PA TRICK MOORE

OLDBOURNE -J.ONDON

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*

FRONTISPIECEColor photograph of the Great Spiral in Andromeda;

48-inch Schmidt telescope, Palomar.

(California Inslilult of Technology, Pasadena, Cat.)

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ASTRONOMY

ASTRONO Y

PATRICK MOORE

OLDBOURNE- LONDON

OLDBOURNE BOOK CO. LTD121 Fleet Street London E.C.4

Oldbourne Book Co. Ltd 1961

Designed, printed, and bound in Great Britainby Jarrold & Sons Ltd Norwich

contents

i the sky above us 13

2 watchers of the stars 19

3 the greek astronomers 24

4 the rebirth of astronomy "2~j

5 the design of the universe 3 1

6 the story of tycho brahe 347 legends in the sky 398 the laws of johannes kepler 439 telescopes and the stars 47io the face of the sun 53ii exploring the solar system 60

12 the genius of newton 69

13 the royal observatory 7714 lomonosov 84

15 the 'father of stellar astronomy' 88

16 the planet-hunters 98

17 great telescopes i07

18 explorers of the moon 121

19 the sun's family 133

20 wanderers in space i5i21 how far are the stars? i5922 exploring the spectrum 1 6523 giants and dwarfs of the sky 1 7424 the variety of the stars 183

25 THE GALAXIES igi

26 THE STORY OF RADIO ASTRONOMY 207

27 LIFE IN THE UNIVERSE 220

28 ROCKETS INTO SPACE 224

29 EARTH SATELLITES 234

30 SPACE PROBES AND LUNIKS 237

CONCLUSION 243

LANDMARKS IN THE STORY OF ASTRONOMY 244

LIST OF ILLUSTRATIONS 246

ACKNOWLEDGMENTS 249INDEX 250

foreword

of all the sciences, astronomy has perhaps the greatestpopular appeal. There are two main reasons for this. First, itforces itself upon the attention of everyone; who has not beenstruck by the beauty of a starlit sky? Secondly, it offers plenty ofscope for the amateur student, who can make himself reallyuseful to professional workers. It is true that amateur observersare less important than they used to be half a century ago, butthere is still a great deal for them to do.

Interest has grown still further since the direct exploration ofspace began in 1957. Within a decade, space-ships have steppedout of fantasy and into fact; few scientists now doubt that theMoon and some of the planets will be reached in the foreseeablefuture, though it should be remembered that true interplanetaryflight is by no means the only aim of space research. Yet I do notbelieve that the remarkable surge of interest is due primarily tothis cause. Astronomy has an appeal all its own, and wouldcontinue to do so even if rocketry had never been developed.

In this book, I have tried to tell the story of astronomy from itsdim beginnings up to the opening of the age of the direct explora-tion of space. My first recorded date is about 2000 B.C., when theChaldeansor possibly the Cretansdivided the stars into con-stellations; my last is 1961, when the Russian space-ship Vostokorbited the Earth with a human passenger and Alan Shepardmade his first flight into space. To give a full coverage of a 4000-year story in a single book is clearly impossible, but I have donemy best to sort out the salient facts and write them down clearlyand concisely, developing each theme as it arises. I hope that theresult will interest at least some people; if not, the fault is minealoneit cannot lie in the story itself.During the compilation of this book I have had help from a

great many people. I am grateful to them all; their names will befound in the list of acknowledgments, and it is necessary to repeatit here, but I must stress that any errors or omissions are to belaid solely at my door. I must however make special mention ofDavid Hardy-, the astronomer-artist who has been responsible formany of the color illustrations; neither must I omit to record mythanks to the publishers and to the printers, with whom I haveworked closely throughout.

I have made no attempt to be technical. This is a book foramateurs, written by an amateur. Ifit persuades even a few peopleto make a hobby of astronomy, I will feel that it has been wellworth writing. Few. men can explore the heavens by rocket;everyone can help in exploring the skies by means of telescopes.

PATRICK MOOREEast Grinstead

GROUP OF NEBULAE IN LEO, pholo-% graphed with the 200-inch Hale reflector at

Palomar Observatory 1 the sky above us

THE ANDROMEDA SPIRAL. One of thenearest of the external galaxies, photographedwith the 300-inch Hale reflector at PalomarObservatory. The two satellite galaxies areclearly shown

Great spiral

Alpheratz

SQUAREOF

PEGASUS

POSITION OF THE ANDROMEDAspiral. The Spiral is just visible to theunaided eye on a clear night

far away IN space, so remote that it looks like nothing morethan a misty patch of light, lies an object which astronomers knowas the Great Spiral. It is just visible without optical aid when thesky is really dark, and a pair of field-glasses will show it clearly.The neighboring stars belong to the group or constellation

known as Andromeda, and are members ofour own stellar system.The Great Spiral is more distant than Andromeda; it is an inde-pendent star-system, and is so far away that its light, moving at186,000 miles per second, takes about two million years to reachthe earth. When we look out into space, we also look backwardin time; we are seeing the Spiral not as it is now, but as it used tobe two million years ago. It is a vast system made up of more than100,000 million suns, many of which are a great deal larger andhotter than our own.

Ideas of this kind are difficult to grasp, and it is tempting toregard the Earth as the most important body in the universe.Nothing could be further from the truth. Every star is a sun;astronomical distances have to be reckoned in millions of millionsof miles, and the Earth proves to be utterly insignificant.The Earth is a typical planet, travelling round the Sun and

completing one journey in 365 J days. Of the eight other planetswhich move round the Sun at various distances, four are smallerthan our world, and the remaining four much larger. Jupiter, thelargest planet, is big enough to contain over 1 ,300 globes the sizeof the Earth. It is not solid and rocky; it is made up of dense gas,so that life there appears to be out of the question. Whereas theEarth has a single satellite, our familiar Moon, Jupiter has twelve.The Sun, like all the other stars, sends out a tremendous quan-

tity of light and heat. The Moon and the planets, however, arenot self-luminous, and shine only because they reflect the Sun'srays. Seen from a distance of thousands of miles, the Earth wouldappear to shine in the same way.

Together with other bodies of lesser importance, the Sun, planetsand satellites make up the Solar System. This system represents onlya very small part of the universe, but is extremely large judged byeveryday standards. The Moon, the Earth's nearest neighbor, isalmost a quarter of a million miles away, while the distance of theSun is 93 million miles. Pluto, the most remote of the nine planetsin the Solar System, moves round the Sun at an average distanceof well over 3,000 million miles.

This may be shown by means of a scale. Let us represent theSun by a globe 2 feet across. The Earth will then be the size ofa pea, placed at a distance of 215 feet; Jupiter will become a largeorange at a distance of one-fifth of a mile, while Pluto will becomea pin's head more than 2 miles away from the model Sun. On thisscale the nearest star, known to astronomers as Proxima Gentauri,will be several thousands of miles off. If the 2-foot Sun is set downin the middle of England, Proxima will have to be taken to Siberiaor the United States in order to represent its distance correctly.

PLANETS

MERCURY

CHA.METE* IN MILESMEAN DISTANCEFHOM THE SLININ HILLIONS OF MILES

COMPARATIVE SIZES OF THE SUN AND planets. The sizes of the Sun and planets, drawn to ike same scale.Drawing by D. A. Hardy

the sky above us 16

star-trails. A time-exposure was madewith the camera painting at ike NorthCelestial Pale; the stars seemed to moveslowly across the sky, so producing trails.This apparent movement of the sky is due tothe real rotation of the Earth on its axis.Photograph by Allan Lanham

amoN, One of the must brilliant constella-tions in the sky, with two first-magnitudestars, Beietgeux and tiigei

All the bodies in the sky seem to move in an east-to-westdirection, giving the false impression (hat (he Earth lies in thecentre of the universe. This daily impression of movement hasnothing to do with the stars or planets, and is due entirely to thefact that the Earth is rotating on its axis from west to east. Thestars arc not fixed in space, as ancient peoples believed; they movein various directions at great speeds, but are so far away that theirindividual motions cannot be detected except over long periods,and the patterns or constellations or stars seem to remain more orless unaltered. There is a good everyday comparison. A high-flyingjet aircraft will seem to crawl across the sky, while a near-by birdwill flash past quickly

yet in reality the jet is the faster of the two.Though the stars arc suns, they are not all alike. Some are

hundreds of millions of miles in diameter, while others are nolarger than the Earth; some are cool and red, others white andextremely hot. Some of the stars brighten and fade over shortperiods; some prove to be made up or 'twins* so close (ogethcrthat to the unaided eye they appear as one; some are surroundedby clouds of gas, and some are spinning round so quickly thatthey arc shaped more like eggs than billiard-balls. It is possible,too, that many of them may have planet-families of their own, andthat some of these planets arc inhabited.On a clear night it is possible to see about two thousand stars

without the aid of a telescope, and the leading constellations areeasy to find. Most people can identify the Great Bear, Orion andthe Little Bear, while Australians and New Zealanders are just asfamiliar with the Ship and the Southern Cross. The powerfulinstruments used by modern astronomers show many additionalstars, and it is now known that our stellar system or Galaxy con-tains about [00,000 million separate suns. Each star is using upenergy as it shines, and there must come a lime when the star willhave used up all its supply of 'fuel', so that it will die. The Sun isno exception. It will last for thousands of millions of years yet,but it will not last for ever.The bright stars visible at night are members of (he Galaxy

in which the Sun and the Earth lie. The Great Spiral is muchmore remote; i( too contains suns of all types, and is a systemlarger than ours. The world's most powerful telescope is capableofshowing 1,000 million galaxus.

Ancient peoples had little idea of the true nature of the universe.They had no telescopes, and had to tcIv solely upon their eyes.Today, large instruments are used for astronomical research;the biggest telescope so far in use, the reflector at Palomar inCalifornia, collects its light by means of a mirror 200 inches indiameter, which acts as a giant 'eye'. Recently the Russians havebegun the construction of an even larger telescope, with a236-inch mirror.

If a stone is dropped into a calm pond, ripples will be formed.The distance from one crest to another is known as the wavelength,and there is a comparison here with light, which may be regardedas a wave-motion. The color of light depends upon its wave-lrngth; red light has a wavelength longer than that or blue, andso on. Radiation of still longer wavelength cannot be seen at all,but it can be studied by means of special instruments known as

16 the sky above us

radio telescopes. The best-known of these radio telescopes is theBritish one at Jodrell Bank, Manchester, and takes the form ofa wire dish 250 feet in diameter. It is used to collect radio wavescoming from the depths of space, and is capable of detectingwaves from distant galaxies which are colliding with each otherand sending out energy in the process.

Radio astronomy is a young science, while more recently therehas been yet another developmentrocket astronomy. Scientificinstruments have been sent above the Earth's atmosphere, andman-made vehicles have travelled to and around the Moon. InFebruary 1961 the Russians launched a probe which passedwithin less than. 100,000 miles of the planet Venus. This wasfollowed by an even greater triumph the first true space-flightby Major Yuri Gagarin; while from Cape Canaveral, Florida,Commander Alan Shepard made the pioneer space-flight by aWestern astronaut. It is clear that man's direct exploration of theSolar System is well under way.

Radio telescopes and space-rockets are very modern, but astro-nomy itself is the oldest science in the world. Even the earliestmenthe cave-dwellers of long agomust have looked up at theskies and wondered just what the stars were; it was natural 10worship the Sun and Moon as gods, and equally natural to believethe Earth to be a flat plain in the exact centre or the universe.Less than 600 years ago it was still believed that the Earth mustbe the most important body of all, and men who lived at the limeof the Spanish Armada refused to believe that our world goesround the Sun. The apparent positions of the stars, and themovements of the planets in the sky, had been measured withgreat care, but their significance remained unknown.

Tn 1609 Galileo Galilei, an Italian Professor of Mathematics,first looked at the heavens through a telescope. It was he whofirst saw the moons ofJupiter, the craters of the Moon, and thethousands or faint stars which make up the Milky Way. His tele-scope was low-powered judged by modern standards, but duringsucceeding years better instruments were built, and the firsttrue observatories came into being. Since the time of Galileo andhis primitive telescope, astronomical knowledge has increasedsteadily. Men have found out the distances and sizes of the stars;they have studied the remote galaxies; they have analyzed theradio waves from space, and they are now making plans to travelto the Moon.

It is impossible to understand and appreciate all this withoutsome idea of what happened in past ages. The story of astronomyis more fascinating than any other branch of history, and bytracing its progress it is possible to gain a real understanding ofthe Earth, the stars and the universe itself.

SCALE OF THE SOLAR SYSTEM. Thedistances of the variitut planetsfrom Ike Sunare shown on this scale drawing

the hale reflector. The 200-inch reflector at Palomar Observatory, sofar the largest telescope in the world

2 watchers of the stars

TOTAL F-CLfPSE OF THE SUN, I9I9.The draining shows m extensive prominencewhich was nicknamed 'the Atitrain Promi-nence'. It is very seldom that a prominence ofthis size is seen during a total eclipse. Draw-ing by D. A. Hardy

PARTIAL ECLJPSE OF THE SUN,June iff, 193(1, idh som. Frnm an observa-tion made by Patrick Alaare, using a 3-inchrefractor, X 100. Three sumpats are shown.Drawing by D. A. Hardy

it is impossible to tell just when astronomy began. The cave-men of thousands of years ago must have looked up into the skyand marvelled at what they saw Uicre, so that in one sense theywere 'observers'; they noticed unusual happenings, such as eclipses,and written records of man's findings and theories go back to thedawn of history.

Early races believed the Earth to be flat and stationary, withthe entire sky revolving round it once a day. The idea that theworld is a globe nearly 8,000 miles in diameter, whirling roundthe Sun at a speed or some 66,000 m.p.h., did not occur to them.Some of their old ideas sound strange to our ears. The Vedicpriests of India believed the Earth to be supported upon twelvemassive pillars; during the hours of darkness the Sun passedunderneath, somehow managing to pass between the pillars with-out hitting them. Even more peculiar was the Hindu theory,according to which the Earth stood on the back of four elephants;the elephants in turn rested upon the shell of a huge tortoise,while the tortoise itself was supported by a serpent floating in alimitless ocean.

Ancient man had to begin at the very beginning, and mistakeswere inevitable, but at least useful observations could be made,and many of the early records have proved to be of tremendousvalue. Probably the first real students of the sky were the Chinese.

20 watchers of the stars

TOTAL ECLIPSE OF THE SUN, February15, 1961. Thl lint of totality extendedacross Southern Europe. The eclipse wasshown on B.B.C. televisionfrom three stationsm Europe in succession; St. Michel (France),Florence [Italy) and Mount Jastrebac (Jugo-slavia). Pictures obtained from France andItaly wire good; conditions in Jugoslavia,where the author was commenting, wereaffected by cloud. This photograph was takenoff the television screen during transmissionfrom France

THEORY OF AN ECLIPSE OF THE SUN.The Moon's shadowjust reaches asfar as theEarth; a partial eclipse is seen to either side

of the bell oftotality

\ I'fa tliawing is sal (a stall]

About 3000 B.C., they adopted a 'year' of 365 days, which enabledthem to work out a calendar. It mattered little to them whetherthe Sun went round the Earth, or the Earth went round the Sun;the 365-day year was correct in either case. The Chinese Emperor'sastronomers produced a reliable calendar, and were also able totell when eclipses were due.The Moon has no light of its own, and is a relatively small body

only 2,160 miles in diameter. The Sun, with a diameter of864,000miles, is so much farther away than the Moon that it appearsalmost exactly the same size in the sky, and this is why 'solareclipses' can take place.

During its monthly journey round the Earth, the Moon mustsometimes pass in front of the Sun. At such times the dark side ofthe Moon is turned towards us, as shown in the diagram; andsince this side docs not shine, the Moon cannot be seen. As itmoves between the Earth and the Sun, blocking out a part of theSun's surface, a 'bite' appears in the edge of die solar disk, andthis bite becomes larger as the eclipse progresses. If the Mooncompletely covers the Sun, the eclipse is total, and the solaratmosphere flashes into view, with startling effect. Normally it isimpossible to sec this solar atmosphere with the unaided eye orwith ordinary telescopes, since it is overpowered by the brillianceof the Sun itself. When the Moon acts as a screen, however, the fullspectacle becomes visiblethe glorious pearly gas known as thecorona, as well as the red prominences which rise from the solarsurface. No total eclipsr lasts for more than about eight minutes.As soon as part of the Sun's disk reappears, the prominences andthe corona are blotted out; the Moon moves steadily in its path,and the eclipse comes to an end.The Chinese had different ideas, and had no thought that the

Moon could be concerned in any way. They believed that adragon was trying to cat the Sun, and their remedy was to scarethe beast away by making as much noise as possible. The wholepopulation took part, shouting and wailing, and bearing gongsand pans to add to the uproar.However, they did know that any eclipse is likely to be followed

by another eighteen years eleven days later, and by reckoningaccording to this periodthe so-called Sorosthey could avoidbeing taken unprepared. Now and again mistakes were made, andthere is one famous slory which relates how two luckless Courtastronomers, Hsi and Ho, were executed because they had failed

watchers of the stars 21

to predict an eclipse. This may be nothing more than a legend,but in any case there can be no doubt that eclipse forecasts werebeing made by men who lived 4,000 years ago.A solar eclipse does not happen every month, because the

Moon's path or orbit is appreciably tilted, and in most cases thedark and therefore invisible Moon passes either above or belowthe Sun in the sky, so that no eclipse occurs.The Chinese did not confine their records to eclipses. They also

recorded comets, which they regarded as unlucky. It is now knownthat a comet is made up ofcomparatively small particles surroundedby an envelope ofgas, and is completely harmless, but the spectacleof a brilliant comet with shining head and long tail was - enoughto strike terror into the hearts of primitive peoples.The Chinese and other early observers, notably the Egyptians,

were content to compile their records without troubling greatlyabout what the various phenomena meant. This was probablybecause the heavenly bodies were regarded as divine, and formany centuries it was impossible to separate astronomy from

comet AREND-R01.AND, 1957. Thiswas one of the two naked-eye comets seen inl957> *' was 1 foirty conspicuous object forsome weeks during May, when it was in thenorthern part of the sky. Photograph by F. J.Acjield, Forest Hall Observatory, Northum-berland

VENUS AND HALLEY'S COMET, I9IO.When this photograph was taken, the cometwas almost at its brightest, but Venus isnecessarily over-exposed. Halley's Comet willnext be brilliant in ig86. Photograph byH. E. Wood, Union Observatory, Johannes-burg

22 watchers of the itan

INCLINATION OP THE EARTH S AXIS.The equator is inclined by 23 \ degrees to theplane ofthe Earth's orbit

APPARENT MOVEMENT OF THENORTH CELESTIAL POLE. The shiftingis due to precession effects, or the slaw move-ment in the direction of the Earth's axis. Inancient times the Pole Star was Thuban mDraco; at present it is Polaris in Ursa Minor;by theyear A.n, 14000 the northern polar starwill be the brilliant Vega, in Lyra

astrologythe superstition of the stars. Even today such confusionis not uncommon, but there is no excuse for it. Astronomy is anexact science, whereas astrology is of no value to anybody.The stars appear to keep to the same patterns in the sky, while

the much nearer Sun, Moon and the planets appear to wanderslowly about. Yet the 'wanderers* do not move irregularly; theykeep to a definite belt in the sky, known as the Zodiac. Accordingto astrologers, a person's character and destiny are affected by thepositions of the Sun and other bodies of the Solar System at thetime of birth. The whole idea is completely baseless, but onlyduring the past two or three centuries has astrology been finallydiscredited. Until then it was regarded as more important thanEras astronomy.Apart from recording startling events such as eclipses and

comets, as well as drawing up a workable calendar, the Chinesemade little progress astronomically. The Egyptians, however,proved to be extremely skilful at measuring the apparent positionsof the stars, and they 'lined up' their famous Great Pyramid inaccordance with what was then the North Pole of the sky. Thisagain is important, because it has given an excellent clue to theage of the Pyramid itself.The Earth's axis of rotation is tilted at an angle of 23J

degrees to the perpendicular. At the present moment the axispoints northward to a position close to the bright star Polaris,which is therefore our Pole Star, When the Pyramid was built,the pole of the sky was in a different position, and the Pole Star ofthose days was a much fainter objectThuban in the constellationof the Dragon. The Earth is not a perfect sphere; it is slightlyflattened, so that the equatorial zone bulges out, and the diametermeasured through the poles is 26 miles shorter than the diametermeasured through the equator. The Sun, Moon and other bodiesexert a pull on this bulge, and the result is that the Earth's axisseems to wobble very slowly, in the manner of a top which isabout to Tall. This causes an effect known as precession. The polarpoint describes a circle in the sky, and has shifted considerablysince the time when the Great Pyramid was set up.

Astronomy was developing. The stars were divided into de-finite constellations; observations began in other countries, andcalendars were improved. Then, about 600 years before Christ,came the Greeksand with them a revolution in scientificknowledge.

an eclipse of the moon, in four stages. Drawings by D. A. Haxdy\

3 the Greek astronomers

THREE PHOTOGRAPHS OF ANeclipse of the moon. It is easy to seethat the Earth's shadow on the lunar surfaceis curved

thales, first of the great Greek astronomers, was bornabout the year 624 B.C. Like the Chinese and the Egyptians, hestudied the stars, but he went further, and tried to explain what hesaw. He may have realized that the Earth is a globe, though sinceall his original writings have been lost it is impossible to be certain.The first definite arguments against the traditional flat-earth

theory were advanced by Aristotle, who lived from about 384 to325 B.C.As Aristotle pointed out, the stars appear to alter in height

above the horizon according to the observer's position on theEarth. From Greece, the Pole Star appears high above the horizon,because Greece is well to the north of the Earth's equator; fromEgypt, the Pole Star is lower down, and from southern latitudesit can never be seen at all, since it never rises. On the other handCanopus, a brilliant yellowish star in the southern part of the sky,can be seen from Egypt but not from Greece. This is easy toexplain on the assumption that the Earth is a globe, but cannotbe accounted for by supposing the Earth to be flat. Aristotle alsonoticed that during a lunar eclipse, when the Earth's shadow fallsacross the Moon, the edge of the shadow is curvedshowing thatthe surface of the Earth must also be curved.The records left by Thales, Aristotle and others were kept in a

large library at Alexandria, in Egypt. Unfortunately this librarywas later destroyeda loss which can never be made good. Formany years the library was in the charge ofEratosthenes ofCyrene,a great scientist in his own right, who has earned his place inhistory as being the first man to measure the size of the Earth.From one of the books in the library he learned that at the timeof the Summer Solsticethe 'longest day' in northern latitudes

the Sun was directly overhead as seen from the town of Syene(the modern Aswan), some way up the Nile, so that at noon thesolar rays would shine directly into a well without casting ashadow. At this moment, however, the Sun was not overhead atAlexandria; it was 7 degrees away from the zenith or overheadpoint. A full circle contains 360 degrees, and 7 is about one-fiftiethof 360, so that if the Earth is a globe its circumference must be50 times the distance from Alexandria to Syene. Eratosthenesmeasured this distance, and worked out that the distance rightround the Earth must be about 24,850 miles. There is someuncertainty about this result, as his figures were calculated not inmiles but in 'stadia', and the precise length of one stadion is notknown; but his figure for the Earth's circumference seems to havebeen correct to within less than 100 miles.The Greeks knew that the world is spherical, and they had an

excellent idea of its size, but they found it difficult to believe thatthe Earth could be anything but the centre of the universe. Thiswas a serious barrier to further progress. Even Aristotle was certainthat the whole sky moves round the Earth. One or two astronomersnotably Aristarchus, who lived from about 310 to 230 b.c.

the Greek astronomers 25

ERATOSTHENES METHOD OF MEA-StfHINU THE SIZE OF THE EARTH . Atnoon at tht limt ofthe summer solstice, the Sunwas vertical at Syenc, but not at Alexandria.Eratosthenes measured the Sun's altitude atthis time, as seen from Alexandria, as 7degrees away from the zenith, and was thusable to measure the circumference of the Earthwith remarkable accuracy. It is significantthat the value which he gave was mart correctthan that used many centuries later byChristopher Columbus on his voyage ofdiscovery to the j\'ew World, which is aremarkable tribute to Eratosthenes' theoreticaland practical skill

the PTOLEMAicTiiRORY. According toPtolemy, a planet moved in a small circle orepicycle, whits the centre of this circle {thedeferent) itself moved round the Earth in aperfect circle. This system was not inventedby Ptolenty, but its greatest development wasdue to him. Ptolemywho was an excellentmnthemaikianrealized that this compara-tively simple arrangement would not explainthe actual movements of the planets in the sky,and he was compelled to introduce extraepicycles, thus making the whole systemclumsy and unwieldy. However, the Ptolemaictheory was almost universally accepted byscientists up to the time ofCopernicus

had the courage to suggest that the Earth revolves round the Sun,but such ideas were highly unpopular, and the last two importantastronomers of ancient times, Hipparchus and Ptolemy, keptfirmly to the older theory.

Hipparchus lived about 1 50 b.c. Details of his life arc unknown,but it is obvious that he was a man of great ability, and he drewup an important and remarkably accurate star catalogue- It wasHipparchus, too, who discovered precession, the apparent move-ment of the celestial pole; and he worked out the distances of theSun and Moon more correctly than had been done before, thoughthe values which he gave were still much too small.

Claudius Ptolcmams, better known as Ptolemy, lived inAlexandria. As with Hipparchus f nothing is known about hiscareer or personality, but science owes him a great debt. He pro-duced a book in which he gave not only his own results, but alsothose of the astronomers who had come before him, and it is thisbook which has provided modern scholars with much of then-information about ancient science. It has reached us by way orits Arab translation, and is generally referred to by its Arab nameof the Almagest, or 'greatest'.

Ptolemy was an excellent observer and mathematician, and heundertook a thorough revision of Hipparchus' star catalogue. Healso investigated the movements of the bodies of the Solar System,and concluded that the Earth lay in the centre, with the wholeheavens moving round it. The Moon was the nearest object inthe sky; then came the planets Mercury and Venus; then the Sun,and then the more remote planets Mars, Jupiter and Saturn,beyond which lay the stars. This arrangement is termed thePtolemaic System, although Ptolemy himself was not the first todescribe it.

26 the Greek astronomers

THE UNIVERSE, ACCORDING TOptolemy. Prom an old print, 1600. Thearrangement of (he aUstial bodies accordingto the Ptolemaic theory is clearly shown,though no attempt has been made to make thedistances even approximately correct. How-ever, Ptolemy realized that the actual motionsof the planets must be complex; as we hateseen, he was compelled (0 introduce numerousepicycles in ait attempt to reach agreement

with the observational data.

REVOLVING TABLE FOR ESTIMAT-ING THE POSITIONS OF THE ZODIA-CAL CONSTELLATIONS III".'!" W I".FN7000 B.C. AND a.d. 7000. Publishedill the famous book Astionomir.umOsamim by P. Apian (Ingolstadt, 1340).The apparent movements of the stars are ofcourse affected by the shifting of the celestialpole due to precession, so that conditions werenot precisely the same in Apian's time as theyhad been in Ptolemy's. Yet even in r^jo, thePtolemaic theory of the universe was stillgenerally accepted, and few scientists eventOtubknd questioning it

The movements of the planets caused him a great deal of diffi-culty. It had always been supposed that the orbits of the heavenlybodies must be circular, since a circle was regarded as the 'perfect'form, and nothing short of perfection could be allowed in the sky.This was Ptolemy's view, but he realized that the observed motionsof the planets could not be explained by the theory that theysimply turned round the Earth in circular paths. ConsequentlyPtolemy worked out a system according to which each bodymoved in a small circle or epicycle, the centre ofwhich itself movedround the Earth in a perfect circle. Even this would not suffice,and more and more epicycles were introduced, until the wholesystem became hopelessly clumsy and artificial.

Ptolemy never solved Uiis problem, and after his death, abouta.d. i8o, the science of astronomy came almost to a standstill.Greece was no longer powerful, and the Roman Empire wascrumbling; the Dark Ages came to Europe, and much of the oldk-armiit:. was forgotten. Many of the boob in the AJexancManLibrary were destroyed, and the remainder scattered. It is truethat in Central America the people known as die Maya weremaking observations and working out an accurate calendar butthere was no communication between the Old and the NewWorlds, and progress in European astronomy was halted.

Centuries passed. Then, slowly and painfully, astronomy wasrebornnot for its own sake, but because of astrology. To maketheir predictions, the astrologers had to learn about the movementsor the planets, and this knowledge could be gained only by carefulobservation. The Arabs took the lead, and some of their starcatalogues were better than Ptolemy's. Then, as the peoples ofEurope gathered into definite nations, the stage was set for dienext phase in the history of astronomy.

4 the rebirth of astronomy

Arabian astrolabe. This astrolabe, atypical example, was made in 1014 tyMustafa Ayyub

4

DIAGRAM TO SHOW THE USE OF ANastrolabe. The axis DA is made level,and the star S is sighted along the directionFCBS; the observer's eye is shown behind F.The altitude of the star is then read off on thescab, h this case, the altitude amounts to30 degrees

THF. ROMANS, WHO RULED much of the civilized world for somany centuries, did very little for astronomy. They did not sharethe Greek love of learning, and were concerned only with practicalaffairs.

One matter which did seem worth troubling about was thestate of the calendar. The true *year1 , or time taken for the Earthto go once round the Sun, is not exactly 3615 days, but more nearly

365I;, so that to draw up a calendar which will not become out ofstep with the seasons is not as easy as it might appear. JuliusCasar realized this, and instructed a Greek astronomer, Sosigenes,to form a more accurate calendar. Sosigenes did his work well;for instance he invented 'leap year', which took care of the extraquarter-day in the Earth's period of revolution. The 'Juliancalendar1 was not perfect, and has been further improvedsince, butit was quite good enough to satisfy the Romans.

In many ways it is unfortunate that scientific progress came to ahalt for a long time after the death of Ptolemy about A.D. 180.It is even more unfortunate that so many of the old books werelost. However, some survived, and one of these was Ptolemy'sAlmagest, which reached Baghdad, capital of the caliphs, in theeighth century. It was translated into Arabic, along with variousother volumes, and serious astronomy began once more.The Arabs themselves proved to be skilful observers, and

founded astronomical centres at Damascus and Baghdad. Theyset up scientific equipment for measuring the positions of the stars,and one Caliph, Al Mamonson of Harun al-Rashid, or /IranianNights famebuilt a fine observatory. It was quite unlike amodern observatory, since telescopes still lay far in the future,but it contained an excellent library. By the time Al Mamon died,in a.d, 833, Baghdad had become the 'astronomical' capital ofEurope.

Probably the most famous astronomer of the Baghdad schoolwas Al-Battaui, who was born about the middle of the ninth cen-tury. He was a particularly good mathematician, and madeobservations which compared favorably with those of Hipparchusand Ptolemy. Al-Battani also wrote an important book, theEnglish tide of which may be given as Tht Movements of the Stars.

Another skilful Arab astronomer was Al-Sufi, who lived from903 to 986. He, too, wrote a book

Uranographia, in which hedealt largely with the apparent brilliancies of the stars. This is animportant matter, and is worth describing in a little more detail.The stars are graded into classes or 'magnitudes' of apparent

brightness. The scale may seem confusing, since the brighteststars have the smallest magnitudes; thus a star of magnitude 1outshines a star of magnitude 2, and so on. The faintest starsvisible to the naked eye, under ordinary conditions, are of magni-tude 6. The scale may be compared with a golfer's handicap; thelower the handicap, the better the golfer.

Rigcl in Orion is very brilliant, and on the modern scale its

23 the rebirth of astronomy

VENUS

- 1

1-4SINUS (brightest star)

VEGA

ANTARES I

POLE STAR 2

Faintest star visibleto the naked eye

Faintest star to berecorded from palomar

star MAnNiTtitiEs. The magnitudes artshown here as disks, though the stars them-selves appear only as points of light

magnitude is reckoned as O'l, or ordy one-tenth fainter thanzero. The four brightest stars in the sky (Sirius, Ganopus, AlphaCcntauri and Arcturus) have negative magnitudes, that of Sinusbeing minus 14. On the same scale, the Sun has a magnitude ofminus 26,Nowadays it is possible to measure star magnitudes very

accurately, and the world's largest telescopes can record objectsof below magnitude plus 20. Al-Sufi had to make his estimatessimply by using his eyes, but most of his values agree well withthose of today. On the other hand there are some interestingdifferences. He ranked Alhcna, a star in the constellation ofGemini (the Twins) as of the third magnitude, but it is nowbrighter than the second, and there are other similar cases inwhich stars have apparently brightened up or faded away. It isdifficult to be sure whether these changes are real, or arc due tomistakes by Al-Sufi and other observers of his time, but we. do atleast know that some of the stars are variable in' brilliancyandwe must not be too quick to accuse Al-Sufi of inaccuracy.Many of the star names now in use are due to the Arabs. A

typical example is Aldebaran, in the Bull, which means 'thefollowing', since Aldebaran seems to follow the famous star-clusterof the Pleiades or Seven Sisters. Astronomical terms such as'zenith' (the overhead point of the sky) arc also Arabic in origin.The Arabs did not confine themselves to measuring star posi-

tions and magnitudes. They carried out many other observations,and were particularly interested in eclipses. They also studiedthe movements of the Moon and planets. In this connection men-lion should be made of King Alphonso X of Castile, who called anumber of Jewish and Arab astronomers to Toledo, and wasresponsible for the publication of the famous Alphonsine Tables,which contained data for planetary positions and the forecastingof eclipses, and which were used throughout Europe for thefollowing 300 years.The Mongol prince Ulugh Bcigh, grandson of the Oriental

conqueror Timor (more generally known as Tamerlane) also hashis place in the history of astronomy. Ulugh neigh founded amagnificent observatory at his capital of Samarkand, equippedwith instruments which were the best of their day. Unfortunatelyfor him, he was a firm believer in astrology, and this led to hisdeath. He cast the horoscope of his eldest son, Abdallatif, andfound to his alarm that the boy was destined to kill him. Hetherefore dismissed his son from Court and sent him into exile.Abdallatif had no wish to be set aside, and rebelled, finallymurdering Ulugh Beigh and becoming king in his place.The murder of Ulugh Beigh put an end to the Arab school of

ULVOH beigh memorial. This mem~orial now stands in Samarkand, Ike site ofUlugh Heights old observatory

the rebirth ot astronomy 29

cemini, the twins. According toAl-Sufi and others ofhis lime. Caster used tobe brighter than Pollux, but il is now half amagnitudefainter. Athena, on the other hand,is now a magnitude brighter than as givenby At-Sufi

c< ) r#PHASES UF THE MOON. The Moon AflJno light of its own, and depends on reflectingthe rays of the Sun, so thai half of il isluminous while the other half h dark, li'henthe Moon has its darkface turned towards us,so that it is almost between the Earth and theSun, we cannot set it a! all, and this is whatthe astronomer terms 'New Moan*. (If thethree bodies are exactly lined up, the result is

an eclipse of the Sun, but owing to the lilt ofthe lunar orbit this does not happen a! everyNew Moon.) In the diagram, New Moon isshown in position t. At other limes the Moonshows as a half {positions 5 and 7) , three-quarter shape (4, S), andfull (5).

nslronomy, but by this time the old wish to learn had revived inEurope and observatories were set up in various places. The firstwas established at Nurnberg, in Germany, byJohann Miillrr, whois belter known by his latinized name of Regiomontanus. Withhis tutor, George von Peuerbach (Purbach) he revised the oldAlphonsine Tables, and after Purbach's death he joined with hisown pupil, Bernard Walther, to introduc c new and better methodsof observation. Printing had now been invented, and Regiomon-tanus set up his own press, so that he could publish astronomicalinformation for the use of others. He continued to do so up to thetime of his death in 1476.

Mention must also be made of 'the universal genius'Leonardoda Vinci, who lived from 1452 to 1519. Leonardo was one of thegreatest painters the world has known, and he was also a brilliantscientist. He was not a true astronomer, but he made one discoverywhich cleared up an old mystery. This concerned the Moon.The cause of the Moon's phases, or apparent changes of shape

each month from new to full, was not in the least mysterious.More puzzling was the fact that when the Moon shows as acrescent, the 'dark' portion can often be seen shining faintly,and giving the appearance known popularly as 'the Old Moonin the Young Moon's arms'. Leonardo realized that this mustbe due to light reflected from the Earth on to the Moon. Earth-shine is in fact nearly always seen whenever the crescent moonshines down from a clear, dark sky.

All this work, carried on in countries all over Europe, showedthat astronomy had really 'woken up'. For instance star cata-logues had been greatly improved, and careful observation hadmade it possible for astronomers to predict the positions of theplanets for years in advance.The main handicap to progress was the false theory that the

Earth must He in the centre of the universe, with all the otherheavenly bodies moving round it in circular orbits. Astronomershad followed Ptolemy rather than Aristarchus, and so their ideasabout the design of the universe were completely wrong,A few scientists had their doubts. One was Nikolaus FCrrbs, the

son of a German wine-grower, who was born in 1401 and died in1464. His boyhood was unhappy; he ran away from home, andstudied first at Heidelberg and then in Italy, becoming a well-known scholar and mathematician. He entered the Church, andbecame a Cardinal; he is often known as 'Nicholas of Cusa 1

,since

hr was bom at Cues on the Moselle. Krebs urged alterations in thecalendar, and in a famous book called De Docta Ignvrantia he sug-gested ihat after all, the Earth might not be lying at rest in theten Ire of the universe.

Not many astronomers of the time paid any attention to him,and for half a century after his death nothing more was heard ofthe theory of the 'moving Earth'. The next stepthe realizationthat our own world is not, after all, an important bodywas onewhich mankind found very hard to take.

Delhi Observatory

These photographs show one of the greatest observatories ofpre-telescopk times, erected at Delhi in India. The right-handpicture shows part uf the building, and a general view of theobservatory is given below. An ancient observatory ofthis kindwas, of course, very different from a modem astronomicalobservatory; all work had to be carried out with the naked eyeonly, and was therefore limited largely to positional measure-ments. Some of the measures made were, nevertheless, ofsurprising accuracy. The Delhi Observatory contained instru-ments which were capable ofyielding eery valuable results.Photographs by W. T. O'Dea

tl=

4 - - - \ >-: L J j - -

1 k. .j i ~ -

M

5 the design of the universe

DEI- HI OBSERVATORY. At* tntftU'T vitW.Photograph by W. T. O'Dea

2ct.nl,

since greek times, when scientists such as Ptolemy had re-jected the idea chat the Earth might move round the Sun, thedevelopment of astronomy had been held up. Aristarchus, whohad hit upon the truth, had met with no support; neither didNikolaus Krebs many centuries later. The man who altered allthis, and finally changed our ideas about the universe, was aPolish cleric named Nicola us Koppcrnigk, better known to us asCopernicus.

Copernicus was born at Thorn, nn the River Vistula, in 1473.He studied at Cracow University, and then in Italy. Later he wentback to his own country, and hecarnc Canon of Fraucnberg inErmland. In addition to his work for the Church he practisedmedicine, but his main interests were astronomical. Yet he was byno means a man who spent hour after hour looking at the stars.He was a theorist first and foremost, and he was concerned mainlywith the design of the Solar System.

Early in his life he became very doubtful whether Ptoh msystem could be correct. The main irouble, as he saw it, was thatthe theory was so complicated. To account for the observedmovements of the planets in the sky il had been necessary to addlarge numbers of small circles, or epicycles, until the scheme hadbecome clumsy and artificial. In science, a simple and straight-forward theory is generally more accurate than a cumbersomeone, and Copernicus looked for some way of avoiding die com-plications which Ptolemy and his followers had been forced tointroduce.

In one way Copernicus was better ofT than Ptolemy; he couldmake use of more accurate measures of the planetary movements,and he could be sure that these measures were not greatly inerror. Finally he came to the conclusion that there was only onesolution, the Earth must no longer be regarded as the centre ofthe universe. If it were assumed that the Earth, together with theother planets, moved round the Sun, then many of the complica-tions would be removed at one stroke.Today, we are so used to thinking of the Sun as the most

important body in the Solar System that we find it hard to con-sider any other idea. Yet Copernicus knew that he was taking 3bold step. He was saying that the astronomy then being taught inall schools and universities was utterly wrong. Worse still, he facedopposition from the Roman Catholic Church, which would cer-tainly object to the idea that our world was not of supremeimportance. Copernicus was himself a priest; and though he hadworked out his Uicory by 1533, and written it down in a bookwhich he called De Revolulionihus Orbium Cattstium ('Concerningthe Revolutions of the Celestial Bodies'), he did not feci inclinedto publish it.

armillary SPHERE, containing twentycircles; ike Jtydiac is clearly shown. From anold print

32 the design of the universe

RF.TRnr.HADE MOVEMENT OF MARS.The apparent path ofMars in the sky is givenat Ihe lop of the diagram, mid the actual rela-tive positions of the Earth and Mars at Iheboltam. It will be seen thai between positions$and ft the Earth catches up Mars and passes it,so thatfor Ihis period Mars seems to move in aretrograde or backward direction among Ihestars. Behavior of this sort was eery difficultto explain an the old theory according to whichIhe Sua moved round the Earth, and was oneof the reasons why Ptolemy wasforced to addjurlher epicycles. Of the planets known inancient limes, Jupiter and Saturn behave insbnitarfashion, but the effects are less obviousbecause both these planets are so muchfartherawayfrom the Sun, and their apparent move-ments in ihe sky are slower

It is important here lo consider the- apparent movements ofihe planets, since this was the basis of all Copernicus' work. Aplanet does not move steadily among ihe stars in a straight line;it may sometimes seem to stand still for a few days, and thenmove 'backwards' for a short period before resuming its wcsl-to-rast journey. This apparent backward motion is known asretrograding.

Mercury and Venus have their own way of behaving, since theyare closer to the Sun than we areand even on Ptolemy's systemthey were assumed to be the closest bodies in the sky apart fromthe Moon. The remaining bright planets, Mars, Jupiter andSaturn, are more remote. Mars is shown in the diagram, but thesame arguments apply to Jupiter and Saturn as well as to Uranus,Neptune and Pluto, which were of course unknown in Copernicus'lime).

For most of the time Mars seems lo move from west to castamong the stars, though the shift is so slow that it can be detectedonly over periods of hours. We now know that it is moving roundthe Sun in a larger orbit than that of the Earth, and also that it istravelling more slowlyonly 15 miles per second, as against18} miles per second for our own world. In the diagram we beginwith the Earth at position 1 and Mars at position 1. By the timethe Earth has moved to 2, Mars has reached a, and so on, Wc cansec thai in positions 4 and 5, Mars is apparently moving acrossthe sky in a retrograde direction, though in fact its real motionround the Sun is unaltered. In positions 6 and 7, the usual wesl-to-east movement has been resumed.A planet which seems to perform a slow 'loop' in the sky was

very hard to explain on Ptolemy's theory, and this was one of thereasons why Copernicus rejected the Earth-centred scheme.Mars is well seen only at intervals separated, on an average, by

780 days (the interval is not quite constant). At such times theEarth is almost directly between Mars and the Sun. Mars there-fore appears opposite to the Sun in the sky, and is said to be atopposition, as shown in the diagram on page 33.

V^-iin wc begin with the Earth al Ei and Mars at Mi thetime of opposition, A year later, the Earth has completed onejourney, and is back al Ei; but Mars, moving more slowly andhaving farther to go, has not made a full circuit, and is in positionMa. The Earth has lo 'caich it up', and does so when the positionsof the two bodies are E2 and M3 respectively, so thai there is an-other opposition. The interval between successive oppositions isknown as the planet's synodic period. When the planet is on the farside of the Sun, it is said to be in conjunction, and is above ihehorizon only during the hours of daylight.

It must not be supposed that Copernicus solved all the pro-blems and drew up a really accurate plan of the Solar System,He certainly look the great step of placing the Sun in the centre,but he still believed that the orbits of the planets (including theEarth) must be circular; a circle was the 'perfect' form, andsurely nothing short of perfection could be allowed in the heavens?This led to new difficulties, and Copernicus was forced to bringback epicycles to account Tor the observed motions or the planets.In fact, he was falling back into the trap which he had tried so

the design of the universe 33

Mars

. -Li , .... *.-^rr-

page from Copernicus' greatbook. A reprint of a typical pagefrom theDe Revolutionibus Orbium CcelestiumofCopernicus, published in 1543

oppositions of mars. When the Earth is at Ei and MarsMi, the Sun, the Earth and Mars are in almost a straight line;Mars is opposite to the Sun in the sky, and is at opposition. Ayear later the Earth has returned to Ei; but Mars, moving moreslowly in a larger orbit, has not completed afull revolution, andlies at M2, so that it is unfavorably placedfor observation. An-other opposition does not occur until the Earth has 'caught Marsup', when the Earth will be at E2 and Mars at Mj. This is whyoppositions of Mars occur only at intervals of about j8o days(the synodic period of Mars)

hard to avoid. He finally reached a scheme according to whichthe Solar System was made up of a central Sun and six planetsmoving round it in circular paths, with the Moon going round theEarth and with the fixed stars beyond the orbit of the mostdistant planet, Saturn.

Copernicus' book was ready, but remained unpublished, simplybecause he knew that it was certain to arouse violent criticismfrom the Church. Many people were aware of its existence,however, and tried hard to make its author give it to the world.Even the Archbishop of Capua, Cardinal von Schonberg, wantedit to be published. Georg Rhsticus, at one time Professor ofMathematics at Wittenberg in Germany, was also highly inter-ested, and went to Frauenberg to hear about the new theory. Hewas forty years younger than Copernicus, and became his pupil,staying at Frauenberg for two years. It appears to have beenlargely because of Rhaticus' urging that Copernicus at last agreedto publish the complete book. Wisely he added a dedicationto the Pope, Paul III, and Rhaeticus took the manuscript toNiirnberg to have it printed.

In 1543 De Revolutionibus appeared, though the publisher,Osiander, had added an announcement to the effect that the Sun-centred theory was merely 'a mathematical fiction' which wouldbe convenient for use in predicting the positions of the planets.Copernicus had not agreed to any such thing, but by now he wasan old man, and seriously ill. In fact it is said, probably withtruth, that the first printed copies of his great work reached himonly a few hours before he died.

It is only too likely that had he lived, Copernicus would havefound himself in serious trouble with the Church. This was whathe had always feared, and his alarm was well-founded. At first onlyRhaeticus and his friend Erasmus Rcinhold, also a Professor ofMathematics in Wittenberg University, dared to come out inopen support of the new theory. The Roman Catholic authoritieswere strongly against it, though the real storm did not break untilover half a century after Copernicus' death.We must admit that Copernicus realized only part of the truth;

he made many mistakes, and parts of his book were unsound.Yet he had found the essential clue, and for this reason alone hemust be regarded as one of the greatest men in the history ofastronomy.

6 the story of Tycho Brahe

FEittcie* TtoioE Brahe Onun D\K1I L1& DC BftfiiHW ST Ma* \hvaoavni; i*^ IH&VLA HEUiOONTl IWOI I NOCC\ FVNIWCRKI l\STR\A\XTlJfr.A\qt' AVlWKCMCCIftV A IAOE*DCmS1M>\ M\EMOHB ET STRVCTCiUSI *.*KnS 5VE.A*0 4Q AWO tKjfffl civvn.

tycho brahe,

the story of Tycho Brahe 35

It also seems that Tycho was a hot-tempered, quarrelsome man.While at Rostock he had a dispute with another Danish nobleman,whose name has not been recorded, and the result was a duel,fought with swords in the middle of the night. The fight endedwhen Tycho had part of his nose cut off. He made himself a newpart with gold, silver and wax, and apparently suffered noill-effects.

So far Tycho's main work had not begun. The start of his truecareer may be said to date from 1572, when a new star appearedin the sky, and held the attention not only of Tycho but also ofastronomers all over the world.One of the most famous of all constellations is Cassiopeia, the

'Lady in the Chair'. It is easy to find, as the Great Bear and Polarismay be used as direction-finders to it; its five chief stars arearranged in the form of a W, and never set over Great Britain. Itwas here that Tycho's Star blazed up, on November 1 1, 1572, andit is worth recording the words of Tycho himself:

In the evening, after sunset, when, according to my habit, I wascontemplating the stars in a clear sky, I noticed that a new and unusualstar, surpassing the other stars in brightness, was shining almost directly

above my head; and since I had, almost from boyhood, known all thestars of the heavens perfectly (there is no great difficulty in attainingthat knowledge), it was quite evident to me that there had never beforebeen any star in that place in the sky, even the smallest, to say nothingof a star so conspicuously bright as this.

Naturally enough he was filled with amazement. It had alwaysbeen thought that the stars were unchanging, and for the momenthe doubted the evidence of his own eyes. When others too saw thestar, he knew that there could be no mistake. He began to makecareful observations, and as the days passed by the star becameeven brighter, until it far outshone even the planet Venus. It wasvisible even during broad daylight. Then, slowly, it faded away.At last it fell below the sixth magnitude; and since telescopes hadnot then been invented, even the keen-eyed Tycho could follow itno further.

We now know what the star was. It was a supernova, a realstellar outburst, sending out as much luminosity as millions ofSuns put together.. Only two other supernova; have been seenin our own star-system during recorded timesthose of 1054,observed by Chinese astronomers, and (probably) the star of1604.

A star, as we know, is a globe made up of intensely hot gas.Normally it shines steadily, and does not alter much over periods ofthousands or even millions of years. In some cases, however, a star

may suffer some tremendous internal disturbance, and will flareup suddenly, increasing its output of light and heat many thou-sandfold before dying back to its original brilliance. When a starbehaves in this manner it is known as a nova, from the Latin wordfor 'new'. (The name is rather misleading, since a nova is not infact a completely new star.) Normal nova? are not particularlyuncommon. Several have been seen during the past few years;there was a bright one in 1934, and others in 1936, 1946 and i960,while fainter nova; are fairly frequent.

position of tycho's star. Tychomade careful observations of the brilliantsupernova of 1572, which appeared inCassiopeia not far from the famous 'W ofstars. This was the brightest supernova of thepast thousand years. Only two others havebeen seen in our own Galaxy during this time;the 1054 star was certainly a supernova, andKepler's Star of 1604 may also have been

SEXTANT USED BY TYCHO BRAHE.This sextant was in use by Tycho in 1577. Itwas one of the elaborate instruments whichwere set up on the island of Hven, whereTycho workedfor so many years

36 the story of Tycho Brahe

THE OBSERVATORY AT HVEN. Tycho'sgreat observatory where most ofhis main workwas carried out. After Tycho left Denmark, r59^> fh* observatory was never used again,andfell into ruins

{>.*: I

-'

-IS 'J ,-:-'?

''Mi ISVs .

l&1 I!W

J^uTycho's Star was an outburst on a much grander scale. Evid-

ently the explosion more or less destroyed the original star andhurled gaseous material into space in all directions. Today thereis no visible trace of the original star itself, though our radiotelescopes can pick up radiations which may possibly come fromthe remnants of the expelled gas.Tycho knew very little about the nature of the stars, and he

could not give a satisfactory reason for such an outburst, but atleast the supernova made him determined to spend the rest of hislife studying astronomy. He wrote a book about the star, De NovaStella, and this book made him well known. By this time he wasmarried, and he considered settling down in Basel, but then hereceived a generous offer from King Frederick II of Denmark.Frederick wanted Tycho to stay in Denmark, and granted himthe little island of Hven in the Baltic, between Elsinore andCopenhagen, together with enough money to build an observatoryand pay for its upkeep.Tycho was quick to accept, and in 1576 he began the construc-

tion of his observatoryUraniborg, the 'Castle of the Heavens'.t lay in the middle of a large square enclosure laid out a"s a

garden, the corners of which pointed north, south, east and west.It contained a library and a chemical laboratory as well as

QUADRANT USED BY TYCHO BRAHE. One of thegreat quadrants used by Tycho at Hvenfor his measures ofstar positions

the story of Tycho Brahc 37

ALTITUDE INSTRUMENT USED BYtycho brahe. This was yet another ofthe complex instruments made by Tycho andused at the observatory on Hven

stjerneborg. This is a recent view ofStjerneborg, the second of Tycho's twoobservatories on Hven. The buildings shownare of course modern, since nothing now re-

mains of the original observatory. Photograph

by Gbsta Persson, /95S

living apartments and the rooms for the instruments themselves.Later, in 1584, he built a second 'Castle of the Stars', Stjerneborg,in which some of the instruments were located below ground-level. The reason for this seems to have been that Tycho experi-enced trouble when the wind blew strongly and shook the instru-ments lying above ground. He also added a printing press and apaper-mill. Hven became a hive of scientific activity, and manydistinguished people from all over the world visited itamongthem James VI of Scotland, who afterwards became James I ofEngland.Tycho lived in magnificent style, and those who visited him

were royally entertained. Banquets, games and hunts were held,and it is said that the guests were entertained by a dwarf whomTycho kept specially for the purpose. On the other hand theislanders themselves were not well treated; Tycho was a harshlandlord. One of the less welcome buildings at Uraniborg was aprison in which he used to lock up those who would not pay theirrents, or who displeased him in other ways.The instruments themselves were by far the best of their time,

and since Tycho was a most careful and accurate observer heobtained excellent results. He measured the positions of 777 stars,and drew up a catalogue; it is said that his star positions werenever in error by more than 1 or 2 minutes of arc. When weremember that he had no telescopes, and that all his work had tobe done with instruments without lenses, we can sec how good anastronomer he must have been. He was still enthusiastic aboutastrology, however, and never began observing without dressinghimself in special robes.As well as drawing up his star catalogue Tycho measured the

apparent movements of the planets, and it was these observationswhich proved to be so useful later on. He was also interested inthe brilliant comet which appeared in 1557, and proved that itmust be much more distant than the Moon. This was a step for-ward, since up to that time it was still thought possible that

comets might be near at hand, and perhaps contained in theupper part of the Earth's atmosphere.Tycho regarded Copernicus' theory that the Earth could

revolve round the Sun as heretical. On the other hand he knewquite well that the movements of the planets could not be explained

on Ptolemy's theory, and he suggested instead that the planetsrevolved round the Sun, while the Sun and Moon revolved roundthe Earth. This was not an entirely new idea, but it satisfied very

few people apart from Tycho himself.King Frederick of Denmark died in 1588, and in 1594 Tycho

lost another of his supporters, the Danish Chancellor Kaas.

Unfortunately he had made himself unpopular everywhere; theislanders of Hven hated him (as they had good reason to do), andthe new Royal Court was much less friendly to him than the oldone had been. Tycho would never see that he could be in thewrong, and eventually his supplies of money were cut off. In 1596,after having worked at Hven for twenty years, he left Denmark inanger, taking with him the more portable instruments fromUraniborg and Stjerneborg, and went to Germany. He neverreturned to his own country.

38 the story of Tycho Brahe

FRONTISPIECE OFTHE RUDOLPHINEtables. These tablesrepresented Kepler's last

astronomical work, andwere published shortly be-fore his death. Kepler's

acknowledgement to Tychois prominently featured onthe title-page. The tableswere so named in honorof Kepler's old benefactor,the Holy Roman EmperorRudolph II, who wasinterested mainly in mysti-cism and astrology, butwho also encouraged astro-nomical science. Rudolphhimself had of coursebeen dead for many years'by the time that the tablesappeared

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128 explorers of the Moon

lunar craters, photographed with the200-inch Hale reflector at Palomar. Amongthose shown are Copernicus (mid right),Stadius (near middle) and Eratosthenes(lower left). The photograph was takenunder low light, so that the Copernicus brightrays are not visible

**rffi mm

>,

copernicus, photographed at MountWilson. This time the crater is shown underhigh light, and the ray-system is prominent

telescopes, and although the evidence in favor of change isreasonably strong it is not conclusive.Schmidt was a German, but spent many years in Greece, and

was Director of the Athens Observatory. He produced a large andgood lunar map in 1878, and others also made their appearance.Edmund Ncison completed one in 1876, Walter Goodacreanother in 19 10, and so on. Later a detailed chart was preparedby Philipp Fauth of Germany, who established a private observa-tory and whose book, Unser Mond, was issued in 1936.The lastso far, at leastof the great amateur observers of the

Moon was a Welshman, Hugh Percival Wilkins, who was born in1896, and became an engineer. He began lunar work in 1909, andlater moved to Kent, where he set up a 15^-inch reflector. He gaveup practical engineering in 1940 to enter the Ministry of Supply,but his main interest was always in astronomy.

Using his own observations, together with photographs takenwith large instruments, Wilkins completed a lunar chart 300inches across. It shows a tremendous amount of detail, and it isnot surprising that the work took him half a century. He retiredfrom the Ministry ofSupply at the end of 1959 in order to spend therest of his life in revising his map, and it was particularly tragicthat he died of a heart attack in the following January.Lunar photography began in 1840, but Arago was wrong when

he said that it would quickly become possible to map the wholeMoon 'in a few minutes'. The changing angle of sunlight causesremarkable apparent changes in the lunar landscape; a craterwhich is shadow-filled and very prominent one night may behardly recognizable twenty-four hours later, particularly if itswalls arc low and incomplete. Various photographic atlases havebeen produced. Some of them are very valuable, and a new set ofphotographic charts has been produced in America by a teamof observers led by G. P. Kuiper and E. A. Whitaker.

Particular mention must be made of the splendid photographstaken by French astronomers at the Pic du Midi. This observa-tory, in the Pyrenees between France and Spain, lies nearly10,000 feet above sea-level, and tht atmosphere there is particu-larly clear. Bernard Lyot, Audouin Dollfus and others have pro-duced pictures which are probably the best ever taken, though thetelescope used, a 24-inch refractor, is much smaller than the giantAmerican reflectors. (Only two lunar photographs have beentaken with the Palomar 200-inch, both of which are reproducedhere, though the 60-inch at Mount Wilson has been more exten-sively used for lunar study.)

Charting the Moon is fascinating work, but it is only a means toan end. We want to know what sort of a world the Moon really is,and wc want to know something about its past history. Forinstance, how were the craters formed?

Originally they were thought to be volcanic, but they are not inthe least like Earth volcanoes such as Vesuvius. Two Englishastronomers of the nineteenth century, James Nasmyth (alsofamous as the inventor of the steam-hammer) andJames Carpenter,wrote a book in which they outlined a 'fiery fountain' theoryaccording to which the walls of a crater might be producedby material spurted out of the central peak. This idea has been

proved to be wrong. There are many reasons for its rejection, oneof which is that the walls of a lunar crater are often very massiveand there is no known case in which a central peak rises as high asthe top of the rampart.Another theory, still popular, was proposed by a German

astronomer named Franz von Paula Gruithuisen. According toGruithuisen, the Moon was once bombarded by pieces of materialfrom spacemeteors, in factwhich left the scars we still see ascraters. Here too there are difficulties to be faced. The lunarsurface is not uniformly peppered with craters; there are certainlaws about distribution. For instance, when one crater breaks intoanother, as often happens, it is always the smaller formation whichruins the larger. It is hardly likely that all the larger meteors fell

first, but the difficulty disappears if we suppose the craters to

be volcanic. As the Moon aged, and became less active, thecraters produced would be smaller.The 'ray' craters, such as Tycho, are particularly significant.

On the meteor theory they must be the youngest formations of all,since the rays pass over all other featurescraters, mountains,valleys and the rest. Yet Tycho, for instance, is over 50 milesacross. If it were produced by a meteor, it would be expected tocause a 'moonquake' which would shake down the walls of anycraters already existing in the area. Nothing of the kind is seen, andsome astronomers believe that the formation of the craters wasmuch less violent, due perhaps to more gradual lifting-up and latersinking of the Moon's crust.

Then, too, there are formations on the Moon which lookremarkably like Earth volcanoes. I have made a close study ofthese, and have listed well over fifty, which shows that they aremuch commoner than used to be believed.Up to 1958 there was no evidence for any activity on the Moon

apart from the rather doubtful case of the change in Linne.Then, however, a remarkable observation was made by NikolaiKozirev, a Russian astronomer working at the Crimean Observa-tory with the 50-inch reflector there.On the night of November 3-4, Kozirev was studying the area

of Alphonsus, which has high walls and a central elevation.Suddenly he saw a reddish 'cloud' close to the central peak, whichmoved slowly but perceptibly.

Kozirev was equal to the occasion. The telescope was equippedwith a spectrograph, used for photographing spectra, and he at oncebrought it into action. Though the 'cloud' lasted for less than halfan hour, the photographs showed that there had been a rise intemperature of at least 2,000 degrees, and that hot carbon gas hadbeen sent out. In fact there had been a volcanic disturbance.The observation was unexpected, and caused a great deal of

argument, but the photographs could not be questioned, and itseems that there must be at least isolated pockets ofheat below theMoon's crust. On October 23, 1959, Kozirev again reported minoractivity in the area, but much less definitely. I was observing thearea at this time, but could see nothing unusual.Though Kozirev's work has shown that the Moon is not com-

pletely inert, we must agree that any lunar activity is on a very

minor scale. No great craters have been produced for many

tt$to^^Q^Kh

iHOVOf

THE LUNAR CRATERS ARZACHEL (upper), ALPETRAGIUS (right) ANDalphonsus (lower), photographed by D. Alter with the 6o-inch reflector at MountWilson Observatory

explorers of the Moon 131

OCCULTATION OF VENUS BY THEmoon, December 20, 1934, 16.30. Photo-graphed by K. Suguki with the 8-inch refrac-tor at the Tokyo Museum of Science

mwfmw* A ' '

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millions of years

perhaps hundreds of millions of years. Fallingmeteors must occasionally produce pits, but that is all.One of the most obvious facts about the Moon is that it shows us

no definite color. Reds, greens and blues are absent, and every-thing seems to be either yellowish-grey or else black shadow. Itis also interesting to find that the Moon reflects only 7 per cent ofthe sunlight falling upon it, and is the poorest reflector of all thebodies in the Solar System.

It may be that the whole surface is coated with dust or ash. Onemodern astronomer, T. Gold, has suggested that the dust may behundreds of feet deep, so that, in his words, 'space-travellers of thefuture will simply sink into the dust with their gear'. Not manyauthorities agree, and it is more generally thought that any dustyor ashy layer is only an inch or two in depth; but we do not reallyknow. The Russian astronomer Nikolai Barabashov believesthe Moon's surface to be covered with a layer of crushed rockystuff not more than three centimetres thick.

Another problem concerns the existence or non-existence of anatmosphere round the Moon. Here again ideas have changedconsiderably during the last few years.There is an easy way to show that the Moon cannot have a

dense 'air'. As it passes across the sky, the Moon sometimes hidesor occults a star; these occultations can be watched with smalltelescopes (provided that the star concerned is a bright one), andare fascinating. The star shines steadily until the very momentwhen the Moon's limb sweeps over it. When this happens, thestar snaps out suddenly. One moment it is there; the next it isnot.

If the Moon were surrounded by a dense atmosphere, the starwould flicker and fade for some seconds before being hidden.Nothing of the sort has been recorded, which is enough to provethat even if the Moon has any air at all, the density must below.

In 1949 a Russian astronomer, Y. N. Lipski, announced thatby an indirect method he had found a lunar atmosphere with aground density of about one-thousandth of that of the Earth.This value is low, and corresponds to what we normally call avacuum, but it now seems that the density is even less than this

perhaps one-hundred-thousandth of that of the Earth's atmosphereat sea-level. To all intents and purposes, then, the Moon is an'airless' world, though the term must not be taken too literally,and probably there is just a trace of a gaseous mantle.We know that the Moon takes 27J days to spin once on its

-the same time that it takes to go once round the Earth.axis-

THE SOUTHERN UPLANDS OF THEmoon. Tycho is seen near the centre of thepicture. Lick Observatory photograph

This means that to anyone standing on the lunar surface, theSun would rise very slowly, and would not set again for a periodequal to almost two Earth weeks. Since there is practically noatmosphere to shield the surface, the heat would be intense.On the Moon's equator, the moon temperature reaches over2 1 6 degrees Fahrenheit, hotter than the boiling-point of water,though at midnight it drops to 250 degrees below zero. (Nearerthe poles the day temperature is much less, since the sunlightfalls at a lower angle.)

Suppose we could go to the Moon? We would find a strange

132 explorers of the Moon

the sinus iridum (Bay of Rainbows),on the border of the Mare Imbrium. This isone of the loveliest formations on the wholeMoon

THE LUNAR CRATER CLAVIUS,photographed with the 200-inch Hale reflectorat Palomar. Clavius is one of the very largestof the Moon's craters, and when on the termi-nator is clearly visible to the naked eye. Thechain of craters inside it is particularlynoticeable. Clavius is prominent even thoughit lies in an extremely crowded region of thelunar surface; its diameter is notfar short of150 miles

world. The sky would be black, since there is no air to scatter thesunlight, and the Earth would be a brilliant object during the longnight, flooding the barren rocks with radiance. The lack ofatmosphere means that there could be no sound; sound-waves arecarried by air, and the airless Moon is plunged in eternal silence.The horizon would be relatively close, and we would be facedwith a landscape pitted with cratcrlets and broken by moundsand peaks. Nowhere would there be any movement; nowherewould there be any life.One last problem concerns the origin of the Moon. In 1898 the

English mathematician George Darwinson of the famousbiologist Charles Darwin, always associated with the theory ofevolution

put forward a theory according to which the Earthand Moon used to be one body, and that the Moon broke away,leaving a scar in the terrestrial crust which is now filled by thePacific Ocean. It is now known that this idea is not correct.Earth and Moon were never one; they were formed at about thesame time, and in the same way. It is better to regard the Earth-Moon system as a double planet rather than as a planet and asatellite.

Though the Moon is our nearest natural neighbor in space, wecannot yet claim to know all about it; our knowledge will be com-plete only when we manage to send space-ships to it. Meanwhileastronomers will go on studying it with their telescopes, just asSchroter, Madler, Schmidt and other great observers used to do inthe past.

19 the Sun's family

map of mercury by G. V. SchiaparelH

map of mercury by E. M. Antoniadidrawn from observations made with theMeudon 33-inch refractor. Antoniadi's nomen-clature is now in general use

phases of mercury. The Earth isassumed to be below the bottom of the page.When at its closest, and therefore largest,Mercury is 'new*, and cannot be seen exceptwhen it is in transit across theface of the Sun

the sun has an interesting family. Its senior members arethe nine planets, all of which have their own special features.We know a great deal about the third planet in order of distance,since it is the Earth on which we live, but with regard to the restour knowledge is still far from satisfactory.Any small modern telescope will show the phases of Venus, the

dark patches and white caps of Mars, the moons of Jupiter andthe rings of Saturn. With larger telescopes, such as are possessedby many serious amateur observers, it is possible to do reallyuseful work

particularly since the great reflectors at Palomarand elsewhere are kept busy on their studies of distant stars.

First, then, let us look briefly at each planet in turn, beginningwith Mercury and working our way outward from the Sun.The trouble about Mercury is that it is relatively close to the

Sun, and never becomes conspicuous. It is always somewhere nearthe Sun in the sky, and is at its highest during daylight, when it isinvisible without a good telescope equipped with setting circles.There is a story that Copernicus never saw Mercury in his life,owing to mists rising from the River Vistula near his home. Thisis probably 'just another tale'after all, Copernicus spent sometime in Italy, where there is little mist or fogbut we have toagree that Mercury is not easy to find with the naked eye. Whenvisible at all, it appears either low in the west after sunset, or low

in the east before sunrise. At its best it looks like a rather pinkishist-magnitudc star.Mercury often twinkles. In the ordinary way a planet twinkles

much less than a star, because it shows a small disk instead ofbeing a mere point of light; twinkling, of course, is due entirelyto the unsteadiness of the Earth's air. Mercury, however, is nevervisible to the naked eye at all except when near the horizon, so thatit is shining through a thicker layer of atmosphere.

Small or even moderate-sized telescopes will show little onMercury, though the phase is easy enough to see. Schrotcr drew it

134 the Sun's family

mercury, April 23, 1936. 15 h., T8'5 in.reflector, power 4J0. Drawing by PatrickMoore

old drawings of venu s by Biaiichiniand Schroter. The markings recorded byBiaiichini, at least, are certainly illusory

often, and also tried to draw a map of the surface markings. Heeven believed that he had detected a mountain 1 1 miles high.This was one of his mistakes; there may well be peaks on Mercury,but it is not likely that they reach so great an altitude.The first reasonably good map was drawn by an Italian astrono-

mer, Giovanni Schiaparelli, more than half a century afterSchroter's time. Schiaparelli, born in 1835, became Director ofthe Brera Observatory, Milan, in i860, and resigned only whenhis eyesight began to fail him; it is tragic to record that he wasblind for several years before his death in 1910.

Schiaparelli was an excellent observer. He decided to studyMercury during the daytime, with the Sun well above the horizon,since the planet would then be higher up and its image would besteadier. He managed to draw various dark patches on the surface,and gave them names. He also found that the axial rotationperiod is equal to the revolution period88 Earth-days. Justas the Moon keeps the same face permanently towards the Earth,so Mercury keeps the same side turned towards the Sun. Thismeans that part of Mercury has permanent 'day', and must beextremely hot, with a temperature ofover 700 degrees Fahrenheit.On the opposite side, over which the Sun never rises, the coldmust be intense.However, Mercury's path round the Sun is more eccentric than

that of the Earth. There are effects similar to the librations of theMoon, and the result is that over a certain area of the planet thereis alternate day and night. Astronomers have nicknamed this areathe 'Twilight Zone'. Unfortunately we cannot find out its extent,because our maps of Mercury are still very approximate.

Schiaparelli was followed by Eugene Antoniadi, who wasGreek by descent but who spent most of his life in France. Anto-niadi mapped Mercury with the help of the 33-inch Meudonrefractor, and his chart, published in 1933, remains the best wehave. The names given to the various features are due toAntoniadi; the largest dark area is called 'Solitudo HermaeTristmegisti'the Wilderness of Hermes (Mercury) the ThriceGreatest.

Mercury is not a great deal larger than the Moon, though moremassive, and it has practically no atmosphere. In 1950 AudouinDollfus, at the high-altitude Pic du Midi Observatory, announcedthat he had found a very thin mantle, but the ground densitycannot be more than one-three-hundredth of that of the Earth'satmosphere at sea-level.Mercury must be the most unfriendly of worlds. Part of it is

scorched, part of it chilled; it is almost airless, and we cannotimagine that any sort of life can exist there.

Venus, the second planet reckoning outward from the Sun, isvery different. It is about the same size as the Earth; indeed, if itis represented by a billiard-ball, the Earth would be another ballso similar to it that the two would have to be carefully weighed tofind out which was which. Venus has a diameter of 7,700 milesas against 7,926 for the Earth.

Like Mercury, Venus shows phases from new to full, since it iscloser to the Sun than we are. (Galileo, as we have seen, used thesephases to prove the truth of the Copernican theory.) Yet, very little

drawing of v enus , made on September'> '959i by W. M. Baxter with a 4-inchrefractor. At this time Venus was practicallyat inferior conjunction. The curve of theterminator is correctly shown as smooth; theserrations drawn by older observers such asSchroler are due to ejects of the Earth'satmosphere, and have nothing directly to dowith Venus itself

six photographs of venus, takenwith the 100-inch reflector at Mount Wilson

the Sun's family 135

else can be made out. As both Schroter and Herschel realized,Venus is covered with atmosphere, and this atmosphere is so'cloudy' that we cannot see through it. Nobody has yet glimpsedthe true surface of Venus.

Schiaparelli believed that Venus behaved in the same way asMercury, keeping the same face permanently sunward. In thiscase the axial rotation period would be 224! Earth-days, which isthe time which Venus takes to go once round the Sun, and therewould be no true 'day' or 'night' there. However, this seems to bewrong. We still do not know the length of Venus' rotation, but itmay be about as long as a terrestrial month, though Russian workcarried out in 1961 has yielded a value of only 10 days.We can at least measure the temperature of the upper part of

the cloud-layer which covers Venus, and between 1923 and 1928two astronomers, Edison Pettit and Seth B. Nicholson, obtainedgood results with the help of the 100-inch Hooker reflector atMount Wilson. They used an instrument known as a thermocouple.A thermocouple consists ofa circuit made up oftwo different wires,soldered end to end. If one of the joins is warmed an electriccurrent will be set up, and the amount of the current is a key to therise in temperature responsible for it. By using the 100-inch reflec-tor to focus the tiny quantity of heat from Venus, Pettit andNicholson found that the upper atmosphere of the planet was wellbelow freezing-point. Later results by W. M. Sinton and J. Strongshow that the temperature there is about 40 degrees Centigrade,

136 the Sun's family

APPARENT SIZE OF VENUS, from WWtofull

^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

and there is little difference between the 'day' and 'night' sides,which seems to indicate that Venus does not keep the same faceturned to the Sun all the time.Used together with a large telescope, a thermocouple is remark-

ably sensitive. To detect the heat of a candle 1,000 miles awaywould be quite possible, and there is no reason to suppose that thevalues given for Venus are very far wrong.

1 1932, also at Mount Wilson, W. S. Adams and TheodoreDunham analyzed the atmosphere of Venus, using powerfulspectroscopes fitted to the 100-inch. The results were rather sur-prising. Most of the Earth's air is made up of the two gases oxygenand nitrogen, but that of Venus proved to be composed chiefly ofcarbon dioxide. There seemed to be so little free oxygen that Adamsand Dunham could not detect it at all.Up to then it had been generally thought that Venus must be a

moist world, not unlike the Earth must have been perhaps 250million years ago. Svante Arrhenius, a Swedish physicist, hadsuggested that 'on Venus everything is dripping wet', and hethought that there might be vegetation, together with primitivecreatures such as amphibians or even reptiles. But when it becameclear that the atmosphere contains so much carbon dioxide,astronomers thought again. Carbon dioxide, they pointed out,tends to blanket in the Sun's heat, in the manner of a greenhouse.The surface, then, must be fiercely hot, and probably a barrendust-desert.

The Earth's atmosphere contains a high amount of water vapor,

photograph of venus, taken onFebruary ig, 1961, by H. E. Dall with hisij\-inch reflector

venus. From observations by Patrick Moore. (Upper) kJuly 17, igjg, i6h. 55m., 24-inch reflector X 350. fThe southern cusp-cap is clearly shown. (Lower) January3, ig$8, i6h. 30m., 12-fj-inch reflector X 250. TheAshen J.ight is shown, but has been somewhat exaggeratedfor the sake of clarity. Drawing by D. A. Hardy

138 the Sun's family

APPARENT OPPOSITION SIZE OFmars. The maximum size far each opposi-tion between 1954 and sgjr is shown

OPPOSITIONS Of MARS, 1956-71. Itwill he mm that the most unfavorableoppositions ate those of 1963 and /goj

and this makes it hard to delect water vapor in the clouds ofVenus. In late 1959, two Americans, Commander Ross (the pilot)and C. B. Moore, wrnt up irv a balloon, and look instruments withthem. They were able to study Venus from a high altitude, abovemuch of the water-vapor in the Earth's air, and they were able toshow that there is moisture over Venus also. There may, in faet> bejust as much as there is in our own clouds. Recent studies by ayoung English astronomer, B, Warner, indicate that the atmosphereof Venus may also contain some free oxygen.

At the moment there arc two main theories ofVenus, In spite ofthe Ross-Moore results, ihe dust-desert idea still has its supporters.More likely,, however, is a suggestion by F. L. Whipple and D. H.Menzel that the clouds consist of H^O and that Venus is largelycovered with water.

Another problem of Venus concerns the so-called Ashen Light,or faint luminosity of the "dark" side when Venus is a crescent.There is a similar effect in