Astronomy-Dyson 1910.Pdf

ASTRONOMY ^ /o A HANDY MANUAL FOR STUDENTS AND OTHERS BY F. W. DYSON, F.R.S. ASTRONOMER ROYAL FOR SCOTLAND ; PROFESSOR OF ASTRONOMY IN THE UNIVERSITY OF EDINBURGH WITH 95 DIAGRAMS AND ILLUSTRATIONS LONDON J. M. DENT & SONS, LTD. 29 & 30 BEDFORD STREET, W.C. 1910 All rights reserved PREFACE IN this little book I have attempted to give an account of the methods employed by astronomers and the reasons for some of the propositions they advance. Astronomical investigations frequently seem complicated owing to the amount of subsidiary detail, but the principles underlying them are simple and usually admit of a clear statement which can be followed by a general reader. In the introduction to his lectures delivered at Ipswich in 1848 Sir George Airy draws attention to an attitude towards Astronomy which is still prevalent. Such questions as the determination of the distance of the Sun or Moon are considered as the instruments with beyond ordinary comprehension ; which astronomical measurements are made are sup- to obscure difficult posed be based on and principles ; therefore the best a layman can do is to accept state- ments on the personal credit of the astronomer making them. He points out that this is an exaggerated view. The principles involved in measuring the distance of the Moon are no more abstruse than those employed to find the distance of a tree on the other side of a river. Astronomical instruments are as simple in principle and far less complicated in detail than a lathe or a steam-engine. It is quite true that in both instruments and methods many subsidiary details must be attended to when great accuracy is required in order that disturbing causes may be allowed for or eliminated. But an explanation of the essential principles may be given which can be readily understood with ordinary care and attention. VI PREFACE As far as possible an historical order has been followed. The reader's attention has been drawn to the desirability of making for himself certain observa- tions of the sky which do not require the use of a telescope. Every educated person ought to see with his own eyes how Sun, Moon, Stars and Planets move in the sky, and be able to infer from his own observa- tions the movement of the Earth about the Sun, the comparative nearness of the Moon, and other common- places of Astronomy. It is difficult to acknowledge fully my obligations to the authors of books and papers which I have con- sulted. Mention should be specially made of Mr. Arthur Berry's History of Astronomy, Prof. Young's General Astronomy, and the articles by Prof. New- comb on and Prof. Hale on "Spec- " "Astronomy" troscopy in the Encyclopaedia Britannica. I am indebted to the Astronomer Royal Sir William Christie, Prof. Barnard, Prof. Ritchey and other astronomers for permission to reproduce photographs published by them, and to Mr. W. B. Blaikie for the map taken from his Monthly Star Maps published by the Scottish Provident Association. I wish to express my thanks to a number of friends for help in various directions. Mr. Heath, first assistant of the Royal Observatory, Edinburgh, copied a number of photographs and supplied me with the at drawing on p. 172 ; Mr. Storey, assistant the Observatory, and Mr. MacGregor assisted with the diagrams. I am much obliged to Mr. Storey and to Dr. J. Reynolds Green, who read the book in proof, for their valuable criticism and suggestions. F. W. DYSON. Royal Observatory, Edinburgh, March 7, 79/0. CONTENTS CHAP. PAGE I ANCIENT ASTRONOMY ..... i II THE COPERNICAN SYSTEM . -3 III THE LAW OF GRAVITATION .... 45 IV ASTRONOMICAL INSTRUMENTS .... 63 V THE SUN'S DISTANCE 85 VI THE SUN .100 VII THE SOLAR SYSTEM . .129 VIII DISTANCES AND MOVEMENTS OF THE STARS . 164 IX STARS AND NEBULAE 193 X DOUBLE STARS AND CLUSTERS .... 208 XI VARIABLE STARS AND NEW STARS . 225 XII THE SIDEREAL UNIVERSE .... 239 CHAPTER I ANCIENT ASTRONOMY ASTRONOMY is the oldest of the sciences. The orientation of the pyramids or of Stonehenge, the mythology of the Greeks and the religious festivals of the Jews are familiar evidences of astronomical knowledge in early civilizations. The constant repetition, day by day, month by month, or year by year, of similar phenomena both invited and aided study of the heavenly bodies. Before the dawn of history men wondered what caused the rising and setting of the Sun and Moon each day, why the Moon went through her phases each month, and why Orion reappeared each year in the sky as surely as the seasons returned. An out-of-door life was favourable to a knowledge of the heavens, and the tribes of Chaldea and Arabia two or three thousand years ago were, perhaps, better acquainted with the appearance of the sky than we are to-day. Modern conditions of life are not favourable to the general cultivation of practical astronomy. Buildings circumscribe the view7 of the sky, clocks and watches save us the trouble of observing the Sun and stars for time, and artificial illumination makes us independent of moonlight. But the following phe- 2 ASTRONOMY nomena of the skies ought not to be merely read of in books, but should be verified by actual observation. (1) The diurnal movement of the Sun; the different positions of the Sun in the sky in summer and winter. The of the the of (2) phases Moon ; rapid movement the Moon among the stars. The diurnal movement of the stars the (3) ; appearance of different constellations at different times of the year. Observation of the Sun. In the latitude of London the Sun rises on midsummer day at about 3 h. 45m. at a point on the horizon considerably north of east. At midday, when due south, it is high in the sky. If we take a stick and point it horizontally and turn it gradually towards the vertical, we shall not be pointing to the Sun till the stick has been turned through rather more than two-thirds of the right- angle between the horizontal and vertical directions. It sets in the evening about 8 h. 19 m. at a point on the horizon considerably north of west. From midsummer to midwinter it rises later and sets earlier each the of the horizon where it rises day ; point gradually shifts from north of east to due east, and then to south of east the it sets shifts ; and point where in the other direction from north of west to due west, and then to south of west. In midwinter the Sun rises at 8 h. 7 m. and sets at 3 h. 51 m., and at noon, instead of being high up in the sky, it is only 15, or one-sixth of a right-angle, above the horizon. ANCIENT ASTRONOMY 3 From midwinter to midsummer the days lengthen and the Sun at noon is higher each day. Midway between midsummer and midwinter, on March 22 and Sep. 22, the Sun rises due east and sets due west, and day and night are equal in length. This succes- sion of phenomena repeats itself year by year, and is more striking in the latitude of the British Isles than in the more southern countries from which our astronomical knowledge was derived. Diagram I illustrates the path of the Sun across the sky at different times of the year. The circle NES is the horizon, and NPZS the meridian or circle of the sky cut by a vertical plane in a direction due north and south. The diagram represents the eastern half of the sky the part above NES being above the horizon and visible, and the part Dj below NES below the horizon and invisible. On midsummer day the Sun travels from A to C between midnight and midday, crossing the horizon at B (i.e. rising) at 3 h. 45111. All points of the circle ABC are 66J from P, i. e. if we divide the arc of the sphere from the pole P to the opposite one into 180 parts, all points of the circle ABC are 66J parts from P. But on Septem- B 2 ber 22 the Sun is 90 from P. At midnight on that day it is at D, rises at E (due east) at 6h. om., and is at its highest point F (due south) at 12 h. om. When we come to December 22 the Sun is 113^ from P. At midnight it is at G, does not reach H, i. e. does not rise till 8 h. 7 m., and is at K (due south) at 12 o'clock. Solar Day. It is not quite correct to say that the Sun is due south exactly at 12 o'clock, but this is suffi- ciently true for the explanation given in the last paragraph. The interval of time between the moment when the Sun is due south one day to the moment when it is due south the next is called the solar day, and is the natural unit of time. The length of the solar day is not quite constant, but varies to the extent of one minute at different times of the year. Its average length throughout the year is the mean solar day, and is exactly the 24 hours of our clocks and watches. Year. The exact time the Sun takes to go through the cycle of changes described above is the year. The length of the year can therefore be found approxi- mately by counting the number of days from a par- ticular day in any year to the day in the following year when the Sun is the same height in the sky at noon.

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