© Copyright, Princeton University Press. No part of this book may be 6 distributed, posted, or reproduced in any form by digital or mechanical Introduction 7 introduction means without prior written permission of the publisher. δ γ γ The night sky is one of the most beautiful and Vega have a bluish tinge. Binoculars β ε sights in nature. Yet many people remain bring out the colours more readily than the δ lost among the jostling crowd of stars, and naked eye does. Section II of this book, start are baffed by the progressively changing ap ing on page 267, explains more fully the dif α β ε pearance of the sky from hour to hour and ferent types of star that exist. from season to season. The charts and de By contrast, planets are cold bodies that scriptions in this book will guide you to the shine by re fecting the Sun’s light. They too α most splendid celestial sights, many of them are described in more detail in Section II, within the range of simple optical equipment from page 304 onwards. The planets are such as binoculars, and all accessible with an constantly on the move as they orbit the average-sized telescope of the type used by Sun. Four of them can be easily seen with amateur astronomers. the naked eye: Venus, Mars, Jupiter and Constellations: Stars in a constellation are usually Mediterranean regions; Hevelius and others It must be emphasized that you do not Saturn. Venus, the brightest of all, appears as unrelated to one another. Above, the stars of invented constel lations to fll in the gaps need a telescope to take up stargazing. Use a dazzling object in the evening or morning Crux, the Southern Cross, are shown as they between the fgures recognized by the Greeks. the charts in this book to fnd your way sky. Charts showing the positions of Mars, appear from Earth, left, and in a 3D view as they The whole process sounds rather arbitrary, among the stars frst with your own eyes, and Jupiter and Saturn for a 5-year period can be actually lie in space, right. (Wil Tirion) and indeed it was. A number of the newly then with the aid of binoculars, which bring found on the HarperCollins website: devised patterns fell into disuse, leaving a the stars more readily into view. Binoculars www.collins.co.uk/starsandplanets total of 88 constellations that were offcially are a worthwhile investment, being relatively About 2000 stars are visible to the naked and Telescopium. Whether large or small, adopted by the International Astronomical cheap, easy to carry and useful for many eye on a clear, dark night, but you will not bright or faint, each constel lation is given a Union (IAU), astronomy’s governing body, purposes other than stargazing. need to learn them all. Start by identifying separate chart and description in this book. in 1922 (see the table on pages 8–9). the brightest stars and major constellations, The main constellations were devised at the As well as the offcially recognized constel and use these as signposts to the fainter, less dawn of history by Middle Eastern peoples lations, you can fnd other patterns among Stars and planets prominent stars and constellations. Once you who fancied that they could see a likeness the stars called asterisms. An asterism can be know the main features of the night sky, you to certain fabled creatures and mytho composed of stars belong ing to one or more In the night sky, stars appear to the naked will never again be lost among the stars. logical heroes among the stars. Of particular constellations. Well-known examples are the eye as spiky, twinkling lights. Those stars importance were the 12 constellations of the Plough or Big Dipper (part of Ursa Major), near the horizon seem to fash and change zodiac, through which the Sun passes during the Square of Pegasus, the Sickle of Leo and colour. The twinkling and fashing effects Constellations its yearly path around the heavens. However, the Teapot of Sagittarius. are due not to the stars themselves but to the it should be realized that the astrological Earth’s atmosphere: turbulent air currents The sky is divided into 88 sections known ‘signs’ of the zodiac are not the same as the cause the stars’ light to dance around. The as constellations which astronomers use as modern astronomical constellations, even Star names steadiness of the atmosphere is referred to as a convenient way of locating and naming though they share the same names. the seeing . Steady air means good seeing. celestial objects. Most of the stars in a Our modern system of constellations derives The main stars in each constellation are The spikiness of star images is due to optical constellation have no real connection with from a catalogue of 48 compiled by the Greek labelled with a letter of the Greek alphabet, effects in the observer’s eye. In reality, stars one another at all; they may lie at vastly astronomer Ptolemy in ad 150. This list was the brightest star usually (but not always!) are spheres of gas similar to our own Sun, differing distances from Earth, and form a expanded by navigators and celestial map being termed α (alpha). Notable exceptions emitting their own heat and light. pattern simply by chance. Incidentally, when makers, notably the Dutchmen Pieter Dirks in which the stars marked β (beta) are in Stars come in various sizes, from giants to astronomers talk of an object being ‘in’ a zoon Keyser (c. 1540–96) and Frederick de fact the brightest include the constellations dwarfs, and in a range of colours according given constellation they mean that it lies in Houtman (1571–1627), the Pole Johannes Orion and Gemini. The entire Greek alpha to their temperature. At frst glance all stars that particular area of sky. Hevelius (see page 166) and the Frenchman bet is given in the table on page 10. appear white, but more careful inspection Some constellations are easier to recognize Nicolas Louis de Lacaille (see page 216). Particularly confusing are the southern reveals that certain ones are somewhat orange, than others, such as the magnifcent Orion Keyser and de Houtman introduced constellations Vela and Puppis, which were notably Betelgeuse, Antares, Aldebaran and or the distinctive Cassiopeia and Crux. 12 new constellations, and Lacaille 14, in once joined with Carina to make the exten Arcturus, while others such as Rigel, Spica Others are faint and obscure, such as Lynx parts of the southern sky not visible from sive fgure of Argo Navis, the ship of the For general queries, contact [email protected]
Stars_Planets_5th_pages.indb 6-7 16/02/2017 10:36 © Copyright, Princeton University Press. No part of this book may be 8 Introduction distributed, posted, or reproduced in any form by digital or mechanical Introduction 9 means without prior written permission of the publisher.
Name Genitive Abbrevn. Area Order Origin* THE 88 CONSTELLATIONS (square of degs.) size Name Genitive Abbrevn. Area Order Origin* (square of degs.) size Libra Librae Lib 538 29 1 Lupus Lupi Lup 334 46 1 Lynx Lyncis Lyn 545 28 5 Andromeda Andromedae And 722 19 1 Lyra Lyrae Lyr 286 52 1 Antlia Antliae Ant 239 62 6 Mensa Mensae Men 153 75 6 Apus Apodis Aps 206 67 3 Microscopium Microscopii Mic 210 66 6 Aquarius Aquarii Aqr 980 10 1 Monoceros Monocerotis Mon 482 35 4 Aquila Aquilae Aql 652 22 1 Musca Muscae Mus 138 77 3 Ara Arae Ara 237 63 1 Norma Normae Nor 165 74 6 Aries Arietis Ari 441 39 1 Octans Octantis Oct 291 50 6 Auriga Aurigae Aur 657 21 1 Ophiuchus Ophiuchi Oph 948 11 1 Boötes Boötis Boo 907 13 1 Orion Orionis Ori 594 26 1 Caelum Caeli Cae 125 81 6 Pavo Pavonis Pav 378 44 3 Camelopardalis Camelopardalis Cam 757 18 4 Pegasus Pegasi Peg 1121 7 1 Cancer Cancri Cnc 506 31 1 Perseus Persei Per 615 24 1 Canes Venatici Canum Venaticorum CVn 465 38 5 Phoenix Phoenicis Phe 469 37 3 Canis Major Canis Majoris CMa 380 43 1 Pictor Pictoris Pic 247 59 6 Canis Minor Canis Minoris CMi 183 71 1 Pisces Piscium Psc 889 14 1 Capricornus Capricorni Cap 414 40 1 Piscis Austrinus Piscis Austrini PsA 245 60 1 Carina Carinae Car 494 34 6 Puppis Puppis Pup 673 20 6 Cassiopeia Cassiopeiae Cas 598 25 1 Pyxis Pyxidis Pyx 221 65 6 Centaurus Centauri Cen 1060 9 1 Reticulum Reticuli Ret 114 82 6 Cepheus Cephei Cep 588 27 1 Sagitta Sagittae Sge 80 86 1 Cetus Ceti Cet 1231 4 1 Sagittarius Sagittarii Sgr 867 15 1 Chamaeleon Chamaeleontis Cha 132 79 3 Scorpius Scorpii Sco 497 33 1 Circinus Circini Cir 93 85 6 Sculptor Sculptoris Scl 475 36 6 Columba Columbae Col 270 54 4 Scutum Scuti Sct 109 84 5 Coma Berenices Comae Berenices Com 386 42 2 Serpens Serpentis Ser 637 23 1 Corona Australis Coronae Australis CrA 128 80 1 Sextans Sextantis Sex 314 47 5 Corona Borealis Coronae Borealis CrB 179 73 1 Taurus Tauri Tau 797 17 1 Corvus Corvi Crv 184 70 1 Telescopium Telescopii Tel 252 57 6 Crater Crateris Crt 282 53 1 Triangulum Trianguli Tri 132 78 1 Crux Crucis Cru 68 88 4 Triangulum Australe Trianguli Australis TrA 110 83 3 Cygnus Cygni Cyg 804 16 1 Tucana Tucanae Tuc 295 48 3 Delphinus Delphini Del 189 69 1 Ursa Major Ursae Majoris UMa 1280 3 1 Dorado Doradus Dor 179 72 3 Ursa Minor Ursae Minoris UMi 256 56 1 Draco Draconis Dra 1083 8 1 Vela Velorum Vel 500 32 6 Equuleus Equulei Equ 72 87 1 Virgo Virginis Vir 1294 2 1 Eridanus Eridani Eri 1138 6 1 Volans Volantis Vol 141 76 3 Fornax Fornacis For 398 41 6 Vulpecula Vulpeculae Vul 268 55 5 Gemini Geminorum Gem 514 30 1 Grus Gruis Gru 366 45 3 Hercules Herculis Her 1225 5 1 * Origin: Horologium Horologii Hor 249 58 6 1 One of the original 48 Greek constellations listed by Ptolemy. The Greek fgure of Argo Navis has since been Hydra Hydrae Hya 1303 1 1 divided into Carina, Puppis and Vela. Hydrus Hydri Hyi 243 61 3 2 Considered by the Greeks as part of Leo; made separate by Caspar Vopel in 1536. Indus Indi Ind 294 49 3 3 The 12 southern constellations of Pieter Dirkszoon Keyser and Frederick de Houtman, c. 1600. Lacerta Lacertae Lac 201 68 5 4 Four constellations added by Petrus Plancius. Leo Leonis Leo 947 12 1 5 Seven constellations of Johannes Hevelius. Leo Minor Leonis Minoris LMi 232 64 5 6 The 14 southern constellations of Nicolas Louis de Lacaille, who also divided the Greeks’ Argo Navis into Lepus Leporis Lep 290 51 1 Carina, Puppis and Vela. For general queries, contact [email protected]
Stars_Planets_5th_pages.indb 8-9 16/02/2017 10:36 © Copyright, Princeton University Press. No part of this book may be 10 Introduction distributed, posted, or reproduced in any form by digital or mechanical Introduction 11 means without prior written permission of the publisher. Argonauts. As a result of Argo’s subsequent the greek alphabet Star brightness magnitude difference trisection, neither Vela nor Puppis possesses converted to stars labelled α or β, and there are gaps in the α alpha ξ xi Stars appear of different brightnesses in the brightness difference sequence of Greek letters in Carina as well. β beta sky, for two reasons. Firstly, they give out ο omicron Difference in Difference in The system of labelling stars with Greek γ gamma different amounts of light. But also, and just π pi magnitude brightness letters was intro duced in the early 17th δ delta as importantly, they lie at vastly differing century by the German astronomer Johann ε epsilon ρ rho distances. Hence, a modest star that is 0.5 1.6 Bayer on his star atlas called Uranometria ζ zeta σ sigma quite close to us can appear brighter than 1.0 2.5 (see page 246), so these designations are η eta τ tau a tremendously powerful star that is a long 1.5 4.0 2.0 6.3 often known as Bayer letters. In heavily θ or ϑ theta υ upsilon way away. ι iota 2.5 10 populated constellations, where Greek letters ϕ or φ phi Astronomers call a star’s brightness its 3.0 16 κ kappa ran out, fainter stars were assigned Roman magnitude. The magnitude scale was intro 3.5 25 λ lambda χ chi letters, both lower-case and capital, such as duced by the Greek astronomer Hipparchus 4.0 40 μ mu ψ psi l Carinae, P Cygni and L Puppis. in 129 bc. Hipparchus divided the naked-eye 5.0 100 ν nu ω omega An additional system of identifying stars stars into six classes of brightness, from 1st 6.0 250 is that of Flamsteed numbers, originating magnitude (the brightest stars) to 6th mag 7.5 1000 from their order in a star cata logue drawn date back to ancient Greek times. Many nitude (the faintest visible to the naked eye). 10 10,000 up at Greenwich Observatory by the frst others, such as Aldebaran, are of Arabic In his day there was no means of measuring 12.5 100,000 Astron omer Royal of England, John Flam origin. Still others were added more recent star brightness precisely, so this rough classi 15 1,000,000 steed (1646–1719). Examples are 61 Cygni ly by European astronomers who borrowed fcation suffced. But with the coming of and 70 Ophiuchi. For more information on Arabic words in corrupted form; an example technology it became possible to measure a Objects more than 250 times brighter Flamsteed numbers see page 178. is Betelgeuse, which in its current form is star’s brightness to a fraction of a magnitude. than 6th magnitude are given negative The genitive (possessive) case of the con meaningless in Arabic. The star names used In 1856 the English astronomer Norman (minus) magnitudes. For example, Sirius, stellation’s name is always used when refer in this book are those offcially recognized by Pogson (1829–91) put the magnitude scale the brightest star in the sky, is of magnitude ring to a star within it; hence Canis Major, the International Astronomical Union. on a precise mathematical footing by defn −1.46. At the other end of the scale, stars for instance, becomes Canis Majoris, and the Star clusters, nebulae and galaxies have a ing a star of magnitude 1 as being exactly fainter than magnitude 6 are given progress name α Canis Majoris means ‘the star α in different system of identifcation. The most 100 times brighter than a star of magnitude ively larger positive magnitudes. The faintest Canis Major’. All constel lation names have prominent of them are given numbers pre 6. Since, on this scale, a difference of fve objects detected by telescopes on Earth are standard three-letter abbreviations; for in fxed by the letter M from a catalogue com magnitudes corresponds to a brightness around magnitude 27. stance, the abbrevi ated form of Canis Major piled in the late 18th century by the French difference of 100 times, a step of one mag Any object of magnitude 1.49 or brighter is CMa. astronomer Charles Messier (1730–1817). nitude is equal to a brightness difference of is said to be of frst magnitude; objects from Before 1930, there were no of fcially For example, M1 is the Crab Nebula and just over 2.5 times (the ffth root of 100). 1.50 to 2.49 are termed second magnitude; recognized constellation boundaries; some M31 the Andromeda Galaxy. constellations overlapped, and some stars Messier’s catalogue contained 103 objects. were shared between constellations. In that A few more were added later by other the ten brightest stars as seen from earth year, the International Astronomical Union astronomers, bringing the total to 110. A published defnitive boundaries for all far more comprehensive listing, containing Star name Constellation Apparent magnitude Absolute magnitude Distance (l.y.) constellations. In the process, certain stars many thousands of objects, is the New Sirius Canis Major −1.46 +1.43 8.60 allocated by the Bayer and Flamsteed systems General Catalogue (NGC) compiled by J.L.E. Canopus Carina −0.74 −5.62 309 to one constellation found themselves trans Dreyer (1852–1926), with two supplements Rigil Kentaurus Centaurus −0.27 +4.12 4.32 ferred to a neighbour, leading to gaps in the called the Index Catalogues (IC). Arcturus Boötes −0.05 −0.31 36.7 sequence of letters and numbers. Messier numbers and NGC/IC numbers Vega Lyra +0.03 +0.60 25.0 Prominent stars also have proper names remain in use by astronomers, and both Capella Auriga +0.08 −0.51 42.8 by which they are commonly known. For systems are used in this book. On the charts, Rigel Orion +0.13 −6.98 863 example, α Canis Majoris, the brightest star such objects are labelled with their Messier Procyon Canis Minor +0.37 +2.64 11.5 number if they have one, or otherwise by in the sky, is better known as Sirius. Stars’ Betelgeuse Orion +0.4 (variable) −5.52 498 proper names originate from several sources. their NGC number (without the ‘NGC’ Achernar Eridanus +0.46 −2.69 139 Some, such as Sirius, Castor and Arcturus, prefx) or IC number (prefxed ‘I’). For general queries, contact [email protected]
Stars_Planets_5th_pages.indb 10-11 16/02/2017 10:36 © Copyright, Princeton University Press. No part of this book may be 12 Introduction distributed, posted, or reproduced in any form by digital or mechanical Introduction 13 means without prior written permission of the publisher. and so on. The magnitude system may magnitudes we deduce that Deneb gives out Parallax: As the Earth sound confus ing at frst, but it works well in about 50,000 times as much light as the Sun moves around its orbit, practice and has the advantage that it can be and hence is exceptionally luminous, even so a nearby star appears extended indefnitely in both directions, to though there is nothing at frst sight to mark to change in position the very bright and the very faint. it out as extraordinary. Star against the celestial When used without further qualifcation, A number of stars actually vary in their background. The shift the term ‘magnitude’ refers to how bright a light output, for various reasons, and are in position is known as star appears to us in the sky; strictly, this is a favourite subject for study by amateur the star’s parallax. The the star’s apparent magnitude. But because astronomers. The nature of such so-called nearer the star is to us, the distance of a star affects how bright it variable stars is discussed on pages 284–287. Parallax the greater its parallax. appears, the apparent magnitude bears little Here, the amount of relation to its actual light output, or absolute parallax is exaggerated magnitude. Star distances for clarity. (Wil Tirion) A star’s absolute magnitude is de fned as January the brightness it would appear to have if it In the Universe, distances are so huge that were at a standard distance from us of 10 astronomers have abandoned the puny kilo parsecs (32.6 light years). The origin of the metre (km) and have invented their own parsec is explained on page 13. The table units. Most familiar of these is the light Sun below shows the apparent and absolute year (l.y.), the distance that a beam of light magnitudes of the ten brightest stars visible travels in one year. Light moves at the fastest in the night sky. Astronomers calculate the known speed in the Universe, 299,792.5 km Earth’s orbit absolute magnitude from knowledge of the per second. A light year is equivalent to 9.46 July star’s nature and its distance. million million km. Absolute magnitude is a good way of On average, stars are several light years comparing the intrinsic brightness of stars. apart. For instance, the closest star to the For instance, our daytime star the Sun has Sun, Proxima Centauri (actually a member an apparent magnitude of −26.7, but an of the α Centauri triple system), is 4.2 light Earth is on one side of the Sun, and then would, in the jargon of astronomers, be said absolute magnitude of 4.8 (when no sign years away. Sirius is 8.6 l.y. away and Deneb remeasured six months later when the Earth to lie at a distance of one parsec , equivalent is given the magnitude is understood to be 1400 l.y. away. has moved around its orbit to the other side to 3.26 light years. In practice, no star is this positive). Deneb (α Cygni) has an apparent The distance of the nearest stars can be of the Sun. When viewed from two widely close; the parallax of Proxima Centauri is magnitude of 1.3, but an absolute magnitude found direct ly in the following way. A star’s differing points in space in this way, a nearby 0ʺ.77. Astronomers frequently use parsecs in of −6.9. From comparison of these absolute position is measured accurately when the star will appear to have shifted slightly in preference to light years because of the ease position with respect to more distant stars of converting parallax into distance: a star’s (see diagram on the facing page). distance in parsecs is simply the inverse of the ten closest stars to the sun This effect is known as parallax, and its parallax in seconds of arc. For example, a applies to any object viewed from two star 2 parsecs away has a parallax of 0ʺ.5, at 4 Star name Constellation Apparent magnitude Absolute magnitude Distance (l.y.) vantage points against a fxed background, parsecs away its parallax is 0ʺ.25, and so on. Proxima Centauri Centaurus 11.13 15.57 4.23 such as a tree against the horizon. A star’s The farther away a star is, the smaller its α Centauri A Centaurus −0.01 4.38 4.32 parallax shift is so small that under normal parallax. Beyond about 50 light years a star’s α Centauri B Centaurus 1.33 5.74 4.32 circumstances it is unnotice able – in the case parallax becomes too small to be measured Barnard’s Star Ophiuchus 9.51 13.21 5.95 of Proxima Centauri, which has the greatest accurately by telescopes on Earth. Before the Wolf 359 Leo 13.51 16.61 7.80 parallax shift of any star, the amount is about launch of the European Space Agency’s Lalande 21185 Ursa Major 7.52 10.49 8.31 the same as the width of a small coin seen at astrometry satellite Hipparcos in 1989, Sirius A Canis Major −1.46 1.43 8.60 a distance of 2 km. Once the star’s parallax astronomers had been able to establish Sirius B Canis Major 8.44 11.33 8.60 shift has been measured, a simple calculation reliable parallaxes for fewer than 1000 stars UV Ceti A Cetus 12.61 15.47 8.73 reveals how far away it is. from Earth; from space, Hipparcos increased UV Ceti B Cetus 13.06 15.93 8.73 An object close enough to us to show the number of reliable parallaxes to over a parallax shift of 1ʺ (one second of arc) 100,000. A newer spacecraft, called Gaia, For general queries, contact [email protected]
Stars_Planets_5th_pages.indb 12-13 16/02/2017 10:36 © Copyright, Princeton University Press. No part of this book may be 14 Introduction distributed, posted, or reproduced in any form by digital or mechanical Introduction 15 means without prior written permission of the publisher. launched in 2013, has extended the number declination and the equivalent of longitude is to more than a billion. called right ascension. Declination is measured Before the days of accurate parallaxes, in degrees, minutes and seconds (abbrevi astronomers had to use an indirect method ated °, ′ and ʺ) of arc from 0° on the celestial of fnding star distances. First they estimated equator to 90° at the celestial poles. The the star’s absolute magnitude by studying the celestial poles lie exactly above the Earth’s spectrum of its light. They then compared poles, while the celestial equator is the pro Alderamin this estimated absolute magnitude with the jection onto the sky of the Earth’s equator. observed apparent magnitude to determine Right ascension is measured in hours, Deneb the star’s distance. The distance obtained in minutes and seconds (abbreviated h, m and this way was open to considerable error, and s), from 0h to 24h. The 0h line of right Polaris the values quoted in various books and cata ascension, the celestial equivalent of the North celestial pole logues often differed widely as a result. Greenwich meridian, is de fned as the point Distances of stars given in this book where the Sun crosses the celestial equator North ecliptic pole are taken from the revised edition of The on its way into the northern hemisphere Hipparcos Catalogue (2007). Most of these each year. Technically, this point is known distances are accu rate to better than 10 per as the vernal (or spring) equinox. cent, with the uncertainties tending to be Vega come greater for the more distant stars. Ecliptic: As the Earth moves along its orbit, the Thuban Sun is seen in different directions against the star Star positions background. The Sun’s path against the stars is known as the ecliptic. The constellations that the 1 To determine positions of objects in the Sun passes in front of during the year are known 23 /2° sky, astronomers use a system of coordinates as the constellations of the zodiac. Below, the similar to latitude and longitude on Earth. Sun’s apparent motion through Gemini, Cancer The celestial equivalent of latitude is called and Leo is shown. (Wil Tirion)
GEMINI CANCER Ecliptic
LEO
Precession: The Earth is very slowly wobbling The Sun’s path around the sky each year Sun in space like a tilted spinning top, a movement is known as the ecliptic. This path is inclined known as precession. As a consequence, the at 23½° to the celestial equator, because E celestial poles trace out a complete circle on the that is the angle at which the Earth’s axis is arth’ s orbit sky every 26,000 years. Only the path of the inclined to the vertical. The most northerly north celestial pole is shown here, but the effect and southerly points that the Sun reaches in applies to the south pole as well. (Wil Tirion) the sky each year are called the solstices, and For general queries, contact [email protected]
Stars_Planets_5th_pages.indb 14-15 16/02/2017 10:36 © Copyright, Princeton University Press. No part of this book may be 16 Introduction distributed, posted, or reproduced in any form by digital or mechanical Introduction 17 means without prior written permission of the publisher. they lie 23½° north and south of the celestial Proper motions equator. If the Earth’s axis were di rect ly upright with respect to its orbit around the All the stars visible in the sky are members Sun, then the celestial equator and eclip tic of a vast wheel ing mass of stars called the would coincide. We would then have no Galaxy. Those stars visible to the naked eye seasons, for the Sun would al ways remain are among the nearest to us in the Galaxy. di rect ly above the Earth’s equator. More dis tant stars in the Galaxy crowd One ad ditional effect that be comes im - together in a hazy band called the Milky port ant over long periods of time is that the Way, which can be seen arching across the Earth is slowly wob bling on its axis, like a sky on dark nights. spin ning top. The axis remains in clined at an The Sun and the other stars are all orbit- angle of 23½°, but the position in the sky to ing the centre of the Galaxy; the Sun takes which the north and south poles of the Earth about 250 million years to com plete one URSA MAJOR (The Plough) In 100,000 years are pointing moves slowly. This wob bling of orbit. Other stars move at dif ferent speeds, the Earth in space is termed pre cession. like cars in dif ferent lanes on a high way. As a As a result of precession, the Earth’s north result, stars are all very slowly chang ing their and south poles de scribe a large circle on the positions relative to one another. sky, tak ing 26,000 years to return to their Such stel lar movement, termed proper start ing places (see the diagram on page 15). motion, is so slight that it is undetect able to Hence the positions of the celes tial poles are the naked eye even over a human life time, al ways chang ing, al beit im per ceptibly, as are but it can be measured through tele scopes. the two points at which the Sun’s path (the As with many other as pects of stel lar positions, Deneb Deneb eclip tic) cuts the celes tial equator. our knowl edge of proper motions has been As an example of the effects caused by radical ly im proved by the Hipparcos and precession, Polaris will not always be the Pole Gaia satellites. Star. Although Polaris currently lies less than If ancient Greek as tronomers could be 1° from the celestial pole, that is just a matter trans ported for ward to the present day, they of chance. In 11,000 years’ time the north would notice little dif ference in the sky, with Albireo Albireo celes tial pole will lie near Vega in the constel- the exception of Arcturus, a fast-mov ing lation Lyra, having moved through Cepheus bright star, which has drifted more than two and Cygnus in the interim. Similarly, the Moon diameters from its position then. Over In 100,000 years vernal equi nox, which lay in Aries be tween very long periods of time the proper motions CYGNUS 1865 bc and 67 bc, now lies in Pisces and in of stars con sider ably dis tort the shapes of all ad 2597 will have reached Aquarius. con stel lations. The diagrams on the facing The effect of pre cession means that the page show some examples of how proper coor dinates of all celes tial objects – the motions will alter some familiar patterns. catalogued positions of stars, galaxies, and An ad ditional long-term effect of stel lar even con stel lation boundaries – are con tinu- motions is to change the appar ent mag ni- al ly drift ing. As tronomers draw up catalogues tudes of stars as they move towards us or and star charts for a standard reference date, away. For example, Sirius will brighten by or epoch, commonly chosen to be the start or 20 per cent over the next 60,000 years as its middle of a century. The epoch of the star Denebola positions in this book is the year 2000. For Regulus Regulus Denebola most general pur poses, pre cession does not Proper motions: Three familiar star patterns in tro duce a notice able error until after about as they appear today, at left of diagram, and as 50 years, so the charts in this book will be they will appear in 100,000 years’ time, right.
us able with out amendment until halfway The changes in appearance are due to the proper LEO In 100,000 years through the 21st century. motions of the stars. (Wil Tirion) For general queries, contact [email protected]
Stars_Planets_5th_pages.indb 16-17 16/02/2017 10:36 © Copyright, Princeton University Press. No part of this book may be 18 Introduction distributed, posted, or reproduced in any form by digital or mechanical Introduction 19 means without prior written permission of the publisher. distance shrinks by 0.8 light years. Then, as celes tial pole is at some inter medi ate al titude N The changing it moves away again, it will be superseded as be tween horizon and zenith, and the stars appearance of the sky the brightest star in the sky by Vega, which closest to it circle around it with out set ting as seen from different will peak at mag nitude −0.8 nearly 300,000 (they are said to be circum polar) while the latitudes on Earth. years from now. rest of the stars rise and set. Circumpolar The exact angle of the celes tial pole above the horizon dep ends on the observer’s Left: For an observer Appearance of the sky latitude. For some one at latitude 50° north, at the Earth’s pole, only for in stance, the north celes tial pole is 50° one half of the sky is 90° Three factors affect the appear ance of the sky: above the north ern horizon (bottom diagram ever visible, the other Observer the time of night, the time of year and your opposite). As an other example, if you were half being permanently latitude on Earth. First ly, let’s con sider the at latitude 30° south, the south celes tial pole below the horizon. Horizo uator effect of latitude. would be 30° above the south ern horizon. In n = celestial eq At one of the Earth’s poles, latitude 90°, other words, the altitude of the celes tial pole an observer would see the celes tial pole di - above the horizon is exact ly equal to your Always invisible
rect ly over head (at the zenith), and as the latitude, a fact long recognized by navigators. r o t
a
Earth turned all stars would circle around As the Earth turns, completing one 360° u
q
e
the celes tial pole with out ris ing or set ting rotation every 24 hours, the stars march
l
a
i (see the top diagram on the facing page). across the heavens at the rate of 15° per At the north pole t S s
e
l
At the other ex treme, an observer stationed hour. There fore the appearance of the sky e
C exact ly on the Earth’s equator, latitude changes with the time of night. An added W 0°, would see the celes tial equator di rect ly complication is that the Earth is also orbit ing
over head, as shown in the middle diagram the Sun each year, so the con stel lations on Right: At the equator, S N oppos ite. The north and south celes tial poles show change with the seasons. by contrast, all the sky Observer Hori would lie on the north and south horizons For example, a con stel lation such as Orion, is visible; as the Earth zon E respect ively, and every part of the sky would splendid ly seen in December and Janu ary, rotates, stars appear to be visible at one time or another. All stars will be in the day time sky six months later rise in the east and set would rise in the east and set in the west as and hence will then be in visible. The maps in the west. the Earth rotated. on pages 26–73 will help you fi nd out which For most observers, the real sky ap pears stars are on view each month of the year, some where be tween these two ex tremes: the wherever you are on Earth. At the equator
+60° North celestial pole Celestial sphere: To C eles +30° tial C understand celestial eq ir Left: At intermediate u cu Circle of declination at m or coordinates and motions, po latitudes, the situation la it helps to think of objects r is between the two W 0° in the sky as lying on extremes. Part of the 50° Circle of right ascension a transparent sphere sky is always above Observer S N h surrounding the Earth, the horizon (the part 18 as shown in the diagram on marked ‘Circumpolar’), h E Horizon 20 the left. Right ascension and but an equal part –30° h
22 Ecliptic declination can be visualized is always below the A lw h as circles on this sphere, a horizon and hence is 0 y h s i 2 along with the celestial nv invisible. Stars between is Celestial equator ib le –60° equator and the ecliptic. these two regions rise (Wil Tirion) At 50° north and set during the night. (Wil Tirion) South celestial pole For general queries, contact [email protected]
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