvel of exact mathe “ mati cal reasoning by the greatest genius the world h as ” ’ ever produced , according to Lagrange s estimate of New ’ ton s intellectual powers . Galileo had experimentally shown before Newton that the rate at which two bodies fall to the ground from equal s heights is independent of their weights . A mas of gold th e and a mass of lead , altho of unequal weight , reach Earth at the same time i f dropped simultaneously from the same height . Newton repeated the experiment very ex actl i s y. He realized as a result that weight ( gravitation ) constant . But because a pound of lead weighs less than two on e pounds of lead ( in other words , is attracted with h alf the force ) merely fo r the reason that it contains les s i s matter, he was forced to the conclusion that gravitation dependent upon quantity of matter as well as distance . Thus he introduced the very difli cult conception of mass as TH E LA W O F GRAVITATION 55 d u ex isting ished from weight, or the force of attraction r b o e ted . a s on it by the Earth The former, of course , is o f lute and constant, but the latter varies with the position ’ the material in question on the Earth s surface or else i n where the universe .

I f the mass o f Venus i s seven times that of Mars , then the force with which the Sun attracts Venus is seven times as great as that with which it would attract Mars if placed at the same distance ; and therefore also the force with which Venus attracts the Sun is seven times as great as that with which Mars would attract the Sun if at an equal distance

. r o from it Hence , in all cases of attraction , the force is p portional not only to the mass of the attracted body, but a lso to that of the attracting body as well as being inversely proportional to the square of the distance . Gravitation thus appears no longer as a property peculiar to the central body of a revolving system , but as belonging to a planet t o in j ust the same way as to the Sun , and to the Moon , or a stone in j ust the same way as to the Earth .

Moreover, the fact that separate bodies on the surface of the Earth are attracted by the Earth and therefore in turn attract it , suggests that this power of attracting other w bodies , which the celestial bodies are sho n to possess , does not belong to each celestial body as a whole , but to the separate particles of which it is composed ; so that , for example , the force with which Jupiter and the Sun attract each other is the result of compounding the forces with w hich the separate particles making up Jupiter attract the separate particles making up the Sun . Thus is suggested ﬁnally the law of gravitation in its most general form : ‘Every particle of matter attracts every other particle with a fOr ce proportional to the mass of each and inversely pro ’ portional to the square of the distance between them . ‘ ’ When Newton completed his Principia astronomy b e came in the fullest sense an exact science . Given the posi tions , velocities and motions of the Sun , Earth , Moon and other planets , then the manner in which they interact on 5 6 ASTRONOMY one another can be learned and even their form and dimen sions determined . In short , astronomy , from a more or less mystical science became in earnest a mathematical science . When the motions and orbits of heavenly bodies were once of observed , the positions these bodies could be computed for future epochs . In his ‘Principia’ Newton conﬁnes himself to the demon str ti n a o of the laws of gravitation . He says nothing about the means by which bodies are made to gravitate toward each other . His mind did not rest at this point . He felt that gravitation itself must be capable of being explained . It is known that he even suggested an explan at ion de on pending . the action of an ethereal medium pervading space . But with that wise moderation which is character i sti c s ecu of all his investigations , he distinguished such p lati on s from what he had established by observation and ‘ ’ demonstration and excluded from his P r incipia all men tion of the cause of gravitation , reserving his thoughts on this subj ect for the ‘Queries ’ printed at the end of his ‘ i k ’ O pt c s . The attempts which have been made since the time of Newton to solve this diffi cult question are few in

- number and have n ot yet led to any well established result . CHAPTER VI — TH E SOLAR SYSTE M PLAN ETARY DISTANCES

To T H E ancients as well as to the moderns the Sun an d the Moon appeared not only the largest but the most im of portant all the celestial bodies . With the Sun and Moon ﬁve other conspicuous spheres eventually were r linked, spheres distinguished by reason of their regula motions . These orbs , Mercury , Venus , Mars , Jupiter and “ “ or Saturn , were named planets wanderers to distin “ ” u g ish them from the ﬁxed stars .

Venus, familiar as the evening star or the morning star, — — was discovered i t is claimed by Pythagoras in the sixth B C a r e century , but even in the poems of Homer there references to both stars without any indication o f their identity . Jupiter, Venus , Mars and Saturn , ranking with the brightest of the stars , and Mercury , occasionally seen near the horizon j ust after sunset or before sunrise , all of were known to the ancients . A study their movements naturally led to the obvious conclusion that all these mov ing stars or planets were related in some way and that the motion of one was more or less dependent on the motions of the others . Hence it may be asserted that the ancient history of astronomy begins with the system of planets that revolve around the Sun . What is the nature of these planets ? Obviously they r a e not all alike in size or distance . Even to the naked eye their appearance seems to reveal conditions that need n expla ation . Early observation and study revealed the S7 5 8 ASTRONOMY f act that the planets occupied a section o f the heavens ” w - ob here there were no so called ﬁxed stars . But later s ervation also revealed that, associated with the planets , a r e a number of smaller bodies of much the same nature “ ” “ known as planetoids , or asteroids , which , with a S ingle exception , occupy the zone of the heavens between M u o f ars and J piter . Lastly there are a large number t emporary visitors to this solar system known as the “ ” comets . They plunge in from space , sweep around the s un and drift away by various paths or orbits , most of I n them ever to return .

Planets , satellites , planetoids and comets comprise the

Solar System . Vast and marvelously complete as that

system is , it must be admitted that it is but a part of the to great universe . It may be , as there is some reason

suppose , that this Solar System is but one of many similar systems scattered throughout the universe and that each of these—including that in which the Earth is situate—is in turn wheeling about some central orb inexpressibly dis i s tant . The Solar System to which the Earth belongs

merely a type and not a unique example of planetary order. The i ntellectual rise in Astronomy is nowhere more clearly revealed than in the history of man ’ s conception t of the Solar System . Perhaps the ﬁrst inquiry that mus have ﬂashed across the mind of a thinking Chaldean or ‘ Greek concerned itself with the distances of the h eav ? e h ly bodies . How far away are the planets How i s their di stan ce measured ? The second question concerned

itself with their motions , Whither do they drift and why ? u Around these questions cluster a group of vague g esses ,

fruitless speculations and poetic fancies , from which at last a scientiﬁc method was evolved for measuring plane e tary distanc s and accounting for planetary movements . It was not until comparatively late in astronomical history that means were devised for ascertaining the physical con

dition of e ach planet . d m The istances of the planets , s all as they seem in com

6 0 ASTRONOMY s v ﬁve e enteen inches , Mars a ﬁfth , at about feet , Jupiter ; a sixth , at nine feet , Saturn ; a seventh , at eighteen feet,

Uranus , and an eighth , Neptune , at the end . The mean distances of the planets from the Sun are as follows

E MIL S .

0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0

O O O O O O O O O O O O O O O O O O O O O O O

O O O O O O O O O O O O O O O O O O O O O

O O O O O O O O O O O O O O O O O O O O

Attempts to measure some o f these distances appr oxi m ately are found in early times . The idea that some of the planets must be nearer the Earth than others must h ave been suggested by eclipses and occultations passage of the Moon over the S un and over a planet o r ete r mi n ﬁxed star . No direct means being available for d i n g the distance , rapidity of motion anciently was em

ployed as a test of probable nearness . The stars being s een above , it was but natural to think of the most distant c elestial bodies as the highest , and accordingly Saturn ,

Jupiter and Mars , being beyond the Sun , were called superior planets ” as distinguished from the two “ infer ior ” planets , Venus and Mercury . Uranus and Neptune are modern additions to the solar system and could not have 8 - 2 2 been included in the hypothesis . Aristotle ( 3 4 3 fo r example , arrived at the conclusion that the planets are farther o ff than the Sun and Moon as the result of a n oc c ultatio n of Mars by the Moon and as the result o f similar o bservations made in the case of other planets by the

Egyptians and Babylonians . Ptolemy ( second century A D ) , altho far more original and daring in his astronomi PLANETA RY DI STANCES 6 1

e t cal conc ptions than Aristotle , was able o add but littl e toward a solution of the problem . He expressly state s that he had no means o f estimating numerically the dis tances of the planets or even of knowing the ord er of th e o f distance the several planets . He followed tradition in conj ecturally accepting rapidity o f motion as a test of

— F I E XTER I O T H E I T O F TH E E ET . F ig . 8 ORB S V R PLAN S

nearness and placed Mars , Jupiter, Saturn (which per ' 2 1 2 an d foi m the circuit of the celestial sphere in about , A 2 . s 9 years , respectively) beyond the Sun in that order

Venus and Mercury accompany the Sun , and may there fore be regarded as on the average performing their revo to r lutions in a year, the test some extent failed in thei 6 2 ASTRONOMY

an case , but Ptolemy again accepted the Opinion of the cient mathematicians " —probably the Chaldeans—that

Mercury and Venus lie between the Sun and Moon , Mer

cury being the nearer to the Earth . o f Copernicus gave the ﬁrst glimpse the truth . To quote “ ” Berry in hi s Short History o f Astronomy : From the fact that Venus and Mercury were never seen very far f rom the Sun , it could be inferred that their paths were o f nearer to the Sun than that the Earth , Mercury being o f so the nearer to the Sun the two , because never seen

far from it in the sky as Venus . The other three planets , ’ being seen at times in a direction opposite t o that o f the

S un , must necessarily revolve round the Sun in orbits l w arger than that of the Earth , a Vie conﬁrmed by the fact that they were brightest when Opposite the Sun ( in

which positions they would be nearest to us ) . The order o f their respective distances from the Sun could be at o nce inferred from the disturbing eff ects produced on their a pparent motions by the motion of the Earth . Saturn f being least a fected , must on the whole be farthest from t he Earth , Jupiter next and Mars next . The Earth thus

became one of six planets revolving round the Sun , the o — rder of distance Mercury, Venus , Earth , Mars , Jupiter, Saturn—being also in accordance with the rates of motion

round the Sun , Mercury performing its revolution most 88 rapidly ( in about days) , Saturn most slowly ( in about 3 0 It was not until John Kepler ( 1 57 1 - 1 630 ) published his ” Epitome of the Copernican Astronomy , his Harmony “ o f the World and a treatise on Comets that a str on o mers were given a deﬁnite formula which enabled them

to determine planetary distances with any exactitude . Kepler ’ s speculative and mystic temperament led him con s tan tly to search for relations between the various numeri

cal quantities occurring in the Solar System . By a happy inspiration he tried to discover a relation between the PLANETARY DI STANCES 63 sizes of the orbits o f the various planets and their times of revolution round the Sun . After a number of unsuccess ful attempts he discovered a simple and important rela ’ tion commonly known as Kepler s third l aw “ The squares o f the times of revolution of any two planets ( including the Earth) about the Sun are pro portional to the cubes o f their mean distances from

the Sun . to In other words , given the periods , there is need only ﬁnd the interval between any two o f t hem in order to infer at once th e ‘ di st an ce separating them all from on e a s another and from the Sun . Here was the plan . What w next to be discovered was the scale upon which the plan was to be drawn . There must be ﬁrst a trustworthy meas th e ure of the distance of a single planet from the Sun ,

Earth , for example , and the problem would be solved . How is this measure to be obtained ? Sir Robert Ball “ ” o f in his Story the Heavens , gives this simple example “ for partial explanation : Stand near a window where you can look at buildings or at any distant obj ect . Place on the glass a thin strip of paper vertically in th e f n e middle o o e o f the panes . Close the right eye and not with the left eye the position of the strip o f paper rela i v l l t e . y to the obj ects in the background Then , while stil an d remaining in the same position , close the left eye again observe the position o f the strip o f paper with th e f on right eye . You will ﬁnd that the position o the paper the background has changed . “ Move closer to the window and repeat the observation , and you ﬁnd that the apparent displacement of the strip increases . Move away from the window and the displace ment decreases . Move to the other side of the room, the displacement is much less , tho probably still visible . We thus see that the change in the apparent place o f the strip o f paper, as viewed with the right eye or the left eye , varies in amount as the distance changes ; but it varies in 64 A STRONOMY t h e to opposite way the distance, for as either becomes greater the other becomes less . We can thus associate with each particular distance a corresponding particular displacement . From this it will be easy to infer that , if we have the means o f measuring the amount of displace m ent, then we have the means of calculating the distance from the observer to the window . It is this principle a pplied on a gigantic scale which enables us to measure the distances of the heavenly bodies . “ Look, for instance, at the planet Venus ; let this corre s on d to on p the strip of paper and let the Sun , which

Venus is seen in the act of transit, be the background .

Instead of the two eyes of the observer, we now place two observatories in distant regions of the Earth ; we look at Venus from one observatory , we also look at it from the other ; we measure the amount of displacement n a d from that we calculate the distance of the planet . All d epends , then , on the means which we have of measuring the displacement o f Venus as viewed from the two differ ” ent stations . Two observers standing upon the Earth must be some thousands of miles apart in order to see the position o f the Moon altered with regard to the starry background to obtain the necessary data upon which to ground their c ff o n e alculations . The change of position thus o ered by ’ s n ot ffi ide of the Earth s surface at a time is su cient , how ever, to displace any but the nearest celestial bodies .

When there is occasion to go farther aﬁeld , a greater change of place must be sought . This can be obtained as ’ a consequence of the Earth s movement around the Sun . w Observations , taken several days apart, will sho the e ffect of the Earth ’ s change of place during the interval upon the positions of the other bodies of our system . But when the depths of space beyond are to be sounded and a n effort is made to reach out for the purpose of measur ing the distance of the nearest star the utmost change of

place is necessitated . This results from the long j ourney PLANETARY DI STANCES 65 o f many millions of miles which the Earth performs

a . round the Sun during the course of each year Still , even t o f his last change place, great as it seems in comparison ffi with terrestrial measurements , is insu cient to Show any t hing more than the tiniest displacements in a paltry forty t hree out o f the entire host of stars . It is thus readily realized with what an enormous di s a dvantage the ancients coped . The measuring instruments a t their command were utterly inadequate to detect such s to mall displacements . It was reserved for the telescope reveal them, and even then it required the great telescopes o f recent times to show the slight changes in the position ’ o f the nearer stars which were caused by the Earth s being a t on e time at one end o f its orbit and some six months — later at the other end stations separated by a gulf o f a bout miles . It was from an opposition of Mars observed in 1 672 by John Richer - 1 696) at Cayenne in concert with Giovanni Domenico Cassini at Paris that the first scientific estimate ’ o f to the Sun s distance was derived . The Sun appeared be n 1 6 6 early miles away . John Flamsteed ( 4

the ﬁrst Astronomer Royal of England , deduced from his independent observations o f the same ’ o 1 2 0 - 1 2 was ccurrence . Jean Picard s ( 6 68 ) later result ’ j ust one- half F lamsteed s Philipp e De L ahi r e thought that the Earth must be separated from the

Sun by at least miles . The transits of Venus 1 6 1 1 6 in 7 and 7 9 were employed, after other attempts had ’ been made, to measure the Sun s distance . 1 6 n ot for The transit of 7 9 is of particular interest, only ’ a fairly good determination of the Sun s distance , but a lso for the reason that the celebrated Captain Cook was c ommissioned to sail to Otaheite for the purpose of wit n e s i n d s . on g the transit of Venus At Otaheite , June 3 , the phenomenon was carefully observed and meas u r e d . Simultaneously with these observations others were of obtained in Europe and elsewhere . From a combination 66 ASTRONOMY a ll the observations , an approximate knowledge of the ’ Sun s distance was gained . The most complete discussion of these observations did not, however, take place for some 1 8 2 time . It was not until the year 4 that the illustrious Johann Franz Encke computed the distance of the Sun and gave as the deﬁnite result miles . Later Urbain Jean Joseph Le Verrier ( 1 870 ) reduced the estimate to

miles , which held good until Prof . Simon New 1 2 1 00 comb in 88 gave the ﬁgure miles . In 9 nearly all the observatories of the world under the di r ec tion of Maurice Loewy and the French Academy of Sci ence began a new computation which will lead to more ’ exact results . The old problem of measuring a planet s distance from the Sun its not yet completely solved . If Sir David Gill ’ s plan of basing a new set of calculations o n 1 1 the Opposition of Eros in 93 is carried into execution , the Sun ’ s distance will be ascertained to within miles . Present knowledge declares the distance of the planets from the Sun with an error n ot exceeding one ti eth of on ﬁf e per cent .

68 ASTRONOMY m ight at times hear . This is the origin of the idea of the music of the spheres which recurs continually in medieval speculation and is found occasionally in modern literature . At a later stage these spheres of Pythagoras were devel oped into a scientiﬁc representation of the motions of the celestial bodies , which remained the basis of astronomy till the time of Kepler .

Philolaus , the Pythagorean , who lived about a century after his master, introduced for the ﬁrst time the idea of the motion of the Earth . He appears to have regarded the Earth , as well as the Sun , Moon and ﬁve planets , as

revolving round some central ﬁre , the Earth rotating on

its own axis as it revolved , apparently in order to insure that the central ﬁre should always remain invisible to the

inhabitants of the known part of the Earth . Altho pure

fancy, the idea of Philolaus was a valuable contribution

to astronomical thought .

Despite the immense inﬂuence of the Pythagoreans , most Greeks shared Plato ’ s idea that any careful study of

celestial motions was degrading rather than elevating , for the whole subj ect smacked too much of the unesthetic

- . 2 B C . section of geometry Still , Plato ( 4 9 347 ) did give

a short account of the celestial bodies , according to which

the Sun , Moon , planets and ﬁxed stars revolve on eight concentric and closely ﬁtting wheels or ci rcles around an

axis passing through the Earth . This idea of Plato ’ s was more or less followed by later

- 6 B C . at philosophers . Thus Eudoxus of Cnidus (409 35 ) tempted to explain the more obvious peculiarities of plane c i r cu . tary motion by means of a combination of uniform t lar motions . The celestial motions were to some exten 2 1 explained by means of a system of 7 Spheres , for the

6 2 0 . stars , for the Sun and Moon , for the planets There is no clear evidence that Eudoxus made any serious a t tempt to arrange either the size or the time of revolution of the spheres so as to produce a precise agreement with w the observed motion of the celestial bodies , tho he kne PLANETARY MOTIONS 69

with considerable accuracy the time required by each planet to return to the same position with respect to the S un ; in other words , his scheme represented the celestial m otions qualitatively but not quantitatively .

Aristotle adopted this scheme of Eudoxus , but need l essly complicated it by treating the spheres as material b 2 2 6 . odies and added more Spheres , thus making 5 in all ’ H e argued against the possibility of the Earth s revolving a round the Sun on the ground that there ought to be a

corresponding apparent motion of the stars , an obj ection ﬁ nally disposed of only during the nineteenth century , w hen it was discovered that this motion can be seen only i n a few cases because of the unutterably great distance o f h t e stars .

No substantial advance can be noted until Hipparchus . 1 60 - 1 2 B ( 5 C . ) made an extensive series of observations w ith all the accuracy that his instruments would permit a n d critically made use of old observations for comparison w ith later ones so as to discover astronomical changes too — s low to be detected in a single lifetime an essentially m odern method . He systematically employed a geometri c al scheme ( that o f eccentrics and epicycles ) for the r ep r e s u c tations of the motions of the Sun and the Moon , a m ode suggested in substance by Apollonius of Perga , who

ﬂourished in the third century B C . The great services rendered to astronomy by Hipparchus can hardly be better expressed than in the words of the

g reat French historian of astronomy , Delambre , who is in “ g eneral no lenient critic : When we consider all that Hip p a r c h us invented or perfected and reﬂect upon the number of h i s works and the mass of calculations which they i mply , we must regard him as one of the most astonishing m en of antiquity and as the greatest of all in the sciences which are not purely speculative and which require a com bination of geometrical knowledge with a knowledge of phenomena to be observed only by diligent attention an d

reﬁned instruments . 70 ASTRONOMY

The last great name encountered in tracing the record o f changing conceptions o f planetary motions is that of 1 00 -1 0 A D on hi s Ptolemy ( 7 ) , whose reputation rests “ ” l Almagest, which may be regarded as the astronomica

o f . gospel the Middle Ages Hipparchus , as we have seen , found the current representations o f the planetary motions

inaccurate and collected a number of new observations . o f own These, with fresh observations his , Ptolemy em

ployed in order to construct an improved planetary system . o f Following the idea Hipparchus , Ptolemy thought that the Sun and Moon moved in circular orbits around th e ’ t o Earth as a center . Ptolemy s chief work was expand the system o f epicycles so that it could explain di sc r ep an

cies between theory and observation , discrepancies over o r n Th e looked ig ored by Hipparchus . deviations of the

planets from the ecliptic , for example , were accounted for

by tilting up the planes of the epicycles . Thus with the h i s aid of the system of Hipparchus , supplemented with

own idea of tilting epicycles , he worked out with great

care and labor the motions o f the planets . Altho the Hip p ar chi an -Ptolemaic doctrine was framed on an extravagant ~ estimate o f the importance o f the Earth in the scheme of

the heavens , yet it must be admitted that the apparent movements o f the celes tial bodies were thus accounted for F r with considerable accuracy . o fourteen centuries the Almagest was regarded as the ﬁnal authority on all ques tions of astronomy and it may be considered as the loftiest

piece o f calculation appertaining to the Ancient World . CHAPTER VIII — T H E SOLAR SYSTE M MODERN INVESTIGATION

T H E Ptolemaic system o f astronomy was discredited only at an epoch nearly simultaneous with that of the di s c over y o f the New World by Columbus . The true ar rangement o f the solar system was then expounded by 1 - 1 De Nicholas Copernicus ( 473 543 ) in the great work, ” ﬁ t R ev oluti on ibus . , to which he devoted his life The rs principle established by these labors showed the diurn al movement of the heavens to be due to the rotation of the l Earth on its axis . Copernicus pointed out the fundamenta difference between real motions and apparent motions . He proved that the appearances presented in the daily r ising and setting of the Sun and the stars could be a c th at th e counted for by the supposition Earth rotated, j ust as satisfactorily as by the more cumbrous supposition o f h t Hipparchus and Ptolemy . He s owed , moreover, tha t e if the ancient supposi ion were true , the stars must hav an almost inﬁnite velocity and de clared that the rotation of the entire universe around the Earth was clearly pre posterons . immor The second great principle , which has conferred t al r u glory upon Copernicus , assigned to the Earth its t e p os ition in the universe . Copernicus transferred the cen ter, about which all the planets revolve , from the Earth to the Sun , and he established the somewhat crushing truth be that the Earth is merely a planet , pursuing a track t ween the paths o i Venus and of Mars and subordinated 7 1 72 A STRONOMY

like all the other planets to the supreme sway of the Sun . This great revolution swept from Astronomy those dis ’ tor ted o f views the Earth s importance which arose , per n ot haps unnaturally , from the fact that the observers th e chanced to live on this particular planet . Whether actual services rendered by Copernicus are commensurate th e with his fame may be doubted . He labored under weight of an e cclesiastical tradition that could not be abandoned without some risk . He was a bold man indeed who dared to overthrow or even to question orthodoxy ’ and to diminish the Earth s overshadowing importance i n the Solar System . The Copernican system was n ot ﬂawless either in theory or ar ti cu logic and many obj ections could be made to it , p la r ly by an astronomer who had observ ed and studied th e f of movements o the heavenly bodies . After the example the ancients , Copernicus assumed as an axiom the uni o f form , circular motion the planets , and , as the only motions which are observed are in a state o f incessant t i n e uali variation , he was obliged , in order o explain the q ties to suppose a different center for each of th e orbits . The Sun was placed within the orbit o f each o f the plan W f o f . ets, but not in the center o any them In other ords ,

. he still adhered to a system o f epicycles . Consequently the Sun performed no other ofli ce than to distribute light n and heat . Excluded from any influence o the system , “ ” th ' moti on the Sun became a stranger to all e s . The fixed stars were alleged to be stationary, and it was necessary to suppose that they were almost infinitely distant, inas much as they always seemed to preserve the same position ’ when viewed from the Opposite sides o f the Earth s orbit . While various astronomers showed some disposition to accept the Copernican teaching, most of them were bitterly c opposed to it on ecclesiastical , traditionary and scientifi - grounds . Tycho B rahe ( 1 546 1 60 1 ) was the most distin ui sh ed of ob g these opponents . Being an indefatigable server and practically the first to realize the value of con TH E S OLAR SYSTEM 73 t i n uous observation , he enriched astronomy by a star catalogue and studies o f the movements of the other c n ce heavenly bodies . Tycho accepted the Copernican o p f tion o a central Sun , but rej ected the idea that the Earth

— F o TH E S OLAR SYSTE M A CCO R p I NG To CO PE RN I CUS . ( r m

th e D e R e v olut I on I bus . )

moved . Thus he sought to eff ect a compromise between the Ptolemaic and Copernican systems . It was the study of a comet in 1 577 that led Tycho to formulate his ideas f o the solar system . He believed that the comet ( X ) as s hown in the accompanying diagram was revolving around 74 A STRONOMY the Sun at a distance greater than that of Venus and as sumed that both the Sun ( C ) and the Earth (A) we r e o f v centers revolving systems , the ﬁve planets revol ing around the Sun and the entire system in turn moving ' t around the Earth . This incorrect proposi ion , which

— ’ 1 TY CH O S S T E O F T H E WO . F o bo k on F ig . 0 YS M RLD ( r m o th e Com e t o f

’ was on e o f the least o f Tycho B r ahe s contributions to i s how astronomical science, significant, showing as it does difficult it was for the principles o f Copernicus firmly to establish themselves and planetary motion to be explained satisfactorily . o n .Whatever Tycho may have thought Of the C pernica

76 ASTRONOMY

—false and contrary to the Holy and Divine Scriptures that the Sun is the center of the world and that it does o not move from east to west, and that the Earth d es move and is not the center of the world ; also that an opinion can be held and supported as probable after it has been ” declared and decreed contrary to the Holy Scriptures . ’ Despite Galileo s abj uration , his general attitude toward the Church and the Bible is contained in his approval of “ B ar on i us the saying of Cardinal , That the intention of the Holy Ghost is to teach us not how the heavens go , h ” but ow to go to heaven . His attempts to explain and demonstrate the Copernican system in his great astronomi “ cal treatise, Dialog on the Two Chief Systems of the ” World, the Ptolemaic and Copernican , led to his trial and conviction before the Inquisition . ’ Kepler, another of Galileo s contemporaries , did more even than the great Italian to bring about a proper con cepti on of the solar system and the motions of the planets .

A student under Tycho, it was but natural that Kepler should have imbibed from his master a respect for syste matic observation , regardless of the correctness or incor r ectn ess of Copernican views . As a result Kepler early b adopted the Copernican doctrine , opposed tho it was y his master . His observations led him to the conclusion , however, that even Copernicus had not revealed all the mysteries of planetary motion and that the hypothetical circles in which the planets revolved around the Sun , according to Copernicus , did not agree with the paths ad observed . Under the instruction of Tycho , Kepler dressed himself to the problems involved in the planet

Mars , whose positions as seen in the sky were a combined h result of its own motion and that of the Earth , as bot move around the Sun . Actual observation of the planet and the consideration of various geometrical theories tha t suggested themselves eventually led to the conception that the path of the planet must be some form of an oval . THE SOLAR SYSTEM 77

Finally Kepler reached the conclusion that instead of b ’ eing circular, the planet s motion must lie in the simple cu rve known as an ellipse and formed by taking an oblique section of a cone . While the circle has but a single cen ter, the ellipse depends for its form upon two ﬁxed points , each of which is termed a focus . It can be drawn by

i — F . 1 1 D W I E I P E g RA NG AN LL S . using two pins stuck in a sheet o f paper and by inserting a pencil within a loop of string that also includes the two pins . The curve may be traced by moving the pencil , while the string is kept taut . It will be found that if the two points are kept close together the curve approaches m in for a circle , while if they are separated the ﬁgure becomes elongated and possesses what the mathematicians term greater eccentricity . At any rate , every point on the curve is such that the sum of its distance from the two foci is always the same . Kepler found that the Sun was 78 ASTRONOMY

a t . t t f one focus When the planet was near ha ocus, it moved with greater velocity than when at the opposi te l . w was a part of its orbit The speed of motion , ho ever, ways proportional to the areas swept out by a straight t line from the Sun in equal intervals of time . In o her n ow ﬁ words , there were formulated the famous rst and second laws of Kepler as follows : 1 i n in on e . S un be The planet describes an ellipse, the g focus .

— F i . 1 2 E A I N E M g QUAL REAS QUAL T I ES .

2 . The straight line j oining the planet to the Sun out equal areas in any two equal intervals of time . s Kepler not only established these laws for Mar , but immediately applied his principle to the Earth and then “ o f claimed (without proof, however) in his Epitome the ” Copernican Astronomy that these two fundamenta l laws applied also to all the planets and to the motions o f the

Moon . Accompanying these two laws was the third, s already discussed , in which it is stated that the quares of the times of revolution of any two planets ( including the Earth ) about the Sun are proportional to the cubes o f their mean distances from the Sun . It was the disclosure of these wonderfully simp le r ela

80 ASTRONOMY

carry the planets round with the Sun . Kepler says him “ ” self i n his Epitome : “ There is , therefore , a conﬂict between the carr ying power of the Sun and the impotence or material slug gi sh n e ss ( inertia) of the planet ; each enjoys some a me sure of victory, for the former moves the planet ’ from i ts position and the latter frees the planet s body to some extent from the bonds in which it i s thus n b held , but only to be captured agai y ” of other portion this rotatory virtue .

— F i 1 V I V E O I T O F E I T I MOTI O N . g . 3 ARY NG L C Y LL P C

Thus is faintly indicated the great theory o f gravitation sati sfac which , as developed by Newton , was to supply a tory explanation of planetary motion and which i s the underlying basis of all modern astronomy . Newton had become convinced that the attracting power f ac o the Earth extended even to the Moon , and that the — celer ati on produced in any body whether it be as distant as the Moon or close to the Earth—was inversely propor ’ tion al to the square of the distance from the Earth s center and also proportional to the mass o f the body . THE SOLAR SYSTEM 8 1

Then he found that the motions of the planets could be r o explained by an attraction toward the Sun , which p duced an acceleration inversely proportional to the square o f the distance from the center of the Sun , not only in ff f it the same planet in di erent parts o s path , but also in ff di erent planets . Again it follows from this that the Sun attracts any planet with a force inversely proportional to the square ’ o f the distance o f the planet from the Sun s center a n d

o f . also proportional t the mass o the planet Accordingly, or t if the Earth Sun attracts a body, the body must exer on or n a similar force the Earth the Sun , and gravitatio i s n ot only a property o f the central body o f a revolving a system , but belongs to every pl net in j ust the same way a s to the Sun and to a Moon or t o a stone j ust as to the

Earth . After Newton had established provisionally the law o f o f for him gravitation and the laws motion , it remained to prove that the observed motions o f the planets agreed o f com with those calculated . A situation the greatest lexit p y, however, was relieved by the fact that the mass of even the largest planets is so very much less than that of the Sun that the motion o f any single planet i s affected to be but slightly by the others , and it may be assumed moving very nearly as it would if the other planets did n ot for or exist, due allowance being made subsequently min n e disturbances or perturbations produced in its path . O on e by the various irregularities observed were explained , and the motion of the Moon and its various eccentricities were computed with accurate numerical results . F or many years the solar system remained in its fancied integrity . There was no change in the original ﬁve planets and the number o f satellites ﬁrst discovered by Galileo and added to by subsequent observers had reached an - 1 apparent culmination when G . D . Cassini ( 1 62 5 1 7 2 ) had detected the second pair of satellites of S aturn in m 1 68 . i 4 Accordingly, when Herschel , following h s custo 82 A STRONOMY o f making a r evIew of the heavens with each new tele 1 1 8 1 scope that he constructed , found March 3 , 7 , with a

Newtonian telescope , seven feet in length , a small star which appeared so much larger than its companions and of t o such uncommon appearance , he suspected it be a m c omet . Further study revealed that it was ore than a oh comet and of far greater interest . When heedfully s n o erved and its path calculated , it was found that ordi n n ary cometary orbit would i any way fit its motion . Anders Johann Lexell ( 1 740 -1 784) first recognised that ’ Herschel s body was not a comet but a new planet r evolv ing around the Sun in a nearly circular path and at a dis tance about nineteen times that o f the Earth and n early to double that o f Saturn . A vain attempt by Herschel name the new planet after his royal patron , George III . , “ ” fi ac Georgium Sidus , nally resulted in the proposal and ceptan ce by British and continental astronomers alike o f n the name Uranus , which harmonized with the ames o f the other planets . a s This discovery was of especial interest, inasmuch l h n n Jo a Daniel Titius a professor at Wittenberg, had pointed out the remarkable symmetry in the disposi 1 2 tion of the planets . In a note , published in 77 , he Showed that the distance of the six known planets from the Sun could be represented with a close approach to accuracy by a certain series of numbers increasing in the regular p r o

r ession o 6 1 2 2 8 . g , , 3 , , , 4, 4 , etc Adding 4 to each number, the results would give the relative distances o f the six known planets from the Sun . In applying this law (which d oes not hold good in the case of Neptune) it was found that the term of the series following that which corresponded with the orbit of Mars was not represented i n the list of planets . Accordingly Johann Elbert Bode 1 h oth eti ( 747 a German astronomer, assumed a yp i cov cal planet to take this place . When Uranus was d s ered its distance was found to fall but slightly short o f e o f im p rfect conformity with the law Titius, and it st u

84 ASTRONOMY was r eached that considerable errors existed or that the

was . planet wandering from its course In fact, these dis tur ban ces had aroused the interest of several math emati ci an s and astronomers , and a young English student , John Couch Adams ( 1 8 1 9 soon after his graduation from 1 8 to Cambridge , communicated in 45 the Astronomer Royal numerical estimates o f the elements and orbits o f the unknown planet which he assumed was acting on

o f . Uranus and was the cause the disturbances In fact , he indicated the actual place in the heavens of the hypo thetical planet . At practically the same time a French a 1 8 1 1 stronomer, Urbain Leverrier ( who had made a careful study of the solar system in response to a request O b from Dominique F . J . Arago , the head of the French se r vator y, prepared an elaborate memoir in which he demonstrated that only an exterior body could produce the disturbances observed and that such a body must occupy a certain and determinate position in the zodiac . He also assigned the orbit of the disturbing body, indicating that it would be as visible and bright as a star of the eighth mag n i tude . of In fact , he supplied data to Professor Galle , the

Berlin Observatory, which enabled that astronomer to ﬁnd i n the 2 1 8 6 heavens on September 3 , 4 , within less than a degree o f the spot indicated an obj ect with a measurable f disk. A reasonably complete map of this portion o the sk y, in which all the stars were noted, proved beyond question that the obj ect was not a star, while its move ment a s predicted was ample conﬁrmation of its planetary ’ n ature . Adams work , which antedated that of Leverrier, had not received attention originally in Great Britain at o f o the hands the Astr nomer Royal , but as the matter as sumed importance the Cambridge Observatory also par ti ci ated on 2 1 8 6 p in the search and September 9, 4 , the

w as di scover ed . planet was seen again . Thus Neptune To show the rapidity of astronomical research in the nine teen th century it may he remarked that it required but T H E SOLAR SYSTEM 8 5 seventeen days for the discovery of a satellite by Lassell - - ( 1 799 1 88 0 ) with a two foot reﬂecting telescope . Astronomers have suspected the existence of still other planets , and the belief has been expressed that such a body exists nearer to the Sun than Mercury, which , as has been seen , enj oys the reputation of being the closest of all the planets to the central luminary . The average dis tance o f Mercury from the Sun is about miles ,

“ F i 1 —T H E T A I T O F V T H E O E I N TR A M ER g 5 R NS ULCAN , SUPP S D I T CUR AL PLAN E .

so that there would be space enough for such a planet .

Its peculiar position in close proximity to the Sun , how e ver , would act against its being observed . A small lumi b nous point in this position would be altogether invisi le , even with the best modern telescope, while its setting

o wi th and rising, simultaneous almost those of the Sun , make it invisible at these times even under the most favor If h able conditions . this planet should pass across t e ’ u s S n disk j ust as do Mercury and Venus , it would be 36 ASTRONOMY

se en . While from its size it would be much less of a spec

t d . acle than the two planets mentioned , it might be detecte Claims have been advanced by astronomers that they have seen such a transit of a small spot . The first suggestion of an intra -Mercurial planet came from the distinguished French astronomer, Leverrier, who in 1 859 advanced such a hypothesis in an attempt to explain the movements of the planet Mercury . His theory involved a body of about the Size of Mercury revolving at somewhat less than half its mean distance from the Sun , or at a greater distance i f of less mass and vice versa , whose motion in great part would explain the i r r egula r i Le sc r bault O r . a ties observed . In the same year Dr at geres maintained that he had observed such a body cross 5 ’ ing the Sun s disk , and the name of Vulcan was bestowed u pon it . Several astronomers claimed to have seen the new d planet . Their observations were not well authenticate . and on the dates fixed for the probable transits no trace of the planet could be found . The strongest test was the S k examination of the y at the time of a solar eclipse , for then the light of the Sun was cut o ff and a strange body D could be readily identified . espite a careful watch at subsequent eclipses and ' an examination of photographic

- plates , only negative results have been obtained . To day the belief that there i s any body of considerable size within the orbit of Mercury is held only by a few a s t r on o mers and very guardedly stated . If the search for an intra - Mercurial planet was un suc ce ssful e n , it has in no way deterred astronomers from dea v o r i n g to ﬁnd other unrecognised members of the solar system . Much interest has been aroused by a h - ypothetical ultra Neptunian planet , which of course would be the furthest from the Sun of all the members of b the solar system . The asis for such a hypothesis is the reduction of observations made of the positions and motions of Uranus and Neptune . Neptune has been under observation for only a small part of a revolution , so that

“ 88 ASTRONOMY ‘

After astronomers had deﬁnitely decided how . far the planets and the Sun are situated from the Earth and how w t on they move ith respect to one another, they began o w der i f perhaps the whole solar system did not in turn t e ﬁ volve around some central orb . The possibility rst occurred to Tobias Mayer John Michel ( 1 767 ) and Joseph Jerome Le F r a n goi s Lalande but the prob lem was not attacked with any degree of success until Sir William Herschel in 1 782 began to draw conclusions from

’ hi s study of the Milky Way and decided that th e entire solar system was drifting toward the constellation Hercules . But Herschel ’ s theory did not meet with general acceptance for h many years . Ot er astronomers suggested various stars as possible central suns controlling the movement o f our A l Solar System . Thus Madler not only proposed that c on e in y , the principal star the Pleiades , should be the o f central sun , because of its situation at a point neutrali z ati on of opposing tendencies and consequently at rest, but even went so far beyond the limits of astronomy as to declare that Here was the seat of the Almighty, the ” M f t o ansion o the Eternal . It is hard even for science quell the imagination and to conﬁne an observer to facts . ’ l r - o f Mad e s t heory was short lived . Further study the ’ stars demonstrated the soundness of Herschel s views . When a modern telescope is turned toward the Milky Way this white girdle of the celestial sphere is resolved into a vast number of stars of which more than f already have been counted on photographs . Each o these stars is a sun like that which governs the Earth , probably surrounded by planets like the Earth , and all these solar systems also are moving, many of them more swiftly than ours . ’ It was inferred from Herschel s measurements o f stellar positions , distances and motions that the solar system was situated comparatively near the center of a universe - e . shap d like a thin , double convex lens This universe was TH E SOLAR SYSTEM 89

to supposed rotate as a unit about its center, with the result that the Sun ( comparatively near that center, but absolutely at an immense distance from it) moved in a circle of dimensions so vast that since the discovery of its motion it had not deviated appreciably from a straight line , but had steadily directed its course toward the constella tion Hercules . s n o b for has Thi simple scheme must w e abandoned, it

. . I 0 0“ s . I o ' . O 0

' 0 0 0 o ‘ . . e 0 0 O \ , \0 0 0 0 Q ' o b c o q' . Q Q . ~ o o s 0

s o " s . s O

1 6 — TI T E O F T E F ig . TH E PLA CE AND DE S NA I O N I N T H E U N I VERS H O T S LAR SYS E M .

’ Ka te n h e d . . b en iscovered by Professor J C p y , a Dutc o di s astronomer, that the visible universe consists of tw i tant parts . The scientiﬁc imagination s compelled to picture two processions o f stars moving in paths which ma 1 1 r o f ke an angle of 5 deg ees with each other . One these stellar streams moves three times as fast as the other . The Sun forms a part of on e o f the streams and is at present at their intersection . Altho it is known in a general way that the entire solar system is moving through space at the rate of 1 2 miles a 90 A STRONOMY

’ second , the shape and size of the Sun s orbit are utterly u nknown . The changes of environment , accordingly, that will accompany the description of it defy conj ecture . The actual course of the solar system is inclined at a small angle to the plane of the Milky Way . Presumably it will become ffi deﬂected , but perhaps not su ciently to keep the system

- clear of entanglement with the galactic star throngs . In the present ignorance of their composition , no forecast of the results can be attempted ; they are uncertain and ex bi l o r tan t . y remote Hence , in a sense , the world knows where it is and in what direction it is moving . But what is the goal , when shall it be reached and what will happen ? then O r, in this crossing of the congested thoroughfares o f the heavens , will the world be shattered by a collision and resolved into a glowing nebula ? This has been the fate of many stars , of several within the period of human — — history and of one Nova Per sei within a few years and witnessed in all its destructive detail by astronomers still living.

92 ASTRONOMY

scribed an annual path on the celestial sphere , which path e cli is a great circle , and this great circle, known as the p tic because eclipses take place only when the Moon is in or near it, is at an angle to the equator of the sphere . This angle is termed the obliquity of the ecliptic and was meas ur d e 1 1 00 B . C . by the Chinese , it is claimed , as early as with remarkable accuracy . Later the same feat was claimed for B C Pythagoras or Anaximander in the sixth century , both of whom probably derived their information from the

Chaldeans or Egyptians . When the Sun crosses the equator the day is equal to the night, which occurs twice a year , at the vernal equinox about March 2 1 and the autumnal equinox about Septem ber 2 3 . When the Sun is at its greatest distance from the equator on the north side the time is known as the summer solstice, and when at its greatest distance on the south side f it is termed the winter solstice . The positions o these points in the heavens were also known to the early Chinese A n aximan astronomers with considerable accuracy, while der, who supposed the Sun to be of equal magnitude with the Earth , used a gnomon or vertical pillar casting a shadow to observe the solstices and equinoxes . Anaximenes is said to have believed that the Sun was a mass of red -hot iron or of heated stone somewhat larger than the Peloponnesus . He looked upon the heavens as a vault of stones , prevented from falling by the rapidity of be its circular motion , while the Sun could not proceed yond the tropics on account of a thick and dense atmos p h er e which compelled it to retrace its course . Later Cr oton a Philolaus of , who was a disciple of Pythagoras and followed his master ’ s teaching that the Earth revolved about the Sun , assumed that the Sun was a disk of glass which reﬂected the light of the universe . Eudoxus of 0 B C Cnidus , about 37 , stated that the diameter of the

Sun was nine times greater than that of the Moon , which marked a triumph over the illusions of sense . About the o f time Alexander, the Great Pytheas , using the gnomon , THE SUN 93 determined the length o f the shadows cast at the summer solstice in various countries . His Observations are the most ancient of those preserved after those made in China . The study of the Sun was undertaken very systemati cally by Hipparchus , who discovered that the solar orbit was eccentric and that the Sun moved at different speeds ff at di erent parts of its j ourney . With his observational data Hipparchus produced the ﬁrst tables of the Sun which are mentioned in astronomy . He was enabled to determine the di fference between the solar day or time as shown by the Sun and the time indicated by some such measuring

- device as the clepsydra or water Clock .

The motion of the Sun was also studied by Ptolemy . He compiled solar tables more extensive than those o f Hip h lb t 1 a r c us A a e n i . 8 p , which were employed until g (b 5 ) made a new compilation of greater accuracy , and which served as a connecting link between the astronomers o f of Alexandria and those of modern Europe . The obliquity the ecliptic was constantly being studied . Ulugh Begh , a

Tartar prince and grandson of the great Tamerlane , at n 1 00 Samarkand, using a g omon feet in height , determined o f the obliquity the ecliptic or precession of the equinoxes , and secured data for the construction of astronomical tables which were of considerable accuracy. The apparent motions of the Sun furnished many diﬂi cult problems for the astronomer , since the observational data lacked accuracy on account of the absence o f satis factory instruments . Measuring angles by the shadow cast and positions in the sky by c r ude for ms of angular measurement were not adequate for exact work . Not until the advent of the quadrant and the telescope with its various adj uncts was scientiﬁc measurement possible . But o f there were from time to time solar eclipses , which a careful record was maintained and which the ancient priests noted in connection with their calendar observa tions . These eclipses played a most important part in ancient astronomy . 94 ASTRONOMY

o f h An eclipse the Sun occurs when Eart , Moon and n e S un are in direct line at the time of w Moon . As the latter lies between the Earth and the Sun its dark body ’ off illumi will pass across the Sun s disk , cutting the direct ff n ation . If the Earth cuts o the sunlight from the Moon there is a lunar eclipse . Solar eclipses are of three kinds , partial , annular and total . In the ﬁrst the Moon , instead of passing directly between the Earth and the Sun , slips past on one side and cuts off from sight only a portion of ’ the Sun s surface . In the annular eclipse the Moon is cen t r all y interposed between the Sun and the Earth , but falls short o f the apparent size required to conceal the solar disk entirely . Consequently at the height of obscuration a bright ring is visible around the Moon . In a total eclipse , however, the Sun completely disappears behind the dark ff body of the Moon . The di erence between a total and an annular eclipse depends upon the fact that the apparent diameters of the Sun and the Moon are so nearly equal as to preponderate alternately one over the other through the slight periodical changes in their respective distances from the Earth . It is the total eclipse that particularly arouses the atten o ff tion of astronomers , for it cuts entirely the light of the

Sun , and in addition to enabling the observation of the various parts of its surface as it passes across its disk to be made , also makes it possible for an observer to see the stars and planets in the daytime even if they are very near the Sun . A total eclipse of the Sun is not visible on the entire Earth , but only along a comparatively narrow band , lying roughly from west to east and measuring about 1 65 miles in width . A partial eclipse is seen for about miles on either side of this band , but otherwise the phe ’ n omen on is not visible on the Earth s surface . Chinese rec ords going back over years describe a solar eclipse which occurred during the twenty -second cen

B . tury C . That eclipse carried with it a distinct moral lesson . H 0 Two bibulous state astronomers, and Hi , unfortunately;

96 A STRONOMY

n o f thick ess and is composed glowing gases , chief among which is hydrogen . The chromosphere is a brilliant scarlet in hue , but the redness is entirely overpowered by the intense white light of the photosphere, which shows f through from behind . The most interesting features o the chromosphere are the red prominences, which are the marks of violent agitation in its upper portions and which

\R e v e r 3 i ng

/ zPﬁ 5 16 if) 6 27 2

F i 1 —T H O T . T g 7 E S LAR S RUC URE.

r are such a notable feature in total solar eclipses . Afte the chromosphere comes the corona , which is the outer envelope of the Sun and consists of a halo O f pearly white light of irregular outline which fades away into the sur rounding sky and extends outward for many millions of Th miles . e corona suff ers so much from the brilliancy of the photosphere that it is only on the occasion of a total solar eclipse that it can be seen in all its remarkable beauty . TH E S U N 97

As the photosphere , the reversing layer and the chromo o f sphere are all sources light , the solar spectrum Observed in spectroscopes is composed of the three separate spectra ff o or combined . For this reason eclipses a ord welcome pp ’ t un i ti es for studying the Sun s surface . When the Moon completely covers the photosphere its brilliant light is cut o ff and the other features of the Sun can be examined visually, and , what is more important, spectroscopically and photographically . Thus when the spectroscope is di r ect ed to the reversing layer during a total eclipse the d a r k lines o f the solar spectrum change into bright lines or are reversed . But this reverse spectrum is a phe n ome n on of a moment only , and as the Moon progresses o an altered spectrum is Obtained . That of the chrom

‘ sphere is o f sufficiently long duration to permit an esti o f i ts i s cov mate depth and nature , and finally , when this ered up, there is the corona, which has a distinct spectrum own of its , containing a strange line, the distinguishing green o f which has not yet been identified with any element known upon the Earth . Modern conceptions of the Sun are due very largely to the use of the telescope , the spectroscope and the spec t r h lio r h o e a . g p , especially the last two instruments With o f i s the telescope , when the intense light the Sun properly reduced , it is possible to examine its surface and obtain a certain amount of information as to its nature or to obtain photographs of that surface by very short exposures . But on the spectroscope and the spectroheliograph the a st r on o mer depends for his knowledge of the constitution and composition of the great center of the solar system . The development and nature of the spectroscope , as used in a solar research , have been already discussed , but it is p p r op r i ate to add here a brief explanation o f the spectro heliograph , for to its use is due not only a large part of

- present day information of the prominences , but more recently an explanation of the sun- spots themselves and d the stu y of various features of the photosphere . 98 ASTRONOMY

The spectroheliograph was ﬁrst devised in successful b . O working form by Prof . George E Hale at Kenwood “ se r r i n str u vat o 1 88 . y, Chicago , in 9 The principle of this ” “ ment is very simple , writes Professor Hale . Its obj ect is to build up on a photographic plate a picture of the solar ﬂames by recording side by Side images of the bright spec tral lines which characterize the luminous gases . In the

ﬁrst place , an image of the Sun is formed by a telescope on the slit of a spectroscope . The light of the Sun after transmission through the spectroscope is spread out into a long band of color, crossed by lines representing the vari w ous elements . At points here the slit of the spectroscope happens to intersect a gaseous prominence the bright lines of hydrogen and helium may be seen extending from the base of the prominence to its outer boundary . If a series of such lines corresponding to diff erent positions of the slit on the image of the prominence were registered side by side on a photographic plate , it is obvious that they would give a representation of the form of the prominence itself . “ To accomplish this result it is necessary to cause the solar image to move at a uniform rate across the ﬁrst slit

o f the spectroscope , and , with the aid of a second slit (which occupies the place of the ordinary eye - piece of the spectroscope ) , to isolate one of the lines , permitting the light from this line and from no other portion of the spec trum to pass through the second slit to a photographic

plate . I f the plate be moved at the same speed with which o f the solar image passes across the ﬁrst slit, an image the

prominence will be recorded upon it . The same method answers for the study o f the sun - spots and other features ’ o f the Sun s surface . As the result of telescopic , spectro sc0p i c and spectroheliographic observation it is now known ’ - that the Sun s principal features are its sun spots, its pho

tos h er e . p , its chromosphere and its corona A sun -Spot when examined through a telescope consists

of a dark central region called the umbra into which long,

1 00 ASTRONOMY

2 2 6 o f of between 5 and days , as well as the general zone

- the sun spots . The next important contribution to sun -spot theory O came from Derham , whose bservations were made during 1 0 - 1 1 1 the years 7 3 7 . He believed that the spots on the

Sun were caused by the action of some new volcano, whose ” smoke and other Opacous matter produced the spots . As they decayed they became half shadows encircling the darker portions and ﬁnally became bright spots . Lalande, the celebrated French astronomer , believed that the spots r e r e were rocky elevations , about which the penumbra p sented shoals or sandbanks , while around ﬂowed enormous ’ w oceans . Lalande s explanation , as ell as that of Derham , was clearly based upon terrestrial analogies , which were employed with considerable frequency by early ast r on o mers . 1 6 1 1 In 7 9 Alexander Wilson , of Glasgow ( 7 4 ex a mi n i n - g the large sun spot visible that year, noted that, as ’ n the Su s rotation carries a spot across its disk , there was f a change in its appearance , and that the same ef ect of perspective was produced as i f it were a saucer- shaped depression , the bottom forming the umbra or central black spot and the sloping sides the penumbra or surrounding portion of half shadow . The penumbra appeared narrow est on the side nearest the center of the Sun and widest on “ the part nearest the edge . Hence Wilson assumed that the great and stupendous body of the Sun is made up of ff two kinds of matter , very di erent in their qualities , that by far their greater part is solid and dark , and that this immense and dark globe i s encompassed by a thin cover ing of that resplendent substance from which the Sun would seem to derive the whole of his vivifying heat and ” energy . Wilson went on to explain that the excavation of spots might be occasioned “ by the working of some sort ” o f elastic vapor which i s generated within the dark globe , and that the luminous material , which was more or l ess THE SUN I O I

a fluid, was cted upon by and tended to throw down and cover the nucleus . Si r William Herschel devoted considerable attention to sun - the Sun , and observing the variation in the spots , ’ reached the conclusion in 1 80 1 that it indicated a certain variability in the total amount of solar radiation, which he assumed might have some connection with terrestrial phe n omen a fi , especially the weather . He endeavored rst in 1 80 1 o f to trace connection between the price wheat, natu ff O f on rally influenced by the e ect weather the crops , and

- the occurrence of sun spots , claiming that when the latter were scarce there was a diminished solar activity, which caused colder weather, with obvious results . Ingenious as f this theory was , there were not su ﬁcient substantiating meteorological data . It ﬁnds , however, a counterpart in ’ o f modern studies the Sun s heat , not necessarily connected

- exclusively with sun spots , however, whereby it is hoped to establish some useful knowledge of the relation between the amount of heat radiated from the Sun and weather con di ti on s on our Earth . ’ Herschel s observations of the Sun and sun -spots were o f continued by his son , Sir John Herschel , at the Cape Good H Op e ( 1 836 John Herschel assumed that their motion was due to ﬂuid circulations similar to those

- producing the trade and anti trade winds on the Earth . ” “ The spots , in this View of the subj ect, he said , would ’ come to be assimilated to those regions on the Earth s surface where for the moment hurricanes and tornadoes prevail , the upper stratum being temporarily carried down ward , displacing by its impetus the two strata of luminous matter beneath , the upper of course to a greater extent n tha the lower, and thus wholly or partially denuding the ” Opaque surface of the Sun below .

- Such observation of sun spots , made with consider tho r n es able o s by the astronomers mentioned , as well as did numerous others , not establish any regularity in ff for h their appearance or e acement . It remained Heinric 1 02 ASTRONOMY

Schwabe ( 1 790-1875 ) at Dessau to announce in 1 843 that the sun- spot phenomena reached a maximum probably in a decennial period . This announcement , altho coming as it t n o did after a patient study of the Sun , a tracted particular attention until a series of sun -Spot statistics were pub ’ “ ” o f li shed in Humboldt s Kosmos . Then the correctness ’ S chwabe s observations and deductions was apparent to a all . When compared by Dr . John L mont and Sir Edward

Sabine with various periodical magnetic disturbances , it was found that the two cycles of changes agreed with ex t r aor i n ar d y exactness . It was a remarkable coincidence that the observations of a number of investigators were in

- complete harmony . A study of sun spot records estab li shed the decennial period more correctly at years . Thus commenced a recognition that magnetic disturbances on the Earth were related in some way to sun -spot phe m omena . For many years no direct connection could be established , altho various theories were forthcoming . Like wise further attempts were made to identify the variations - t of sun spots wi h meteorological phenomena, but without 1 8 success , until Wolf in 59 by an examination of the Zii r i h 1 - 1 80 A D c 000 0 . en chronicles ( . ) found data which abled occurrences of the Aurora Borealis to be correlated with a disturbed condition of the Sun . From this time on the inﬂuence of the Sun on terrestrial conditions assumed new importance . The beautiful phe n omen on of the aurora , which consists of a glow in the

sky about the north and south poles , had been observed for t e ages , but the first scientific connection of importance 1 1 6 corded was in 7 , when Halley stated that the Northern “ ” r r Lights were due to magnetic effluv i a . In 1 741 H i o te at ' Upsala observed that they produced an agitati on o f the

magnetic needle . This connection was further demon st r ated by Arago so that by the middle of the nineteenth century the connection of the Aurora with the Sun and in turn with terrestrial magnetism was as evident f as it was insu ﬁciently explained .

1 04 A STRONOMY

which have been conﬁrmed by Newcomb , show that the average temperature of the Earth determined by the com bination of a great number of thermometric obser vations ° made at several stations indicated a ﬂuctuation of 0 3 to 1 1 - - C . during the year sun spot period . In other words , the temperature of the Earth ’ s atmosphere indicates small

- ﬂuctuations which correspond with the sun spot period , thus indicating that the solar heat radiation varies with the - r a number of the sun spots . The mean tempe ture of the Earth is greatest at the time of minimum sun - spots and - eter mi lowest at time of maximum sun spots . Hence the d nation o f the amount of heat radiated by the Sun at vari o us - i s times , especially at sun spot maxima and minima , a matter of considerable terrestrial importance . The study of the sun -spots carried on by Professor Hale with the spectroheliograph and other apparatus , including

- special red sensitive plates of considerable speed , reached 1 08 an interesting stage in 9 , when it was demonstrated that sun - spots are centers of attraction which draw toward them the hydrogen of the solar atmosphere . Subsequently it was found that these spots are the seats of great cyclones , I n which cool hydrogen gas is set whirling and i s sucked down in the great maelstrom of the Sun , rushing into the center of the spot at a rate of about 60 miles a second . Consequently the spots are the center o f great solar dis - A tur ban ce s which are of an electro magnetic nature . c cording to the modern electronic theory o f matter and ” electricity , electrons , or minute particles of matter, in their terriﬁc cyclonic velocity produce magnetic lines o f force . It was found by Professor Zeeman that , when light is passed through a strong magnetic ﬁeld , the lines of the spectrum are subdivided and appear double . This Zeeman effect Professor Hale has found in the spectrum o f the

- sun spots . If one looks at the center of a spot the light travels in the direction of the axis of the whirl or cyclone , while in viewing a spot at the edge of the Sun the di r e c tion is at right angles to the axi s and i s manifested accord THE SUN 1 05 i n l ha g y in the spectrum . This theory s been thor oly con d - - firme , so that to day it is known that sun spots are mag netic fields of great intensity . The important discoveries made by Pro fessor Hale and his associates may thus be : summarized as follows First , that the spots are cooler than the surrounding region ; second , that they are centers o f violent cyclones , and , third , that they are magnetic fields o f great intensity . In addition to the sun -spots the photosphere includes other interesting features , notable among which are the ‘ ’ “ ” a facul e, little torches, so named by Father Scheiner .

- These bright , globular obj ects , besides the sun spots , are the only other phenomena of the Sun ’ s surface visible by S ch r Ot e r ae direct observation . Showed that the facul are

- heaped up ridges of the disturbed photospheric matter . Secchi and Young assumed that the faculae are the result of

- violent eruptive action of the sun spots , but it remained for the Spectroheliograph to give a clear idea of their nature . Professor Hale states that “ they are usually most numer ’ - ous in the vicinity of sun spots , and near the Sun s limb they are sometimes very conspicuous brilliant obj ects cov ering large areas . Near the center of the Sun , however , they are practically invisible , tho faint traces of them can sometimes be made out on photographs taken with a suit i th e able exposure . This increase o brightness toward ’ Sun s limb is assumed to b e due to the elevation of the faculae above the photosphe r i c level and their escape from a considerable part of that absorption which so materially

reduces the brightness of the photosphere . Rising above the denser part of the absorbing veil and thus suffering ’ ut b little diminution of light , they appear near the Sun s

limb as bright obj ects on a less luminous background . The chief difference of the faculae from the rest of the photo

sphere lies in their greater altitude , as photographs have shown that they may be resolved into granular elements

similar to those constituting the photosphere . But they are the regions from which immense masses of vapors rise to 1 06 ASTRONOMY the solar surface and for that reason are important in the s olar mechanism . “ Near the edge of the Sun their summits lie above the lower and denser part of that absorbing atmosphere which ’ s o greatly reduces the Sun s light near the limb, and in this region the faculae may be seen visually . At times they may be traced to considerable distances from the limb , but as a rule they are inconspicuous or wholly invisible toward the central part of the solar disk . The Kenwood experiments had shown that the calcium vapor coincides closely in form a and position with the facul e, and hence the calcium clouds were long spoken of under this name . In the new work at the Yerkes Observato r y the differences between the cal c ium clouds and the underlying faculae became so marked that a distinctive name for the vaporous clouds appeared n flocculi ecessary . They were therefore designated , a name chosen without reference to their particular nature , but suggested by the flocculen t appearance o f the photo ” graphs . “ With the spectroheliograph , Professor Hale relates , n ot it was at once found possible to record the forms , only of the brilliant clouds of calcium vapor associated with the a - facul e and occurring in the vicinity of the sun spots , but a lso of a reticulated structure extending over the entire surface of the Sun . From a systematic study of spectroheliograph negatives , in the course of which the heliographic latitude and longitude of the calcium clouds , ’ o r flocculi , in many parts of the Sun s disk were measured from day to day (by Fox ) , a new determination of the rate o f the solar rotation in various latitudes has been made . flocculi - This shows that the calcium , like the sun spots , complete a rotation in much shorter time at the solar equa tor a t than points nearer the poles . In other words , the

Sun does not rotate as a solid body would do , but rather n ot like a ball of vapor, subj ect to laws which are yet ” understood .

1 08 ASTRONOMY

graphic evidence was forthcoming . During the eclipses of 1 898 and 1 90 0 abundant corroborative material was ob tai n ed , and the reversing layer as a reality was conclusively demonstrated . The total depth of this reversing layer has 00 0 been placed at from 5 to 60 miles . It continues in a normal state of tranquillity , for little change is produced in f the aspect o the dark lines . The chromosphere or envelope o f glowing gases which covers the Sun completely was detected by observers of eclipses in the eighteenth and nineteenth centuries . There S tan n an is a record in a letter from Captain y to Flamsteed , the British Astronomer Royal , describing an eclipse wit

n essed 1 . 1 06 at Berne on May ( O 7 , in which he states that the Sun ’ s “getting out of the eclipse was preceded by ” - a blood red streak of light from its left limb . Halley and 1 1 De Louville in 7 5 noted a similar phenomena , and it was 1 1 8 also observed during annular eclipses of 737 and 74 , but “ ” “ with the ruby brilliancy toned down to brown or dusky red by the surviving sunlight . During the eclipses of 1 82 0 1 8 6 18 8 , 3 and 3 similar observations were made , but it 8 th 1 8 2 was not until the eclipse of the of July, 4 , that the virtual discovery of the chromosphere as a solar appendage 1 868 may be said to have been made . The eclipse of , which was observed spectroscopically and photographically as well as with the telescope , served to make clear the nature of the chromosphere and to reveal that it is a continuous o f envelope hydrogen and other incandescent gases , some thousands of miles in thickness and of the same eruptive nature as the prominences which are shot out from it . In o ther words, it seems to be a collection of minute flames set close together and giving it the appearance of a large c on fla r a ti on g . The summits of the flames of fire , which ’ incline when the Sun s activity is greatest, are erect during its phase of tranquillity . The chromosphere is m arked by an irregular di st r ibu ’ tion over the Sun s surface , which in no way partakes of o f 1 8 the character an atmosphere . Professor Hale in 97 S OLAR ENERGY 1 9 9

r discovered a low stratum of carbon vapor . S uch are metals as gallium and scandium have been discovered with the spectroscope . The vapors of magnesium , iron and several other substances are conspicuously represented in the spectrum of the chromosphere , and with the Yerkes telescope the fine bright lines due to the vapor o f carbon also may be seen . The solar prominences are conspicuous eruptive or flame like emanations from the chromosphere which are seen at total eclipses of the Sun . They proj ect like red flames beyond the dark edge of the Moon , and were first de V a ss en i us o f scribed by Lector , a Swedish professor 2 Gothenburg, who observed the total eclipse of May f 1 733 . One of these reddish clouds outside o the solar disk was so large that it could be detected with the naked eye . The phenomenon excited his admiration 1 8 and wonder . The prominences were also Observed in 77 who by the Spanish Admiral Don Antonio Ulloa , was convinced o f their connection with the Sun on account o f thei r color and magnitude . By some observers the solar prominences were regarded as the illuminated sum o f mits lunar mountains , and by Arago they were de scribed as solar clouds shining by reflected light . Abbé P e tal 1 8 2 - y , in 4 , spoke of them as self luminous and as ' a third or outer solar envelope composed o f the glowing substance of the bright rose tint which produced moun ’ tains , j ust as clouds were piled above the Earth s surface . 1 8 1 In 5 Hind, an English astronomer , noted on the south “ - limb o f the Moon a long range of rose colored flames , which Dawes Spoke o f as a low ridge o f red prominences resembling in outline the tops of a very irregular range of hills . Airy also noted this rugged line of proj ections , “ ” and spoke of its brilliancy and nearly scarlet color . But the truly solar origin of the phen omena was n ot con e 1 860 lusively demonstrated until , when the prominences w were photographed by Secchi and De la Rue , and sho n to o f be independent the motion of the Moon . In 1 868, 1 10 ASTRONOMY

h wit the growth o f spectroscopy and solar chemistry , the gaseous nature of the prominences and their connection with the Sun was made evident . They were found to c onsist o f immense masses o f hydrogen and helium gas rising from the chromosphere and reaching an altitude o f hundreds of thousands o f miles . — The Coron a The corona is a beauti ful lustrous solar wrapping, which can be observed only during a total eclipse . The winged circle , the winged disk, or the ring with wings , as it is variously called , found upon Assyrian and

Egyptian monuments , may be reproductions of the phe n omen on . The first definite mention of a solar corona is to be found in Plutarch , in connection with 1 A the eclipse which probably took place in 7 D . He “ writes that the obscuration caused by the Moon has no time to last and no extensiveness, but some light shows itsel f around the Sun ’ s circumference which does not allow the darkness to become deep and complete . Kepler mentions a ray o f light seen around the eclipsed Sun in 1 6 5 7, and ascribes it to some sort of luminous atmosphere 1 06 around the Sun . In 7 Cassini , observing an eclipse o f “ ” the Sun in France , saw the crown o f pale light around i llumi the lunar disk , and stated that it was caused by the o f nation the zodiacal light . Halley, observing an eclipse 1 1 in London in 7 5 , describes minutely the phenomena of the luminous ring rising around the Moon to a great height, and showing considerable brilliancy . The eclipse ’ o f 1 842 was the first to indicate the corona s great i m portance to astronomers , and from that date it received careful attention and earnest study . 3 In 1 869 Professor Harkness and Pro fessor Young dis covered a bright line of unknown origin in the coronal spectrum , showing that it consists in large part of glowing gases . With the advent of astronomical photography , and with the development o f the spectroscope , more attention than ever was given to the careful study o f the coron a

1 1 2 A STRONOMY

’ 1 8 ff v o f century . In 59 Kircho s great disco ery the ex planation O f the Fraunhofer lines in the solar spectrum made it possible to ascertain the chemical c ompo si ton of

- the Sun . Thus , as has been shown , the bright colored band formed o f light from a small hole in Newton ’ s shut ter passing through a prism made that prism the forerun ner o f an instrument able to teach the nature and con sti tu tion o f bodies far distant in the heavens . Thomas Mel vill had examined with a prism various ﬂames in which ff di erent substances had been introduced , and had reached the conclusion by the middle of the eighteenth century a that cert in vapors , notably sodium , contained light which ’ M elvi ll had a deﬁnite place in the spectrum . s deep yel low ray became the sodium line in the Spectroscope o f r aun ho fer F . When Kirchoff noted th e identity of certain lines char ac ter i sti c o f terrestrial elements , and then assumed their presence in the Sun , he laid the foundation of the modern a o science o f solar chemistry . Wherever light c n be h t ai n ed from a heavenly body it is now possible to resolve i t into its spectral elements and thus to identi fy the sub stances . In fact, as substances such as helium were found n o in the Sun by Lockyer, which had terrestrial counter part, hypothetical elements were assumed . The spectro scope has shown in the Sun the presence not only o f gases a s such hydrogen and helium , but i ron , sodium , mag n esi um , calcium , and many other substances . Hence the chemical composition o f the Earth and the Sun are much the same , altho there is evidence o f the existence in o f the Sun substances not yet found in the Earth . When the spectroscope was applied to the analysis of the eb ro mosph e r e and its prominences it was found that they are composed of the vapor of calcium and o f the light gases - d helium and hydrogen . Sun spots , too , have been foun to have a characteristic chemical composition , while the corona emits rays which probably indicate the presence o f in it very light and tenuous gases . CO O O F T H E T K E D I TO T E I Y e ke R NA SUN , A N UR NG AL CL PSE . ( r s b e v a o O s r t ry . )

1 1 4 ASTRONOMY

are o ften observed . I f they have approximately the c r iti cal diameter they will float above the Sun in the form o f beautiful Carmine clouds , balanced in Space by the equal and oppositely acting forces of gravitation and radiation pressure . These clouds have hitherto been par ti cular l i n y puzzling , for the absence of a dense solar atmosphere thei r existence seemed a celestial paradox . I f the condensed drops are smaller than the critical diame ter they will be proj ected by the pressure of light far beyond the Sun , to form the beauti ful pearly corona . From the fact that comets have passed through the corona without any very apparent retardation , some idea o f its tenuity may be gained . Assuming that the corona consists o f particles of such size that the radiation pres on sure each exactly equals its weight , Arrhenius finds that the enti re corona weighs no more than long tons , which is equivalent to four hundred large trans u atlantic steamers , and is not more than the amo nt o f coal burned on the Earth every week . Compared with the infinity of the space in which it i s poised, the Earth is smaller than a vanishingly small Speck on a sheet of paper having an area o f many square i s miles . So far as the Earth is concerned, the Sun very much in the position o f a man who throws away all but a single cent of a fortune consisting of twenty three million dollars ; for only of his radiated energy reaches this globe . What, then , becomes o f the huge number of corpuscles which are shot from the Sun and which never strike the Earth ? It is conceived by Arrhenius and his followers that many of them must col lide with corpuscles discharged by suns other than that — f of thi s solar system suns ine fably distant , so that their light reaches the Earth only a fter the lapse of countless

centuries , and so that they are seen not as they gleam n ow , but as they gleamed when Egypt was young and

Gr eece was a wilderness inhabited by savages . Such col lisions must result in the formation of larger masses up SOLAR ENERGY 1 1 5 to a limit determined by the electrical charges carried by the corpuscles .

- Solar Energy . The Earth depends upon the Sun for its supply of heat and light . The Sun is transmitting heat r e to the Earth , and unless the supply of energy is being plenished in some way it is obvious that it must be losing in heat and temperature . From its effect on the Earth an estimate can be had of the amount of heat radiated by the Sun annually . I f it be assumed that the Sun has the same heat capacity as water , and hydrogen is the only s i ubstance with a greater heat capacity , it would fall n ° temperature about 4 F . annually . I f , therefore , the great o ff luminary were simply a hot body , cooling , its present rate o f radi ation could not be maintained for more than

years . That the Sun has been radiating a much longer period than this is obvious from geological and biological evidence , so that some other cause must be sought . Up to the nineteenth century the doctrine o f inﬁnite durability was generally held by astronomers and geolo gists . For that reason no particular attention was paid to the nature of the heat of the Sun and its source ; but with the formulation of the doctrine o f the conservation o f energy it was realized that the energy of the Earth must proceed from the Sun for the greater part , and con sequently it became necessary in turn to question the source of solar heat . Robert Mayer , to whom is due the earliest conception of the conservation of energy , asked ? : o ff himself I f the Sun is hot , why does it not cool In 1 848 Mayer published some answers to his own ques tion in a pape r which failed to receive the approval o f the French Academy of Sciences . His conclusions were as follows : The Sun cannot be a glowing mass sending out radiation without compensation . Solar heat cannot be due enti rely to chemical changes , nor can it be due to was o f solar rotation . In his Opinion it the result meteors 1 1 6 ASTRONOMY

falling into the Sun . He did not overlook the fact that the resulting increase in the mass of the Sun would i n crease its attraction for the planets and would shorten ob the sidereal year . This was contrary to the facts of con ce servation , and Mayer was forced to an incorrect p tion o f the undulatory theory of light to explain the situa ' tion . Six years later William Thomson , subsequently

Lord Kelvin , reached independently almost the same con e lusion as Mayer , but he was able to explain the increase ’ i n the Sun s mass resulting from meteoric showers , for according to the gravitation theory the ‘‘added matter is n draw from space , where it acts on the planets with very nearly the same force as when incorporated in the Sun . Lord Kelvin then ventured an estimate of the age of the Sun , which was the ﬁrst attempt in this direction made by a physicist . He assumed that the solar energy of rota th e in eteor s tion was derived from fallen , which , allowing for the constant loss o f solar energy by radiation , could be acquired in years . Taking into consideration the limited amount o f meteoric matter available near the “ Sun , he concluded that sunlight cannot last as at present ” for three hundred thousand years . This theory attracted 1 8 little attention when promulgated by him in 54, and was abandoned by him later .

But in the same year Helmholtz , working along the o f lines the nebular hypothesis of Kant and Laplace , de rived the heat o f the Sun from the contraction o f the nebula from which the Sun and planets were formed .

He asserted also a further contraction o f the Sun , now assumed to be in progress , by which the kinetic energy e obtained was conv rted into heat , and compensated for

o f . the loss solar heat by radiation Accordingly , i f the Sun contracts one ten - thousandth part of its radius each year, enough heat would be generated to supply radia ’ tion for years . Helmholtz computation gives

- twenty two million years as the probable age of the Sun , based on a uni form radiation and homogeneous density

‘ 1 1 8 A STRONOMY

' radium , and when the extraordinary properties o f this n ew substance were kn own it was stated that a bare frac tion o f a per cent . of radium present in the Sun would account for and make good the heat that i s annually lost en by radiation . Should this hypothesis hold good, an ti r ely new aspect is given to the problem of solar heat and to the heat of the Earth . This innovation , advanced by members of the younger school of physicists , all of whom had prosecuted with vigor researches in r adi oacti v in ity , did not appeal to Lord Kelvin , who maintained 1 906 that the gravitation theory was still suﬂi c i en t to ’ account for the Sun s heat . No evidence has been pro d uc ed of the presence of radium in the Sun , altho helium has been found there . As helium is obtained from radium , the existence of radium in the Sun i s quite probable . f Sir George H . Darwin , in discussing the e fect of these “ : recent discoveries on solar age , says Knowing, as we n ow do , that an atom of matter is capable of containing an enormous store of energy in itself , I think we have no right to assume that the Sun is incapable of liberating atomic energy to a degree at least comparable with that d o which it would i f made of radium . Accordingly , I see no reason for doubting the possibility o f augmenting the estimate of solar heat, as derived from the theory o f ” gravitation , by some such factor as ten or twenty .

It i s Obvious , therefore , that while the contraction the ory explains the origin of a vast amount o f the Sun ’ s heat , yet there are other sources of internal energy which recent discoveries plainly indicate are of great importance, so that the scientist at this stage is unable to declare positively the age of the Sun or to make any accurate estimate as to the probable duration of the time through which it will afford light and heat to the Earth and the other planets . Nevertheless , it seems assured that millions of years hence , how many cannot even roughly be d etermined , the Sun will be reduced from a ball of glowing v apor to a gi gantic black cinder rushing through space . SOLAR ENERGY 1 1 9

Unwarmed by any central luminary , its crust will be washed by oceans of air liqueﬁed by a cold too intense for n a y living creature to endure . The light of the Sun is obviously more intense than

any other luminant known to man . If compared with ‘ the full Moon , times as much light is received

from the Sun . Expressed in another way , the Sun gives as over . times as much illumination a standard

candle at a distance of one yard . But not all of the light and heat which is radiated from the Sun comes to the

Earth . Professor Langley, in his experiments at Mt . 1 88 1 Whitney in , found that a Clear atmosphere would o ff 0 cut 4 per cent . of the rays coming perpendicularly ’ to the Earth s surface . Gases in the atmosphere , such off as carbon dioxide , cut even a greater amount , and the general absorption i s greater at the violet end than at

the red . It is for this reason that high altitudes and a clear atmosphere are essential for solar investigation ;

and for this reason , too , that the setting Sun appears red , the bluish rays being absorbed in traversing a greater amount of terrestrial atmosphere than when the Sun is

high in the sky . The temperature o f the Sun can be estimated from its

brilliancy and from spectroscopic and bolometric studies . It is a common experience that a ﬁlament o f an incan d escent lamp emits more light and glows more brightly

when the amount of current i s increased . At ﬁrst the

ﬁlament is red , but as more current is permitted to ﬂow

it becomes yellow, and ﬁnally a brilliant white . This i s

marked by an increase in temperature ; and secondly, the

temperature depends upon the brilliancy of the glow. The s ame analogy holds good in the case of the Sun and the

' th e r adi ati on i s w stars . I f the wave length of kno n , and the color which emits the greatest amount o f heat in the spectrum—and this can be measured by the bolometer — by a Si mple law i t is sible to calculate the absolute ° pgs n 2 0 th e temperature o f a star . en by deducti g 7 from 1 2 0 ASTRONOMY result its temperature in the ordinary Centigrade degrees i s obtained . Thus in the case of the Sun the maximum

- heat radiation occurs in the greenish yellow light , which gives a temperature for the rotating disk of the Sun O f ° ooo C about 5 , . , or F. The atmospheric absorption already referred to serves to cut down the intensity o f the i con rad ation , so that, taking this and other amounts into ’ sideration , the temperature o f the Sun s disk can be esti °

at 6 z oo . mated about , C

Similar investigation in the case o f Sirius and Vega , which are white , or younger stars , give a temperature ° 1 ooo about , C . higher than that of the Sun , while the red star , Betelgeux , which is a declining star , Older than ° 2 oo the Sun , has a temperature some , 5 C . less . The f temperature of the Sun furnishes di ferent results , de pending on the manner in which the problem i s attacked . “ ” Arrhenius , in his work on Worlds in the Making, sum “ ma r i z es : i n t en recent work , and states that From the s i t y of the radiation , Christiansen , and afterward War ° 6 ooo burg, calculated a temperature o f about , C . Wilson ° 6 2 oo and Gray found for the center of the Sun , , which they afterward corrected into Owing to the absorption o f light by the terrestrial and the solar a t mo s h er e s a p , too low values are always found . That p plies to a still greater extent to any estimate based upon the determination of that wave length for which the heat emission from the solar spectrum is maximum . Le Cha t eli er compared the intensity o f sunlight ﬁltered through red glass with the intensities of light from several ter r estr i al sources of fai rly well -known temperatures treated in the same way . These estimates yielded to him a solar abso temperature of C . Most scientists accept an ° o f 6 oo lute temperature , 5 , corresponding with about “ C . That is what is known as the effective tempera ” ture of the Sun . I f the solar rays were not partially absorbed this tem p er atur e would correspond with that o f the clouds of the

1 2 2 ASTRONOMY

which , when fully absorbed for one minute over a square centimeter of area placed at right angles to the ray , would produce suffi cient heat to raise the temperature o f a ° 1 gram of water C . I f once the original quantity and kind of heat emitted ff by the Sun be known , its e ect on the constituents o f a tIn O S h er e the p on its j ourney to the Earth , how much o f it reaches the soil , how through the change of the atmosphere it maintains the surface temperature o f o ur globe , and finally , how in diminished quantity , or altered d kind, it i s returned to outer space , it woul be possible to predict nearly all of the phenomena of the weather. Thus it has been known that when there is a small de crease i n the solar radiation there follows a marked and w general decline in temperature . So that , kno ing the to d t variation of radiation , it should be possible pre ic changes in climate . These data are secured by measuring the total intensity ’ h o f the radiation , as it arrives at the Earth s surface, wit o r the pyrheliometer , thermometer with blackened bulb, ca re fully protected from all other influences except the direct rays o f the Sun ; and in the second place by meas uring the heat or energy in di fferent parts o f the solar - o f spectrum with the spectro bolometer . The absorption the atmosphere nearer the Earth for di fferent areas re for a t quires measurements to be made at several stations , - i n ten sI t o f n Washington , near the sea level , the y radiatio actually observed is only about three - fourths as great as

that observed in the clear atmosphere of Mount Wilson ,

at a height of feet . Recent observations made at t Mount Wilson and Washing on by Mr . Abbot , of the

Astrophysical Laboratory of the Smithsonian Institution , indicate that heat sent out to the Earth from the Sun in the course o f a year is capable o f melting an ice shell 1 1 4 feet thick over the whole surface of the Earth . The h solar radiation is not a constant quantity , but varies wit m the decrease in solar distance , the changes occurring fro SOLAR ENERGY 1 2 3 m t on th to month and from year o year . The variation i s due to the chan ges in the source o f radiation rather than ff ur to the e ects o f o atmosphere or external causes . The distance and position of the Sun as regards t he other members of the Solar System has been considered in a previous chapter . It is known that its apparent diameter ’ 2 or 1 0 is 3 which corresponds with miles , 9% times the diameter of the Earth . This would give it an area times that of the Earth and a volume i s times greater . The actual mass of the Sun i s times that of the Earth , but its average density only about a quarter as great , so that the Sun , which has a density of as compared with water, is four times as large as it would have to be i f its density were the same as that of the Earth . Taking into consideration this light ness as well as the high temperatures prevailing in the Sun , one is forced to the conclusion that the body of the Sun must be in a gaseous state . The conditions under which the gases are found must be quite diff erent from those with which we are acquainted on the Earth . Gravity at the surface of the Sun exceeds by more than twenty - seven times gravity on the Earth . The motion of the Sun as regards the other stars in the to heavens has been treated elsewhere , but it is proper here refer to its rotation on its axis from west to east, which takes place in a period of about 2 5 days and is apparent

- from the motion of the sun spots , tho it can also be de t ected by directing a spectroscope toward the edges of the limb and noting by Doppler ’ s principle how one side is approaching the observer and the other is retreating . The m o f a ti e rotation is not the same for all p rts of the disk, but depends upon the position of the spots selected . Those m i nearest the equator show the ost rapid rotat on . CHAPTER XI

M ERCURY

ANCI ENT records make no mention o f the di scove r y of w Mercury, yet the existence of the planet was surely kno n even in the days of Nineveh , when a chief astronomer directly refers to the planet in a report which he made to

King Assurbanipal o f Assyria . The planet is occasionally mentioned in early and medieval astronomical literature . It i s stated that Copernicus regretted that he had never been able to observe it properly in the high altitude o f

Frauenburg . That the planet should have been overlooked by the ancients is n ot strange when it is considered that it is never visible in the higher altitudes except occasionally near the horizon , j ust after sunset or before sunrise . In the clear sky over an eastern desert the primeval a st r o n o mer doubtless saw the bright star in that part of the hori zon where the setting Sun was still shedding its beams . Its luster diminishes as the planet draws near the horizon at sunset, until ﬁnally it sets so soon after the Sun that o o r it is invisible . Years may elapse before a similar pp tun i t f y is a forded .

I f a similar phenomenon took place at sunrise , the primi tive astronomer might have inferred that Venus and Mer cury were identical , especially as a long series of observa tions would establish the fact that one of these bodies was never seen until the other had disappeared . Accordingly some of the an cIen t astronomers assumed that there was

1 2 6 ASTRONOMY

H oayaﬂw

! ar m

- 1 T . F i g . 8 ORB I T AND PHAS E S O F AN I N F ER I O R PLA N E C orre spon di n g v i e ws o f t h e s ame si tua ti o n s o f a n i n fe ri o r plan e t a s e e n f o th e e a h h o i n con e e n hases a nd a te r m r , s w g s q u t p l r s . t I I at on s n apparen t Si z e . MERCURY 1 2 7

’ est E r aste n Elongation . Again when it is passing nearest ‘ ’ to us i s it said to be in Inferior Conj unction ; and finally , w i s hen it has drawn as far as it can to the right hand , it ‘ ’ o f spoken as being at its Greatest Western Elongation . The continual variation in the distance o f the inferior planets , Venus and Mercury, from the Earth , during their of revolution around the Sun , will course be productive of n o great alterations in apparent size , which doubt also its ff had e ect in bewildering the ancients . At superior c t to onj unc ion , being then farthest away , Mercury ought be show the smallest disk ; while at inferior conj unction , ing the nearest , it should appear much larger . When at or greatest elongation , whether eastern western , it should naturally present a n appearance midway in size b etween the two . From these considerations it would seem that the best time for studying the surface of Venus or Mercury i s at or i s to inferior conj unction , when the planet nearest the i s n ot a Earth . But that this the case will at once appe r ’ if it i s considered that the Sun s light is then falling upon un il the side most distant, leaving the proximate side on lumined . In superior conj unction , the other hand, th e light falls full upon the side Of the planet facing the Earth ; but the disk is then so small and the view besides ' is so da z z led by the proximity o f the Sun that observa o f th e tions are little avail . In the elongations , however, so see - of sunlight comes from the side , and we one half the planet lit up ; the right half at eastern elongation and the left half at western elongation . Piecing together the results given at these more favorable views , it is possible , bit by bit, to gather some small knowledge concerning the f r surface o Mercury o Venus . Consequently it will be seen that the in ferior planets Show various phases comparable with the wax ing and waning of the Moon in its monthly round . to n Superior conjunction is , in fact, similar full Moo , and in ferior conj unction to new Moon ; while the eastern 1 28 ASTRONOMY an d western elongations may be compared respectively ’ with the Moon s first and last quarters . When these t elesc0 i c phases were first seen by the early p observers , the Copernican theory was felt to be immeasurably strengthened ; for it had been pointed out that if this sys tem were correct, the planets Venus and Mercury , to n e were it possible see them more distinctly, would of cessi ty present phases when viewed from the Earth . The apparent swing of an inferior planet from side to o f on e on side the Sun , at time the side , then passing ” east ’ to on into and lost in the Sun s rays , appear once more the i on e west side , s the explanation o f what is meant when “ ” “ speaks of an evening star or a morning star . Mercury “ ” o r Venus is called an evening star when it i s at its east — t o f ern elongation that is o say , on the left hand the Sun ; o r on set f , being then the eastern side , it will after the Sun as n sets , both sink in their turn below the western horizo o f i s i ts at the close day. Similarly, when either planet at — to to o f the western elongation that is say, the right hand —i t so n Sun will precede him, and will rise above the easter horizon before the Sun, receiving therefore the designa “ i o f t on morning star . ’ t Mercury s motions were early studied . With the plane was associated the first of the remarkable astronomi cal n o e predictions that w have become almost commonplac , so carefully are they worked out by astronomers . Kep o f ler, from his studies the motions of the planet, was the to first realize that i f the orbit of Mercury, which , as has been seen , lies within that of the Earth and thus nearer o f the Sun , were exactly circular and in the plane the f ’ d ecliptic, a transit o Mercury across the Sun s disk woul o r occur once in each synodical revolution , period between two successive conj unctions of the planet with the Sun as seen from the Earth, the epoch of the transit could be cal ’ culated very easily . But Mercury s orbit is inclined 7 degrees to the ecliptic and is very eccentric , for which c reason his calculations were only approximately corre t.

A STRONOMY

ex swiftest i n its movements of all the planets . With the ce tion o f o f r p some the satellites, it has an orbit that depa ts —i n r most from the circular form other words , an o bit marked not only by the greatest eccentricity , but also by the greatest inclination of all planetary orbits to the ecliptic . This eccentricity o f the orbit is such that the Sun is - f seven and one half million miles out o its center , while the actual distance of the planet from the Sun ranges from t - - or about wenty nine million miles to forty three million ,

- a mean distance of about thirty six millions o f miles . The v -ﬁv elocity varies from thirty e miles a second, when the planet i s at perihelion or nearest the Sun at the pointed o f t - sec end its oval course, o about twenty three miles a on d at aphelion . ’ To é explain the perturbations in Mercury s motion , Abb Moreux has recently advanced the following hypothesis : “ The Sun i s unquestionably surrounded by matter which Pho gives rise to the observed phenomena of the corona . togr aph s made during the eclipse of 1 90 5 showed an outer corona extending to a hitherto unsuspected distance from the Sun . This outer corona is ellipsoidal and its maj or axis i s very nearly coincident with that of the zodiacal light . This region , then , is ﬁlled with a resisting medium , composed o f matter which is gradually falling in toward

the . i s Sun The mechanism o f this fall , or contraction , v o f ery complex , and as we have no idea of the density the

i s fi . matter, it dif cult to calculate its changes in form “ The resistance which the medium Opposes to a mov d ing bo y is likewise difficult to estimate , but we know from the observed acceleration of comets that this r e si stan i max ce s by no means negligible . At the epochs of imum coronal development this resisting medium may easily extend to the orbit of Mercury . Hence that planet in its revolution around the Sun must traverse masses o f rarefied gas and swarms of minute particles by which i ts i course s modified . We are far from knowing the quan t itati ve f t of ef ec of the resisting medium , and much study MERCURY 1 3 1 the corona will be required to complete the solution o f this ” interesting problem . Because o f the lack of surface markings th e r otation M r . s cH a periods are very uncertain Recently Profe sor g , i 2 f . 8 . n o . France , has deduced a rotation 4 h m G Schia p ar elli has put forward the View that both V enus and Mercury rotate in a time equal to the individual period S un of revolution around the , and thus always turn the same face toward the Sun . Such a motion , which is o f analogous to that the Moon around the Earth , could be easily explained as the result o f tidal action at some past time when the planets were , to a great extent , fluid . o f This may be true, as Mercury is one the planets which has n o Moon and consequently will be influenced directly and solely by the Sun in so far as tidal phenomen a f are concerned . These tidal e fects must be especially se vere on account o f the proximity o f Mercury to the Sun . Because o f this lack o f a satellite the mass o f the planet is f t o m very di ficult determine . At various ti es Laplace , Encke and Leverr ier have given values from to ’ o f tu the Earth s mass . The first convenient Oppor nity o f placing this planet in a gravitational balance and o f de ter mi n in 1 8 g its mass was presented in August, 35 , when ’ Encke s comet in the course o f its eccentric path through the solar system penetrated within the orbit o f Mercu r y f at its perihelion . The attractive power o the planet prod uced only a slight deviation from the regular course o f sufli ci en t the comet, , however, to Show that the value ’ previously assumed for Mercury s mass was nearly twice t oo large . At the same time the calculations made from these data have not removed the uncertainty that sti ll prevails upon this point ; Simon Newcomb decided on a o f i l a value of about that the Earth , wh le Wil i m Harkness made it Beyond its motion comparatively little i s known o f M ercury . The spectroscope seems to indicate that the atIn O Sph er e o f Mercury contains water vapor j ust as does 1 32 A STRONOMY

the Earth. But the studies of Professor Lowell at Flag ff n o r obscu sta , Arizona , do not show any sig s of clouds rations , and there are no indications of any atmospheric e nvelope . In fact , the surface of Mercury has been termed “ ” colorless , a geography in black and white . The ﬁrst student of the surface of Mercury was Johann S ch r oter 1 Hieronymus ( 745 who , as will be s was een , a pioneer in the study of the topography of the h r . 1 2 S c ii ter Moon In April , 79 , concluded from the grad ual degradation of light on its brightly illuminated disk that Mercury possessed a tolerably dense atmosphere . 1 During the transit of May 7 , 799, he was struck with the appearance of a ring of softened luminosity circling the ” planet to a height of 3 and about a quarter of its own d iameter .

Referring to this ring of softened luminosity , Agnes ‘ ’ Clerke writes in her History of Astronomy : Altho a ‘ ’ t v i s mere hought in texture , it remained persistently i ble bo th with the seven - foot and the thirteen - foot r e

2 88 . a ﬂectors , armed with powers up to A similar p Plan tade pendage had been noticed by De at Montpellier , 1 1 1 6 1 86 1 8 November , 73 , and again in 7 and 7 9 by Pros F lau er ues p erin and g g ; but Herschel , on November 9, 1 8 02 saw , the preceding limb of the planet proj ected on t he Sun cut the luminous solar clouds with the most per ‘ ’ feet sharpness . The presence , however, of a halo was u a 1 8 2 nmist kable in 3 , when Professor Moll , of Utrecht , described it as a ‘nebulous ring of a darker tinge approach ’ ing to the violet color . Again to Sir William Huggins on 1 868 and Stone , November 5 , , it showed as lucid and m ost distinct . “ No o f o r change in the color the glasses used , the pow ers applied, could get rid of it , and it lasted throughout th e transit . It was next seen by Christie and Dunkin at 6 1 8 8 Greenwich , May , 7 , and with much precision of de tail by Trouvelot at Cambridge , Mass . Professor Holden , o n -on - l the other hand , noted at Hastings Hudson the tota

CHAPTER XII

VE NUS

F ox all time Venus has been known as evening and m a was to t o orning star . By the ncients it supposed be w s eparate bodi es which were named Phosphorus ( Luci fer ) , or Morning Star and Hesperus ( Vesper ) o r Evening h B C F o r t . . Star . Pythagoras in e sixth century is claimed the honor of discovering the identity o f the two stars , but i t was probable that he restated the views of Eastern a stronomers . A s of the most brilliant star the heavens , Venus is cer tai n l y the one that has always been most observed . Glit tering in the sky like a clear diamond, its pure white light , which , on a night when there i s no Moon , is strong enough to s cast a shadow, naturally would impress all whose live ar e out spent of doors . Hence Venus has always figured “ ’ ” o f in literature as the Shepherds Star . Homer speaks “ Kalli sto s— the planet as the Beauti ful , and as a type o f i ts beauty worship figures in all mythologies . At a time when the planets were personified as gods and goddesses it i s easy to understand why this star was selected to typ i f y Love . on o Not only account f its splendor in the evening sky, but for other phenomena, Venus is familiar in history and

- . 1 1 6 2 8 B C literature The historian Varro ( . . ) states that “ JEn eas on his voyage from Troy to Italy saw this planet ” h i s constantly above the horizon , and the same istorian b quoted y St . Augustine as speaking o f a change in the 1 34 VENUS 1 35

an 1 o c olo r d brilliancy o f Venus . In 7 1 6 the visibility f Venus in full daylight was hailed as a marvel by the peo o f 1 0 ple London , and in 75 its appearance at noon aroused 1 n general astonishment in Paris . Again , in 797, whe Napoleon returned to Paris from his conquest o f Italy h e found the attention o f the populace divided between hi s reappearance and a similar striking midday phenomenon .

In fact, Bonaparte always associated this star with his

on e fortunes , and evening while engaged in viewing the o to starry heavens , pointing t the planet, said Prince Tal “ le r an d see ? ! S o y , Do you That is my star long as it ” shines I will have no doubt of success . In more recent times many brilliant appearances o f the n planet have been observed, the interval betwee periods o f a moun in on e greatest vi sibility t g t o eight years . At o f 1 0 s these, in April , 9 5 , at Cherbourg, Venu appeared a s o a an a bright meteor f ppreciable size and full form, e ff to o f ect which was due partly the formation a halo . A cor di n to s was a a s g the Ptolemaic theory , Venu lw y between the Earth and the supposed orbit o f the Sun . o f i ts Hence it was possible that , at the most, but half illuminated sur face could be visible to the Earth . When 1 61 0 t he Galileo , with his telescope , in , made striking discover y that Venus appeared in various phases j ust a s as as the Moon , exhibiting the gibbous well the crescent u n eces phase, it was a strong and almost the last arg ment sary to establish beyond question the Copernican theory s that the planets revolve around the Sun . Were V enu for as large as Jupiter, example, the phases would read i l be . , y g discerned by the unaided eye Indeed Sir Robert Ball has wondered what would have been the effect on the history o f astronomy had Venus been o f the size o f Jupi ter so that its crescent form could have been seen wi thout a telescope . Then the elementary truth would have been apparent that Venus was a dark body revolving around the S un and our t u have . The analogy betw een it Ear h wo ld 1 36 ASTRONOMY been at once perceived and the theory o f Copernicus long since might have been established . The mean distance of Venus from the Sun i s miles . With the exception o f the Moon and an occasional comet n o other heavenly body comes so near the Earth at n a y time . The orbit is marked by the smallest eccentricity in the planetary system , and i s there fore more nearly cir ' a t s a c ul t than that o f any other plane . The planet gre t

- = i . E F g 1 9 T I1 1: F O UR PRI NC I PAL PHAS S O F V ENUS . est an d least distances from the Sun var y from th e mean o nly by about miles each way . as i s Venus is the nearest planet to the Earth , there o nly miles between the orbits o f the two at i o r nferior conj unction their nearest approach , yet Venus i s not as well known as Mars , since when Venus passes n so earest the Earth it is then between it and the Sun, t hat the hemisphere which is illuminated is n ot visible to the Earth . The appearance o f Venus in the sky as the evening and the morning star was n o less impressive to the ancients

1 38 ASTRONOMY.

1 6 1 u a s Dec. They have occ rred on the following d te : 7, 3 2 1 8 . 1 6 1 61 1 6 De c. Dec 4, 39 ; June 5 , 7 ; June 3 , 7 9 ; 9, 74 ;

. 6 1 88 2 on u 8 2 00 Dec , ; and will occur again J ne , 4, and un 6 2 01 2 J e , . ﬁ f 1 6 was The rst observed transit, namely, that o 39, watched in England by two persons , Jeremiah Horrocks n d a a friend , William Crabtree, whom Horrocks had fore warned of its occurrence . That the transit was observed at all was due entirely to the remarkable ability of Hor r to th l o f n o t ocks. According e calcu ations Kepler, transi

— m i . 0 T R I T r T A c aos s TH E S F g 2 TH E ANS o V E NUS . PA H UN ' 1 11 3 Tm n s u s O F 1 8 74 AND 1 8 8 2 . VENUS 1 39 could take place that year as the planet would j ust pass clear of the lower edge of the Sun . Horrocks , how ever, worked the question out for himself , and came to the conclusion that the planet would actually traverse the ’ lower portion O f the Su n s disk . The event proved him to be right . Horrocks , who was said to have been a veritable prodigy o f astronomical skill , unfortunately died about two years after this celebrated transit, in his twenty second year . The transits of Venus next Observed in 1 761 and 1 769 were taken advantage of by Edmund Halley to suggest a means of ascertaining the distance of the Sun . The idea had originated in rather vague form with Kepler , but was 1 suggested more deﬁnitely by James Gregory in 663 . After Halley had O bserved the transits of Mercury in 1 677 he realized the advantages of the method and published sev ‘ eral papers urging preparations for observing the transit . “ ” He pointed out , says Berry , treating of this point in hi “ ” “ s Short History of Astronomy , that the desired r esult could be deduced from a comparison of the durations of the transit o f Venus as seen from different

i . e . stations of the Earth , , o f the intervals between the ﬁrst appearance of Venus on the Sun ’ s disk and the ﬁnal dis f appearance , as seen at two or more dif erent stations . He estimated , moreover , that this interval of time , which would be several hours in length , could be measured with con se-t an error of only about two seconds , and that in quen ce the method might be relied upon to give the dis

: tance of the Sun to about part of its true value . As the current estimates of the Sun ’ s distance differed 2 0 0 among one another by or 3 per cent . , the new method, ’ expounded with Halley s customary lucidity and en thusi asm , not unnaturally stimulated astronomers to take great ’ r trouble to carry out Halley s recommendations . The e ’ sult s , however, were by no means equal to Halley s ex ” ecta ti on s p .

Immense trouble was taken by governments , academies 1 40 ASTRONOMY and private persons in arranging for the observation of

1 61 1 6 . the transits of 7 and 7 9 For the former , observing l parties were sent as far as to Tobolsk , St . He ena , the

Cape of Good Hope and India , while Observations were also made by astronomers at Greenwich , Paris , Vienna ,

Upsala and elsewhere in Europe . The next following O b sc r v ed transit was on an even larger scale , the stations selected ranging from Siberia to Cali fornia , from the Varanger Fj ord to Otaheite (where no less famous a per ’ son than Captain Cook was placed) , and from Hudson s

Bay to Madras . The expeditions organized on this occasion by the 'American Philosophical Society may be regarded as the ﬁrst of the contributions made by America to the science which has since owed so m uch to her ; while the Empress Catherine of Russia bore witness to the newly acquir e d civilizati on o f her country by establishing a number o f observing stations on the soil of her Empire . A variety of causes prevented the moments of contact between the discs of Venus and the Sun from being ob served with the precision that had been hoped . The values o f the parallax of the Sun deduced from the earlier of ” ” the two transits ranged between 8 and 1 0 while those 6 b e Obtained in I 7 9, tho much more consistent , still varied tween 8 ” and corresponding with a variation of about

miles in the distance of the Sun . The whole set o f observations was subsequently very elaborately dis cussed i n 1 8 2 2 -4 and again in 1 835 by Johann Franz

Encke, who deduced a parallax O f corresponding r with a distance of miles , a numbe which long remained classical . The uncertainty of the data is shown by the fact that other equally competent astronomers have deduced from the O bservations of 1 769 parallaxes of a n d The transits of Venus in 1 874 and 1 882 were memorable as the ﬁrst in which photographic methods and the h eli om eter were applied , in addition to the Older methods of time

1 42 ASTRONOMY

fl c ted e to incident light , is In other words , the planet reflects three - fourths o f the light that falls upon it from the Sun , a proportion which is little exceeded by freshly fallen snow . This indicates that Venus is surrounded by a dense and cloudy atmosphere . Similar indications are given by the spectroscope , pointing to the existence of an atmosphere containing water vapor and generally similar to our own , but denser . to This atmosphere of Venus , owing its great refractive power , sometimes appears as a luminous ring in transits of the planet . From observations made at the transit o f 1 88 2 it has been computed that the density of the ’ atmosphere O f Venus is that of the Earth s a t mO S h er e p , which produces a refractive displacement o f ’ Ma edler s According to calculations, the atmosphere o f Venus is times as dense as that of the Earth . Many astronomers believe therefore that Venus is sur rounded by a dense atmosphere into which the Sun ’ s rays do not penetrate to a depth sufficient to cause much loss by absorption—probably because they are reflected by

Opaque clouds . From this it would follow that the mark ings which have been observed on the disc of Venus thro ugh the telescope for centuries may be purely atmos her i c p . As the displacement of the surface markings of a planet when viewed through a telescope furnish th e o f basis for estimates o f the period rotation , it may be that calculations so made are without j ustiﬁcation . These markings of Venus are altogether di fferent from those of any other planet . They are faint, indistinct and apparently variable , for the drawings of Venus made by di fferent Observers before the advent of photography are very unlike . The ﬁrst astronomer to Observe any mark ings on Venus was Domenico Cassini , who discovered a 1 bright spot in 666. In the following year he discovered a second spot , from which he deduced a period of rotation 2 2 o f about 4 hours , an estimate which was reduced to 3 ’ hours and 2 2 minutes in Jacques Cassini s revision of hi s VENUS 143

’ h Wh o father s work . On the other hand, Bianc ini , studied 1 2 6- 1 2 the planet in 7 7 7, deduced from his observations a period of rotation equal to 2 4 days and 8 hours . Thus began the long and violent controversy in regard ob ser to the rotation o f Venus . From more than v a ti on s made between 1 830 and 1 841 De Vico computed a 2 2 1 mean value of 3 hours , minutes and seconds for the period of rotation of Venus . This value , furthermore , agrees very closely with the period o f 2 3 hours and 2 1 minutes deduced by S ch r Ot er from observ ations o f the deformation of the horns o f Venus ( 1 788 and it has consequently been adopted in most treatises on astron m 1 8 0 o . y But in 9 , however , belief in the short period was ’ shaken by Schiaparelli s discoveries . Before this Schia par e lli had made the surprising announcement that the period o f rotation of Mercury was equal to the period o f its revolution about the Sun , and he now asserted that the t same law governed the motions of Venus, a statemen f 1 8 - 8 based on observations made in the winter o 77 7 , which showed that the bright Spots then visible never v m or aried their positions with respect to the ter inator, o bse r boundary line between the planet and its shad w . O v ati on s made in 1 895 gave additional support t o th e view that Venus rotates on her axis in the period o f her r evolu 2 2 s tion about the Sun ( 5 days ) , and consequently alway

turns the same hemisphere toward the Sun , j ust as the t Moon always presents the same face o the Earth .

Thi s theory , which on its announcement rested solely ’ on Schiaparelli s authority , soon obtained strong inde pendent support . It has apparently been completely con r L ff ﬁ med by the observations made by owell at Flagsta , 8 - 1 6. Arizona , in 95 Lowell saw markings that have been or seen by no other astronomer , be fore since, but the most surprising thing about his discoveries i s that he saw n o

spots , but only bands or lines bearing a superﬁcial resem

blance to the canals of Mars . The whole conﬁguration r e n ot mained unchanged for hours, which could be the case 1 44 ASTRONOMY

2 i f the period o f rotation were only 4 hours . Further m fi to ore , the markings remained xed relatively the ter min ator . Lowell denies the existence Of clouds on Venus , altho he finds in certain phenomena O f twilight evidence of the presence o f a very dense atmosphere . Many astronomers have refused to accept the conclu ad sions of Schiaparelli , Perrotin and Lowell , and have S chi a a hered to the theory of a short period of rotation . p ’ r elli s first announcement was vigorously disputed , soon a fter its publication , by the French astronomer , Trouvelot . s 1 8 During his residence at Cambridge , Mas , from 75 to 1 882 , Trouvelot pointed out seventeen changes in the markings which Schiaparelli regarded as fixed . Trouve lot regarded the bright spots O n the limb of the planet as very high mountains , rising above the atmosphere . The observed changes in the appearance of the horns of the crescent planet have also been ascribed to moun ch r Oter tains . In the eighteenth century S announced the 1 8 discovery o f several mountains on Venus . In 7 9 the s outhern horn of the crescent appeared blunted and near

.i t on the dark disc o f the planet shone a bright peak , the height o f which was estimated by S ch r Oter as 2 2 n b ser . o w o feet, or miles Trouvelot deduced from his v ati on s a period o f rotation of about 2 4 hours . It is very remarkable that two such experienced and trustworthy ob servers as Schiaparelli and Trouvelot could be led to such widely di ffering results from their frequent observations o f the same Obj ect during the same period . It may be regarded as certain that the problem of the rotation of O Venus cannot be solved by bserving markings , which is not surprising i f they are purely atmospheric phenomena .

Sir William Herschel , who has j ustly been called the greatest observer of all time , doubted the reality o f the markings of Venus and regarded them as a t mos h er i c p phenomena , but this view was not shared by Ma edle r any o f his contemporaries . Beer and , who were also most excellent observers , studied Venus repeatedly

146 ASTRONOMY lar ly lengthened to equality with their periods of r evolu tion about the Sun , which exerts a far greater tidal action u t pon these in ferior planets than upon the Ear h .

I f Venus has already reached this stage , all the water on the planet must be accumulated in the form of ice on A n the dark and intensely cold hemisphere . Hence , as t on i adi has o f r pointed out, the presence clouds and wate vapor on the illuminated hemisphere casts grave doubt ’ on e the correctness of Schiaparelli s theory . The presenc o f clouds in the atmosphere of Venus i s denied only by who saw - di s Lowell , the peculiar canal like markings t i n c tly whenever terrestrial atmospheric conditions per mitted .

H an sk No trace o f markings , by the way, was seen by y

' summer o f and Stefanik, who studied the planet in the 1 0 9 7 at the Mont Blanc observatory, which is more ff c elevated than Flagsta , under very favorable atmospheri o f s conditions , and deduced a period rotation slightly les 2 o f o f i s than 4 hours . So the problem the rotation Venus o f to still unsolved, altho a maj ority astronomers appear h O e t share the view of Schiaparelli and Lowell . The p tha it can be solved by Observing markings h as proven illu sory , but there is good reason to believe that the spectro scope will ultimately furnish the solution . Its Venus in size is almost identical with the Earth . diameter expressed in miles i s as compared with It for the Earth . s circumference consequently to amounts miles , as compared with miles , o f on u that of the Earth . The period revoluti aro nd the

Sun is 2 2 5 days . For many years observers have been looking for satel O f 1 6 2 lites Venus . Fontana Cassini ( 7 and and Montaigne among others , imagined that they v for a had made such a disco ery , but these have m ny years been considered optical illusions . CHAPTER XIII

T H E EART H

TH E conception of the Earth formulated by the anci ents a assumed it to be a ﬂat disk ﬂoating on water, which ide persisted in some form or other despite various phe h omena clearly impossible of explanation upon such

a hypothesis . After some more or less vague Spec ulati on on the shape of the Earth by Thales 5 46 a clear and rational idea was advanced by

B C . Pythagoras , who ﬂourished in the sixth century . and taught that the Earth , in common with the heavenly bodies , was a sphere and that it received no sup the port at the center o f the universe . The idea of sphericity of the Earth was doubtless derived by analogy an from the appearance of the Moon . The theory became established part of Greek science and philosophy and con sequently has continued to the present day, anticipating by some years the acceptance of the belief in th e ’

. s Earth s rotation In fact , the pherical form of the Earth fi r n gured p ominently in Greek astronomy , and the divisio o f the Earth into zones and the idea of poles was promulgated by the Alexandrian philosophers and even e b fore their time . Indeed , not only did they realize the s 2 6 hape o f the Earth , but also its size . Eratosthenes ( 7 1 95 or 1 96 whom we have already mentioned r etelesc0 i c f in our chapter on p p work, made one o the fi a t e rst scientific measurements of the Earth , obtaining s fir s ult which must be considered the . t good geod etic 1 47 1 48 ASTRONOMY

of e measurement . He found from the application simpl geometry that the angular distance of the Earth ’ s surface between Alexandria and Syene must be of the circum th e ference of the Earth . Posidonius , who was born about end of the life of Hipparchus , made a new measurement o f the circumference o f the Earth , much after the fashion o f

Er ath osth en e s t oo . that of , but reaching a value small One of the crucial arguments for the sphericity of the Earth is that when a ship sails away the hull first dis s appears from view while the masts are visible . Thi was first advanced by Pliny ( 2 3 - 79 From the Greeks and Romans to the Arabs may be a far j ourney , but little was done in the study of the Earth until of the Arab astronomers at Bagdad , under the patronage a Caliph Al Mamun , made a series of measurements of meridian of the Earth which agreed with those of Ptolemy . These measurements virtually suffi ced until Wi lleb r od Snell ( 1 59 1 a Dutch mathematician who had dis covered the law of the refraction of light , made a series ’ o f measurements of the Earth s surface from which h e computed the length of a degree of a meridian to be about 6 6 7 miles, an estimate subsequently corrected to about 9 miles by one of his pupils and di ff ering but a few hundred feet from the value now accepted . A measurement by R ichard Norwood - 1 675 ) of the distance from 1 6 6 e London to York , made in 3 , enabled a degree of latitud t o be obtained with an error of less than half a mile , and fi 1 6 1 nally Picard, in 7 , made a measurement of this quan s ufli ci en tl tity, which we have seen was y accurate to be used by Newton in computing the mass and motion of the

Earth and in demonstrating hi s law of gravitation . This outline shows that the spherical shape of the Earth was constantly held in mind by scientists for about ’ years . Other views of the Earth s shape also Obtained . The idea of a ﬂat Earth had to be vigorously Opposed

by arguments founded on experiment , observation and ' discovery . After the phenomenon of a disappearing ship ,

1 50 ASTRONOMY

out h the Earth bulged at the equatorial regions . O n t e 1 other hand , in the following century ( in 745 ) the Cas sinis , who were leading astronomers of France and deeply ’ interested in the measurement of the Earth s surface , main t ai n ed that the Earth was contracted at the equator and th e bulged at the poles , or in other words was prolate , difference in the two ideas being concisely expressed by ’ Professor Moulton, who likens Newton s idea of the Earth ’ s ﬁgure to an orange and that of the Cassinis to a lemon . It is interesting that Newton ’ s proof of the oblateness o f the Earth anticipated the direct Observations of th e

- astronomer and geodesist . To day it can be demonstrated in simple form by means of observations of the time o f v s stemati ibration of a pendulum , such as are carried on y cally by such scientiﬁc agencies as the United States Coast n a d Geodetic Survey . It is possible to supply various mathematical proofs of the oblateness O f the Earth in connection with its motions and with its relations to the Moon and to the solar system . we But these hardly concern us in the present pages , and may dismiss the matter with the assertion that the e llip t i city of the Earth as determined by Harkness in 1 89 1 amounts to as compared with a value of estab li sh d 1 866 e e in by Colonel Clarke , of the English Ordnanc

Survey, and accepted for many years as a standard , and as was given by Newton for the ellipticity of a me ’ r idi an section of this oblate Spheroid . Clarke s spheroid o f 1 866 , which is generally adopted in geodetic and other computations , would give a radius at the equator of

miles and miles at the poles , these ﬁgures having been slightly corrected in 1 878 in the sec ond place of decimals . The radius of the Earth is in many ways a fundamental measure , especially in astronomy, and for that reason its determination is a matter of no small ﬁn d importance . Thus observations made with a view of i n d g the distance of the Moon , when discussed and reduce , THE EARTH 1 5 1 s imply give us this distance in terms of the equatorial i n radius of the Earth , so that to determine the distance miles we must know the number of miles in the Earth ’ s radius . As a consequence of the spheroidal shape of the Earth a a degree of latitude , as well as of longitude , v ries with the point at which it is measured . Thus at the equator one degree of latitude is equal to miles , but at the pole f i n a similar degree is equal to miles . The di ference a degree measured on the parallels of latitude , which are small circles , is of course obvious , and while at the equator a degree of longitude amounts to miles , at latitude ° or approximately that of New York (40 it would equal miles and at the north pole it would decrease, of course , to zero .

Newton in his researches on gravitation , by comparing the attraction exerted by the Earth with that of the Sun a n d other bodies , was able to connect its mass with that of the planets and Sun . The problem of determining the mass of the whole Earth in terms of a given terrestrial — — body that is, in tons , pounds or kilograms was indeed a serious one , and it was ﬁrst attacked in Great Britain by

Nevil Maskelyne ( 1 73 2 who was Astronomer Royal . r e I f we know the volume of the Earth and its density, of ferred to some standard as water, air or iron , it is course e i ts asy to determine its mass . But the determination of d ensity is no less a problem than the determination of i ts m ass directly . It is possible , of course , to compute the v relati e amount of water, but this gives only a small a mount of the mere surface of the Earth . We can de scend in mine shafts a mile or more and investigate the n f to ature of the material , but this again a fords no clue t h e thousands of feet of materials below , so that an esti mate o f the aver age den sity based on such data cannot be

. considered at all trustworthy Newton , taking. into con s e ideration the various elem nts of the Earth , estimated its d ensity at between five and six times that of water . Mas 1 5 2 A S TRONO MY kel n e i 1n of y , however, bear ng mind the phenomenon the deflection of the plumb - line observed by Bouguer and La Condamine in their expedition to measure an arc of latitude in Peru ( a phenomenon caused by the attraction of the mountain Chimborazo ) , selected the narrow ridge of S ch eh alli en in Perthshire and found that the attraction ” of the mountain caused a deflection of about 1 2 on each side of the ridge . This attraction of the mountain as compared with the attraction of the Earth on the plumb bob enabled a value for the density of the Earth of about

4% times that of water to be obtained . It remained for the famous Cavendish experiment, carried out by Henry Cavendish ( 1 73 1 to substitute for the mountain a pair of heavy balls . This gave a value for the density of the Earth of 5 % times that of water, a value conﬁrmed by ’ numerous repetitions of Cavendish s classical experiment .

Expressed in pounds , the mass of the Earth is a little 1 more than 3 billion billion , a figure that the human mind utterly fails to grasp . There must have been a time when the Earth contained much more heat than at present . The Sun is not supply ing heat enough to make good the losses sustained by the globe . These losses must have been taking place o over a period of countless years . One is f rced to believe that not only once was our Earth much hotter th an at pres ent, but that its temperature was a red heat . Going back still further, the now solid globe must have been actually difli cult a molten mass . Being a molten mass , it is not to realize that it readily assumed a globular form on its j our ney through space . A falling drop of rain is a globe ; a drop of oil suspended in a liquid with which it does not mix has a spherical shape . Therefore if a mass of material as large as the Earth were so soft that its individual par t i cles obeyed the forces of attraction exerted by each part of the mass on all the other parts , it is quite obvious that s it would assume a globular hape , especially under the f influence o motion . I f this great sphere were caused to

1 5 4 ASTRONOMY

E arth by no means explains the apparent motion of the

d . Sun , Moon and stars as regar s the planet As the nature o f the solar system was developed by Copernicus and n Newton , people were compelled to recog ise that the Earth r r o evolved around the Sun , and, furthermore , that it t ated daily about its axis . These motions are necessary to explain various phe n omen a . For example, when a body moves in a circle a r o force is needed to act on it, pulling toward the center p p ortionally to the square of the rate of spin and propor t i n ll o a y to the distance . Every pound of mass in the Moon would need a pull equal to about the weight of 3 o unces to keep it in a circle which it traveled around once i n 2 4 hours . Every pound in the Sun would need a pull

- a bout three quarters of a hundredweight . The nearest fi xed star would need a pull on every pound , o f about t ons . One cannot imagine that the Earth could exert such forces and so are obliged to think that the Earth is r evolv i n g and not the sky . Final proof of the rotation of the Earth was fur h ished in the middle o f the last century by Foucault . H is experiments are constantly repeated in physical labora t of ories and experimental lectures . From the dome the Pantheon in Paris he suspended a heavy ball by a fi n e cord and caused it to swing in a single plane. The was p ath o f the ball across the floor could be traced, and it found that after it had been in vibration for some time it w a s swinging in a di ff erent direction from that in which it h ad started . The reason was that the floor had revolved

under the ball with the rotation o f the Earth . The daily rotation of the Earth on its axis and its yearly r otation about the Sun supply us with our means o f meas “ uring time . According to Professor Poynting, The Earth i s a clock, the line to the Sun is the ﬁnger and the sky is t h e . face But the Sun is not a regular timekeeper . Our twenty - four- hour day is only the average between suc

c essi ve noons or times when the Sun is due south . If TH E EARTH 1 55

compared with a good clock , the Sun is in parts of the year too soon and in other parts too late , sometimes as o much as a quarter of an hour . This cannot be due t ’ change in the Earth s rate of Spin, for to change a Spin there must be either a force acting at one side of the cen ter of gravity or a change of shape . The forces on the Earth exerted by the Sun and Moon act almost exactly through the center of gravity and S 0 affect the rate o f Spi n fh hardly at all . The Earth does not change its shape su l c ien t y to account for the variation in the solar day . “ The variation in the solar day is due partly to the i n ’ c li n ati on of the Earth s axis to the plane in which it moves ’ around the Sun , partly to variation of the Earth s motion round the Sun at di fferent times of the year “ The ﬁxed stars keep good time , getting round in about

2 . 4 minutes less than 4 hours . By them clocks are rated

Their day is the true time of one revolution of the Earth . Among the movements of the Earth there is one that i s so minute as to be of peculiar interest to the astronomer . It is a small displacement or wobbling of the axis of th e

Earth or theoretical pole . This pole shifts its place through a circle of some few yards in diameter in the course of a period of somewhat over a year , and as a result ’ there is a variation of latitude on the Earth s surface . Kii stn er This was discovered ﬁrst in 1 884 and 1 88 5 by D r . at Berlin , and after observations made at various stations by members of the International Geodetic Association in 1 888 o c , it was found that this synchronous variation was i n curring a periodic manner at a number of stations . The general nature of the occurrence was deﬁned by Prof . 1 8 1 S . . C Chandler in 9 , who stated that the pole of the Earth might be supposed to describe a circle with a radius o f 30 feet in a period of fourteen months . One reason as signed for this phenomenon is a change in the distribution o f mass , due to the temporary occurrence of heavy falls

' of snow or rain limited to one continent , or to the transpor t ati on of great bodies of air and water from place to place 1 56 ASTRONOMY

by atmospheric or ocean currents , so that the globe is made slightly lopsided and temporarily to forsake its normal

abso axis . Also it has been supposed that the Earth is not lut l - e y rigid . As a quasi plastic mass it would yield to cer tain strains which tend to protract the time of circulation of the displaced pole . In fact , in all consideration of the various phenomena of gravitation and tides we have to recognise that in practice the astronomer of to - day is n ot dealing with the theoretically rigid body of Newton and the earlier mathematicians . One of the important conditions of the rotation of the Earth is that it moves around an axis which is not vertical but which is inclined toward the plane of its orbit . Dol ” - mage in his volume , Astronomy of To day , tells us that “ ‘ ’ I f the axi s o f the Earth stood straight up , so to speak , while the Earth revolved in its orbit , the Sun would plainly e ui va keep always on a level with the equator . This is q lent to stating that , in such circumstances , a person at the equator would see it rise each morning exactly in the east , — pass through the zenith that is , the point directly over head of him at midday—and set in the evening due in the A S west . this would go on unchangingly at the equator every day throughout the year, it should be clear that , at any particular place upon the Earth , the Sun would in these conditions always be seen to move in an unvarying manner across the sky at a certain altitude , depending upon the latitude of the place . Thus the more north one went upon the Earth ’ s surface the more southerly in the ’ o sky would the Sun s path lie , while at the north p le itself the Sun would always run round and round the horizon . Similarly the more south one went from the th e equator more northerly would the path of the Sun lie , while at the south pole it would be seen to skirt the hori t zone in the same manner as at the north pole . The resul of such an arrangement would be that each place upon the

Earth would always have one unvarying climate , in which

1 58 ASTRONOMY the Sun and the Moon to the north and south sides o f the equator respectively in the manner we have been d e scr ib ing . Were the Earth , however, a perfect sphere such f change of position would not produce any e fect . “ At present the north pole O f the heavens is quite close to a bright star in the tail of the constellation of the Little

Bear , which is consequently known as the pole star , but i n early Greek times it was at least ten times as far away from this star as it is now . After some years the pole will point to the constellation of Lyra , and Vega , the most brilliant star in that constellation , will then be con s i de r ed as the pole star . This slow twisting o f the Earth i s technically known as precession , or the precession of the equinoxes . “ We have seen that the orbit of the Earth is an ellipse and that the Sun is situated at what is called the focus , a point not in the middle o f the ellipse, but rather toward one o f its ends . Therefore during the course of the year the distance of the Earth from the Sun varies . The Sun , in consequence of this , is about miles nearer to us in our northern winter than it is in our northern summer, a statement which sounds somewhat paradoxical . This variation in distance , large as it appears in ﬁgures , can , however, not be productive of much alteration in the amount of solar heat which we receive , for during the ﬁrst week in January, when the distance is least , the Sun only looks about one - eighteenth broader than at the commence t ment of July , when the distance is greatest . The grea disparity in temperature between winter and summer de pends, as we have seen , upon causes of quite another kind, and varies between such wide limits that the eff ects of this slight alteration in the distance of the Sun from the Earth ” may be neglected for practical purposes . The most striking e ffect of gravitation in its universal a O f spect is seen on the Earth in the case the tides , which a r e due to the attraction of the Moon on the Earth and e specially on the waters o f its oceans . While the ancients TH E EARTH 1 59 could not explain the cause of the daily change in th e tides , the phenomenon was obvious to all mariners or those Th e dwelling by the sea or even on great rivers . i s connection of the Moon with this phenomenon , it ‘ th e 1 000 B . C . claimed, was understood as early as by

Chinese, for at full and new moon exceptionally high tides were observed, and the name of spring tides was applied as contrasted with the minimum high water which was observed at the neap tides at the second and fourth quarters o f th e Moon . Not only the Chinese but o the Greeks and Romans noticed this same phen menon , “ ” and Julius Caesar in his Commentaries tells us that when he was embarking his troops for Britain the tide was high e because the Moon was full , while both Pliny and Aristotl connect the time of high water with the age of the Moon . The connection between the Moon and the tide was thus early established and practically applied by navigators , who were quick to realize the nature of the phenomenon , even tho they and the astronomers were unable to present any satisfactory explanation . A general explanation of the tides as due to the disturb ing action of the Moon and Sun , especially the former, ’ as was put forward by Newton . Newton s explanation , ‘ f ’ described by Berry in his Short History o Astronomy, “ was as follows : I f the Earth be regarded as made of a c the solid Spherical nucleus , overed by the ocean , then f r Moon attracts di ferent parts unequally, and in particula on the attraction , measured by the acceleration produced th e the water nearest to the Moon , is greater than that on solid Eart h and that on the water farthest from the Moon

‘ o f is less . Consequently the water moves on the surface th e the Earth , the general character o f the motion being same as if the portion of the ocean on the side toward the

Moon were attracted and that on the opposite Side repelled . ’ Owing to the rotation of the Earth and the Moon s motion , the Moon returns to nearly the same position with respect t o any place on the Earth in a period which exceeds a day 1 60 ASTRONOMY

0 by ( on the average ) about 5 minutes , and consequently ’ Newton s argument showed that low tides (or high tides ) , due to the Moon , would follow one another at any given place at intervals equal to about half this period ; or, in other words , that two tides would in general occur daily , but that on each day any particular phase of the tides would occur on the average about 5 0 minutes later than

o n . the preceding day , a result agreeing with observation S imilar but smaller tides were shown by the same argu ment to arise from the action of the Sun and the actual tide to be due to the combination of the two . It was shown that at new and full moon the lunar and solar tides would be added together , whereas at the half moon they would tend to counteract one another , so that the observed fact of greater tides every fortnight received an explanation . A number of other peculiarities of the tides were also shown to result from the same principles . From Newton ’ s time to the present the tides have sup plied material for astronomical and mathematical research , so that through the efforts of many astronomers and other scientists their theory and occurrence are now well under stood, and the various governments make ample provision through hydrographic ofﬁces and otherwise to provide such information for mariners . In the theoretical discussion in modern times one name stands out prominently , that of

Sir George H . Darwin , of England , whose researches on t h e tides of the Earth have developed to a point where they have a most important bearing on cosmical theory . It is desirable here to outline the chief features of tides “ ” ﬂood - in their everyday occurrence . The term tide is ” - applied to the rising water , which reaches high water mark at the moment when the tide is highest , while in fall “ ” i n g the tide is said to ebb until low water , or the lowest mark, is reached . Near the times of new and full moon “ ” occur the spring tides , which are the highest tides of the “ - month as distinguished from the neap tides , which are the smallest, occurring when the Moon is in quadrature ,

1 62 ASTRONOMY

idly is rent asunder with violence . Here , therefore , we discern in the remote past a barrier which stops the pres ent argument . There i s a certain critical velocity which i s the greatest that the Earth could bear without risk of rup ture , but the exact amount of that velocity is a question not very easy to answer . It depends on the nature of the materials of the Earth ; it depends upon the temperature ; it depends upon the effect of pressure and on other details not accurately known to us . An estimate of the critical velocity has, however , been made, and it has been shown mathematically that the shortest period of rotation that the Earth could have, without ﬂying into pieces , is about h as three or four hours . The doctrine of tidal evolution thus conducted us to the conclusion that at some i n con c ei vably remote epoch the Earth was Spinning around i ts r axis in a pe iod approximating to three or four hours .

The existence of tides in the solid Earth , as well as in

the 1 86 . oceans, was ﬁrst indicated by Lord Kelvin in 7 The ﬁrst numerical estimate of the amount of the tidal oscillations of the solid Earth was made by Sir George 1 882 H . Darwin in and was based on an indirect method and resulted from observation of the tides of the ocean .

Fur ther indirect methods were successfully applied later, with more complete data .

Sir George H . Darwin has expressed the Opinion that we may now feel conﬁdent that the Earth yields to the tidal forces to the same degree as would a steel globe . The ’ amazing accuracy of Dr . Hecker s recent observations 1 08 ( 9 ) are marked by certain collateral considerations , ﬂexur among which may be mentioned the e of coast lines , e f due to the varying pressure of oceanic tides , and the fects o f high and low barometric pressure in bending the ’ Earth s surface . It is impossible to give an accurate measurement of the amount of the vertical motion of the ’ Earth s surface, but it is probable that at spring tides in this latitude the surface of the solid Earth moves up a n d down through about six inches . CHAPTER XIV

T H E MOON

THAT the Moon is the nearest the Earth of all the beav e n ly bodies is one of the most obvious facts which con

- front the star gazer . Its regular motion must have been early appreciated by primitive man , after he had realized that the rising and setting of the Sun marks regularly t recurrent intervals of time . As he was able to reﬂec u o pon his observations and properly to co rdinate them , he doubtless noted the connection between the form of the

Moon and its position in the S ky with respect to the Sun . For by keeping count with pebbles or rude notches cut in a stick he would learn that the interval o f time between r e curring full moons was always the same , and that the series of changes he could observe followed in regular o rder . Thus when the Moon appeared after sunset near the place where the Sun had disappeared he saw a thin crescent, the hollow side of which was turned away from the Sun . A little later the Moon set . The next night he observed the Moon further from the set Sun , with a thicker crescent , and noted also that it l ff ater, an e ect that gradually increased until the semi ﬂat circular disk , with the side turned away from the Sun , was seen in rather less than a week after the ﬁrst appear a - en nce of the crescent . In another week the semi circle lar ged to a complete disk and the Moon rose about sunset a n d set about sunrise , being in a direction nearly opposite to that of the Sun . From this time on the size again 1 63 1 64 ASTRONOMY

’ Di re cti on from wh i ch th e Sun s rays a r e comi n g . I l l !

V ari ous p osi ti on s an d i llum i n a ti on s o f th e M o o n b y th e S un e n E h d uri n g h e r r vo lut i o a roun d t h e a rt .

- F i . 2 1 O R 13 1T E O F T O O g AND PHAS S H E M N . The c o e on di n o i t i o n a s v i ewe d f o th e E a th h owi n rr sp g p s s r m r , s g h n n h t e c o seq ue t p a s e s . diminished ; the semi - circular form was seen once more with the ﬂat side still turned away from the Sun and to ward the west instead o f the east as the Moon approached the Sun on the other side, rising before it and setting in the daytime . Again he saw the crescent and marked that the time of rising approached that of sunrise , until the Moon became altogether invisible . Two or three nights inter vened and the new Moon reappeared , whereupon the whole

1 66 ASTRONOMY made a similar clear and definite statement of the reason for the phases of the Moon . Perhaps the first systematic study of the Moon was that of Aristarchus , a famous member of the Alexandrin e school , who flourished in the first half of the third cen tury and wrote a treatise “ On the Magnitudes and Dis ” tances of the Sun and Moon , which still survives . Tak t o ing the Moon when it was hal f full , so that a line drawn it from the Sun made a right angle with a line from the the Moon to the Earth , by measuring angle between the Moon and the Sun he was able to determine the ratio of the sides of this triangle and the relative distance of th e

Moon and the Sun from the Earth . While the method of

Aristarchus was ingenious, yet the result he obtained , that the Sun was 1 8 to 2 0 times as far distant as the

Moon , was sadly in error , for the actual distance is nearly fﬁ four hundred times . Still the di culties in the way of accurate observation were enormous for lack of prope r instruments . Aristarchus also took advantage of the solar eclipse to ascertain the distances of these two bodies , and reasoned correctly that when the Moon sometimes rather more than hides the surface of the Sun and some times does not quite cover it, their diameters must vary ob as their real distances . He even obtained by eclipse s e r vati on s a value for the diameter of the Moon in terms of that of the Earth . What would have been an excellent approximation was marred , however, by an incorrect esti mate of the apparent size of the Moon , which he took as

- two degrees instead of one half a degree . Nevertheless this work was an important advance in practical as

t r o n om . y It paved the way for Hipparchus , who discussed the motions of the Moon , motions which , long before his time , were known to be irregular and much more com plicated than those of the Sun . The path of the Moon i s always changing and its motions are subj ect to variation . Hipparchus notes that the part of the Moon ’ s path in which the motion is most rapid is not a lways in the THE MOON 1 67

on same position the celestial sphere , but moves continu o usl f y. He was able to realize the di ferent motions of the

Moon , to distinguish the various months based upon them and to employ the Chaldean and other early eclipse oh s e r vati on s for determining the position of the Moon at various earlier epochs . Hipparchus really evolved a most complicated set of motions . He was aware of the short o f comings his theory, but was unable to reconstruct it in a satisfactory manner . Following out the eclipse method of Aristarchus, Hipparchus made a determination of the relation between the distances of the Sun and Moon , measuring their angular diameters , and found that the distance of the Moon is nearly 59 times the radius o f the

Earth . Combining this ﬁgure for the distance of the

Sun with that of Aristarchus , a value of times the radius of the Earth was obtained , which value was em ployed for many centuries . 1n Ptolemy, the fourth book of the Almagest, discusses the length of the month with the theory of the Moon an d makes the important announcement of a further inequal i t ’ y in the Moon s motion , which Hipparchus had only suspected and which was due in large part to its position “ ” with respect to the Sun . This was later termed evection a n d involved a still further complication of the math emati c al computations o f Hipparchus because it meant the use 'i of an epicycle and a deferent, which was itself a mov ng e ccentric circle , the center of which revolved around the ’ Earth . Ptolemy s mathematics and ingenuity were able t o ﬁt his theory to observations . Altho his work showed m was any inconsistencies which , great as he was , he unable to control, nevertheless it represented a notable de v e lo me n t p in astronomy . To Ptolemy is due the parallax method for obtaining the distance of the Moon by obse r v ’ i n g its direction from two points on the Earth s surface a n d by ﬁnding the distance between these two points in ’ terms of the Earth s radius . This distance of the Moon h e h estimated was 59 times the radius of the Earth . Wit 1 68 ASTRONOMY

to this value, according the method of Hipparchus j ust mentioned , he computed the distance from the Earth to the Sun . He found that the distance was times the radius of the Earth , which value was equally in error as compared with the modern ﬁgure . The actual distance is about twenty times this amount . So readily were the motions of the Moon observed and so carefully were the records maintained that even at an early date much was known about its motions . These data enabled much calculation of the Moon ’ s motion to be car ried on , so that early mathematical astronomy dealt with the Moon in no small degree . Various irregularities of its motion and appearance were discussed, and practically all of the notable astronomers made some contributions to our knowledge o f the satellite . Even an outline of these discussions would lead the reader far aﬁeld into the depths o f mathematics , for which reason it is possible to mention only a few of the important researches and to indicate merely their general nature . The study of the i Moon had a pract cal importance , however , as lunar posi tions were early used to determine longitude in navigation and lunar tables were calculated by government and other e astronom rs . Nowhere was there more interest manifested in the problem of the motion of the Moon than at Greenwich

Observatory, where the matter had always been a spe “ ” ci alt y of that institution . It is a curious fact , states the late Professor Newcomb in his Reminiscences of an ” “ a ll Astronomer, that while that observatory supplied the Observations of the Moon , the investigations based upon these observations were made almost entirely by n foreig ers , who also constructed the tables by which the ’ Moon s motion was mapped out in advance . The most

perfect tables made were those of Hansen , the greatest master of mathematical astronomy during the middle o f

the century , whose tables of the Moon were published by 1 the British Government in 857 . They were based on a

1 76 ASTRONOMY c ombined together . They were claimed to explain the ’ ' ﬁ whole dif culty . The Moon , having followed Hansen s t heory for one hundred years , would not be likely to de i t v a e . from it Now it was deviating . What could it m ean ? “ ’ Taking it for granted , on Hansen s authority, that his t ables represented the motions of the Moon perfectly s 1 0 ince 75 , was there no possibility of learning anything from observations before that date ? As I have already s aid, the published observations with the usual instruments were not of that reﬁned character which would decide a ” q uestion like this . But observations of stars which might b e available, and which had been made in many instances , w ould have an important bearing on this work . For if a n occultation or passage o f the Moon between a star and t h e Earth occurred and its time registered , the path of the

. Moon in the heavens and the time at which it arrives a t each point of the path would be determined i f the time o f the occultation was known within one or two seconds . “ P rofessor Newcomb continues : It was not until after t he middle of the century ( seventeenth ) , when the tele s cope had been made part of astronomical instruments for

ﬁ nding the altitude of a heavenly body, and after the p endulum clock had been invented by Huygens , that the t ime o f an occultation could be ﬁxed with the required 1 6 0 1 6 0 e xactness . Thus it happens that from 4 to 7 some

what coarse observations o f the kind are available , and a fter the latter epoch those m ade by the French a str on o

mers become almos t equal to the modern ones in precision . “ The question that occurred to me was , Is it not pos s ible that such observations were made by astronomers l ong before 1 75 0 ? Searching the published memoirs of the French Academy of Sciences and the Philosophical O T ransactions , I found that a few such bservations were r e a ctually made between 1 660 and 1 700 . I computed and ’

d uced a few of them , finding with surprise that Hansen s B ut t ables were evidently much in error at that time . THE MOON 1 7 1 n either the cause, amount or nature of the error could be well determined without more observations than these . Was it not possible that these astronomers had made more than they had published ? The hope that material of this sort existed was encouraged by the discovery at the Pul kowa Observatory of an old manuscript by the French astronomer Delisle , containing some observations of thi s kind . I therefore planned a thoro search of the old rec ords in Europe to see what could be learned . By good fortune suitable observations were found at the Paris Observatory and their relations and the method o f making were studied and everythi n g necessary was copied . This work took some six weeks , but Professor N “ ewcomb says , The material I carried away proved the r greatest find I ever made . Three o four years were Spent in making all the calculations I have described . Then it -fiv e was found that seventy years were added, at a single ’ step , to the period during which the history of the Moon s w as motion could be written . Previously this history s upposed to commence with the observations of Bradley 1 0 n ow at Greenwich , about 75 ; it was extended back to 1 6 75 , and with a less degree of accuracy thirty years far ’ ther still . Hansen s tables were found to deviate from the truth in 1 675 and subsequent years to a surprising i s n ot un extent, but the cause of the deviation entirely folded even now . “ One curious result of this work is that the longitude o f the Moon may now be said to be known with greate r a ccuracy through the last quarter of the seventeenth cen Th e tury than during the ninety years from 1 75 0 to 1 840 . n o f e reason is that, for this more modern period, e fectiv comparison has been made between observations and Han ’ sen s tables . As the Moon rotates around the Earth while the Earth t s is passing in its orbit abou the Sun , it will be obviou that twice in its j ourney the Sun must come into a line o f ’ i ntersection of the Moon s orbit with that o f the Earth . 1 72 ASTRONOMY When this happens at the time o f full moon the Earth will l i e directly between the Moon and the Sun , so that the l ight from the Sun is intercepted and a shadow is formed ’ o n the Moon s surface . Sometimes the Moon only par ’ t i ally enters the Earth s shadow and then the eclipse, as thi s phenomenon is termed, is partial . I f , however , the S un is situated on the line of intersection at the time of n e w moon , then the Sun will be eclipsed, and a solar e clipse so rich in astronomical significance occurs . Solar e clipses have been discussed elsewhere and their i m p ortance explained . Lunar eclipses , besides enabling us

’ 2 —F O O F T H E E T O W OW I T H E E F ig . 3 RM AR H S SHAD , SH NG P NUM I O WI T I B A O R A T I ALLY SHADE D REG N . H N T H E E NU M B A R , P R P R T H E MOO N I s V I S I BLE ; I N T H E SHADO W IT I s N EARLY I N v1 s

I BLE . t o check the motions o f the M oon and furnishing an i ff nteresting spectacle , a ord little scientiﬁc information . When the black Shadow on the Moon is ﬁrst detected in t he o f case a total lunar eclipse , it is interesting to watch i ts encroachment until the entire surface of the satellite is

c . on n o overed Even the Moon , which direct sunlight can

- fall , is often visible , glowing with a copper colored hue , s uﬂi c i en tly bright to enable several of the markings on the s urface to be seen . This is due to refraction of the o atm sphere , which bends the sunbeams that have j ust grazed the Earth and permits them to fall within the s hadow. In their j ourney through the denser atmosphere they have become rich in red rays , which gives to the disk 3 ruddy or copper -like hue analogous to that of the Sun a t sunrise or sunset . Whether the eclipse of the Moon is total or partial d epends on the extent to which it passes

1 74 ASTRONOMY

tides could no longer exist, and the Moon would be eman c i ated p from tidal control . It seems impossible to predi c ate h ow far the Moon can ever con form to the circum s tances of a rigid body , but it may be conceivable that at some future time the tidal control shall have practically c e eas d, There would then be no longer any necessary i dentity between the period o f rotation and that of r evolu t ion . A gleam of hope is thus proj ected over the astron o m y o f the distant future . We know that the time o f

, revolution of the Moon is increasing, and so long as the t idal governor could act, the time of rotation must increase sympathetically . There will then be nothing to prevent t h e a s o f rotation remaining at present, while the period o f r evolution is increasing. The privilege seeing the o o f o ther side the M on , which has been withheld from all previous astronomers , may thus in the distant future be ” g ranted to their successors . o f i a While study the Moon and ts motions continued, b eginning was made in the Renaissance to examine the s o f 1 2 -1 1 urface this satellite . Leonardo da Vinci ( 45 5 9) w as the ﬁrst to explain correctly the dim illumination seen o ver the rest o f the surface o f the Moon when the bright part was only a thin crescent . This he maintained is due t o the earthshine o r slight illumination o f the Moon by l i s ight reﬂected from the Earth , j ust as moonshine able t o illuminate the Earth . Galileo ’ s lunar observations through his telescope were

- e poch making . Not only was he able to disprove many common conceptions of the nature of the satellite and its s ur face, but also to present a mass o f evidence of a positive c o f haracter. In spite the familiar dark markings , the

Moon was really suppos ed to be a smooth Sphere . After t h e introduction of the telescope, however , it was recog ’ n i sed by Galileo that the surface of the Earth s satellite wa s dotted with various inequalities , which he assumed to a c be mountains , valleys and seas . Thus he correctly i h c . ounted, part at least, for the unevenness of the surface THE MOON 75

He was not content with mere Observ ation of the feature s ’ th e e o f the Moon s surface, but measured height of som o f the more conspicuous lunar mountains and Obtained e for them an estimated elevation of four miles , a ﬁgur which agrees fairly well with modern estimates . Having ’ on i t seen and measured mountains the Moon s surface , e seemed natural that there should be water . The larg wa s dark Spots he erroneously regarded as seas , altho he n ot responsible for the corresponding names applied to these supposed expanses o f water by some o f hi s suc c esso r s . f , and still preserved in lunar maps The chie ’ marks of astronomical progress as revealed by Galileo s observation of the Moon were that it was a body in many i t was t respects similar to the Earth , that not a perfec ff b e Sphere, and that there is no fundamental di erence o ur own r tween celestial bodies and Earth , either in thei o r wa s motions in their general nature, which important in the ﬁnal establishment Of the Copernican theory. ’ One oth er discovery o f Galileo s in connection with the

Moon is of great importance . It had been known for many years that as the Moon revolved around the Earth n the same marki gs were constantly seen . With the tele scope these marki n gs could be studied so much more dis ti n ctly that it occurred to Galileo to ascertain whether ’ i n or e i ts there was any change the Moon s disk, wheth r

' wa H t appearance s always exactly the same . e found tha as the Moon moves in i ts orbit around the Earth that d i t . slight changes are seen in s appearance In other wor s, small portions of the hemisphere alternately on its north o f h ern and southern hal f are exposed . The simplest t e mot ions of the Moon in this way subsequently came to be ” known as librations . Kepler in his Epitome o f the Copernican astronomy demonstrated that his planetary laws applied to the motion s of the Moon around the Earth , despite irregularitie which introduced enormous complications . In thi s work, t o f the however, he devotes much attention o the theory 1 76 ASTRONOMY

Moon , explaining in considerable detail both evection and v ariation . Galileo established the fact that the Moon was similar t o the Earth in many respects . The analogy was carried s omewhat further by certain of the pioneer workers with l arge telescopes . Even Herschel held that because the

Moon closely resembled the Earth , it might be a suitable h abitat for human beings . The dark spots once taken for s eas and bearing that name on lunar maps are , in reality, l t th e ava , while h e craters which dot the surface of on e or satellite, with two possible exceptions , belong to v olcanoes long since extinct . The dark lines once known “ ” a s rills, which it was assumed were rivers , are plainly w ithout water . I f there is a lunar atmosphere its density m ust be very small, in fact less than that of the atmosphere far above the Earth . That there is a very rare lunar a a ssum tmosphere seems to be probable . In fact, the p t ion of an atmosphere is necessary for th e explanation of c ertain phenomena . After Galileo ’ s lunar Studies the next important work w as H evel 1 61 1 that of John , Of Danzig ( who pub li shed 1 6 S elen o r a hi a in 47 his g p , in which not only the t ext but the plates were prepared by him . Here he sys t emati cally describes and names the chief features of the n Moo , the immense craters and seas , employing many n ames taken from the Earth , such as Apennines , Alps , a n d Mare Serenitatis for the Paciﬁc Ocean , all designa tions still found on modern lunar maps . Not all of his n ames have survived . John Baptist Riccioli ( 1 598 i n a treatise on astronomy called The New Almagest, 1 6 1 5 , gave to various mountains and craters the names o f distinguished men of science and philosophers . Hence th e o f names Plato , Archimedes , Tycho and Copernicus a r e found on lunar maps . These names have survived in c - onsiderable number . More modern map makers , such as B M aedler 1 8 eer and , whose map was published in 37, and S o f wh chmidt Athens , o published a map o f the Moon

1 78 ASTRONOMY i s i s so miles , while that o f the Moon miles , that the diameters stand very nearly in the relation o f 1 o 4 to , while the superﬁcial area of the Moon is equal t about part of the surface of the Earth . The average d istance of the Moon fr om the E ar th i s also fairly con mo e stant, and the average ﬂuctuations do not exceed r t han about miles on either side of i ts mean value f o miles .

- T MOO i . T I Z T E T H E N F g 2 4 CO MPARA I VE S ES O F H E AR H AND .

The Moon is essentially a dead planet in the eyes o f m ost astronomers . Its ﬁres long since have been ex ti n ui sh ed g . It is a great globe o f chilled slag. Its craters have no counterpart on the Earth . The lunar crater i s 0 1 00 a great circular plane, 5 or even miles in diameter , around which rises a precipice perhaps ﬁve or ten thou i n two sand feet , while the center there may be a hill or about half as high . Thousands of these volcanoes are visible in the telescope . How these craters were formed is im a puzzle . Some astronomers hold that they mark the O f to be p act countless meteorites . Others assume them TH E MOON 1 79 t he product of gigantic bubbles in a once molten mass that burst . Again it is claimed that they are volcanoes resem bling those of the Earth . Most astronomers tell us that the craters have long been dead, that the Moon has had for centuries no atmos p h er e and therefore cannot have water or support plant

o r . . animal li fe Prof . William H Pickering, however , tells us of an exceedingly rare lunar atmosphere and main ’ n t ains that the Moon s craters are not all extinct . He eve claims that certain great white expanses are snow and ice , and, furthermore , that there is evidence of the growth t To f and decay of vege ation . support his views o vanish i n gly feeble volcanic acti vity he calls attention to the little

a . crater named Linné , after the famous naturalist , Linn eus In modern times this crater has unquestionably undergone

- ’ i t a changes . Only a few centuries ago w s described on the old maps as “ a crater of moderate size ” and later as “a ” v n ery small rou d brilliant spot . A dead volcano cannot alter its shape . I f there are still a few intermittently active volcanoes t W hey must expel ater and carbonic acid gas , j udging e by earthly volcano s . But water cannot be found on the i s Moon as a liquid, for the temperature o f its sur face ° 60 b . probably not far from 4 F . elow zero Many of the elevated peaks are capped with a silver glow, which also ’ characterizes the lining of the craterlets . In Pickering s e a c yes that white sheen is snow . He believes also in cumulations of snow and ice at the poles . I f carbonic acid is expelled by lunar volcanoes it must cling to the Moon w ith great tenacity because of its weight . Since it is the o fo d o f plant li fe, it may possibly support vegetation on the Moon . Professor Pickering sees in variable dark

spots on the planet organic life resembling vegetation . He ﬁgures that since certain lichens grow in certain regions o f the Earth where the temperature never rises above the

freezing point , there is no reason why that vegetation may ’ n ot ﬂourish on the Moon s surface . CHAPTER XV

MARS

MARS is another instance o f a celestial body whose dis cove r y long antedates written history, for which reason it is natural to assume that its appearance in the heavens an d It its motions have always been known to mankind . s ruddy light naturally associated it in the Greek mind with o f o f s the God War, j ust as the soft, warm glow Venu was considered appropriate to the Goddess of Love . To the Chaldean and to other ancient star - gazers the mo tions of the planet were indeed of interest . No planet h a s received greater attention from astronomers or has been used more for the solution O f important astronomical and cosmical problems . Thus it was from an observation of the passage of the

Moon in front of Mars , or what is termed by astronomers t an occultation of the planet, that Aristotle concluded tha the planets are more distant than the Sun and Moon . What reason he had for including the Sun it is di fﬁ cult to conceive , as an occultation by that body would be quite o impossible . Later it was fr m the study of the motions o f the planet made for Tycho Brahe that Kepler was able t o d erive his great laws o f planetary motion . By observing “ ” - the great Hour glass Sea , or Syrtis Maj or, on Mars , Huygens in 1 659 noted the rotation of the planet on i t s i t s axis , and from a measurement of the planet made at Opposition Richer was able to Obtain a reasonably accurate o f estimate the distance o f the Sun in 1 673 .

1 80

1 82 ASTRONOMY

be the existence of such satellites . Still that they might - found is forcibly reﬂected in eighteenth century literature . “ ’ ” In Gulliver s Travels ( 1 72 6) is the striking statement that an astronomer on the ﬂoating island of Laputa had discovered two tiny satellites of Mars . Dean Swift even advanced the extraordinary statement that one of these 1 0 a moons revolved around the planet in hours , the p proximate correctness o f which will presently be appar “ ” Mi cr o mé a s ent . Furthermore , Voltaire in his g , published “ ’ 1 2 in 75 and Obviously modeled on Gulliver s Travels , a n d also speaks o f these small bodies . Altho literature d imagination had preceded observation , astronomers coul not detect any companions to the ruddy planet until August 1 1 1 8 2 6- h , 77, when Professor Asaph Hall , using the inc re fractor o f the U . S . Naval Observatory at Washington , which at the time of its construction ( 1 873 ) was the lar s gest telescope in the world, studied the vicinity of Mar

w . 1 8 S O ith unusual care In 77 it was at conj unction , that conditions were favorable for an exhaustive inquiry; On August 1 1 Professor Hall detected a minute speck t O f light near the planet, and a few days later, Augus 1 6 , he ascertained positively that it was a satellite . On be the following evening a second body was discovered , wilder i n l n two g y rapid in its motio . Since that time the satellites have been frequently observed at opposition even 88 . 1 8 18 with much smaller instruments On July , , the large Lick telescope was able t o reveal them when thei r brightness was only about one -eighth that at the time of their discovery . n satellites The names appropriately assig ed to the new f

Deimos and Phobos ( Fear and Panic) , were taken from the Iliad and represent the companions in battle of the

God of War . The satellites are very small , having diame ters respectively of six and seven miles , as photometrically measured at Harvard University soon after their discovery r by Professor Pickering. Phobos , the inner, is the large 2 i . 2 . d and traverses an elliptical orbit in 7 h . 39 m s at a s MARS 1 83

t . ance of only miles Deimos , its companion , moves m i n 0 1 8 . a nearly circular orbit in 3 h . m at a distance fro t he planet of miles . It should be remembered in all discussions of the nature o f the surface of the planet Mars and in developing theo ries based thereon that astronomers must deal with a little d isk which once in ﬁfteen years has a maximum of about

the area of the full Moon . When nearest the Earth Mars is at a distance o f miles and when Ob s erved with a telescope magnifying times it appears o nly as large as the Moon with an ordinary field glass , e nlarging six or seven times . In other words , Mars is f brought to a distance of miles . Hence it is di ficult to distinguish minute details . Consequently visual and p hotographic observation of Mars must be o f the most r i s efined character, for especially in the former there O pportunity for psychological or subj ective phenomena to o ccur in connection with the observation . While it is neither rational nor scientific to deny what credible obser v e r s claim they have seen through the telescope in exami n i n Obse r g the planet , yet it must be recognised that visual v ati on s are attended by necessary limitations and should be r eceived in a spirit of caution rather than of enthusiasm .

The mapping of Mars is no recent matter, for even in 1 659 a rough sketch of the surface of the planet was made by Huygens in which the V -shaped marking at the equator pointing to the north can be identiﬁed as the Syrtis Maj or . This was followed by rough sketches from time to time 1 8 0 Maedler d own to 4 , when ﬁrst began a systematic 1 86 charting of the planet . His map was followed in 4 by ’ ’ ’

1 8 6 1 8 . Kaiser s , by Flammarion s in 7 and Green s in 77 D rawings of various parts of the planet were made during

. these intervals , but were not combined into good charts Observations made by Professor Lowell and his staff ff at the observatory at Flagsta , Arizona, have had the oh study of this planet especially in view . The Lowell s er vator y itself is situated feet above the sea and 1 84 ASTRONOMY contains an excellent 2 4-inch telescope with wh i ch it i s claimed that the faintest stars shown on the chart made at the Lick Observatory with a 36- inch instrument ar e perfectly visible . The atmosphere is well adapted for col telescopic work , so that Professor Lowell and his leagues possess singular advantages over other Observers . h as On the other hand , in many important respects there been a lack o f corroboration of their observations .

The surface of Mars , as seen in the telescope , is com o f a posed two white polar caps , which wane with the p proach of summer ; orange areas , which are supposed by - Lowell and his followers to be deserts , and blue green areas , which change their hue to orange during the Mar tian autumn and winter and reassume their verdant tint in spring. The planet is covered with a network of ﬁne ﬁ 1 8 lines, rst discovered by Schiaparelli in 77 and called by “ ”

w . him canals , a designation by which they are still kno n These canals connect the polar caps with the temperate and equatorial zones . According to Professor Lowell , they may be regarded as planetary irrigation ditches which serve the purpose of leading the melting water of th e poles to those desert regions which would still blossom i f properly watered . The canals disappear with the approach o f winter and creep down from the poles toward the equa i n astr on o tor summer , a phenomenon which long puzzled mers until Pickering ingeniously suggested that we see not the canals themselves ( for they are much too narrow) , but the vegetation which fringes their banks , which with ers as the cold of winter descends and which ﬂourishes with the melting of the snows . It was Schiaparelli who also ﬁrst announced the curiou s

doubling o f the canals in certain instances , an announce ment which was at ﬁrst received with derision but which an d has since been conﬁrmed by his disciple , Lowell ,

the astronomers of the great observatories . This curi o us gemmation has not been satisfactorily explained , altho Professor Lowell attributes it in part to vegetal

1 86 ASTRONOMY

(7 ) The extensive shrinkage of the polar snows Shows t heir quality to be inconsiderable and points to scanty d eposition due to dearth of water . ( 8 ) The melting takes place locally after the same gen e ral order and method, Martian year after year, both in t he south cap

’ n And in the north o e . This is evidenced by th e r ecurrence of rifts in the same places annually in each . T he water thus let loose can, therefore, be locally c ounted on . That the south polar cap is given t o greater ex t on e o f remes than the north implies again , in view the o f eccentricity of the orbit and the tilt the axis , that depo s itiou in both caps is light . 1 1 ) The polar seas at the edges o f the caps being t f emporary a fairs , the water from them must be fresh . The melting o f the caps on the one hand and their r e- forming on the other aﬂi r m the presence of water vapor i n o f the Martian atmosphere, whatever else that air c onsist . o f molec Since water vapor is present, which the u 1 8 lar weight is , it follows from the kinetic theory of g ases that nitrogen , oxygen and carbonic acid, of molecular w 28 2 8 too eights , 3 and 3 respectively, are probably there , , owing to being heavier . - The limb light bears testimony to this atmosphere . ’ ( 1 5 ) The planet s low albedo points to a density for t he atmosphere very much less than our own . The apparent evidence o f a twilight goes to afﬁrm

w ( 1 7) Permanent markings sho upon the disk , proving

that the surface itself is visible . Outside o f the polar cap the surface i s divided

- - - i nto red ocher and blue green regions . The red ocher stretches have the same appearance as our deserts seen f rom afar, MARS 1 87

1 a s ff ( 9) And behave such , being but little a ected by change . The blue -green areas were once thought t o be seas . But they cannot be such because they change in tint, th e according to the Martian season , and the area and amount of the lightening is not offset at the time by corre sp on di n g darkening elsewhere Nor by any augmentation of the other polar cap or due precipitation into cloud . It cannot, therefore , be to shift of substance . an d Furthermore , they are all seamed by lines i n spots darker than themselves , which are permanent place, so that there can be no bodies of water on the planet . - i s On the other hand , their color, blue green ,

. out that of vegetation This regularly fades , as vegetation s would , to ocher for the most part, but in places change o - t a chocolate brown . The change that comes over them i s seasonal in period , as that of vegetation would be . Each hemisphere undergoes this metamorphosis in turn . ( 2 6) That it i s recurrent i s again proof positive of an atmosphere . on e The changes are metabolic , since those in direction are later reversed to a restoration of the original . on status . Anabolic as well as katabolic processes thus go — t . s here that is , growth as well as decay takes place Thi proves them of vegetal origin . The existence of vegetation shows that carbonic th e acid, oxygen and undoubtedly nitrogen are present in i e Ma tian atmosphere, since plants give out oxygen and tak in carbonic acid . The changes in the dark areas follow upon the t melting of the polar caps , not occurring until after tha melting is under way , 0 e ( 3 ) And not immediately then , but only after the laps o f a certain time . 1 88 ASTRONOMY

1 n ow d (3 ) Tho not seas , from their look the ark areas s O ld On a uggest sea bottoms , and when the terminator p p ear as depressions ( whether because really at a lower level or because of less illumination is not certain ) . 32 ) That they are now the parts of the planet to sup oﬂi ce p ort vegetation hints the same past , as water would n aturally drain into them . That such a metamorphosis s hould occur with planetary aging is in keeping with the kinetic theory of gases . Terminator observations prove conclusively that t here are no mountains on Mars , but that the surface is s urprisingly ﬂat . 34) But they do reveal clouds which are usually rare a n d - are Often , i f not always , dust storms . un ( 35 ) White spots are occasionally visible , lasting c f r hanged o weeks in the tropic and temperate regions , s howing that the climate is apparently cold , But at the same time proving that most of the

- s urface has a temperature above the freezing point . In winter the temperate zones are more or less - c overed by a whitish veil , which may be hoar frost or may be cloud . A spring haze surrounds the north polar cap dur i ng the weeks that follow its most extensive melting.

( 39) Otherwise the Martian sky is perfectly clear, like ” t hat o f a dry and desert land . r ea These facts , according to Professor Lowell , make son ably evident on Mars the presence o f

Days and seasons substantially like our own .

( 2 ) An atmosphere containing water vapor, carbonic a cid and oxygen .

3 ) Water in great scarcity .

(4) A temperature colder than ours , but above the

- Fahrenheit freezing point, except in winter and in the ex treme polar regions . ” 5 ) Vegetation . While all of Professor Lowell ’ s observations and results

1 90 ASTRONOMY j ust quoted were further developed by him in the course “ o f as a few years succeeding its publication , and in Mars ” t h e o f 1 08 Abode Li fe, published in 9 , he expresses him s elf as even more ﬁrmly convinced that Mars is inhabited b y a race o f intelligent beings . Additional study o f t o i n Martian phenomena , according Professor Lowell , d i cates that the canals and oases as he sees them are proof t hat li fe of no mean order of intelligence prevails on the : planet . He suggests “ Part and parcel of this information is the order o f i n t lli en ce e g involved in the beings thus disclosed . Peculiarly i mpressive is the thought that life on another world should thus have made its presence known by its exercise of mind . T hat intelligence should thus mutely communicate its ex i sten ce of r e to us across the far stretches space , itself m aining hid, appeals to all that is highest and most far reaching in man himself . More satisfactory than strange t for n o of t his , in other way could the habitation the plane h ave been revealed . It simply shows again the supremacy f ar e o mind . Men live after they dead by what they have w o f t ritten while they are alive , and the inhabitants a plane t ell of themselves across Space as do individuals athwart ” t ime by the same imprinting o f their mind . T o this he adds the statement that conditions for life o n the planet are approaching an end, as it is rapidly dry i n o f g up , and that the energi es the inhabitants are being “ the slowly diminishing supply o f water . The drying up o f the planet is certain to proceed until its surface can ” s th e upport no li fe at all . So that, as compared with o f olu Earth, Mars presents a distinctly later period ev t 0 ion . N such danger at present confronts our own p lanet . But assuming that these explanations are correct , i t is not improbable that the coOp er ati ve action o f all th e nations o f the world may be required at s ome future d ate to deal with a similar problem . si In oppo ng the idea of canals on the surface of Mars , much stress h as been laid on the alleged fact that the MARS 191

finer markings and some o f the apparent doublings a r e based upon Optical illusions or psychological phenomena . e Thus, to prove that instinctively the eye would arrang in the form o f straight lines vague suggestions of mark o f ings, E . W . Maunder, the Greenwich Observatory, o f l England, and J . E . Evans , the Royal Hospital Schoo 1 02 a at Greenwich , in 9 performed an experiment with was number o f s choolboys . A circular disk exhibited on was fiv e o r six inches in diameter, which represented with some accuracy the shaded area o f the planet as it o f a few might appear in the telescope . Instead canals , o f e faint, wavy lines and a larger number faint dots wer to fill h inserted promiscuously . The boys were told in wit pencil on small circles on sheets o f paper the details o f th e t a s a s obj ect exhibited o them , exercising much care pos f d sible . The Obj ect o the experiment was n ot communicate to f o o f the boys . None o them had any idea f the nature ’ a th e the planet s surface . The result w s that boys at greatest distance from the Obj ect, where the faint lines and dots were j ust beyond the limits of separate visibility , drew canals strikingly like those noted by the t elescO pi c o f Observers the planet . Hence it was supposed that the eye in some way integrated such faint stimuli a s irregular scattered dots and faint, wavy lines into straight a s lines which have no obj ective existence . Much h been made o f thi s test , but it may hardly be called con e for elusiv , Flammarion when repeating the experiment with French boys was unable to secure in their sketches lines resembling canals . Moreover, photographs taken during the O pposition o f 1 90 5 at Flagstaff bring out a large number of the more prominent canals as straight

lines . i s It possible that psychology plays an important part ,

and Professor Andrew Ellicott Douglass , o f the Uni versity of Arizona , who has had Opportunities to observe t the planet at several favorable oppositions , believes tha 1 92 ASTRONOMY t h e subj ective phenomena have much to do with Martian o bservations . He says “ One may confidently say that such realities do exist . But with the very faint canals whose numbers reach occa s i on all su y well into the hundreds , discordance reigns preme, and it is frequently found that different drawings by the same artist antagonize each other across the page . Considerations along these lines led me to study seri o usly the origin of these inconsistent faint canals by the m ethods of experimental psychology , and the application o f these methods has resulted in a new Optical illusion and ” n e - w adaptations o f old and well known phenomena . The most important of these phenomena was the halo effect where secondary images are produced under unusual con di ti on s f , which images a fect the minute details of the sur face . Professor Douglass also found that the irregular refraction of the eye produced apparent rays as from a s mall spot , which could be obviated in part by changing h ’ t e . position of the observer s head The ray illusion , he “ s o f ays , is to me a very satisfactory explanation many faint canals radiating from those small spots on Mars ‘ ’ ‘ ’ c o . alled lakes r oases . The only obj ective reality in ” such cases is the spot from which they start . Referring again to the halo he reaches the conclusion t hat “ The halo with i ts light area and secondary image ac c for ounts details which have no obj ective reality, such as o f bright limbs definite width , canals paralleling the limb or dark areas , numerous light margins along dark areas o f and light areas in the midst dark, abundantly exempli ’ fi ed in Schiaparelli s map of 1 88 1 ad Professor Lowell , it must be said , has the unique or - vantage , misfortune it may be, to see canal like mark i ngs that are not visible to other astronomers . Thus on Venus he saw bands or lines which he considered bore a superficial resemblance to the canals of Mars and were ap ar en tl t o p y permanent and not due clouds . Again he claims

CHAPTER XVI

J UPITER

A s Jupiter comes next to Venus in point o f brilliancy o f the heavenly bodies , it is but natural that it should have been known to the ancients from a remote antiquity and that its discovery or early observation should be lost o f in a far distant past . The brightness Jupiter varies with its position , and the relative brightness of the planet at an average conj unction at the nearest and most remote

1 0 2 1 8 . Oppositions is respectively as the numbers , 7, and The orbit of the planet is but slightly inclined to the ° 1 or ecliptic, or and the planet itself moves with an bital velocity o f about eight miles a second in the sidereal n period of years , which is the time of its revolutio as around the Sun from a star to the same star again , o f seen from the Sun . The mean distance o f the orbit the c planet from the Sun is miles . The ecen t r i ci t - y i s nearly one twentieth , the greatest and least dis tances from the Sun being and miles respectively . The average distance o f the planet at from the Earth at opposition is miles , while conj unction it is miles . The minimum opposi 6 6 tion distance, occurring about October , amounts to 3 9, i s miles , while at aphelion in April the distance greater by about miles . Jupiter is larger than all the rest o f the planets in the W solar system , hether its bulk or its mass be taken into

consideration . Its surface is 1 1 9 times and its volum e JUPITER 5

times that o f the Earth . The mean diameter

m . iles , or about eleven times that of the Earth B r elative masses and volumes of p ared, it will be found that J o n e - quarter that of the a s that of the Sun . A body on Jupiter w t imes as much as upon the surface of the Earth , the mean superﬁcial gravity of the planet is g reat . Owing to its rapid rotations and its elliptic t h e di fference between the force of gravity at the and the pole is much greater and amounts to e quatorial gravity, where on the Earth it is as

The planet is brightest, as is also Saturn , in o f the disk, which it will be recalled is the cas

Sun , but not with Mars , Venus and Mercury . o f this resemblance to the Sun , the idea has been sug to g ested that the planet may be, some extent, self l uminous . The planet receives too small an amount o f heat from th e Sun to account for the rapid changes which beyond G q uestion are taking place on its visible surface . onse q uently, to produce these changes the heat must com e from the planet itself . Probably the body is at a temper a n o ture little below that of incandescence , t having solid i ﬁed to any appreciable extent . The most striking feature o f Jupiter is its system of b f right satellites , four of which were the ﬁrst fruits o astronomical discovery as it is now understood, and were revealed to Galileo when he directed his small telescope

t . o f oward the planet In fact, this historic observation 1 61 0 t o January 7, , meant much astronomy . When the g i' eat Italian scientist determined the periods of these “ s e a trange bodies , or Medic an stars , as he termed them , n ew era was opened in astronomy . The four satellites , in addition to being numbered in the order of their distance from the planet, are also known by the mythological names o f I o n , Europa , Ganymede and Callisto , and revolve i 1 96 ASTRONOMY

i 1 1 8 1 6 sidereal per ods , ranging from day , % hours to days , 1 1 6 75 hours at relati v e distances of between and

miles . From the small telescope of Galileo in the Opening years of the seventeenth century to the Lick refractor at the close of the nineteenth i s indeed a far l cry , but the four satellites of Jupiter remained alone unti a ﬁfth was added to their number by Professor E . E . Bar n 1 8 2 ard at Mt . Hamilton in September , 9 . This dis cov er y was as much a triumph for the modern telescope as the original detection of the four moons was an “ O ti ck i s achievement for the p Tube , for the satellite visible only with telescopes having a greater aperture 8 2 1 1 1 0 . than or inches It has a period of hours , and minutes , its nearness to the planet, miles , f makes it additionally di ﬁcult to see . e e But this was by no means the end, for where the y failed the photographic plate was available . In Decem 1 0 1 0 ber , 9 4, and January , 9 5 , Professor C . D . Perrine of

Lick Observatory added , by photography , two new moons ’ to Jupiter s system . Both o f these bodies revolve at a l greater distance than the older known satellites . Stil more recently P . Melotte of Greenwich Observatory, while examining a photograph made there on February 2 8 1 08 a n , 9 , found a faint obj ect which proved to be eighth satellite o f Jupiter , photographed several times at Th Greenwich; at Heidelberg and at Lick Observatory . e movement i s retrograde, which anomaly is of vast cosmi cal importance . The discovery of the satellites of Jupiter by Galileo was still another point which brought him in conﬂict with the 1 61 1 h Church . In there was published a tract in whic it was mentioned that the satellites of Jupiter were n u th e scriptural . This , apparently , was a minor issue with e ecclesiastical authorities , for the evidence of the telescop was incontrovertible . Galileo believed that it was possible to determine th e l s a ongitude at e by means of the satellites of Jupiter,

1 98 ASTRONOMY

the Accordingly, i f Jupiter is approaching the Earth , time which the light from one o f his moons takes to reach this planet must be gradually decreasing, and consequently there is less interval between successive eclipses as seen from the Earth than when the great planet i s departing f from it . Now the di ference of the intervals thus observed , together with the known rates of motion of Jupiter and of i t the Earth , which , of course , could be calculated , made possible to form a rough estimate of the speed at which suﬂi ci en t at light travels . Roemer had not observations th or ol hi s command to investigate this problem very y, but he was able to compute the apparent retardation of the eclipses between opposition and conj unction and thus to obtain a value for twice the time required for light t o come from the Sun to the Earth , which time was very 00 nearly 5 seconds , or eight minutes and twenty seconds . — “ This was the ﬁrst work on the so called equation o f ” light . It was many years before astronomers accepted Roemer ’ s really wonderful method for Obtaining the dis

- tance from the Sun . To day the process is reversed . By elaborate physical experiments made on the Earth ’ s sur face it is possible to obtain an accurate value for th e velocity o f light and then by means of the light equation to deduce the distance from the Sun . James Bradley ( 1 693 the third Astronome r

Royal of Great Britain , also devoted himself to the study ’ of the satellites of Jupiter . With Cassini s observations , h a s whic he used as the basis for some tables , as well many of his own dealing with the eclipses o f the satel a th e lites , he noted large number of discrepancies between ecu observations and the tables , and found even more p li a r i ti e s in their motions than did the early observers . Using Roemer ’ s suggestion of the ﬁnite time consumed by light in traveling from Jupiter to the Earth , which Cas s ini and other astronomers of his time had rej ected , Brad ley was able to make a series o f new and valuable table s ’ f 1 1 o Jupiter s satellites , which were printed in 7 9 in Hal JUPITER 1 99

’ “ ’ ley s Planetary and Lunar Tables . Bradley s knowledge o f the satellites of the planet was applied to the method of the determination of longitude suggested by Galileo , and with great accuracy he found the longitudes of Lisbon a n d o f N ew York . ° on Jupiter rotates its axis , which is inclined about 3 to the orbit, once in about 9 hours and 55 minutes , a time ffi for which is di cult to obtain more than approximately, when di fferent Spots are observed di fferent results are 1 66 obtained . These Spots were ﬁrst observed in 5 by Gio vanni Domenico Cassini ( 1 62 5 who was also the

- ﬁrst to study the so called belts . H e was able to report t h e discovery of the rotation o f the planet by watching the movement of the spots when observed through the tele “ ” s o f i s cope . One these spots the famous great red Spot

ﬁrst observed in modern times by Professor C . W.

1 8 8 . Pritchett in Glasgow, Missouri , in July , 7 This is a rosy cloud attached to the whitish z one beneath the dark southern equatorial band . Of enormous size, measuring some miles in longitude and somewhat l ess than miles in latitude , it was seen by several: o f it s bservers in Europe in the year o discovery , and in the following year was observed by almost every astr on o mer possessed of a telescope . For three years the red S was th e pot conspicuous . Then it began to fade . When ’ planet returned to opposition in 1 88 2 and 1 883 Rica s 1 1 88 observations of it at Palermo , May 3 , 3 , were expected to be the last . It began to recover , however , toward the e n d 1 886 t o o f the year, and at the beginning of , according

W . F . Denning, an English observer, had much the same a 1 88 2 spect as in October, . Before the “ great red spot astronomers had noticed ' th e lan et we various markings on p , one of which , as have 1 seen , was recorded by Cassini in 665 as having a rotation period o f 9 hours and 56 minutes . This spot reappeared and vanished eight times within the next forty- three years a nd M r l 1 1 a a da . was last seen by in 7 3 It was , however, 2 00 ASTRONOMY very much smaller than the recent obj ect and showed n o ‘ o f unusual color . Agnes M . Clerke , from whose History ’ Astronomy is abstracted this brief description of th e “ ” d great red spot , further iscusses the phenomenon as fol “ ‘ lows : The assiduous observations made on the Great ’ Red Spot by Mr . Denning at Bristol and by Professor Hough at Chicago afforded ground only for negative con clusi on s as to its nature . It certainly did not represent the outpourings of a Jovian volcano ; it was in no sense at — t ach ed to the Jovian soil i f the phrase have any applica tion to that planet ; it was not a mere disclosure of a glow ing mass elsewhere seethed over by rolling vapors . It f- was , indeed, certainly not sel luminous , a satellite pro j ected upon it in transit having been seen to show as b right as upon the dusky equatorial bands . “ A fundamental obj ection to all three hypotheses is that the rotation of the spot was variable . It did not then ride at anchor, but ﬂoated free . Some held that its surface was depressed below the average cloud level and that th e cavity was ﬁlled with vapors . Professor Wilson , on the 1 6- other hand, observing with the inch equatorial of the G oodsell Observatory in Minnesota , received a persistent impression of the obj ect’ s ‘being at a higher level than the ’ other markings . “ Mr A crucial experiment on this point was proposed by . 1 8 0 S tanley Williams in 9 . A dark spot moving faster along the same parallel was timed to overtake the red spot a toward the end of July . An unique opportunity hence p p ea r ed to be at hand for determining the relative vertical i n e vi t depths of the two formations , one of which must

i t . ably , was thought , pass above the other No forecast included a third alternative, which was nevertheless adopted by the dark spot . It evaded the obstacle in its path by skirting around the southern edge . “ d Nothing, then , was gained by the conj unction beyon an additional proof of the singular repellent inﬂuenc e exerted by the red spot over the markings in its vicinity .

CHAPTER XVII

SATURN

T H E planet Saturn was considered by the ancients to d be the most distant of the moving heavenly bo ies , a posi tion i t retained even after the tr iumph of the Copernican i deas an d the establishment of th e modern conception of s the solar ystem . The reason for this was that the period o f its oscillato r y motion to and fro in the heavens was longes t of all of the planets . The ancients noted that it took 2 972 years for Saturn to return to the same place 1 2 among the stars , as compared with years for Jupiter , and correspondingly less for the other planets down to

88 days for Mercury . Accordingly they considered that r Satu n was the most distant . 0 - 6 Eudoxus (4 9 35 as has been shown , believed that the motion o f Saturn , as in the case of Mars t and Jupi er , could be represented roughly by supposing that each planet oscillated to and fro on each side of a ﬁctitious planet which moved uni formly aroun d the celes th e tial sphere in or near ecliptic . The Slow period of Saturn also made it the most distant of the planets in the a s system devised by Copernicus , who computed its dis th e tance from Sun as nine times that of the Earth , which may be compared to his credit with the modern ﬁgure o f

times . After the development of observational astronomy Tycho Brahe in 1 5 63 made his ﬁrst recorded observation at o f the University o f Leipzig, noting the close approach

2 02 SATURN 2 03

Jupiter and Saturn , which he found was quite a month in error in the prediction of the Alfonsine Tables , published

‘ in Spain in 1 2 5 2 and in general use by astronomers o f throughout Europe . The next important observation Saturn was indeed of epoch - making signiﬁcance in astron m o y. With his new telescope it was but natural that

Galileo should examine Saturn as he did the other planets . Turning his telescope toward Saturn he observed that that planet, too , was not single and complete , but appar ently consisted of three parts , or , as it appeared in a draw two ing made at the time by him , of a central body and n to satellites in close proximity , which aturally seemed resemble those of Jupiter . At subsequent observations he h n failed to see more t an the central and larger portio , f and, consequently , completely ba ﬂed, he left the problem as a legacy to his successors . The two appendages were seen and described under varying conditions by a number of astronomers , but the true solution was ﬁrst furnished by Huygens ( 1 62 9 when he studied with one of his powerful telescopes the appearance of the planet . Huygens announced in 1 655 that he had discovered a single satellite, which he named

Titan . With a still more powerful instrument he found that the effect of two component bodies observed by Gali leo was due to the ﬁne ring which surrounded the planet and was inclined at a considerable angle to the plane of the ecliptic and consequently to the plane in which

Saturn proceeds around the Sun . As the ring was ex t r em el w a s y ﬁne it became invisible, either when its area di rectly opposite to the observer or when it was directed toward the Sun , as in that case it received no light for reﬂection . Near this opposition or invisibility the ring ap pears to he foreshortened and presents the appearance o f two arms proj ecting from the body of the planet . The ring, o f course, gradually widens from its opposition or l invisibility and the Opening becomes visib e, a period o f 2 04 ASTRONOMY seven years e -asp1n g between such a state and when the ring is seen at its widest . With the observations o f Huygens the reasons for Gali ’ O leo s varied bservations were furnished . To make the matter more conclusive Huygens collected and published a series of early drawings by various observers , which drawings he compared with his own observations . Thus what Galileo conceived as two satellites was really the

— E T i 2 E TI E S I z E s O F S T TH E . F g . 5 R LA V A URN AND AR H

ring when seen with its greatest breadth . The di sap p ear an ce o f these satellites occurred when the edge o f th e the ring was presented to his view, the revolution of planet giving to an observer on the Earth a series of phases in which the appearance of the planet is remark ff ably di erent . What Huygens saw is now familiar to every one who has observed S aturn through a telescope . Surrounding ’ the central body are rings parallel to the planet s equator, but inclined about 2 7 degrees to the plane of its orbit and

2 06 A STRONOMY

at variance with any such hypothesis . He and other as t r on omer s had frequently detected in both o f the bright rings fine dark lines of division , and as these frequently lapsed into impercep tibility the condition was due in his opinion to the real mobility of their particles and indicated a fluid formation . The known solidity of the rin g s was then demonstrated on abstract grounds by Professor Ben m j a in Peirce of Harvard University , who maintained that they were formed by streams of some fluid denser than 1 8 water . In 57 , in England, James Clerk Maxwell , the famous mathematician and physicist , presented a mathe matic al discussion of the subj ect , in which he stated that n either solid nor fluid rings could exist and that the sys tem could be composed only of a great multitude of un col lected particles which revolved independently in a period corresponding with their distance from the planet . This o f idea a satellite formation , remarkably enough , had been l n severa times entertained and lost sight of , so that whe h advanced by Maxwell it was a virtual novelty . The y othe i l s O p s s met the test of t e e c pi c observation . The mathematical theory of the ring system found an

analogy in the assemblage of planetoids , both visible and invisible , which are known to be revolving around the Sun with orbits situated between Mars and Jupiter . I f seen from a considerable distance such a swarm o f these ' small particles would give the impression of a continuous solid

body, so that on its physical basis this theory did not

seem improbable . It was pointed out by Kirkwood in 1 86 ﬁ 7 that the division between the two main orbits , rst

made by Cassini , could be explained by the perturba

tions due to certain of the satellites , j ust as the corre spo n di n g gaps o f the minor planets are explained by a -t the c ion of Jupiter . But in modern astronomy the ff probability o f a mathematical theory is not su icient, and its acceptance must depend upon direct and conclusive evi

dence from telescope , spectroscope or photographic plate .

This was supplied most effectively by Professor James E . SATURN 2 6 7

18 Keeler ( 57 at Allegheny Observatory, Pittsburg, th e an d in 1 895 . He pointed his spectroscope to planet found by examining the light waves from opposite sides on that the main body was in rotation . The light from e side was approaching the Earth and from the other it was receding . This rotation of the planet , o f course , had been realized for some years , but the axial rotation o f the rings had never before been demonstrated , and i n this Keeler proved , revealing the strange fact that the r i o t e r part o f the rings rotated faster than the exterior , which of course would not hold true in the case of a solid o ff body . The motion Slowed outward in agreement with i n the diminishing speed of particles traveling freely, each o n its w orbit . The visibility o f the rings when the Sun and the Earth are on Opposite Sides of their plane is explained by Pro fe s sor Barnard as due to the ﬁltration of sunlight through th e a cloud o f cosmical dust . H e regards the knots as result of the radiation of parts of the clouds which are k n denser , but not necessarily thic er than the rest u der the illumination o f sunlight , which gives to them their adj a o f cent portions less density . Percival Lowell , however, believes that the rings of Saturn are not ﬂat and of uni o n i o f f rm thick ess , but rather resemble a concentric ser es tores or anchor rings , and that the knots represent their

ﬁxed portions .

Professor Si r G . H . Darwin explains the rings of S a an a turn by considering them bortive satellite , scattered by tidal action into annular form , for they lie closer to the planet than i s consisten t with the integrity of a revolving

. a body of reciprocal bulk This interesting appendage, c a cording to Professor Darwin , will eventually disappear, s the constituent particles will be dispersed inward in part and will be gathered to the surface o f the planet , while in part they will scatter outward where they may coalesce unhindered by the strain of unequal attraction . Then on e modest planet revolving within Mimas would be all 2 08 ASTRONOMY that would rema i n of appurtenances which lend char

acter to the planet . The dimensions of these wonderful rings o f Saturn ’ doubtless will arouse the reader s curiosity . The planet itself has an equatorial diameter of miles . Out side o f this ﬁrst comes the crape ring at a distance of

from nine to ten thousand miles of clear space , and

somewhat less than miles in width . The crape

ring is j oined to the second ring, which is the most brilliant and is about miles in width . Then comes ' about 1 6oo a gap of , miles , and there lies the outer ring,

miles in width , with an exterior diameter of about h miles . Hence the entire ring system has a widt f o between and miles . A model of the outer

ring, constructed on the scale o f miles to the inch , could be made with an approximation to accuracy from a a m sheet of writing paper ne rly seventeen inches in dia eter . Cassini ’ s discovery of the dark markings in Saturn ’ s ring was one result of a series of telescopic observations

which he made of the planet , in which he discovered four n ew — 1 6 1 1 6 2 satellites Japetus in 7 , Rhea in 7 , and Dione 1 and Thetis in 684. This list of satellites was increased by 1 8 o - two more in 7 9, when Herschel , using his 4 foot tele 2 8 th scope for the ﬁrst time , August , detected a sixth 1 satellite of Saturn , Enceladus , and on September 7th dis

a . covered fainter satellite, Mimas Both of these were nearer to the planet than any of the ﬁve previously ob 1 8 8 served . In September , 4 , W . C . Bond, of Cambridge ,

Mass . , discovered Hyperion , and two days later this same satellite was also observed at Liverpool by William Las

sell .

' o A ninth satellite o f Saturn , Ph ebe , was discovered by s Professor W. H . Pickering on photographic plate taken

at the Harvard Observatory at Arequipa , Peru , and was a 1 0 nnounced in July , 9 4, the satellite being seen on a num o f ber recent photographs . This satellite is much smaller

than any of the existing moons ; so much so , in fact, that

CHAPTER XVIII

URANUS A N D N EPT UNE

T H E discovery of Uranus distinctly represents one of the most important results of modern methods in astron o m y. The other planets considered were known from pre o f historic times . Even the least conspicuous them could be observed with the naked eye under favorable conditions . Just as the satellites of Jupiter were the first fruits of th e telescopic discovery in the heavens , so Uranus was first planet to be telescopically added to the list which through centuries had been known and studied . 1 1 8 1 Its discovery was made on March 3 , 7 , by Si r William Herschel while engaged in the systematic ex ami nation o f every stellar body visible with his 7 - foot r efle ct : ing telescope . Herschel states that In examining the ‘ ’ small stars in the neighborhood of H . Geminorum I per cei ved one that appeared visibly larger than the rest being struck with its uncommon appearance I compared it ‘ ’ b t o H . Geminorum and the small star in the quartile e ‘ ’ ‘ ’ r tween Auriga and Gemini , and finding it so much large ” than either of them I suspected it to be a comet .

Though it appeared as a star of the sixth magnitude, - its di fference from the other stars was at once a ppr e a ci a ted by Herschel and is evidence of that keenness o f

sight which was so characteristic of him . Observations o f the new body and study of its orbit failed to establish it

as a comet . Within three or four months of its discovery the conclusion was reached ﬁrst by Anders Johann Lexell

2 1 0 URANUS AND NEPTU N E 2 1 1

( 1 740 - 1 784) that it was a new planet which revolved around the Sun in an orbit nearly circular at a di stance o f about nineteen times that of the Earth and nearly double that of Saturn . Herschel ’ s discovery at once won for him a national r e reputation and royal honors , which he attempted to c i r ocat p e by con ferring the name of his royal patron ,

George III . , on the new planet , calling it Georgium Sidus .

But the name never gained currency outside of England . ’ After a vain attempt to apply Herschel s name to it, the old mythological nomenclature was observed and the new su p lanet became permanently known as Uranus , at the g g estion of Bode , after the father of Saturn and the grand be father o f Jupiter . The name means Heaven itself , yond which it was supposed nothing further could b e found . Q

Following its discovery by Herschel , with a reﬂecting 0 - telescope 4 feet in length and of 4 foot aperture , came 1 1 1 8 the detection on January , 7 7, of two moons or satel a lites of Uranus , to which the names of Oberon and Titani were subsequently given . Herschel discovered that these moons moved almost at right angles to the ecliptic in a direction contrary to that of all previously known mem r sus be s of the solar kingdom except the comets . He ected p the existence o f four more such satellites , but he was not able to assure himsel f positively of their exist

e . nce In fact , it was only his large telescope and his keen

eye that enabled the ﬁrst two moons to be observed . But with the progress of astronomy and the improvement o f

instruments other discoveries were bound to come .

- Within the paths of Oberon and Titania , Ariel and Um 2 1 8 1 briel were found , October 4, 5 , by William Lassell, a wealthy brewer, who during his life devoted himself

assiduously and with great success to astronomy, espe

a ll t ele sc0 i c . c i y p observation These satellites , altho not

easily visible in a telescope on account of their distance, a r e much larger than the satellites o f Mars or in fact 2 1 2 ASTRONOMY

many of the planetoids . It is estimated that their diame

00 . ters are between 5 and miles Oberon , which is dis tant from the planet miles , has a period of rota 1 tion of 3% days ; Titania , the largest and brightest , dis tant miles , has a period of days ; Umbriel , dis t ant miles , has a period of days ; and Ari el , dis tant miles , has a period o f days . These satel i n lites all move the same plane , which is inclined about 8 ° ’ 9 to the plane of the planet s orbit . The satellites re volve in a retrograde direction . The surface markings observed on Uranus by many astronomers have been vague and transitory , so that any determination o f the period o f rotation is but app r oxi

. a 1 0 1 2 mate Nevertheless , period of or hours has been indicated . It is stated that the plane of the equato r i s inclined something like 1 0 to 30 degrees to the plane of the orbit of the satellite . The disk of the planet shows a ﬂat tening at the poles , so that it has an elliptical section . The appearance o f Uranus to the naked eye is that of a small star o f about the sixth magnitude . It was on this account that a high power telescope was required to di f fer en ti at e it from the myriad other stars of this size to establish it as a planet . It is so far away that there is but little change in its position whether it is in opposition or In t h e quadrature . Measuring the disc , which appears - th e telescope to be of a sea green color , the diameter of planet is found to be about miles , or four times as great as that of the Earth , which would give it a volume 6 4 times greater . But , like the other distant planets , 6 Uranus is composed of lighter materials , so that while 4 1 times as large its mass is but 5 times that of the Earth , or, in other words , Uranus would compare with the Earth in about the same proportion as regards volumes as does the Moon with the Earth . The elliptical orbit in which Uranus moves at a mean 1 8 00 o r e distance from the Sun of nearly , , ooo , oo miles 8 of quires 4 years for its passage, and the diameter this

2 1 4 ASTRONOMY

h mathematical work, assiduously addressed himsel f to t e 1 8 problem , and in 45 sent to the Astronomer Royal at Greenwich numerical estimates of the elements and mass of the unknown planet , together with an indication of its ’ actual place in the heavens . Un fortunately , Adams work , for various reasons , was not taken up by the government astronomers , and in the meantime Urbain Jean Joseph Leverrier ( 1 8 1 1 as a result of the study of the stability of the solar system , and especially of the Uranian diﬂi cult 1 8 y, to which his attention had been directed in 45 by Arago, announced before the French Academy that f only an exterior planet could produce the Observed e fects . Such an announcement aroused astronomers to a point d o f expectancy , and in fact , as Sir John Herschel declare to the British Association regarding the hypothetical new “ planet, We see it as Columbus saw America from the ” coast o f Spain . In less than two weeks from the time o f this utterance the message quoted was sent to Galle at e Berlin . Within a week N ptune was also observed in

England, where delays lost the honor of priority for

Adams .

Once the existence of the new planet was established, a few weeks ’ observation made possible the computation of its orbit and its identiﬁcation with what had been con sider ed a ﬁxed star . Neptune Supplied the exception which proved the rule ’ in the case o f Bode s law , discussed in the chapter on planetoids , for its mean distance from the Sun was 6 miles instead of 3 , oo , ooo , ooo as would be required under the terms of the law . Furthermore , Neptune has an orbit with an eccentricity of only so that its path is more nearly circular than that of any other member o f the solar system except Mercury . But so large is its orbit that the small eccentricity makes a variation of over miles in the distance of the planet from the Sun ff at di erent parts of its orbit . Moving with an orbital URANUS A ND NEPTUNE 2 1 5

1 6 velocity o f about 3 % miles a second, it requires 4 years for its j ourney around the Sun .

Neptune has but one satellite , which Lassell found in 1 6 84 , within a month o f the original discovery of the planet . Its distance is about miles and its period

2 1 . o f revolution is 5 days and hours It i s a small body, about miles in diameter , or about the size of the ’ Earth s Moon , and it moves backward j ust as the satel ° o f 1 to t lites Uranus , in an orbit that is inclined 45 tha o f the planet . i s Like Uranus , Neptune varies little, as its distance s o great that any variation by change in the position o f the planet would not aff ect its appearance on the Earth .

Its diameter is estimated at about miles , which would give a volume 85 times that of the Earth ( or ac cording to Pro fessor T . J . J . See, U . S . N miles ) but, as also the case with Uranus , it is much lighter than A S the Earth . its density is probably about its m o f i ts ass , which astronomers compute from the motion satellite , is about seventeen times that of the Earth . On account of i ts great distance it receives from the Sun about the amount of heat that falls upon the Earth . I f its capacity for absorbing and retaining heat are the °

60 . same , the theoretical temperature would be about 3 F ° ( 2 2 0 or between the temperature o f liquid air and m the boiling point of hydrogen , a te perature so cold that on the Earth complex methods to produce it have to be adopted in a physical laboratory . Nevertheless , the amount o f m light received fro the Sun is not insigniﬁcant, and the noonday illumination of the planet would be some 700 times that of brightest moonlight . I f the Sun were placed

w 68 . at Neptune , its light ould equal that of 7 full moons

No surface markings have yet been seen on Neptune . Consequently astronomers are unable to determine its rate of rotation by direct observation ; but from various i n t r i c ate processes Neptune is believed to have a slower

rotation than Jupiter or Saturn . CHAPTER XIX

TH E PLANETOIDS

I N the chapter on the Solar System it has been Shown how Titius in 1 772 pointed out that if 4 should be added to the following series ( irregular in that the ﬁrst number 0 0 6 1 2 2 8 should be V2 instead of ) , 3 , , , 4, 4 , a sequence of numbers would be obtained that indicated the relative distances of the six planets , with the striking exception that next after Mars there was no representative . Bode

ﬁlled this gap with a hypothetical planet , and his name is often associated with that of Titius in the statement of this law . After the discovery of Uranus by Herschel in 1 78 1 the law of Titius or Bode , with which this planet conformed , c gained onsiderable respect from astronomers , altho no mathematical or other reason was known to uphold the singular relation between the distances of the planets from the Sun . At any rate , the conviction that there must be an undiscovered planet between the orbits of

Mars and Jupiter, as was even suspected by Kepler in his “ ” Co smo r a hi cum 1 6 Mysterium g p of 59 , was strengthened , and accordingly it was proposed by a voluntary association of astronomers in 1 789 to undertake a systematic search for the missing planet . An organization was arranged on a fairly methodical basis by Baron Von Zach ; but before ” these celestial police , as the astronomers enlisted for this purpose were humorously termed by their organizer, had secured results from their plan of cooperation the

2 1 6

2 18 ASTRONOMY

e a v ry position assigned by Gauss to the runaway planet, strange star was discovered , which ﬁlled the gap in the series of Titius , and in the following year ( March , 1 8 - 1 8 0 v at Bremen , Dr . Heinrich Olbers ( 75 4 ) also obser ed a o f similar star . The ﬁrst of these , at the request Piazzi,

— 2 T F I O ET . F i g . 6 PA HS O M N R PLAN S w as o f named Ceres , after the protecting divinity Sicily, m while the second , which furnished an additional proble of for Gauss , was named Pallas . Harding, the assistant ch r Ot r 1 80 S e at Lilienthal , discovered ( 4) Juno as the 1 80 third , and Olbers ( 7 ) Vesta , the brightest of all the — of p lanetoids , as the fourth all lying between the orbits THE PLANETOID S 2 19

. the Mars and Jupiter There were thus , in contradiction to the law of Titius , instead of one planet , four present in zone between Mars and Jupiter . They were designated “ ” tho n asteroids by Sir William Herschel , the more moder “ ” term planetoids is to be preferred .

Thus was inaugurated the discovery of minor planets , which continued during the nineteenth century and is still

‘ in progress . Where one new planet might be anticipated according to older doctrines , many were found . Various theories have . been advanced to explain their existence and occurrence . That Ceres and Pallas were parts of an ex p loded planet was an ingenious theory which received for a time considerable acceptance , but which was disproved by r Professor Newcomb . The recognition of the mino planets became of the highest importance in astronomical an d science, and led to the construction of star maps to further interest in the observations of the heavens . The discovery of the ﬁfth planetoid did not occur for a number of years and then fell to the lot of an amateur i n astronomer, Postmaster Hencke , at Driessen , who was dust r i o usly searching with his telescope the quarter of the k s y lying opposite the Sun . He made a practice of map ping all the little stars and of comparing them again on 1 8 n ew the following nights . At last ( 45 ) he found a a 1 8 p lanetoid , to which he gave the name Astr ea , and in 47 a second , Hebe . Since then no year has passed without be the discovery of one or more new planets . This may o f hi s attributed not only to the example Hencke , with systematic industry , but also to the perfecting of the telescope and to the publication of printed special sta r charts upon the basis of the localizations of numerous e ﬁxed stars . Thus by the year 1 868 the number of littl 1 00 1 8 2 00 1 8 0 00 1 8 planets had increased to ; in 79, ; 9 , 3 ; 95 ,

00 1 0 00 1 06 600 1 0 00 1 08 8 00 . u 4 ; 9 3 , 5 ; 9 , ; 9 7 , 7 ; 9 , Co se - c quently, instead of one looked for planet, suspe ted by nd Kepler, a complete stream is present between Mars a

I u i ter . 2 6. J p , as indicated in Fig 2 2 0 ASTRONOMY

The zeal of astronomers was directed toward the ob servation of the numerous newly - discovered heavenly bodies , and the most fortunate discoverers achieving suc cess in this ﬁeld were Hind ( at London ) , Goldschmidt , an a mateur ( at Paris ) Gasparis ( at Naples) , Robert Luther Dii sseldor f ( at Bilk near ) , Chacornac and the brothers

— 2 7 CO M PARA TI VE S I Z E S O F T H E MO O N AND T HR E E MI N O R T PLAN E S .

P P aul and Prosper Henry , who discovered at aris , during

the construction of stellar charts of the zodiac , a number o f planets among the numerous charted small ﬁxed stars ; a n d from the United States , Watson at Ann Arbor, and

C . H . F . Peters at Clinton , who astonished the world by

their numerous discoveries . Johann Palisa should be n O amed above all . While director of the bservatory at P a 8 ol he discovered 3 new planets , the maj ority with a

2 2 2 ASTRONOMY t he microscope after development and ﬁxing. In this way more planets have been discovered in Heidelberg since 1 8 2 9 than by all previous observers together . Charlois a t Nice emulated Professor Wol ff in applying the photo r so 1 8 2 g aphic method , that there has developed since 9 , e f with great succ ss , a new ﬁeld o astronomical photog r a h p y.

An American astronomer, the Rev . Joel Metcalf, at

Taunton , Mass . , has improved the technique of observation .

In 1 905 he found two and in 1 906 twelve n ew planetoids . S ince 1 892 newly discovered planets have received pro v isional designations of the letters A , B to Z ; AA , AB to 2 2 A Z . ; BA , etc , to , and with the number of the year preceding . If it has once been established that they can be observed sufficiently long to guarantee ﬁnding them a gain , and that they are identical with none of the earlier p lanets , they receive the next numeral and name , and are t un n ec e s hus included in the system . In this manner an s ary increase in the lost planets of the system i s avoided . In 1 907 the crucial moment came in which the planet 2 2 w as discovered , and the series of provisional designations began again . Up to 1 896 all the 432 planetoids moved in the zone b o f i n etween the solar orbits Mars and Jupiter ; and,

- d . eed , all with the same right handed motion Consequently n o 1 1 8 8 little attention was attracted when on August 3 , 9 , d Witt, at the Urania at Berlin , photographically discovere ' a planet with an unusually rapid motion . The computa t o f ion its orbit showed great proximity to the Earth , and a mean solar distance of times the distance o f the E arth ; hence less than that of Mars Was thi s c elestial body to be counted in with the stream o f planet o ids between Mars and Jupiter ? Since the p ossibility e xisted that still other planets might be discovered which w ere not conﬁned to the zone between the orbits o f Mars a n d to Jupiter, it was determined include it with the THE PLANETO IDS 2 2 3

n planetoids with the umeral 433 , but to distinguish it by the masculine name Eros . The orbit of this planetoid was found to lie in that forbidden territory within the path of Mars and be o tween it and the Earth . With the single exception f the Moon it is the nearest large heavenly body to th e

. s Earth This little planet, hardly more than twenty mile i ts o in diameter, in addition to an malous position , pos t sesses remarkable characteristics , among them an orbi o f t o n great eccentricity, which causes it be at aphelio some miles beyond the mean distance o f Mars .

An examination of Fig . 2 8 will reveal how very interesting i ts n orbit is , for on the rare occasions o which the planet o id comes nearest to the Earth it i s closer to the Earth t or han Mars Venus can ever be . n ot i s but Furthermore, this little planet only variable v as aries in its variability, which has given rise to the s on umption that either rapid changes are taking place it, or its light reﬂecting power varies with its position as r e the gards the Earth and Sun , as might be case if the two r e planetoid were really double, with components or volving around a center of gravity, i f the body itsel f n were unequally reﬂective in its various parts . The mea o f at distance Eros from the Sun is miles , but dis aphelion it is miles away. Its perihelion or s tance is only miles , about mile greater than the mean distance of the Earth . On account o f the large inclination o f the plane of its orbit to the plane of the ecliptic Eros never approaches the Earth nearer than about miles . Th ese near approaches occur only when Eros is in op at s position its perihelion , which unfortunately happen

i t. rarely . Its sidereal period is years, from which follows that its synodic period is years . I f an oppo iti n l s o occurs at perihelion , then in 37 years another wil occur very nearly at perihelion , for 37 is almost evenly divisible by and The next most favorable 0p 2 2 4 ASTRONOMY

1 1 position will occur in 93 , and will furnish an unexcelled opportunity for obtaining the solar parallax . Because of its eccentric orbit Eros acquires a highly practical signiﬁcance in respect of the determination of h t e true size of the entire planetary system . From the laws o f the orbits is found , in fact , very accurately (to six d ecimal places ) the relations of the planetary distances .

On the other hand , these distances themselves are very little known . They are inversely proportional to the solar parallax of which one can be conﬁdent of but

2 F ig . 8 ORB I T O F ERO S RE FE RRED To T H OS E OF TH E EART H M AND ARS . two decimal places . We have already seen that by means o f o f o f o f observation the transits Venus , observation the n o f o f v e earer planetoids and Mars , determinations the locit of of y light and the parallactic lunar equation , great pains were taken in the last century to ascertain with more x o f exactness the solar parallax , and thus the e tent our entire planetary system . It appears that the Observation o f Eros yields this quantity with three times the exactness o f O b the earlier methods , and that nothing else than the s ervation of the lunar orbit fo r the determination of the parallactic equation can compete with it . Thus the dis c over y of Eros performed a useful service for astronomy . Two months after the discovery of Eros , Wolf, still at

' 2 2 6 ASTRONOMY

of t one line, for each planet, time and place Opposi ion and t the daily variation in its position . For selected , importan and interesting little planets Opposition ephemerides were f h printed , which gave the position o the planets daily wit f r exactness o six weeks o f their best visibility . The maj ority o f the planetoids appear as mere points and are immeasurably small . However, Weiss estimates , on f 2 the basis of brilliancy, the largest o them at 34 kilome ters ( 2 1 2 miles ) and the smallest at and 1 0 kilome 1 0 ters ( and miles) in diameter . There is n o doubt that many small bodies are in motion a mong them whose diameters are o f 1 kilometer o f 1 o f 1 feet) , perhaps meter inches) and even on t millimeter inch) , and that the maj ority accoun o f t their fain ness are n ot visible to Earth . The planet oids f l are o n o immediate use to mankind . Their orbita calculations can be used neither for the determination o f o th e time n r for the guidance o f mariners . But ideal beauty o f these planets is that their reappear a nce is continually a test of the correctness o f math emati cal assumptions and that they exhibit over and over again ’ the validity of Newton s law o f the universal attraction of a ll o b dies . CHAPTER XX

ET ETE ETE TE COM S, M ORS AND M ORI S

T H E or term comet , derived from the Latin , coma , h a air , applied to celestial bodies which appear to have

to . hairy appendage , goes back the time of the Romans

A similar word, cometa , or cometes, was used by Cicero ,

Tibullus and other ancient writers . The modern as t r on omer speaks of the coma of a comet as distinct from t h e to sur tail , and applies the term the misty hazy light r on h ounding every side a small central bright spot, whic o f he calls the nucleus the comet . While the ancients distinguished between comets an d m or o f eteors , shooting stars , yet they believed them to be ’ th e same nature, and to be found in the Earth s atmos h er e o r h p not far above the clouds , at all events muc Th e lower than the Moon . earlier and Pythagorean v t o iew, however, was much more correct, according modern doctrine ; for it held that comets were bodies o f s with long periods revolution , which idea, like other to attributed Pythagoras , probably came from eastern the philosophers of unknown nationality . Apollonius , M n di an for y , believed that the Chaldeans were responsible this notion of the comets ; for they spoke o f them as trav ele r s that penetrated far into the upper or most distant f e . o celestial space Similar views , typical the imitativ

faculty of the Romans, says Humboldt, were held by

Seneca and Pliny . The Greek philosophers in many cases to l preferred disregard Observations . Hence the fancifu 2 2 8 ASTRONOMY

r theories of Aristotle as regards comets , as well as othe astronomical matters , were prevalent for many centuries . Aristotle even believed that the Milky Way was a vast comet which continually reproduced itself .

The comet , with its brilliant head , ﬂaming tail and uncertain appearance , could not be regarded otherwise than as a divine omen to announce some remarkable event or to forebode evil , particularly pestilence and war .

Indeed, for many years the death of monarchs was be li eved o , especially by those to whom the wish was father f the deed , to be announced by these brilliant messengers i n the sky . In some cases comets were associated with mis fortunes , not merely as anticipating or announcing them , ’ “ but as the actual causes . Seneca s statement that Thi s comet was anxiously observed by every one, because o f some great catastrophe which it produced as soon as it e” appeared, the submersion of Bura and Helic referred to

1 B . a very brilliant comet which appeared in 37 C . about the same time that these two towns of Achaia were swallowed u a p by the sea during an earthquake . A comet which p ear e d B C p in 43 . . was generally believed to be the soul o f

Caesar on its way to heaven . Josephus in forms us that the destruction o f Jerusalem was announced by several prod i i es 6 A D - g in 9 , among them a sword shaped comet which is said to have hovered over the city for the space of a year ! Another classical instance may be quoted from “ ” - 2 A D. Pliny ( 3 79 . ) in his Natural History where he “ says : A comet is ordinarily a very fearful star ; it an n oun e ff an c s no small e usion o f blood . We have seen ” example of this during the civil commotion of Octavius .

A D . When the comet o f 79 appeared, the Roman Emperor Vespasian refused to be intimidated by the frightening i n “ t er r e ta ti on p placed upon it. This hairy star does not “ concern me , he is reported to have said ; it menaces

rather the King of the Parthians , for he i s hairy and I ” h e a m bald . Not long after the appearance of the comet

2 30 ASTRONOMY

bent arm , holding in its hand a great sword, as i f about t o e n d strike . At the of the point there were three stars . On both sides of the rays of this comet were seen a great n - umber of axes , knives , blood colored swords , among h which were a great number o f hideous human faces , wit ” beards and bristling hair . 1 2 t o The comet o f 47 , apparently, was the first comet receive scientific study and not be regarded solely as a cause of superstitious terror . A series of observations Mii ller Kii n i sber were made in Franconia by Johann , of g g, n k own as Regiomontanus . n ot By the time o f Tycho Brahe, while comets were on satisfactorily explained, yet they were being considered A r i stote a more rational basis , and the correctness of the lian doctrine was , as in other matters , being questioned . It was believed before this that comets were generated in 1 0 th e the higher regions of the atmosphere . But in 5 7, on e appearance of a brilliant comet, Tycho, in an elaborat e series of observations , satisfied himsel f that the strang body was at least three times as far o ff as the Moon and also that it was revolving around the Sun in a circular s orbit at a distance greater than that of Venus . Comet H i s s ubsequently were observed by Tycho and his pupils . observations in this field led to those ideas of the solar system o f his which we have discussed . It was but natural o f that Kepler , as a follower Tycho , should have paid 1 60 especial attention to comets . In 7 he observed the ’ comet now known as Halley s comet . Kepler believed that comets were celestial bodies which move in straight lines and a fter having passed the Earth recede i n defi n i tel y into space . Assuming that these strangers in the t heavens would never reappear, he did not consider tha their paths required serious study , for which reason he made no observation s to ascertain their movements and F r aca sto r 1 8 test his theory . Before Kepler , Jerome ( 4 3 1 543 ) and Peter Apian ( 1 495 - 1 552 ) had observed that a ’ S un r comet s tail always points away from the , no matte A ND S 2 1 COMETS , METEORS METEORITE 3

in what direction it may be traveling, and with this observation Kepler agreed, adding as an explanation the supposition that the tail was formed by rays of the Sun penetrating the body of the comet and carrying away with them some portion of its substance . This theory , after due allowance has been made for the change in our con e ti on c p of the nature of light, is of interest as an anticipa ’ tion o f the modern theory of comets tails . Kepler found “ 1 61 i n himself compelled in his Treatise on Comets , 9, which the foregoing observations were published, to refer to the meaning o f the appearance of a comet and its i n ff fluen ce on human a airs . At this time there were striking events enough in the affairs o f Europe to prove any theory o f the influence of comets on human li fe . He realized , how “ e e ver, that comets are very numerous , for he states , Ther a r e as many arguments to prove the motion of the Earth ” a round the Sun as there are comets in the heavens . The motion of the comets was also studied by Galileo i n 1 62 3 as a part o f the motion of the Earth in the Co p er n i c an f t theory o the solar system . But the first and mos i mportant contribution to the true explanation came from DOr fel 1 68 1 of Saxony, who proved from the comet of that t h e orbits of comets are either very elongated ovals o r f p arabolas and that the Sun occupied a focus o the curve . i n Newton , discussing this subj ect his Principia, reached i ndependently the same conclusion a few years later an d e stablished it as a universal law by incontrovertible mathe m a ti c al proof . By the seventeenth century a considerable n umber o f 1 61 1 c . H ev el omets had been recorded John , of Danzig (

published two large books on comets , Prodromus Cometi cus ( 1 654) and Co metogr aphi a which c ontained the ﬁrst systematic account of all recorded c omets . ’ It was a brilliant thought of Newton s that led him to c onsider whether gravitation toward the Sun could n ot ’ e a o f xplain a comet s motion j ust as well s that the planet, 2 32 ASTRONOMY

so and i f , as he took pains to prove in the beginning o f the Principia, such a body must move either along an o f ellipse or in one two other allied curves , the parabola and hyperbola . Edmund Halley ( 1 656 who had been a friend and active associate of Newton ’ s and had assisted him for sev o f eral years in the preparation the Principia , followed

i 2 - A N E O TE E I E O . F g . 9 L NGA D LL PS PARAB LA

’ om Newton s principles in the observation of comets . He c ut ed 1 680 1 682 p the paths of the comets of and , and especially one of 1 53 1 whose appearance was recorded by l Appian . His studies contributed much to the materia in dealing with this subj ect the Principia , especially the 1 0 later editions . In 7 5 he published a Synopsis of Comet 2 ary Astronomy , in which he calculated 4 cometary orbits .

Discussing in detail a number o f these, he was struck with the resemblance between the paths described by the comets o f 1 1 1 60 1 68 2 53 , 7 and , and the approximate equality in the intervals between their respective appearances and

S 2 COMETS , METEOR AND METEORITES 33

1 6 e that o f a fourth comet observed in 45 . Moreover , ther 1 80 i n was historical record of a comet in 3 , as well as 1 e 305 . He at once concluded that all four comets wer really diff erent appearances of the same comet , which moved around the Sun in an elongated ellipse in a period 6 l of about 75 or 7 years , and he accounted for the smal di fferences in the di fferent intervals between the appear anecs of the comet by perturbations caused by planets i n n th e whose neighborhood the comet passed . He the made

— ’ - H E O ET I E 1 8 1 1 0 . F ig . 3 0 ALL Y S C M DUR NG CYCL 3 5 9 f N ot e : The pla ce s gi ven a r e fo r J a n ua ry 1 o th e d at es i n di cated .

e o f m t ﬁrst prediction of the probabl reappearance the co e , n 1 8 n a assig ing the date , 75 , when next it would be see fter - 6 . n o s a 7 year interval Halley , les than modern astron o f a e o f omers , was aware the disturb nce that the presenc ho the planets might work in a comet and its orbit, and w

. the its - time might be altered He conﬁdently announced hi s actual appearance of the comet, but left shortly before 8 death , at the age of 5 years , the following quaintly “ : worded statement in regard to the comet Wherefore , i f n according to what we have already said , it should retur 1 8 e again about the year 75 , candid posterity will not re fus to acknowledge that this was ﬁrst discovered by an Eng ' 2 34 ASTRONOMY

l ishman . When the time arrived the comet was looked

for by astronomers ; the French Savant , Alexis Claude C lairaut ( 1 7 1 3 computed the various perturba f t ions which might have a fected its j ourney . As its path lay through the orbits of Jupiter and Saturn a n d as it passed close to both of these great planets, h i s calculations showed that there might be expected a ’ r etardation of 1 00 days on Saturn s account and 5 1 8 days 1 8 for Jupiter . On Christmas Day , 75 , a month and a day b an efore the date assigned by Clairaut, and in the year n oun ce d was a hal f century before by Halley , the comet a ctually discovered by George Palitzsch ( 1 72 3 o f

S axony , and the great astronomical prophecy was thus “

fulﬁlled . A new member was added to the solar system ; t h e wandering and fear -inspiring comet was thus brought i nto harmony with the other members and made subj ect t o the fundamental calculations of the astronomer . What e ver superstition had attached to these wonderful appa r iti on s to had now all but passed, and comets were found p resent problems no less interesting than other celestial b odies when their fundamental motions were known . ’ In 1 835 Halley s comet duly reappeared and passed t hrough its perihelion within a few days o f the time set as s for it by astronomers . It w observed, among other , b o y Sir John Herschel at the Cape f Good Hope . In 1 91 0 this comet returns again along the orbit shown in i 0 F . o f g. 3 It was ﬁrst discovered by Wolf, Heidelberg,

n 1 1 1 0 . o September , 9 9 ’ In the study of Halley s comet in connection with i ts a ppearance , much attention has been devoted by astron o mers to its earlier history , particularly that recorded in C hinese and European annals . Messrs . Cowell and Crom m elin , at the Greenwich Observatory , have carefully ex a mi n ed the previous work o f Hind and have found it in ’ t h e main correct . Halley s comet unquestionably must be i 1 066 dentiﬁed with one that occurred in , in the year o f the n o Norman Conquest, a representation o f which is w.

2 36 ASTRONOMY

director of the Berlin Observatory . It was discovered by 2 6 1 8 1 8 calcu Pons o f Marseilles , November , , but in the l ation o f its orbit and other elements Encke found that i t revolved about the Sun in a period o f 3 % years , which i s considerably shorter than that of any other known

— ’ i 2 T H E B I T O F E KE CO ET. F g . 3 OR NC S M

h c omet. Furthermore, he established its identity wit c e 1 86 omets seen by M chain in 7 , by Caroline Herschel in ’

1 v 1 80 . 795 , and by Pons , Huth and Bou ard in 5 Encke s c alculations , after establishing its periodicity , assigned the d 2 1 82 2 ate of May 4, , for its next return to perihelion , a n d tho on account of the position of the Earth a t 2 COMETS , METEORS AND METEORITES 37

that time it was invisible in the northern hemisphere, ’ it was detected at Sir Thomas Brisbane s observatory at Paramatta by R ii mker very nearly in the position indi th e c a ted by Encke . This was only second instance o f ’ k the recognised return o f a comet, so that Encke s wor as an astronomical achievement should be considered with that of Halley . ’ O ﬂi cer Biela s comet, discovered by an Austrian o f that o se h stadt 2 1 8 2 6 name at J p , in Bohemia , February 7, , pre n sents as interesting features as that of Encke . It was see ten days later at Marseilles by the French astronomer an Gambart . Both observers announced its discovery d the computation o f its orbit in the same issue o f the

Astronomische Nachrichten . Tho the comet was iden r h 1 2 1 80 was i ed with similar appearances in 77 and 5 , it In not visible after the latter date with the naked eye . 1 832 Sir John Herschel observed it a s a conspicuous n nebula without a tail . While the day of active s uperstitio i n regard to the appearance o f comets had passed with ’ ’ s the demonstration o f Halley s theory , nevertheless Biela comet occasioned widespread popular excitement, founded, o f en a col moreover , on the statements scientiﬁc m that li si on i s on e with the Earth might occur . The possibility o f the remotest of cosmical happenings . ’ 1 8 6 an d ﬁ In 4 Biela s comet reappeared, when rst seen , 2 8 November , presented no unusual appearance . Grad uall n y it became distorted , however, and elongated . Withi wo h two months it divided into t separate bodies , whic 1 6 s were visible until April th of the following year . Thi striking phenomenon of a double comet was noted by ff s many astronomers at di erent observatories , and thu established what Seneca had reproved Ephorus for sup C posing to have taken place in 373 B . . and what Kepler had n 1 61 8 c on v mcm at oted in , but without g astronomers o large of the correctness of his impression . These tw Biela comets contained a small amount o f matter an d p erformed their revolutions around the Sun independently 2 38 ASTRONOMY

w ithout experiencing any appreciable mutual disturbance , which indicated that at an interval of only miles t heir attractive power was virtually inoperative . Various i nteresting phenomena showing internal agitation wer e o bserved and variations of brilliancy and form were dis ’ n tl 1 8 2 i n t i c y evident . In 5 Biela s comet again appeared i ts . double form , but since that time has not been observed The disruption occasioned by its proximity to Jupiter in 1 841 is believed t o have been the beginning of the dis i ntegrating process which resul ted i n its disappearance . The greatest comet of the nineteenth century was that o f 2 Donati , which was seen by him at Florence , June , 8 8 1 . o f 5 By the end September , when the comet had r eached its perihelion , the tail had attained full develop m 1 0 ent, and on October it stretched in a maximum curve o o f r e r e ver more than a third the visible hemisphere , p

‘

1 1 2 . s enting a length o f miles . For the days during which it was visible to the naked eye the fullest o bservations were made . The comet stands by itself , as it i s not possible to identi fy it with any other . At aphelion i ts orbit extended out into space to 5 % times the distance

from the Sun to Neptune, and for its circuit, which is e ff ected in a retrograde direction , requires more than so years , that its next return should be about the

000 A . D . year 4 . . It was computed by M Faye that the v olume of this comet was about 500 times that of the Sun . O n the other hand , he calculated that the quantity of mat ’ t er it contained was only a fraction o f the Earth s mass . T his shows how almost inconceivably tenuous the material forming the comet must have been—much more rareﬁed i ndeed than the most perfect vacuum which can be p r o d uced in an air pump . This tenuity is shown by the fact “ t hat stars were seen through the tail as i f the tail did ” o n t exist . A mist o f a few hundred yards in thickness i s ﬁ h suf cient to hide the stars from our view, w ile a thick n ess of thousands of miles o f cometary matter does n ot s ffi u ce even to dim their brilliancy .

2 40 ASTRONOMY

A s the center . the comet approaches the Sun th e di sk a on e lengthens , and i f the comet is to be tailed , traces ’ o f b egin to be seen a streakiness in the comet s light. m Gradually a tail is formed, which is turned always fro

he . t Sun The tail grows brighter and longer, and the h ead becomes developed into a coma surrounding a di s t in ctly marked nucleus . Presently the comet is lost to v iew through its near approach to the Sun . But after a w hile it is again seen , sometimes wonderfully changed in f r a spect through the e fects o f solar heat . Some comets a e brighter and more striking after passing their point o f n earest approach to the Sun ( or perihelion ) than before ; o thers are quite shorn o f their splendor when they reap 1 861 p ear . On the other hand, the comet o f burst upon t us in i s full Splendor after perihelion passage . “ A s a comet approaches the Sun a change takes place i n o f e the appearance the coma and nucleus, and in som i ob nstances a tail i s generated . The process actually s erved is generally this : In the forward part o f the s to nucleu a turbulent action is seen be in progress , lead i n g to the propulsion toward the Sun of j ets or stream s o f - or en misty looking matter . Sometimes a regular cap v elope i s seen to be proj ected in this manner toward the Th e S un or a set o f on e . , even envelopes within the other matter thus thrown O ff is not suff ered t o pass very far un i s s from the nucleus toward the S , but wept away, as f ast as formed , in the contrary direction . I f the funnel o f s o f u a team engine were directed forward, instead p ward, then the appearance presented by the emitted steam a s o to the engine rushed n ( against a hurricane, suppose , make the illustration more perfect; would exempli fy the process which seems to be taking place around the front o f the nucleus and far behind it as the matter formed i s n c ontinually swept away from the S u . The same Sun which attracts the nucleus seems to repulse the emitted m atter with inconceivable energy . “ When we see the tail o f a comet occupying a volume S 2 1 COMETS , METEO RS AND METEORITE 4

o f o f thousands times greater than that the Sun itself, ‘ th e question naturally suggests itsel f : How does i t hap pen that so vast a body can sweep through the solar ’ system without deranging the motion o f every planet ? Conceding even an extreme tenuity to the substance com t o posing so vast a volume, one would still expect its mass e be tremendous . For instance, i f we supposed the whol mass of the tail o f the comet o f 1 843 to consist o f hydro en n g gas (the lightest substance known to us) , yet eve then the mass o f the tail would have largely exceeded

— F i . TH E I O F O T T g 3 3 TA L A C M ET D I REC ED F RO M H E S U N .

f un that o the S . Every planet would have been dragged so s e from its orbit by vast a mass passing o near . W ff know, on the contrary , that no such e ects were produced . o f our The length year did not change by a single second, showing that o ur Earth had been neither hastened n or we r etarded in its steady motion round the Sun . Thus are forced to admit that the actual substance o f the comet

was inconceivably rare . A j arful o f air would probably have outweighed hundreds of cubic miles o f that vast a ppendage which blazed across our skies to the terror o f h t e ignorant and superstitious . “ The dread o f the possible evils which might accrue i f 2 42 ASTRONOMY the Earth encountered a comet will possibly be diminished by the consideration of the extreme tenuity o f these oh ects s j . But the feeling may still remain that influence other than those due to mere weight or mass might b e exerted upon terrestrial races in the course o f such an o f i s encounter . On account their enormous volumes , it n ot so utterly improbable that we should encounter them a s that we should meet the comparatively minute nuclei . o f In fact , the Earth actually did pass through the tail f 1 861 the comet o . At about the hour when it was cal culated that the encounter should have taken place a strange auroral glare was seen in the atmosphere, but beyond this no eff ect was perceptible . In distinction to the comets moving in regular orbits o f on e around the Sun , the possible portions much larger cometary body which became dispersed by gravitational action or through violent encounter with the suns sur rounding must be mentioned . These comets , which appar ently have been seized by the gravitative attraction o f planets , are compelled to revolve in short ellipses around the Sun well within the limits o f the solar system . These “ ” o f comets are spoken as captures , and while Jupiter, o f s Saturn , Uranus and Neptune each possess families thi a s kind, it i s the first named which are the most important, 0 they number about 3 , and in this family may be included e not only the bodies that Jupiter has attracted, but thos t that have been robbed from other planets . Come families are not found in the case o f the terrestrial o f planets , because the gravitative power the Sun in thei r vicinity is so much greater than any attractive force which they could mani fest . In addition , when a comet enters the inner portion of the solar system , it has such velocity that the gravitational attraction of the planets within these regions is not powerful enough to cause any n appreciable deflection . I f a captured comet i s acted upo d by further disturbing causes , its new orbit may be isar

2 44 ASTRONOMY

a least, metallic core resembling the meteoric masses which ” occasionally reach the Earth from planetary space . The ﬁrst serious study of the physical nature of comets ’ 1 8 1 1 tails was undertaken in by Dr . Heinrich Olbers , the astronomer of Bremen . He assumed that the formation of a tail was due to expelled vapors on which two forces , solar and cometary , acted and balanced each other. In

’ ’ — O T I B R ED1 C H 1 N S T H EO RY C M E S TA LS .

other words , he believed that the tails were emanations , n o t appendages , and consisted of rapid outflows of highly rarefied matter which , in great part , had become perma n en tly detached from the nucleus . This theory is especially interesting in the light o f l modern investigation . It served for many years unti B r edi chi n s that of , who in an examination of variou ’ comets tails found that the curvilinear shapes o f the out 2 COMETS , METEORS AND METEORITES 45 line fall into one or another of three special types a s 1 indicated in the accompanying illustration . Type , or the straightest , is most probably due to the element hydrogen . In Type 2 a number of hydrocarbons are present in the o b dy of the comet, while in the third type iron or some element with high atomic weight was assumed . Some comets may have tails of more than one of these forms , as , ’ for example , in the case of Donati s comet, which had a 2 straight as well as a curved tail . Of Type , comets with a number o f tails have been recorded , such as the one 1 B r edi chi n o f the year 744. calculated that a repulsive force adequate to produce the straight tail of Type 1 n eed only be about 1 9 times as much as the attraction of gravitation , while tails of the second type a repulsive force about equal to to times that gravitation ffi would su ce , while those of the third type would require 1 a repulsive force about to times that o f g ravitation . These parts are nearly inversely propor tional to the a o f tomic weights hydrogen , hydrocarbon gas and iron v B r edi chi n n apor, which ratio suggested to the compositio to o f the various types of tail . While he was unable d emonstrate that the tails were the result of electrical ac tion , yet he assumed some hypothetical repulsive force which electrical action seemed to explain better than a n y other . 1 0 Professor E . E . Barnard in 9 5 stated that a repellent i nfluence of some sort must come from the Sun , and with it he included an ej ecting force proceeding from th e comet itsel f and a resistant force of some kind . The repellent force from the Sun may be found in the pres

r e o f . su light, which Professor J Clerk Maxwell assumed must be exerted by light rays according to mathematical “ ” reasoning . Radiation pressure , as it is termed , was not 1 00 - 1 0 1 e xperimentally proven for many years , but in 9 9 i t was established as a scientific fact by the Russian phys i ci st Lebedev and in America by Nichols and Hull . This principle thus demonstrated, Professor Svante Arrhenius 2 46 ASTRONOMY applied cosmically and held responsible for the gener ’ ation o f streams of matter flowing from the comet s head . As the comet approaches the Sun this pressure exceeds the force of gravity and acts upon the cometary substance so as to drive out multitudes of the minute particles in a direction away from the Sun . Such a swarm of particles receiving the light from the Sun would appear as th e ’ familiar luminous streamer recognised in the comets tails . ’ l When examined with the spectroscope , a comet s tai shows a faint continuous spectrum , produced doubtless by to the sunlight reflected by the small particles , in addition spectral bands due to gaseous hydrocarbons and cyanogen . s Cyanogen is due to electric discharges , for such discharge are observed in comets whose distance from the Sun is so great that they cannot appear luminous owing to their own high temperatures . In other words , the composition of a comet is not unlike the blue flame of a gas stove , which is a combination of hydrogen and carbon . As the comet dashes toward the Sun and its temperature consequently rises , the spectroscope reveals the presence of iron , mag

n esi um . r , and other metals in the nucleus With a close approach to the Sun the hydrocarbons split up into hydro

- gen gas and hydrocarbons of a higher boiling point .

Finally , a time comes when these more refractory hydro carbons i n turn decompose into free carbon in the form of soot . Because interstellar space is airless the soot cannot burn , but must accompany the comet in the form m of a very ﬁne dust . This dust, propelled away fro the Sun by radiation pressure , constitutes the tail of many a comet . Some of the soot particles may be larger than the critical size . They will be j erked forward toward the Sun in advance of the comet to form what is known ’ “ as the comet s beard , a rather rare phenomenon . This phenomenon o f the pressure of light is able to explain the fact that the minute particles ej ected from the nucleus of a comet can pass over great distances i n s t mall intervals o f time , which was one of the hardes

2 48 ASTRONOMY

2 A D the famous revolt of Nika in the year 53 . . , “ s ays : During the same year a great fall of stars came ” E en i ti an from the even i ng till the dawn . M . g s notes 2 A D other early reference to these meteors in 75 , during the reign o f the eastern Emperor Constantine CO p r on y Ni c e h or us mous . Writing of that year , p , a Patriarch o f : Constantinople , states All the stars appeared to be de k B ut tached from the s y and t o fall upon the Earth .

— I T o r O O F MET 6 T H E EO . F ig . 3 ORB A SH AL RS it was n ot until the nineteenth century that Bielids aroused th much attention , and then it was in great part due to e fact that apparently the same orbit was occupied by them ’ as by Biela s comet, which we have seen was not observed

1 8 2 . a fter its appearance in 5 The Bielid shower, however , was since that time has shown increased activity , which s i n e pecially true in the years in which the comet , were it th existence , would have been scheduled to pass near e

Earth . o f e In the case the Leonids , records o f their occurrenc ” o 02 A D g back as far as 9 , which is called year o f stars , 1 2 because on the night of October , while the Moorish Ibr ahi n Ben Ahmed was dying before Cosenza in Calabria , “ a multitude of falling stars scattered themselves across the A D ME S 2 COMETS , METEORS N TEORITE 49 sk y like rain , and naturally aroused great excitement con among those who beheld the phenomenon , which they id er e s d a celestial portent of unusual signiﬁcance . In 1 6 8 9 modern history o f the Leonids began . A maximum Leonid shower has occurred with considerable regularity 1 at periods of about 33 years from that date . In 799, on

— T H H T L I . F i g . 3 7 E I S O RY O F T H E EO N DS

1 1 th B on lan dt d the o f November , Humboldt and p witnesse 1 2 a notable display in South America . On November , 1 8 33 , meteors were said to have fallen as thickly as snow ﬂakes in seven hours were estimated to have to appeared . The radiant from which the meteors seemed con stel come was found to . be situated in the head of the 2 5 6 ASTRONOMY

o f Leo lation , from which circumstance the name

Leonids results . Professor Dennison Olmstead, o f Yale th e University , assigned to this cloud of cosmical particles path o f a narrow ellipse in an orbit around the Sun and intersecting that o f the Earth . This marked the beginning 1 8 o f an important depa r tment o f astronomy . In 37 Olbers established the periodicity of the maximum shower which indicated a regular distribution of the meteoric supply . In 1 866 m , as conj ectured by Olbers , another time of maximu o ccurrence took place , which seemed to demonstrate that while the Earth cut through the orbit each year about - a the same date, at the 33 year period the swarm was at o f 1 8 point maximum density in the orbit . In 99, however , much disappointment was caused by the failure o f the a Leonid shower to take place on the scheduled date, failure which was explained as due to the attraction o f o n e o f the larger planets which had diverted the orbit from its old position so that the Earth failed to pass through the swarm . The cometary connection in the case o f the Leonids is shown by the fact that they seem to ’ 1 866 travel in the orbit of Tempel s comet of .

to 8 1 1 A . D. The Perseids date back the year , and derive c on stellatio their name from the o f Perseus , where their l oth o f radiant point is situated . They are seen on the i s of August in continental Europe . A s this the day w o f t Saint Lawrence , they are kno n as the tears Sain ” Lawrence . But this date is not th e only one on which meteors from this swarm are to be observed, for they fall in greater or less numbers from about July 8 th to August 2 2 d . They are very rapid in their motion and the trails often persist for a minute or two before they are di ssemi n d ate . The Perseids have an easterly motion , shifting each night by a small amount . Their orbit cuts the orbit of the

Earth almost perpendicularly, and they are supposed to be the débris of an ancient comet which traveled the

. o f i n same path Various comets , especially that Tuttle

2 5 2 ASTRONOMY it is usually consumed before it reaches the Earth ’ s s urface . The products of oxidation and disintegration con ’ sist simply of dust , which falls on the Earth s surface or is d istributed throughout space . In the main, meteors simply contribute dust to the Earth . The energy of the meteor as well as its mass can readily be ascertained if its distance , the duration of its luminosity , a n d its brightness are known , for the total amount of light radiated can be calculated on the basis that its entire e nergy is thus transformed . The mass of a meteor is not particularly large and is usually but a small fraction of an o unce . For the most part it is not larger than a pea or p ebble . It is the atmosphere that n ot only heats these r apidly moving bodies but acts as a protection to the

Earth , for i f meteors were not thus disintegrated they would fall upon its surface in a constant bombardment . In a ddition to the meteors seen with the naked eye , estimated by the late Professor Simon Newcomb at not less than 1 46 billion per annum , there are doubtless ten times as many which pass merely as streaks of light in the ﬁeld o f the ’ o bserver s telescope . In addition to the extremely ﬁne dust which settles on the Earth as the result of the disintegration o f various c o f elestial bodies , there are from time to time masses ’ a t greater or less size which , rushing into the Earth s m o sph er e with a brilliant glow due to the heat generated ’ b or y friction , fall to the Earth s surface and become more a cc om less embedded . The appearance i s most striking, p an i ed as it O ften is by a loud roar like a waterfall and occasionally violent explosions . Thus it is stated that at 1 02 Cairo , in August , 9, many stars passed with a great

n . oise and a brilliant light These bodies , a number o f ’ which come to the Earth s surface yearly, are termed m ur an oli th s eteorites , siderites , or aerolites , and appar e ntly are the connecting links between the Earth and out

side space . Thei r nature is none too well known and they present many unsolved problems . It i s interesting to 2 COMETS , METEORS AND METEORITES 53 know that the great Mexican meteorites at the time o f th e Spanish invasion were considered holy bodies by th e

Indians , so that it is inferred that their fall from the heavens was known and was regarded as a supernatural occurrance . In the Greek and Roman records similar e attention was paid to the palladium o f Troy, to the imag o o f o f Diana at Ephesus and t the sacred shield Numa , all o f which were said to have fallen from the heavens and were no doubt meteorites . t o Meteorites are usually divided into w classes , those ' composed chiefly of iron and those compo sed chi efly of stone . Of the 2 92 actually observ ed meteoric falls that took place during the last century, only twelve , or about 4 per cent . , belonged to the first class , yet in our cabinets the two classes are represented in nearly equal numbers . The explanation o f this strange anomaly lies in the fact that unless the fall has been actually witnessed close at

few o f . hand , very the stony meteorites are ever found 2 8 0 O f 3 in the collection of the British Museum , 3 5 , or

93 per cent . , were seen to fall . This is partly because these bodies to ordinary inspection appear very like com mon stones , and therefore are not recognised as meteorites , and partly because owing to their physical and chemical structure they are readily decomposed by the action of th e elements . It is the custom to associate meteorites with falling stars , and to say therefore that they are of cometary ori n gin . This relationship , however, is not as obvious , whe we begin to examine into the case, as at first sight appears . A prominent diffi culty is that the distribution of the meteorites throughout the year differs very materially from t tha of the falling stars and fireballs . While these last two are about twice as numerous during the latter half of the year as during the first half, the meteorites are more numerous during the first half of the year . From this we should infer that while perhaps all meteorites are fir e

. balls , only comparatively few ﬁreballs become meteorites 2 54 ASTRONOMY

The dividing line between meteorites and falling stars then lies among the fireballs , the swiftly moving ones being allied to the falling stars and the slowly moving ones to the meteorites . It is now generally accepted that the crystalline and often conglomerate structure of these bodies proves them to be but the fragments of much larger bodies that have in some manner been destroyed or from which they have r M- othe wise become separated . any believe that the crys t alli n e structure of the iron meteorites indicates a Slow “ cooling, while some say that the structures of the chon dres” of the stony meteorites must certainly have been produced by a very rapid crystallization due to a sudden exposure to a lower temperature . It was formerly thought by some that these bodies might have been expelled from the Sun . Altho it is quite pos sible that solar explosions in past ages were suffi ciently violent to proj ect these bodies with the necessary cometary velocities, yet we cannot believe the Sun to be the direct source of them , since it is improbable that either solid stone or iron should ever have existed upon its surface or within its interior. Nor is it easy to explain how with such an origin the meteorites should have acquired their present orbits . Some of the earlier cosmogonists referred their origin to the terrestrial or lunar volcanoes . This is manifestly impossible in the case of the Earth , since even prehistoric volcanoes could not have expelled their products with such force that after leaving the confines of our atmosphere they should still retain a velocity of over seven miles per im second . Yet this is the speed required to prevent an ’ o mediate return to the Earth s surface . Moreover, alth volcanic eruptions in prehistoric times were undoubtedly n o more frequent and voluminous than at present , it is by means certain for theoretical reasons that they were then

- any more violent than they are to day . Meteors escaping from lunar volcanoes would not have

2 56 ASTRONOMY

have been found . The six which occur most frequently in ’ f the Earth s crust, named in the order o their abundance , a r e oxygen , silicon , aluminium , iron, calcium and mag n i m es u . The eight most commonly found in the stony meteorites are these six , besides nickel and sulphur . Nearly all the stony meteorites contain some metallic i ron , and some of them contain large quantities of it . But this is also true of some of our basaltic lavas . Indeed , large masses of iron have been found in ledges upon the 6 Greenland coast . Some of this iron contains over per cent . of nickel , but much larger proportions have been discovered in New Zealand , Piedmont and Oregon , where considerable quantities of the nickel iron alloys have been

- found . According to Farrington , of the twenty one min e r als recognised in meteorites , fourteen have been found in our volcanic products . It appears to Professor Pickering that the iron and stony meteorites di ffer from one another in other ways besides their composition . That some of the former are associated with falling stars , and therefore with comets , certainly seems plausible . That the latter are not associated with co them seems probable , and if so , whence can they have me i f not from our own Earth ? CHAPTER XXI — — T H E STARS TH E CONSTELLAT IONS M ETHODS OF NOTI NG T H E STA RS

T H E ancients assumed that the celestial sphere of the heavens was a reality and even considered it a solid Sphere o f crystal , on which they observed and marked the posi

- tions of the various stars , j ust as to day the astronomer assumes for the same purpose an ideal celestial sphere of which the observer is the center and in which declination corresponds with latitude on the Earth and right ascension with longitude . They appreciated that altho the stars moved across the sky and apparently with different rates of motion , yet the distances between any two remained unchanged . For this reason they imagined that the stars were ﬁxed on the celestial sphere . The motion of some of the stars about a center or pole which coincided with the

Pole Star was observed , and the two poles in the heavens were early assumed . But the most important observation of the ancients was the relative position of the Sun and the stars . A succession of such ob servations early showed that the stars were gradually changing their position with respect to the Sun or that the Sun was changing its posi tion with respect to the stars . Thus the stars obviously vary in position and mag n itude to , yet in ancient times little was conceived as their nature or possible origin . There was , of course , t h e fundamental distinction between the planets or mov

' i n ﬁxi t g bodies and the idea of the y of the stars , which 2 57 2 58 ASTRONOMY

was early established and persisted for many centuries , 1 600 even tho Giordano Bruno ( d . ) vaguely suggested that 1 18 the suns of space move . In 7 Halley had announced a shifting in the sky of Sirius , Aldebaran , Betelgeux and ’ Arcturus since the time of Ptolemy s catalogue , and similar conclusions were reached by various other astronomers . But it was only in 1 838 when Bessel with the heliometer was able to detect a motion of the star 6 1 Cygni that it was clearly and conclusively demonstrated that the stars move through space as well as other bodies in the universe . The stars were supposed by the ancients to be situated on the celestial sphere at a distance greater, it is true , than the planets ; yet as they were observed year after year in essentially the same positions they were held as ﬁxed an d immovable and a bodily rotation of the celestial sphere itself was assumed . Just as the ancient mind had given to the planets the names of gods and goddesses , so the wise men of antiquity assigned to the stars similar names or l those of anima s , the natural result of their vivid imagina tion . This they did also with groups of stars with even greater play of the imagination . The idea of grouping the stars into constellations dates from the earliest times . In fact the names long ago given to

- many have persisted until to day , tho it must be confessed that they have often proved a cause of embarrassment to the ffi student of astronomy . The modern mind finds it di cult to group in imagination a series of bright points of light in m the Shape of so e mythological hero , bear, dog, serpent or other animal . In most cases the choice had been made in a m ost arbitrary manner , and Sir John Herschel has truly “ remarked : The constellations seem to have been purposely named and delineated to cause as much confusion and i n convenience as possible . Innumerable snakes twine through long and contorted areas of the heavens where no memory fi can follow them ; bears , lions and shes , large and small, ” confuse all nomenclature . The names of the constellations as we know them are

2 60 ASTRONOMY

n ot tions , and only tells the mythological stories but indi cates the positions and numbers of stars in every ﬁgure, f di fering from Aratus only in a few particulars . - s Ptolemy gave forty eight constellations . The ﬁgure were the same as the old constellations of Aratus with a

. a few additions The st rs , however, were marked in their proper places and deﬁned as to latitude , longitude and magnitude . “After Ptolemy a long period ensued during which the astronomical charts were unchanged . It is to the Arabs in the eighth century that the next advance is due . The al Caliphs of this period, among whom was Haroun ‘ ’ Raschid of Arabian Nights fame , were friends to science and gathered around them men of learning, such as the F er an i El- famous astronomers Ulug Bekh , g , Batan and

Abdelrahman Sufi . To a great extent they were satisfied ’ with Ptolemy s work, and altho they retained a great many o f the Greek star names, they added a number derived by tradition from the ancient Arab names . Abdelrahman Sufi wrote a detailed and exhaustive account of the Greek

constellations , carefully following Ptolemy , and at the

same time he treated of the ancient Arabian heavens . “ S O strong was their obj ection to the personal element that when the Greek Zodiac was incorporated by the Arabian astronomers they indicated the names of the obj ects carried by the characters instead of the characters t themselves . Thus Virgo was called the Ears , on accoun o f the wheat she held in her hand ; Sagittarius was not the - r Archer but the Bow, and Aquarius not the Water beare

but the Well Bucket . “ When the great mixture o f Arabian folk - lore was combined with the Greek sky many of the star names

were retained , but occasionally the Greek names were t changed ; for instance, the beauti ful red Antares in Sco ’ pio was approximately called the Scorpion s Heart . “ In 1 433 Ulug Bekh made at his observatory in Samar

kand the most correct catalogue of stars up to that period . THE CONSTELLATIONS 2 61

The famous astronomical tables compiled under Alphonso

. 1 2 2 as X o f Castile date from 5 , and next in importance w the great catalogue o f Tycho B r a hé ( 1 546 “ The southern hemisphere , which was uncharted by the ancients , is of far less interest than the northern , partly because the changes have been frequent and unimportant and partly because the only constellation visible to us is the Dove, introduced early in the Sixteenth century . In

— i . F g 38 T H E P LE I AD E S . the old books it is called Columba N oae because it is near the ship , represented sometimes at that period as ’ Noah s Ark . The regions around the Ship and the North Pole have been subj ect to the most frequent changes Since s the eventeenth century.

The most familiar constellation is the Great Bear, known to Americans as “ the Dipper three stars form the tail of the animal and four others part of his body . To the more prosaic American mind the analogy of the dipper seems far more apt than that of a bear . In Cas s i o ei a h e p , which is across t pole from the Dipper, the 2 62 A STRONOMY

brighter stars form the chair in which a lady is seated . In many cases the position of a figure can be reproduced e with a fair degree of certainty, altho it is hard to realiz how the names were originally given . Most of the constellations familiar by observation or legend are those of the northern sky , because until within modern times there were no recorded observations of the southern heavens . Two of the southern constella tions, however, are noteworthy, one of which is the Cen fir st - n taur, containing two mag itude stars , Alpha and e Beta Centauri , the first of which , as we shall see , is notabl

— i T F g . 39 C A S O N AND P o L L ux.

as being the nearest of all the stars to our Earth , while the second constellation is the famous Southern Cross , or ﬁr - st . Crux, having also a magnitude star, Alpha Crucis Some years ago the Southern Cross rose above the English horizon and was j ust visible in the latitude of

London , but through the centuries it has had a southerly motion and has not been seen for many years even in the “ ” south of Europe . Perhaps the chambers o f the South in the Book of Job ( ix , 9) may be this constellation , for the Southern Cross must have been a feat ure in the sky ' thi s bo k of Palestine when o was written . The designation of individual stars probably antedated the idea of constellations ; this we may infer from the two allusion to the star Arcturus in Job ( ix, The

2 64 ASTRONOMY

the most important to us is Polaris , the North Star or “ ” Pole Star , which i s in a straight line with the two stars marking the bottom of the Dipper, which are termed “ ” ﬁv the pointers . It is e times as far away as the interval between the pointers and very nearly occupies that point of the heavens toward which the north pole of the Earth ’ s axis is directed . To the observer on the Earth it appears

— F i . 1 O I S N E I O I CO TE TIO g 4 P LAR AND GHB R NG NS LLA NS .

its to mark the north . Its position , however, varies with i n ' r l . c e r ta ci cu a r latitude It has a motion , due to the ’ k slow shi fting of the direction o f the Earth s axis , nown o f as precession , so that in the course some twelve thou sand years it will be displaced from its position as pole n i n star and Vega , a pale blue star of the ﬁrst mag itude the constellation Lyra, will assume its place . With the conception of the celestial sphere held by the ancients it was not difﬁ cult to assume that the stars were TH E CONSTELLATION S 2 65

studded about the sky at a considerable di stance . Yet beyond noting their apparent distances from one another was as expressed in angular measure , but little attention paid by the early astronomers to their remoteness from the Earth . Indeed, they assumed them to be very remote , “ and Aristotle remarks , quoting the opinion of the mathe ” maticians , that the stars must be at least nine times as far o ff A r i st llus as the Sun . Timocharis and y , both of whom

B . C . flourished in the early part of the third century , were the first to ascertain and to record the positions of the chief stars by means of numerical measures of their dis tances from fixed positions on the sky, or in other words , to supply data for the first real star catalogue . But this did not afford any adequate idea of the absolute or relative distance . After the death o f Hipparchus it was recorded in an early text-book of astronomy that the stars need not necessarily be on the surface of the sphere , but may be at ff di erent distances from the Earth , which , however, could not be determined with the means at hand . In the works o f this period the conj ecture is also hazarded that the Sun and stars are so far off that the Earth would be a mere point when seen from the Sun and quite invisible from the stars . In the Almagest Pt alemy in his series of postulates

( Book I , chap . ii ) states that the Earth is merely a point h e in comparison with the distance of the ﬁxed stars , and believes that it is more rational to assume that bodies like n f the stars , which seem to be of the ature o ﬁre , are more likely to move than the solid Earth . Copernicus believed that the stars must be at a very a of gre t distance as compared with the size the Earth , because the horizon apparently divides the celestial sphere into two equal parts , and the observer appears to be at the center of this sphere , no matter how much he moves on the Earth , so that the distance moved is imperceptible as compared with the distance of the stars . He maintained that the stars were all at the same distance from the 2 66 ASTRONOMY

Earth , and according to his theory some annual motion o f the ﬁxed stars should be observed , due to the alteration of ’ the Earth s position in its orbit . As this was not noticed, Copernicus assumed that the distance of the stars was too great to cause any appreciable motion . The instruments of those days were quite incapable of detecting such a r eﬁn e small amount of motion , requiring as it did greater ments of observation . The positions of the stars were carefully noted by Tycho

B rahe on a great celestial globe 5 feet in diameter, which , in those days , served as a star chart . With a quadrant 1 or quarter circle having a radius of about 9 feet, by the which angles could be read to single minutes , he made a number of observations of the chief star in Cassiopeia, as interest in this constellation was aroused by the appear n 1 2 h ew . e ance in it of a brilliant star in November, 57 As was unable to determine any perceptible parallax , he as sumed that the star must certainly be farther off than the

Moon . This discovery was important as indicating that changes could occur in that far - distant realm of the fixed stars which hitherto was believed to be constant both in its appearance and constitution . This was corroborated in 1 604 by Galileo in the case of the new star in S e r pen ta r ius , which he demonstrated was at least more distant “ than the planets . Galileo in his Dialogue on the Two Chief Systems of the World ” brings out the fact that the n angular mag itudes of the fixed stars , which were the ifli cult most d to determine , are in reality almost entirely l k - i lusory , and indicates a method nown as the double star or diff erential method of parallax by which the motio n and the distances of two stars at different distances from the Earth can be measured . At the beginning of the eighteenth century , tho thousands of fixed stars had been observed and their positions noted , nothing was known of thei r distance beyond the fact that it was so very great that they exerted no sensible influence . It was not until the time of Sir William Herschel that a

.2 68 ASTRONOMY

would take more than six years to reach the Earth . But 6 1 Cygni is not our nearest star . Subsequent investigation showed that this place belonged to Alpha Centauri , which is four and a third light - years from the Earth and prob ably ten billions of miles nearer to us than any other member of the sidereal system . Sirius is twice this dis - 0 tance , or eight and a half light years ; Vega is about 3

2 1 00 . years , Capella about 3 and Arcturus about

But not for every star are these distances known . They are determined by measuring the parallax wherever pos “ sible . To quote Dr . Otto Klotz , Parallax is the apparent displacement a body suffers by change of place in the ob server . With the highly reﬁned instruments and meth ds d eﬁ o , notably the heliometric and spectrographic , nite results are being attained for a comparatively few stars of the myriads that strew the sky and mark a mile

stone in achievement of practical astronomy . Bold indeed was the undertaking when man ﬁrst attempted to measure the dimensions of the Earth , but bolder was it when he

proj ected his measuring rod to the Sun . What shall o ff we say when we ﬁnd him rushing , measuring rod in

hand, at the rate of miles a second , and , after 4%

years, reaching the nearest star, Alpha Centauri , a bright star of the southern hemisphere ? This tremendous dis

tance , quite beyond our grasp when stated in miles , is ex pressed by saying that its parallax is Many of the

brightest stars are found to have no sensible parallax, while the maj ority o f those ascertained to be nearest to the Earth ” are of ﬁfth , sixth or even ninth magnitudes .

If the parallax and distance of a star are determined , it is possible to make some approximation of its mass in the

case o f binary stars revolving in known orbits . In other a r e words , if we can determine the number of seconds of which separate the two members of a binary system and translate this distance into millions of miles and then

measure the period of rotation , we can ﬁnd their combined

mass in terms of that of the Sun . For example, the com THE CONSTELLATIONS 2 69

on en ts p of the double star Alpha Centauri, to which we have referred , revolve around their common center of gravity at a mean distance of nearly twenty -five times th e ’ 8 radius of the Earth s orbit . As they require years for a period of revolution , the attractive force of the two to gether must be twice that of the Sun . With single stars such a computation cannot be made . Knowing their to parallax , however, it is possible estimate from their size and brilliancy some of the splendid stars in the b eav ens , such as Canopus , Betelgeux and Rigel , and to realize that they are thousands of times greater than the Sun and suffer in comparison by their infinite distances . For it must always be remembered that the brilliancy of a star depends not only upon its intrinsic brightness but also upon its distance from the Earth . For five other of the visual binary stars data are avail able for computing their masses . Thus Sirius , which radi 2 ates 3 times as much light as the Sun, is supposed to have a combined mass for its two constituent stars of that o f the Sun ; Procyon , that o f the Sun ; Cassiopeia , 0 O hi uch i 8 7 p , and 5 Pegasi , The average dis tance of the stars of a pair of those in the above list from each other is 2 3 times the distance o f the Earth from the

Sun , or a little greater than the mean distance of Uranus from the Sun . It is probable , however, that most double t stars are farther apar than this . But it is evident from the stars considered th at the average mass of a pair is times that of the Sun , while the average radiating power is nearly six times as much . CHAPTER XXII

TH E MOTIONS A N D BRIGH T N ESS OF TH E STARS

TH E fi a term xed st r, which has survived from ancient times , we have already found is but relative and that all ta s rs have some motion . I f the position of one of these so-c alled fixed stars is noted by observing the time and the height at which it crosses the south point in the sky, and this observation is compared with accurate records 2 00 which we have , going back nearly years , it will be found that quite a number of the stars are moving slowly r o er moti on across the sky . This movement is termed p p , which , as seen from the Earth , is at the best an exceed i n l g y minute quantity . Thus one star, and that the most the rapid , has moved about the diameter of the Moon in 00 o r last 3 years , an amount which the telescope is quite f incapable o detecting. It may be said in passing, how e ver, that the actual observation of fixed stars from the 1 00 Earth after an interval of, let us say, years , would f s w . o ho more difference Yet this is due , not to motions ’ to the stars , but alterations in the direction of the Earth s axis and other causes which give apparent and common motions . The maximum proper motion is that o f the eighth -mag n i tude 2 star No . 43 of the fifth hour in the Cordova zone

catalogue . The star has an apparent drift of annually, ° which would carry it round a circle of 360 in years if it moved uniformly at its present rate . This amount can be appreciated when it is stated that two cen 2 70

2 72 ASTRONOMY stars through space has thus been ascertained to be about 1 0 2 0 or miles a second . How is it known that stars are moving in the line of sight ? Millions an d millions of miles away a star may be approaching the Earth , and yet through the telescope there is no change of position and no appreciable change in magnitude . But here use is made of an ingenious prin c i le o p , which takes its name fr m its discoverer , Christian 1 8 0 Doppler ( 3 a physicist of Prague , who made an experiment with quite another obj ect in view . Placing some musicians on a railway car and taking his stand on w as the platform , he noticed that the pitch of the sound raised as the moving train approached him and was low ered after it had passed and was receding . If this could

happen in sound , why not in light , altho the vibrations

occur inﬁnitely faster, so that the color of a luminous body ( equivalent to pitch in sound ) would be affected by i ts motion ? In 1 868 Sir William Huggins successfully ap ’ or plied Doppler s principle . I f a star is coming toward us receding there occurs a displacement in the spectra of the manifold light waves varying from the fundamental value a of the velocity of light , miles a second . When

star approaches , the light waves are crowded together ; r when it recedes they are drawn apart . Now as the numbe o f the vibrations of the light waves and their length deter mine the color o f light or the position o f waves in the o spectrum , by microscopically comparing Spectrum phot graphs o f the same star and noting the displacement of the Fraunhofer lines it can be determined whether that star

is approaching or receding . ’ Doppler s law has been explained very simply by Prof . “ 2 Edgar Larkin as follows : In the diagram ( Fig . 4 ) i A s a ray of light from the star S , falling on the side of

the prism P , which has the property of separating any

mixture of light into separate waves . Light from the Sun

or stars is made up of a vast number of colors , all appear

ing between the limits red and violet . Had the pencil A MOTIONS O F TH E STARS 2 73

n ot B . B ut encountered the glass , it would have passed to . the prism separates the white light into colors which can be proj ected on any white surface . The red is invariably bent out of its original direction the least , the violet most, and will respectively pass to R and V, with every other color between . The shorter the waves the greater thei r deﬂection from a straight course . Red waves run to an inch and violet An eye at E would see all the colors between R and V direct and at H by deﬂection , i f a screen is allowed to receive the light from R to V . Solar and stellar spectra are crossed at right angles by

b . lack Fraunhofer lines Take any one , say F, anywhere in the spectrum and measure its position with a mi cr ome um ter. Then the eye , either at E or H , would see a spectr

’ F i 2 —Dop p LE R g . 4 s PRI NCI PLE . f i i h t A . i n f en c B . T he o c o o the t a . w ou d a t o p g , , g , S p l l m r m s r , l ss i f h i t e B w e e e o e d . Th e a e a a te t h e t e pr sm , , r r m v gl ss s p r s s l la r i h t i n t o a ll th c o o i m n i Th e r e d a a r e l g e l rs t ay co t a n . r ys

n R . a n h b e t t o d t e v i o et t o V . fo i n wi th th e co o b e l , rm g l rs twe e n th e e c t o f h t e t a . , sp rum s r a s 2 outlined in the upper diagram of Fig . 4 , extending like

1 2 . as R , , F, , T , V Let the prism move at great speed , such o r that due to the velocity of the Earth , toward the star, h e d let t star move towar the Earth . Then the line F will m 2 h t e . ove to , or toward violet end But if the Earth and to 1 stars move in opposite directions , the line F will move , ” or toward the red . After the more brilliant stars of the heavens had been identiﬁed and their positions determined with as much accuracy as possible with the methods of observation and 2 74 ASTRONOMY

t instruments available to the ancient astronomer, the nex development was to compare their relative brightness . In

1 B . C . at s 34 , the time of Hipparchus , a catalogue of star was prepared which was said to have been suggested by the sudden appearance of a new star in the constellation of the Scorpion . The stars were divided into six mag n i tude s , according to their brightness . The catalogue a h a s of Hipparchus , cont ining as it did stars, e since proved most valuable to astronomers . Next cam ’ ‘ ’ h 1 8 A . D. Ptolemy s great Almagest, published in 3 , whic

. contained a catalogue of stars , doubtless based on that of Hipparchus . Ptolemy used a scale of stellar mag i t n ude s . , which has continued in use to the present time

The brightest stars of the sky , such as Sirius and Arc turus, were regarded as of the ﬁrst magnitude , and the - l faintest visible to the naked eye the sixth . To day a smal telescope will render visible stars down to the ninth mag n itude , of which there are over Ptolemy em ployed the ﬁrst six letters of the Greek alphabet for this r purpose and then subdivided the classes he made . I f a sta 1 seemed bright for its class , he added the Greek letter 1

f i z uez am Me on . ( Mu) , standing for j e ( ) , large or bright I f a for the star was faint, he added ( Epsilon ) , standing m el a o o v . ( Elasson ) , small or faint These estimates were ’ carefully made . If Ptolemy s original manuscript were at hand his magnitudes would be useful to modern a st r on o mers in determining the secular variation of the bright

ness of the stars . But the errors in the various copies and transcripts of the Almagest which have come down to

modern times are so great that the positions , magnitudes and identiﬁcations of about two- thirds of the stars listed A l are uncertain . Indeed , the oldest manuscript of the n magest dates only to the ni th century . The Persian as t r on omer Abd - al- rahman A l- suﬁ ( 903 - 986) reobserved ’

A D. Ptolemy s stars in 964 . and noted cases where he found f a di ference . This work survived in Arabic, and trans lated into French by S chj eller up ( 1 874) is available for

2 76 ASTRONOMY method by using numbers instead of points o f punctuation to denote the intermediate brightness between the various magnitudes , a method known by his name . His catalogue, the great Bonn Durchmusterung ( 1 799 contains as many as stars visible in the northern hemisphere . t After mere j udgment with the eye , it was but natural tha some more accurate means should be employed , and vari ous photometers , to which we have referred elsewhere , were eventually adopted for the purpose of gaging stellar brightness . In 1 8 56 Pogson showed that the scale of magnitudes of d Ptolemy, which is still in use , could be nearly represente by assuming the unit to be the constant ratio which has been adopted as the basis of the standard photometric scale . Thus an increase of four units in number would express the magnitude corresponding with a division of — the light by one hundred , and a Sixth magnitude star would have but one - hundredth the brightness of one of the ﬁrst magnitude . Photometric observations have been undertaken for many years and are now in progress on a large scale at

Harvard University, at Potsdam and at Oxford . Various forms of photometers are employed . Simple photo metric work takes into consideration only the total light of a star in so far as it affects the eye . This light may ff consist of rays of many di erent wave lengths . In red stars one color predominates and in the blue another . Hence the preferred method is to compare the light of a given wave length ( color) in di ff erent stars and then to determine the relative intensity of the rays of different f wave lengths in dif erent stars , or at least in stars whose spectra are of different types . The brightness of the stars may also be measured in a simpler but less satisfactory method by determining the total light in a photographic image , a method open to the same obj ection as eye photometry . In other words , the rays of different colors are combined and affect the MOTIONS OF THE STARS 2 77

ff photographic plate di erently . Consequently blue stars appear brighter than the red . Still photographic pho t omet r y is extensively used and various proportions an d corrections are employed so that satisfactory results are obtained . As a result of modern methods of classification the number of stars of the first six magnitudes visible to the naked eye is about These are grouped in the fol : 2 0 6 lowing order First magnitude, ; second magnitude , 5 1 0 2 third magnitude , 9 ; fourth magnitude , 4 5 fifth magni

2 0 . tude , sixth magnitude , 3 It has been estimated that over stars are visible within the range of present visual and photographic instruments . The ﬁr st - magnitude stars number only about 2 0 and on account o f their conspicuous brightness serve as land marks in the study of the heavens . Their names , con stellation , magnitude and color are given in the following table :

M d C s . on tellation agnitu e . Color . a Canis Majori s B luish white a B ooti s O range Pale b lue Y ellowish White B luis h a Can is Mi n or is White Rud dy a C en ta ur i White a E r i d a n i White Y ellowish R ed D eep red O ra n ge

When a star outsh i n es a star of th e ﬁrst magn i tude i t i s n o lon ger p os d i n a te i t s br i h t n e b 1 H n he n u e i ca ex e i on sible t o es g g ss y . e ce t m r l pr ss s a n i n the fo e oi n t ab e d r g g l . 2 78 ASTRONOMY

C a M ud onstell tion . agnit e

ﬂ Centa uri a Crucis a Piscis Australis

In connection with stellar photometry , it has probably o ccurred to the reader that photographic charts would prove very serviceable, as the brightness of the photo graphed image could be used . Such indeed is the case . Astronomers interested in stellar photometry have devoted n o little attention to th e study of these charts and plates .

2 8 0 ASTRONOMY the star in the co n stellation Scorpion discovered by 2 Hipparchus ; ( ) variable stars of long period , ﬂuctuating in light by large amounts during periods of several months ; ( 3 ) stars whose variations are small and i r r egu lar ; ( 4) variable stars of short period; and ( 5 ) the so called Algol variables , which are usually of full bright ness and at regular intervals grow faint owing to the i n t e r p osi ti on of a dark companion between the star and the

Earth .

Variable stars have long been known , but only about 2 5 0 were recognised by astronomers until photography and

spectroscopy were applied to their discovery . Three r e

markable discoveries , Prof . W . H . Pickering, of Harvard

College Observatory , states , are responsible for greatly im “ creasing the number . The ﬁrst was by Mrs . Fleming at

Harvard College Observatory , who , in studying the photo

graphs of the Henry Draper memorial , found that the

stars of the third type , in which the hydrogen lines are

bright , are variables of long period . From this property she has discovered 1 2 8 new variables and has also shown f how they may be classiﬁed from their spectra . The di fer en c es th e between ﬁrst , second and third types of spectra are not so great as those between the spectra of di fferent

variables of long period . The second discovery is that of

Professor Bailey , who found that certain globular clusters

contain large n umbers o f variable stars o f short period . 0 6 He has discovered 5 9 new variables , 39 of them in four t clusters . The hird discovery , made by Professor Wolf , a of Heidelberg , that variables occur in large nebul e , has 6 led to his disclosure of 5 variables . By similar work Miss

2 . Leavitt , of Harvard , has found 95 new variable The total number of variable stars discovered by photography during the last ﬁfteen years is probably ﬁve times the

entire n umber found visually up to the present time . Hundreds of thousands of photometric measures will be

- required to determine the light curves , periods and laws ” regulating the changes these obj ects undergo . VARIABLE AND BINARY STARS 2 8 1

o f a The discovery nov e, or new, temporary stars ( the

ﬁrst class mentioned) , continued from the time of Hip hu or ar c s . p to the invention of the telescope Four new, temporary, stars were discovered in the interval between the catalogue of this ancient astronomer and the beginning of the sixteenth century . The Star of Bethlehem may have 1 2 been of this character . In November, 57 , a brilliant new star appeared suddenly in the constellation of Cassiopeia , which was observed by Tycho Brahe during the sixteen

months of its life . During this time it rivaled Venus at its ’ Tv cho s brightest and revived interest in astronomy, which t a the time was beginning to wane . He wrote at consider

able length a description of the star, published in I 573 , and Kepler subsequently remarked that “ if that star did noth as ing else , at least it announced and produced a great ” tronomer . In modern times the most striking nova was a

star which was discovered in Perseus in 1 90 1 by the Rev . 1 8 2 Dr . Anderson , of Edinburgh , an amateur , who in 9

had discovered a nova in the constellation Auriga . In the Per sei sudden appearance of Nova , suggests Arrhenius , we evidently witnessed the magniﬁcent termination by collision o f the independent existence o f two heavenly

bodies . Typical o f the second class o f variable stars which ex b ibit marked irregularities in period and in brightness i s similar to those o f the new stars Eta in Argus , which in 1 677 was classed as a star o f the fourth magnitude and in 1 687 and 1 75 1 of the second and in 1 8 2 7 of the first be magnitude . Then Herschel found that it fluctuated 1 8 i n tween the first and second magnitudes . In 37 it crea sed rapidly i n brilliancy and in 1 838 so far outshone the typical fi r st -magnitude stars that its magnitude was denoted by But in the following year it declined to 1 8 the first magnitude and there remained until 43 , when it rapidly brightened until it outshone every star except Sirius (magnitude Thereafter it slowly declined to the sixth magnitude and since 1 869 has fluctuated be 2 82 ASTRONOMY

tween the sixth and seventh . These observed changes 1 1 point to a great collision in 743 and a smaller one in 838 . The smaller collision may be compared to the fall of the f to Earth into the Sun , which would develop heat su ficient 1 00 maintain solar radiation during years . From the older o bservations it appears probable that the star suffered at least one earlier collision . or Another star of this type is Mira , Omicron Ceti , which was the first star to be recognised as a variable , 1 1 6 having been discovered August 3 , 59 , by David Fabri cius and described minutely by a Dutch astronomer, Pho c lides H olwar d a 1 1 1 6 y ( 6 8 in 1 639. In 6 7 its period o f B oulli au about eleven months was fixed by Ismael , or Bullialdus ( 1 605 altho it was found that its fluctua 1 8 0 tions varied considerably . It was described in 7 by 1 Herschel , who had observed it in 779, when it was nearly w as as bright as Aldebaran . Four days later the star invisible even through his telescope . The maximum brightness of this star varies from the first to the fifth magnitude . At the minimum it falls below the sixth mag n i tude , becoming invisible to the naked eye , and occasion a t ally below the ninth , so that its maximum it has times the luminosity of its minimum appearance . The spectrum of the star indicates that it is surrounded by three nebulous envelopes . The innermost of the surround ing envelopes is uni formly distributed and the others form a ring with two points of maximum density corresponding

with the traces of two eruptive streams . This ring revolves in 2 2 months and has a linear velocity or rotational period f o miles per second . Hence it follows that the diame ter of the ring is times that of the Earth ’ s orbit and that the mass of the central stars is slightly less than that o f the Sun . Mira Ceti is typical of most of the variable stars in that they are red and give continuous spectra crossed by dark bands and bright hydrogen lines . The third class of variable stars comprise those of ir “ ” f so- regular period , which dif er from the called new stars

2 84 ASTRONOMY ser vato r 1 8 8 . y in 9 Therefore it is two stars , or a spec r 0 i t osc p c binary . Galileo and subsequent observers noticed that in many instances stars which appeared to the naked eye as singl e really were double . When these stars were separated under a high magnifying power it was found that they n varied in distance , mag itude and color . Thus , if two stars are almost in the same line of sight , they will appear to the observer to be very closely related , altho one of the pair may be much nearer to him than the other and thei r proximity merely accidental . Such a pair of stars is “ ” “ ” known as a double star, or as an optical double , and i s to be distinguished from a pair of stars at app r oxi ffi mately the same distance from the Earth , but so a liated that they revolve about a common center of gravity . In other words , their relative positions would resemble those of the Moon and the Earth . When thus paired the combi nation is termed a binary system . The discovery of binary di scov stars was ﬁrst made by Sir William Herschel . He ered that there were changes in the relative positions of the two stars which obviously were not connected with the motion of the Earth , but indicated an actual circling move ment of the bodies themselves under a mutual attraction . He found that there was a regular progressive change in their motion which indicated that one of the stars was slowly describing a regular orbit around the other . Indeed , it seemed that gravitation had its effect beyond the solar wa s system , as the orbit of each of such a pair of stars found to be an ellipse with the common center of gravity two at the focus . That is , the stars were moving in ellipses which were precisely similar, except that the one described by the smaller star was larger than the other in ’ inverse proportion to the star s mass . While Herschel was the ﬁrst actually to see such binary stars , yet thei r existence had been deemed probable by the Rev . John

Michell , who lived a short time before the great observer . Not only are there double stars to the number of VARIABLE A ND BINARY STARS 2 85 but triple and quadruple stars and even multiple stars in a single system . The distances between these stars gener ” ally amounts to from 30 to as double stars nearer than the latter ﬁgure can be separated by only the most powerful telescopes . But the spectroscope enables stars much closer together to be resolved , and in fact the impor tant class known as sp ect r osc0 pi c binaries previously t e ferred to can be studied and their physical and optical prop

e r ti s . e determined by elaborate measurements Thus Polaris , which in a moderate - sized telescope appears as a double o f star , including one of less than the second and one less

than the ninth magnitude , is really quite complex , for the brighter star revealed by the spectroscope has three stars

very close together and all in circulation about one another . i n o f Again , in the case of b aries a spectrum with two sets

lines is seen in the spectroscope . With the telescope the

image is single , but nothing can explain the double spec

trum except the existence of two separate bodies . Accord i n l as g y, connected pairs of stars such as these are known

spectroscopic binaries . Often they make up a system of

double stars visible in the telescope . Such an example would

be Mizar, in the handle of the Plow, which , seen with a

small telescope , appears as a double star, one of its com on en t s p being white and the other greenish . In reality these two stars are situated so distant from each other that from one of them the other would appear merely as an ordinary

‘ bright star . But the telescope shows that the brighter o f O f these stars is again a binary system two huge suns, 2 revolving around each other in a period of about 0 days . Powerful spectroscopes now are able not only to t e t solve what appears as one point into two , but to detec

motions of the two suns around their gravitation center .

This has been simply explained by Prof . Larkin . He states that by the application of Doppler ’ s principle “ the times of v revolution can be obser ed , and when the distant suns have a sensible parallax the distance between them can be deter i n n s mined miles . With time and distance k own , velocitie 2 86 ASTRONOMY

in their orbits follow at once . Then with velocity and dis — tance the mass of both can be computed that is , both suns ‘ ’ can be weighed in terms of the mass of our Sun . For it is known from the laws of gravity and motion how much matter is required at any given distance and velocity to set up centrifugal tendency equal and Opposite to gravita tion . “ The Doppler principle applied to the discovery of

—A B I T F ig . 43 NARY S AR .

hi di a a . i s t h e a ce o f th e o e a i e sun far n d I n t s gr m S pl m r m ss v , a n id e o f t h e c en t o f t h bi h a wa y t o o e s e r e o r t . T e o rbi t a n d ti o n o f th e e v o i n s un a r e e e e n f o p o g p t e d b A . C . ur r s r lv r r s y , , i n E h o a a e e . a n d F T e tw C . n P . G. G o t t owa d h e , p r ll l l s , , p i r t i Wh e n t h e ov n s un s a t G. c o n a t h . t ow e r m i g , mi g a rd t h e e a th a i n e i n t h e e ct wi h i ft t owa d t h e i o e r , l sp rum ll s r v l t ; a n d

wh e n a t G. oi n awa f o th e e a th th e i n e i e , g g y r m r , l w ll m ov t owa rd t h e r e d .

n binaries is show in the diagram , where S is a sun , far and away to one side of the center of the orbit of its compau

i n . ion , as shown four positions , A , C , P , G As seen from

the Earth when at G, the ﬂying sun will send fewer waves per second into the spectroscope and the Fr aun ho fer lines

will shift toward the red . The point A is apastron and P

2 88 ASTRONOMY

’ speed of the Sun s revolution in its orbit, and this entirely by means of the known relation between shifting of spec tral lines and the speeds of the ﬂying sun at times of ap proach and recession . And by direct observation he notes o f on the time e revolution . He multiplies velocities per second by the number of seconds and thus ﬁnds the cir c umfe r en ce and radius of the orbit . He at once knows ' how many times farther ap art the two suns are than our

Sun and Earth . Then it becomes simple arithmetic to ’ ” a ﬁn pply Kepler s law and d mass . The distribution of the stars in the heavens seemed to h ‘ h l t e w o e . the ancients to be fairly even on It was noted , however, that there was a long, relatively thin segment of space extending across the sky in which the stars appeared i ts more numerous than elsewhere , and on account of “ ” brilliancy this was termed the Milky Way . Aristotle discusses it among other astronomical phenomena , and Greek astronomers looked upon it as a great circle of the celestial sphere . From those days to the present no satis factory explanation has been offered for the existence of

such a segment with the Earth apparently at its center , nor indeed for any of its characteristic peculiarities of aspect and its relationship to the stellar universe . Galileo , with o f con his telescope , found that portions the Milky Way sisted o f multitudes of faint stars clustered together . It was recognised that the stars in the heavens were more and more closely massed as the Milky Way was ap h p r oac ed . With the realization that all stars were n ot at the same d istance , there naturally followed speculation as regards con their arrangement and distribution . The ﬁrst positive t r ibuti on to the various theories and the apparent di st r ibu o f tion of the stars in Space came from Thomas Wright, “ Durham ( 1 7 1 1 published in his Theory of the so Universe This theory is of interest , not but much in its original form as propounded by Wright , for the fact that it was taken up ﬁve years later by the VARIABLE AND BINARY STARS 2 89

great philosopher , Kant , and by him developed in philo

Sophically explaining the origin of the universe . Further i n more , the hands of Sir William Herschel it became an important astronomical theory which received serious con ’ sideration for many years . Herschel s hypothesis was that the space occupied by the stars resembled in form a thick “ disk or grindstone , close to the central part of which the solar system was situated . When such a disk was looked through lengthwise more stars were seen naturally than when it was looked through breadthwise . But at the same time there were vacant spaces or holes in the ground of work the Milky Way , so that we can apparently see or through the collection of stars . These holes clefts are iﬂi l d cu t to explain . e The Milky Way has been described by Prof . Georg “ t C . Comstock as a belt approximately following a grea f circle of the sky, but broad and di fuse throughout one half of its course, while relatively narrow and well deﬁned f o n the opposite side . The broad half o the belt is cleft in two by a dark lane running along its axis , and in addi th e tion contains numerous ri fts and holes , from which o f narrow half is relatively free . The number stars per unit area of the sky is a maximum in the Milky Way and

o n . diminishes progressively either hand, while the inverse ae relation is true for the nebul , their frequency increasing with increasing distance from the Milky Way . But even the most superﬁcial observer is forced to the conclusion that the stellar system is o f limited extent and d F or oes not extend to an inﬁnite distance . if stars and suns exist through an unlimited space , their luminous radi

‘ a tion s f , which do not su fer in intensity in their passage through the ether, would reach the known universe with little diminution and the entire heavens would always blaze

with light . That such is not the case is known from experience and from the fact that the combined illumina tion furnished by all the stars is only about on e-hun dredth h as part of that obtained from the full Moon . The theory 2 90 ASTRONOMY

Wa l been advanced that the dark holes in the Milky y, coa ” n sacks as they have been termed, co sist merely of dark n stars or extinguished suns , such as the one we have see occasions the eclipse of Algol . But this was disputed by n o the late Professor Newcomb , who says that there is evidence that the light from the stars in the Milky Way, which are apparently the most distant bodies visible from the Earth , can be intercepted by dark bodies or dark m atter, and that the stars are seen j ust as they are dis tributed in space . Furthermore , recent photographic work indicates that there is a limit to stellar distribution and that there i s “a darkness behind the stars ” which long exposure and powerful instruments cannot pierce .

The problem is one of striking immensity . Prof . E . C . Pickering speaks of the distribution of the stars and the constitution of the stellar universe as perhaps the greatest problem in astronomy . In a recent discussion he remarks No one can look at the heavens and see such clusters as the Pleiades , Hyades and Coma Berenices without being

convinced that the distribution is not due to chance . This view is strengthened by the clusters and doubles seen in even a small telescope . We also see at once that the stars must be of di ff erent sizes and that the faint stars are not necessarily the most distant . If the number of stars were infinite and distributed according to the laws of chance throughout infinite and empty space , the background of the sky would be as bright as the surface of the Sun . This is far from being the case . While we can thus draw gen be eral conclusions , but little definite information can s obtained without accurate quantitative measures , and thi is one of the greatest obj ects of stellar photometry . If we consider two spheres, with the Sun as common center and on e having ten times the radius of the other, the volume of the first will be one thousand times as great as that of the

second . It will , therefore , contain a thousand times as many stars . But the most distant stars in the ﬁrst sphere o ff would be ten times as far as those in the second Sphere,

CHAPTER XXIV

N EBULJE A N D STAR CLUSTERS

N EBULE are masses of di ffused shining gas which are scattered through space and which undoubtedly consist of the matter out of which stars have been and are being f formed . They di fer from star clusters in that the highest powered telescopes yet constructed are unable to resolve r them into separate component stars , yet without doubt sta clusters are evolved from nebulae and their connection is F r most intimate . o the first record o f a nebula we go back “ to Huygens ( 1 62 9- 1 695 ) and find in his Systema Satur ” nium not only a description but a rough drawing . The description is of the nebula Orion and is as follows “ There is one phenomenon among the fixed stars worthy of mention which , so far as I know, has hitherto been noticed by no one , and , indeed , cannot be well observed except with large telescopes . In the sword of Orion are three

. 1 6 6 stars quite close together In 5 , as I chanced to be viewing the middle one of these with the telescope , instead of a single star twelve showed themselves ( a not un com mon circumstance ) . Three of them almost touched one another, and , with four others , shone through a nebula , so that the space around them seemed far brighter than a the rest of the heavens , which was entirely clear, and p ear ed f p quite black, the e fect being that of an opening in ” the sky , through which a brighter region was visible . The work thus inaugurated by Huygens for some reason or other did not seem to attract the attention of astr on o 2 92 NEBULE AND STAR CLUSTERS 2 93 m ers for many years , altho Lacaille , while at the Cape of 1 0 - 1 2 a Good Hope , 75 754, observed and described 4 nebul e, n ebular stars and star clusters , and altho Charles Messier ( 1 730 who devoted himself to the detection of ae comets , found he was liable to mistake nebul for comets , 1 1 and recorded in 78 the positions of 1 03 of the former . In 1 h ilo so the meantime , in 75 5 Immanuel Kant , the famous p s ecu pher, advanced the theory on purely theoretical and p lative grounds that a single nebula or star cluster was an o f assemblage stars , comparable in magnitude and strue ture with the aggregation which we now term the Milky A o Way and the other separate stars which can be seen . on e cording to this theory, the Sun would be but star of a cluster and every nebula a system of the same order . This “ ” was known as the island universe theory and was first a ccepted by Sir William Herschel . In the course o f his indefatigable investigation of the stars with his large telescopes Herschel inaugurated a sys t emati c a r h study of the nebul e and sta clusters . Altho e found it difli cult to draw a line between nebulae and star clusters , yet he was able to state positively and correctly that they were not identical . Herschel noted the position o f each nebula and its general appearance and marked the positions on a star map . He published catalogues , the first 1 86 ae of which , prepared in 7 , contained nebul and 1 8 clusters , the second in 7 9, of about the same extent, and ’ 1 802 00 . a third in , comprizing 5 Herschel s observation s o f nebulae enabled him to note their differences in bright ness and apparent structure so that he could divide them into eight classes . In 1 786 he published the following interesting account of the varieties in form which he had observed : “ a I have seen double and treble nebul e, variously ar w ranged ; large ones with small , seeming attendants ; narro a but much extended , lucid nebul e or bright dashes ; some o f the shape of a fan resembling an electric brush , issuing a from a lucid point ; others of the cometic shape, with 2 94 ASTRONOMY

seeming nucleus in the center ; or like cloudy stars , sur rounded with a nebulous atmosphere ; a di fferent sort again contain a nebulosity o f the milky kind , like that wonderful inexplicable phenomenon about 9 Orionis ; while others shine with a fainter mottled kind of light, which denotes their being resolvable into stars . Herschel ’ s great problem was to determine the relation a ff between nebul e and star clusters . O ften the di erence between the two was made apparent only by the use of a telescope of suffi cient power to resolve a bright glow in the heavens into clusters of stars . But at the same time there were bright places that still remained nebulous . “ Hence Herschel wrote : Nebulae can be selected so that an insensible gradation shall take place from a coars e cluster like the Pleiades down to a milky nebulosity like ” that in Orion , every intermediate step being represented . To Herschel it seemed that the power of the telescope was the important consideration , and the gradation mentioned, “ he writes , tends to conﬁrm the hypothesis that all are ” composed of stars more or less remote . As Herschel progressed with his investigations the views o f other astronomers , as well as those ﬁrst entertained by 1 1 him , did not seem tenable . By 79 he reached the point o f view that in certain cases at least the nebulae were essentially di fferent from star clusters . Referring to a “ : s certain nebulous star , he wrote Cast your eye on thi cloudy star and the result will be no less decisive .

Your j udgment, I may venture to say, will be that the

nebulosity about the star is not of a starry nature . Her schel reasoned that i f the phenomenon were due to an aggregation of far- distant stars that there must be one central star of extraordinary dimensions or that something ff “ radically di erent , such as a shining ﬂuid of a nature ” totally unknown to us , must be called upon to explain the

appearance . His observations proved that an individual nebula was usually surrounded by a region of the sky com ar ati ve l e p y free from stars , and that where clusters wer

2 96 ASTRONOMY

’ of Herschel s observations to his theory . Laplace had inferred that the planets and their satellites must have been derived from some common source , and he suggested that either they might have been condensed from a body and be regarded as a sun with a vast atmosphere filling the space now occupied by the solar system or that they represented the results of condensation of a fluid mass which now possessed a more or less condensed central nucleus which at one time was not in existence . The nebula of Herschel accordingly suggested to Laplace a suitable fluid mass from which a solar system could have o f been condensed , and , furthermore , the evolution the i n fixed stars could be explained on a similar basis . This ’ gen i ous theory o f Laplace s was rather a scientific specula tion than an accurate conclusion founded on data he had or n ot himself observed . As a theory , whether accepted , it has proved of the most vital importance to science . John Frederick William Herschel ( 1 792 - 1 87 1 ) published u 1 8 o f a o f a catalog e ( 33 ) about nebul , which some ’ 00 5 were new and were his father s, a few being due to 00 other observers , and later reobserved about 5 known nebula while at the Cape of Good Hope ( 1 833 i n cluding the nebula surrounding the variable star Eta of a Argus and the wonderful collection nebul , clusters and stars known as the Nebucula or Magellanic Clouds . In 1 864 Herschel was able to present to the Royal Society a a valuable catalogue of all known nebul and clusters , amounting to Later this great catalogue , which o f contained a condensed description each body, was super ’ catolo ue seded by Dr . Dreyer s general g , which was a s based upon it , and contained nebul and cluster subse known up to the end o f 1 887 . A supplementary list quently published by the same authority contained

entries of discoveries made between 1 888 and 1 894. Hence the two Herschels are responsible for more than half o f the total number o f nebula and star clusters n ow known to

a stronomers . A L I S P I R AL N E B ULA I N ME SS I E R 3 3 T R I N GU .

2 98 ASTRONOMY width of the slit and occupying in the instrument 3 position a t that part of the spectrum to which its light belongs in refrangibility . A little closer looking showed two other b right lines on the side toward the blue , all three lines b eing separated by intervals relatively dark . The riddle o f a the nebul was solved . The answer , which had come to : us in the light itself, read Not an aggregation of stars , b u t a luminous gas . This discovery marked a new era in astronomical p rogress . Its importance was at once appreciated , and t h e ability of the spectroscope to distinguish between a glowing gas and a star-like mass of partially c ondensed vapors solved the problem that had so seriously c oncerned the elder Herschel and immediately brought the spectroscope forward as the chief instrument that must be e mployed in the larger problem of the evolution of the s tars in the universe . It was at once apparent that these a morphous nebula might supply the material from which the stars were formed and that the process was one of evo l ution and possibly mere condensation . What the spectroscope actually has accomplished for r esearch on the nature of the nebula was excellently s ummed up by Sir David Gill in his presidential address b efore the British Association for the Advancement o f “ ’ h e S cience at Leicester Huggins spectroscope, “ s a aid, has shown that many nebul are not stars at all ; - a that many well condensed nebul , as well as vast patches o f nebulous light in the sky, are but inchoate masses of to luminous gas . Evidence upon evidence has accumulated s how that such —nebula consist of the matter out of which s tars suns have been and are being evolved . The different types of star spectra form such a complete and g radual sequence ( from simple spectra resembling those o f nebula onward through types of gradually increasing

complexity) as to suggest that we have before us , written

in the cryptograms of these spectra , the complete story of the evolution of suns from the inchoate nebula onward to NEBULE A ND STAR CLUSTERS 2 99 the most active sun ( like our own ) and then downward to the almost heatless and invisible ball . The period during which human life has existed upon our globe is probably too short—even if our ﬁrst parents had begun the work to afford observational proof of such a cycle of change i n th e any particular star ; but the fact of such evolution , with ”

f . evidence be ore us , can hardly be doubted a After the spectroscope , photography has been found most valuable method of nebular research , and many It discoveries have been recorded through its assistance . will be appreciated readily that an impression left by visual observation in a matter of this kind would necessarily be e very vague , and , furthermore, that the record on a sensitiv plate by long exposure would show far more detail than even the eye of the observer can perceive . Accordingly all the great observatories of the world , particularly those equipped with good reﬂecting telescopes , have been at work o n 1 88 0 nebular photographs since about , tho it was not until 1 883 that the ﬁrst really excellent pictures were ob

t ai n ed . It was early seen that the light o f nebula was strongly photographic , that it was really more actinic than visual . A photograph with a long exposure showed the Merope nebula in the Pleiades j ust as the best observers had drawn it, and at the same time ﬁlled the entire group o f stars with an entangling system of nebulous matter which seemed to bind together the different stars of the group with misty wreaths and streams of ﬁlmy light , nearly all of which detail is entirely beyond the keenest vision and the most powerful telescope . A fter Sir William Huggins with his spectroscope had endeavored in vain to determine the radial movement o f a nebul , Professor j ames E . Keeler repeated the attempt

- at Lick Observatory in 1 890 9 1 and achieved great success . Ten planetary nebula showed satisfactory evidence of line o f- - sight motion , one of which , the well known greenish h globe in Draco, was found to be moving toward the Eart 3 00 ASTRONOMY a t 0 was a rate of 4 miles a second, while the Orion nebula ’ receding at a rate of 1 1 miles . Keeler s work demonstrated t hat no longer Should nebular ﬁx i ty be looked upon as a fact and that nebula have a motion which can be studied .

Again the sensitive plate had scored a triumph . The study of nebula has led to their classification under v arious heads into spiral , planetary , ring and i rregular n a ebula . The first spiral nebul was discovered by Lord Rosse ( 1 800 - 1 867 ) with his great 6- foot reflecting tele hi h ‘ S w c was . cope , mounted at Parsonstown , Ireland A t ypical spiral nebula consists of a central disk - shaped p ortion or nucleus , with two long, curved arms proj ecting f from opposite sides , giving the e fect of rapid rotary move “ ” ment and slightly suggesting the familiar pin -wheel of

ﬁrework displays . The chief characteristic of the spiral n ebula is their white color as compared with the greenish

o f . tinge other types Furthermore , they are more numer o us than all other types combined , it having been estimated

by the late Professor Keeler , of Lick Observatory, that at l east o f these Spirals were within the grasp of th e o f Crossley reﬂector that institution , while Professor Per to rine , at the same observatory , increases the estimate h alf a million and believes that with a more sensitive pho togr aphi c plate and a long exposure over a million could be

o . btained According to their spectra , they are largely in a

solid or liquid condition , but are of a tenuous character and very transparent so that it is inferred that they consist o f v i r ast swarms of ncandescent solid o liquid material , sur i n a rounded by gaseous material . The nebula Andromed i he s t greatest spiral known . Its diameter is more than

times the distance from the Earth to the Sun , or 8 - Dvi sible light years . It is to the naked eye and is second ft c o nly to that of Orion in size and splendor . It has

- quently been mistaken for a comet . If it were one twenty millionth as condensed as the Sun it would attract the r Earth as much as the Sun does . The extreme rarity o t enuity of nebula is evident from the fact that there i s

3 02 ASTRONOMY i t in any direction . A speculative explanation advanced is that the two regions of maximum nebula represent p laces where they have not yet condensed into stars . but Not only the spectroscopic evidence of bright lines , t h e aspect of nebula themselves shows that they are trans p arent through and through . This is remarkable when

taken in connection with their inconceivable size . Leaving o ut f a the large di fused nebul which we have mentioned, t hese obj ects are frequently several minutes in diameter .

O f their distance we know nothing, except that they are p robably situated in the distant stellar regions . Their p arallax can be but a small fraction of a second . We shall p robably err greatly in excess i f we assume that it varies

- - between one hundredth and one tenth of a second . To as sign this parallax is the same thing as saying that at the ’ d istance of nebula the dimensions of the Earth s orbit would show a diameter which might range between one fi fti eth -fifth and one of a second , while that of Neptune would be more or less than one second . Great numbers of n a ebul are , therefore , thousands of times the dimensions of ’ the Earth s orbit, and probably most of them are thousands f o times the dimensions of the whole solar system . That they should be completely transparent through such enor m ous dimensions Shows their extreme tenuity . Were our s olar system placed in the midst of one of them , it is prob a ble that we should n ot be able to find any evidence of i ts e xistence .

Considerably less conspicuous than the constellations , but composed of a far greater number of stars , are various c lusters and systems which often contain several thousand i ndividual members . It was the telescope of Galileo that ﬁ saw rst made possible the study of these clusters , for he i n the Pleiades 36 stars where the ordinary eye could dis t i n i h i x u s S . g but In other parts o f the heavens , as in por

tions of the Milky Way, he could observe clustered together Pr a se e co n stel multitudes of ﬁne stars , and in p , in the l 0 ad ation of the Crab , he was able to count 4 stars . With NEB U LZE AND STAR CLUSTERS 303 di ti on al telescopic power further clusters were noted, and 1 6 6 r Halley ( 5 for example , discovered a great sta 1 r cluster in Hercules , which , now known as Messier 3 , late derived its name from the catalogue of the famous French 1 0 B ut astronomer and comet hunter , Messier ( 73

- it was Herschel , with his high power telescope and his keen powers of observation , who was able to give the sub ect j further research , and he became , as we have seen , greatly interested in the distinction btween nebula and a star clusters . Herschel found that many of the nebul recorded by Messier could be resolved and he immediately pushed observation in this ﬁeld . The most recent theory of the formation of star clusters “ ” is due to Arrhenius , who states in Worlds in the Making that most of the clusters are found in the neighborhood o f the Milky Way, where also the visible stars are unusually crowded and where almost every year some new star i s n discovered . It is in this region that collisions betwee stellar bodies are most likely to occur and where gaseou s ”

a . a s nebul would be produced The nebul , Arrhenius goe “ two on to state , which are produced by collisions between h suns , are soon crossed by migrating celestial bodies , suc as meteorites or comets , which there occur in large num bers by the condensing action of these intruders they ar e b then transformed into star clusters . In parts of the eav ens where stars are relatively sparse ( at a great distance from the Milky Way) most of the nebula observed exhibit so far stellar spectra . They are nothing but star clusters ' removed from us th at th e separate stars can no longer be a so distinguished . That single stars and gaseous nebul are rarely perceived in these regions is no doubt due to thei r ” great distance . 1 00 e About star clusters are known , and they compriz either groups of the brighter stars or densely packed gath e r i n s be g where a large number of faint stars , as a rule tween the twelfth and sixteenth magnitudes , are collected . Of the ﬁrst class the Pleiades are typical and are perhap s 3 04 ASTRONOMY

th o Pr a se e the best known , the Hyades , Coma Berenices , p in Cancer and Orion are also representatives of this form f o cluster . The Pleiades contain seven visible bright stars o f about the fourth magnitude and cover nearly three square degrees , in addition to which there are a large n umber of fainter stars , some of which were ﬁrst seen by

Galileo . O f these 53 have been examined and their mo ‘ n in tions determi ed . Nearly fainter stars are cluded in the cluster . They are less numerous here than i n most parts of the sky and even in the immediate neigh h b or ood . The Pleiades , it must be remembered, are distant 2 6 - r 7 light years , at which distance the Sun would appea to the Earth as a star of the ninth magnitude ; and while t hese perhaps appear as a cluster, they are really something like on e- hundredth as far from each other as the group i s

- f . 1 8 rom us , or two or three light years In 59 Wilhelm T of empel discovered a faint nebulosity near Merope , one the 1 18 0 brighter stars of the group , and on November 4, 9 , B arnard at the Lick Observatory noticed a second nebulous c ompanion to this star and reached the Opinion that the whole cluster contained a number of faint nebulosities .

This Opinion was conﬁrmed by photographic observations, which Showed that the entire group was embedded in a n ebulous matrix which extended over a large section o f the heavens and indicated that the stars , many of which , as our own brilliant as Sun , cannot be seen without high power telescopes , are intimately related in their origin and

i n . their evolution In Coma Berenices , containing seven ﬁ fth -magn itude stars visible to th e ° n aked eye and lying ea or o f st, south west the zenith on a Spring or summer e vening, there is an unusual grouping of lucid stars within a small area and in addition a large number of fainter

s . P r a se e tars The cluster p , in the constellation of Cancer , when seen by the naked eye , appears as a patch of nebulous o f light , but is really a condensed group stars of which the brightest are of the seventh magnitude or j ust beyond the o f range visibility . An interesting cluster is contained in

36 6 ASTRONOMY members of these clusters are variable but the reason for this variability has not yet been explained . The late Professor Newcomb says that Perhaps the most important problem connected with clusters is the mutual gravitation of their component stars . Where thou o f sands stars are condensed into a Space so small , what prevents them from all falling together into one confused mass ? Are they really doi n g so and will they ultimately form a single body ? These are questions which can be i s ats factor ily answered only by centuries of observation . to They must, therefore, be left the astronomers of the ” future . CHAPTER XXV

COSMOGONY A N D STELLAR EVOLUTION

T H E striking regularity and uniformity which prevai ls in the planetary system was a mystery to Newton . He was aware that the then kn own planets and satellites all move in the same direction , all nearly in the same plane — A S the ecliptic and all in almost circular orbits . Newton did not believe in any vortex movement , such as that sug d gested by Descartes, to carry the celestial bo ies along with it, he could not understand this peculiar regularity, the less so because the comets , whose orbits seemed like a fr e wise to be dependent upon the attr ction of the Sun , quently did not at all move in the same direction as the planets . Without any j ustiﬁcation , Newton drew the con clusi on that the regularity of the planetary movements could not have any primal cause . The ﬁrst who seems to have aspired to such an explana “ ff o f tion was Bu on , the ingenious author the Histoire Naturelle From the outset he emphasized the extraordinary improbability that the inclination of the ecliptic to the plane of the planetary orbits would, by itsel f and merely owing to chance , never exceed or one twenty - fourth of the largest possible inclination o f

That point had already been accentuated by Bernoulli . The probability that this inclination is mere chance amounts for every single planet only to one twenty f ourth . For the ﬁve then known planets taken alto 4 2 or gether, the probability assumed the value of 4 , one 30 7 36 8 A STRONOMY

- h eight millionth . They had to consider in addition that t e — s satellites , so far as then known namely , the ﬁve moon about Saturn , four about Jupiter, the one of the Earth and the ring of Saturn— were all moving in planes which devi ated little from the ecliptic . A mechanical cause had to be found in explanation of these facts . f In order to explain the movement of the planets , Buf on supposed that they had resulted from a collision between the Sun and comets . Assuming that a small amount of the ’ o ff fan c1ed Sun s mass was knocked , he that as the result o f such collisions planets and their moons were formed . The p ossibility ‘ o f such an impact between a comet and the 1 680 e Sun seemed to him proved by the comet of , whos ’ s orbit, Newton had calculated , passed the Sun s luminou

- surface at a distance of only one third the solar radius , and on its return 600 years later would fall into the Sun . The fragments of the Sun formed from such a collision o ff h would not drop back into that body , but would ﬂy wit a rotary motion in the same direction . The fragments which possessed the smallest den s1ty would attain the greatest velocity and would therefore be hurled farthest away from the Sun before their orbits began to curve . The enormous heat produced by the collision would probably render the planets liquid , but they would rapidly cool owing to their small Size , tho remaining incandescent for longer ff a or shorter periods . Thus to the Earth Bu on assigned period of years for cooling down to its actual tem e r atur e p , for the Moon years , for Jupiter and for Saturn while the Sun required ten times as long again as Jupiter . In passing through the atmos ph er e of the Sun after their separation the planets would absorb from it air and water from which seas would later o f on be condensed . This atmosphere would resemble that the Earth , for , except that he thought the Sun was a solid f t e incandescent body , Bu fon regarded it as otherwise sembling the Earth in its essential characteristics . The Earth he considered had ceased to be incandescent because

3 1 0 ASTRONOMY central body would be sp eciﬁcally lighter than those neare r

. h to it In this respect he was in error, as we see in t e case of the Earth and the Moon . Kant adva n ced various explanations for such points as the deviation of the planetary orbits from circles and their ’ inclination to the ecliptic, for the formation of Saturn s ibli rings , for the interpretation of the deluge and other b cal facts . Finally he explained the extinction of the Sun , which , according to the View of that age , was a celestial as body in the process of burning . The end of the Sun w destined to ensue from want of air and from accumulations o f ashes . In this process of burning the Sun had lost its o f most volatile and ﬁnest particles , constituting the cloud dust which he assumed to be the seat of the Zodiacal “ i h L ght . Kant makes the Signiﬁcant observation that t e law concerning the extinction o f the Sun includes a germ r for the reunion of dispersed particles , even if the latte ” should have intermingled with the chaos . This is inter esting as indicating that matter passed through a cycl e n o n ow of evolution , being w condensed into suns and t scattered into chaos , the idea partaking somewhat of tha advanced centuries before by Democritus , who believed that matter was in constant motion and that the atoms were eternal and indestructible . In short , the cosmogony of Kant is typical of those theories which assume the planetary system to have originated from cosmic dust o r from a collection of small meteorites . In more modern times ideas based on this hypothesis have been advanced r n ki Dar N o de s Old . by and Lockyer , while Sir George H win has devoted much thought to the mathematical con

sideration of such a theory . Few philosophers or astronomers were in a better posi

tion to frame a basis for cosmical theory than Laplace , whose work in this ﬁeld must be mentioned after that of “ ” 1 6 h e Kant . At the end of his Systeme d u Monde ( 79 ) advances a mechanical explanation of the evolution of the

solar system , accompanying it, however, with the explana COSMOGONY AND STELLAR EVOLUTION 3 1 1 tion that it had not been subj ected to rigorous math ema ti cal analysis and proof and was advanced in a more or less tentative way . Despite the hesitancy with which Laplace ff o ered this contribution to cosmical theory, it soon se cured wide consideration and was found acceptable as a fundamental basis to explain the origin and evolution of the universe . In fact , this nebular hypothesis was , without doubt, during the nineteenth century the guiding idea in cosmical philosophy as well as the philosophical basis u o nderlying astron my . The work itself has been likened very properly to that of Charles Darwin . Laplace starts with the assumption of a glowing mass o f gas which from the very ﬁrst was in vortex motion from right to left ( as viewed from the north ) about an “ axis passing through its center of gravity . In its primi tive assumed condition the Sun resembled those nebula which are shown by the telescope to be composed of a more or less brilliant nucleus surrounded by a nebulosity which is condensing upon the nucleus , transforming it into ” i n deﬁ a star . The solar mass cannot have extended out n itel y. Its limits were the points at which the centrifugal force due to its motion of rotation were balanced by the

' n a gravitation of its action . O cooling the nebul would contract , and in this contraction of the glowing mass of gas a gaseous disk would be Split o ff when the centrifugal force of the rotating mass would be equal to the inwardly a directed force of gravitation , so that the solar nebul were divided into rings of glowing gases which would rotate as a whole and cool down in a solid or liquid ring . “ Laplace says : It may be conceived that the endless v ariety in the temperature and density of the various parts of this great mass produced eccentricity of their orbits and

deviation of thei r motion from the plane of its equator .

Laplace mentions the presence of comets , which he states

are strangers to the planetary system , as colliding with the planets during their process of formation and causing

them to deviate , while other comets entering the solar 3 1 2 ASTRONOMY

system near the condensation of gaseous matter, so nearly i n completed , became retarded in their motion and were co r or ated p into the solar system . His conclusion was as follows : Whatever be the masses of the planets , it is only owing to the circumstance that they all move in the same direction and in almost t circular orbits , at small inclinations to one another, tha the secular variations in their orbits are periodical and w o scil within narro limits , and that the system therefore lates about a mean condition , from which it never deviates ” by more than an insigniﬁcant amount . ’ While Laplace s theory was Open to criticism and obj e c tion at numerous points, yet it played a useful and ew remarkable part in the development of science . The N t on i an doctrine of the wonderful stability of the solar sys tem was more ﬁrmly established and the permanence of the planetary system was guaranteed . While it was con

fined only to the solar system , yet the idea of the nebular hypothesis was readily applicable to the universe itself and formed what long was considered a satisfactory means of accounting for its origin and evolution . ’ In its mechanical aspect Laplace s idea of the segrega tion of small dust particles during the cooling of nebula and their aggregation by the condensation of gases into a ring or a single big mass was found physically impossible by later astronomers , notable among whom were Stock h . h ot e well and Newcomb There would result, on this yp a sis , collection of small meteorites such as circulated in the Saturn rings , and the transformation of the Neptune belt into a planet would have required no less than 1 2 0 million years . The retrograde movement of the moons of Uranus and Neptune and also that of the most remote moons of Saturn a n d Jupiter formed another serious 0b ecti o n j to the thesis of Laplace . The first important cosmical theory seriously to modify the nebular hypothesis of Laplace was that first presented 8 1 1 8 . on November 7 , 7 , by Sir Norman Lockyer This

3 1 4 ASTRONOMY

i t extended would revolve as a solid . But such ideas as those of Lockyer and Darwin involved modiﬁcations of the nebular hypothesis rather than an overthrow . A t the very end of the nineteenth century F . R . Moulton and T . C . Chamberlin, of the University of Chicago , de nied some o f the fundamental ideas involved in the nebular hypothesis . Not only was their work destructive in that they advanced phenomena which the theory of Laplace could not explain , but which were in fact controverted di r ectl y by observation, but they constructed a cosmical the ory in which these elements were taken into consideration and in which , in large part, the conditions were met . The of as chief obj ections to the theory Laplace, stated by

Professor Moulton , are as follows “ 1 of . The considerable mutual inclinations o f the planes the planetary orbits and the inclination o f the plane of the ’ Sun s equator to the general plane of the system are n ot to be expected on the basis of the ring theory . “ 2 to . The eccentricities of the planetary orbits are not f be expected on the basis o the ring theory . “ f 3 . The orbits o the planetoids contradict the ring theory . “ f 4. The rapid revolution o Phobos and of the particles o f the inner ring of Saturn cannot be satisfactorily ex plained . “ f n ot to 5 . The presence o light elements in the Earth is be expected . “ ff o . 6. A series of rings could not have been left

7 . A ring could not have been condensed into a planet . “ i s 8 . The moment of momentum of the present system less than of that of the supposed initial nebula . “ 9. The retrograde revolutions of the ninth satellite of Saturn and ( probably) of the seventh satellite of Jupiter

ﬂatly contradict the theory . Of these obj ections one of the most important is the question of the leaving o ff of the rings at certain interval s f Pr o during the contraction o the nebula . On this point COSMOGONY AND STELLAR EVOLUTION 3 1 5 fessor Moulton writes in his Introduction to Astronomy “ It is easy to overlook the fact that the postulated nebula h must have been excessively rare . According to the y oth eses p made, it must have been denser at its center than near its periphery . But i f we suppose that it was homo ’ e n eous g and that it reached out to Neptune s orbit , we ﬁnd that its density was only that of air at the sea level . ’ Neptune s ring could not have been so dense as this , which is many times rarer than the best vacuum yet produced i n our laboratories . Now a ring of such rarity would have had no cohesion and would not have separated except particle by particle . When the process was once started it seems that it Should have been continuous instead of i n ter mitten t as the theory supposes . The theory postulates that when a ring was left behind , the nebula was made a t stable for a long period of contraction . Roche has tempted to Show that rings of considerable dimension s on would be abandoned at certain intervals , but his work on this point is far from conclusive . Every other writer

f . the subj ect has keenly felt this di ﬁculty Thus Faye , in his modiﬁcation of the Laplacian theory , supposed that the whole nebula broke up into rings simultaneously . “ s We may assume for the sake of argument, he say , that rings are abandoned and inquire whether they will unite into planets or not . The matter of the ring would be very widely spread out and mutual gravitation of its parts would be very feeble . The appropriate investigation shows that the tidal forces coming from the interior mass would more strongly tend to scatter the material than its gravita tion would to gather it together into a plane . Consequently a x ing could not even start to condense into a planet . It would be something like a comet which becomes utterly dissipated by tidal forces . “ To give every possible advantage to the ring theory, we may assume that all the matter has been gathered into a planet except a ring of very small particles , and then ask ourselves whether this minute remainder will be brought 3 1 6 ASTRONOMY

t o the planet . Investigation shows a strong probability ° that only that part of the ring which is within 60 of the p lanet could be brought on to it in any time however long .

That is, i f we assume that the process of formation of a planet out of a ring is almost ﬁnished , we ﬁnd that it can not complete itself . This shows the strong improbability that the assumed stage could ever h aVe been reached by ” condensation from a more uniform ring . Then it must be considered that the ninth satellite of Saturn and the seventh satellite of Jupiter revolve in a d irection contrary to the other revolutions a n d rotations o f the solar system , which would be an impossibility if the

Laplacian doctrine were true . Finally, notwithstanding a c a h oto r a areful and thoro study of nebul , developed by p g phy to a point of great reﬁnement, there has been found no trace of a nebula which is in the course of breaking up i a nto concentric rings . The spiral nebul , such as the t heory propounded by Professors Chamberlin and Moulton a required , are of the normal type of nebul , and it is this fact, brought out in the brilliant discoveries of Professor K eeler, that has largely produced the new theory which has been termed by its originators the “planetesimal the r o y. This planetesimal hypothesis to- day is attracting wide s pread and favorable attention from many astronomers and g eologists . While it explains the formation of the solar s n ystem and other systems , it does not eliminate the questio o f primitive matter, which doubtless must have been nebu l ous . While spiral nebula may have been formed by the collision or interaction of large suns , yet these in turn d emand for their origin a similar occurrence, so that one is led rather strongly to the belief that the formation of s ystems of suns and worlds may be a continuous process . The planetesimal hypothesis has been summarized in the following simple statement by Prof . James F . Kemp in the c ourse of a lecture in which its application to the origin an d development of the world is discussed . He writes :

3 1 8 ASTRONOMY

lous source, and it is not clear that we have yet escaped the necessity of at least the essential features of the ” nebular hypothesis . t Whatever theory is studied or adopted , it is clear tha

' the building of suns and the building of worlds is a process of evolution I n which the original matter must undergo transformation . The process may be continuous and may ma extend through inﬁnite time . The collision of suns y have produced nebula and these nebula in turn may grad uall y develop themselves into suns again . It seems reason ably certain that nebula are the stuff from which the stars are made . Of the many types of nebula the Spiral forms are considered the more primitive . There is indeed evi dence to Show that even such apparently irregu lar, nebula as that in Orion still preserve traces of original spiral formation .

Professor Dewar has experimentally proved that , at the low temperature prevailing in the outer regions of a to a t nebula , it is one of the properties of a dust particle tract and condense on its surface whatever gas may be within its immediate vicinity . It may be safely assumed , therefore , that each particle of dust hurled out by the ex t plosion of two colliding suns bears its charge of gas . Tha these gas - laden dust particles should collide with one an other would follow from their motion and their excessive number, and that these particles should be cemented to gether by the ﬁlm of liquid gases on their surfaces would in turn follow from the fact of their collision . Thus , in o f the course of centuries , large grains or aggregations matter constituting meteorites are formed , which , by gravi t a ti on al attraction , gather about them part of the remain ing dust and gas . Stars , comets and meteorites from other systems will also wander into the nebula , attracting the gas and dust of the nebula . Evidence of this draining of a nebula by an immigrant star is to be found in many a constellation , notably in the Swan , where a distinct lane h a s been plowed through a nebula by an errant star . By COSMOGONY AND STELLAR EVOLUTION 3 19 this upbuilding of meteorites from colliding gas - laden dust particles and the concentration of nebular matter by bodies a s which have strayed into the nebula , the entire mass of g and dust is converted in the lapse of ages into clusters of stars , slowly revolving about the central Sun in periods of thousands of years . Such star clusters are familiar obj ects in the heavens . They may be found in the Pleiades , in

Pegasus and in other constellations . Each star in a cluster may be regarded as either the — center of a new solar system or as a new - born world not

- - a rock bound , sea swept world as yet , such as the Earth , but a glowing chaotic mass , an embryo which will cool and shrink as it cools and which will eventually chill into a solid sphere . The central Sun , because of its greater s on ize , will blaze for centuries after the clusters about it have congealed into planets . Because the shrinking and cooling are not of a year ’ s nor even of a century ’ s dura tion , but are extended over millions of years , it is possible to study the various stages of the process in stars that have been formed from nebula at widely different epochs . The method of investigation bears some relation to the system devised by the evolutionists in tracing the develop ment of life on this Earth . With the. aid of fossils dug out o f the Earth the paleontologist has ingeniously bound the living present to the dead past with the chain of evolution , a n d shown very convincingly how the creatures that n ow roam the Earth are inevitably linked with the extinct ani mals of prehistoric time . An analogous system of stellar classiﬁcation , formulated by Prof . W . W . Campbell , throws n ot a little light on the ancestry of the myriad of stars whose rays pierce the heavens . Such a search indubitably i po nts to nebula as the stuff of which worlds are fashioned . In estimating the period which probably elapses before a star in a cluster will shrink into an incrusted world ’ - the color of the star s light plays no small part . Red hot

- i ron is not nearly so hot as white hot iron . The color of the molten metal gives the iron - founder a ' clue to its tem 3 2 0 ASTRONOMY

’ er t r p a u e . Similarly a star s temperature may be gaged

. r by its tint Since temperature , and therefore color , alte with age , it is possible to state in a rough way that red stars are the oldest of luminous bodies and that the orange, yellow and white stars follow in respective antiquity .

- Youngest and hottest of all are the bluish white stars . The spectroscope draws nicer distinctions than those l which can be drawn by color alone . Professor Campbel has discovered from his study of stellar spectra that when the spectrum of a star is rich in the bright lines of the gases hydrogen and helium it is certain that the star i n s question is young . Indeed , the presence of nebulou masses about many stars in this stage of their evolution ’ proves how true is Professor Campbell s conception . In the constellation of Orion , for example , is a nebula in which stars are plunged . The spectra of the nebula and of t the stars exhibit the same lines . Can there be any doub that the stars were formed from the nebula ? In passing from the white stage of infancy the star th e shrinks more and more . The result is a change in r spectrum . The metals calcium and iron appear in thei s characteristic lines . Vega and Sirius are stars of thi th e type . As the star ages the hydrogen lines thin out and o lines of the metals become stronger and more numer us . When a stage is reached corresponding with that of our own th e Sun , a yellow stage , which may be considered t c very summit of stellar life , some twenty thousand me alli i t lines appear. Then comes a gradual cooling . As l changes in hue from yellow to red , more complex chemica combinations are found in the star, and carbon makes its the appearance . Then follows a stage represented by planet Jupiter, still gaseous , still hot , but no longer mark ’ e dly luminous without the aid of the Sun s borrowed light . the A further step brings with it external solidiﬁcation , formation of a crust enclosing a hot interior, at which c point the Earth has arrived . The last and most patheti — period is represented by the Moon frozen , desolate, dead .