Table Of ContentChapter 1
Naked eye observations
1.1 Introduction
Theetymologyoftheword‘Astronomy’impliesthatitwasthe disciplineinvolvedin ‘thearranging
of the stars’. Today we might say that astronomy is our attempt to study and understand celestial
phenomena, part of the never-ending urge to discover order in nature. We do not know who were
thefirstastronomers—whatwedoknowisthatthescienceofastronomywaswelladvancedinparts
of Europe by the middle of the third millennium BC and that the Chinese people had astronomical
schoolsasearlyas2000BC. Inallages, fromthe burgeoningof man’sintelligence,therehavebeen
people fascinated by the heavensand their changingaspect and these people, as far as their cultural
environmenthasallowedthem,havetriedtoformulatecosmologies.Wearenodifferenttoday.
Nowadays,the word‘Astrophysics’isalso usedtodescribethe studyofthecelestialbodies. In
fact, many astronomers use both terms quite generally and it is not infrequent to find Departments
of Astronomyand Astrophysicswithin educationalestablishments. Thequestion maywell be asked
‘What is the difference between Astronomy and Astrophysics?’ Very loosely, Astronomy might be
definedasthe subjectof the‘where andwhen’ relatedto the descriptionof a celestialbodywith the
‘whyandhow’beingcoveredmorebyAstrophysics.Ratherthantryingtoprovideahardandfastrule
fortheterminology,wewillsimplyuseAstronomytocoverallaspectsofthedescriptionoftheskies
andtheUniverse.
IfourcurrenttheoriesoftheUniversearenearerthetruth,itisprobablynotthatourintelligence
hasincreasedinthepastsixmillennia.Itismorelikelythatthemainfactorhasbeenthediscoveryand
developmentofthe‘scientificmethod’,whichhasledtoourpresentcivilizationbasedonthefloodof
technologicaladvantagesprovidedbythismethod. Thishasenabledscientistsinfargreaternumbers
than ever before to devote their lives to the study of the heavens, assisted by telescopes, computers,
space vehicles and a multitude of other equipment. Their attempts to interpret and understand the
wealthofnewinformationprovidedbythesenewinstrumentshavebeenaidedbyalliedsciencessuch
asphysics,chemistry,geology,mathematicsandsoon.
We must remember, however, that for more than nine-tenths of the last five thousand years of
our study of the heavens, we have had to rely on the unaided eye. The Mediterranean people who
set the constellations in the sky, the Babylonians, Egyptians and Greeks, the Arabian astronomers
whoflourishedduringtheDarkAgesofPost-RomanEurope,theChinese,theMayanandotherearly
Americanastronomers,allbuilttheirtheoriesoftheUniverseonnakedeyeobservations. Andsowe
beginby followingin their footstepsand seeing whatthey saw as they observedovera few minutes
(see section1.2),overafewhours(seesection1.3),overa month(see section1.4)oroveratleast a
year(seesection1.5).Inthisway,wewillfinditeasiertounderstandwhytheircosmologicaltheories
wereformulatedintheirparticularways.
3
4 Nakedeyeobservations
1.2 Instantaneous phenomena
1.2.1 Day
Duringthedayavarietyofphenomenamaybeseen. InaparticulardirectionliestheSun,sobrightit
isimpossible(anddangerous)tolookdirectlyatit. Ingeneral,theskybackgroundisblue. TheMoon
mayalsobevisible,havingadistinctshapethoughcertainlynotcircular. IftheSunhasjustsetorif
dawnisnotfaraway,thereissufficientdaylighttoseeclearly.Wecallthisconditiontwilight.
On the horizon opposite to the twilight glow, a dark purple band is sometimes seen. This area
correspondstoazoneontheskywhichiscutofffromthedirectsunlightbytheEarthandisreceiving
verylittlelightbyscatteringfromtheatomsandmoleculesintheatmosphere. Itcorresponds,infact,
totheshadowoftheEarthinthesky. Itspresencetellsusoftheextremepurityandlowhumidityof
thelocalatmosphere.Needlesstosay,itisveryrarelyseeninBritain.
To the ancients, clouds, wind, rain, hail and other atmospheric phenomena were inadequately
distinguished from what we term celestial events. Our civilization includesthem in meteorology, a
sciencequitedistinctfromastronomy,sothatweneednotconsiderthemfurther,excepttoremarkthat
astronomers’observationshave,untilrecently,beendependententirelyupongoodweatherconditions
being available. With the developmentof radio telescopes and the fact that other equipmentcan be
placedinartificialsatellitesandoperatedabovetheEarth’satmosphere,thisdependenceisnolonger
complete.
1.2.2 Night
If seeing conditions are favourable, a view of the night sky providesa far wider variety of celestial
phenomena.IftheMoonisvisible,itsbrightnesswilldominatethatofallotherobjects. Itsshapewill
becrescentorgibbousorevencircular.Atthelastcondition,itsapparentdiameterisveryclosetothat
oftheSun.Toanyonewithreasonableeyesight,itssurfacewillnotbeevenlybright.Areasdarkerthan
theirsurroundingswillbenoticed,sothatthefancyofprimitivemancouldseea‘ManintheMoon’,a
‘BeautifulLady’ora‘Rabbit’,sketchedoutbythesefeatures.
InadditiontotheMoon,sometwotothreethousandtiny,twinklingpointsoflight—thestars—are
seen,ranginginbrightnessfromoneseasilyvisiblejustaftersunsettoonesjustvisiblewhentheMoon
is below the horizonand the sky backgroundis darkest. Carefulcomparisonof one brightstar with
another shows that stars have differentcolours; for example, in the star pattern of Orion, one of the
manyconstellations,BetelgeuseisaredstarincontrasttotheblueofRigel. Theapparentdistribution
ofstarsacrossthevaultofheavenseemsrandom.
Withtheeyesbecomingaccustomedtothedarkness,afaintbandoflight,theMilkyWay,catches
the observer’s attention. Modern astronomers, with the aid of telescopes, know that this luminous
region stretching from horizonto horizonacross the sky in a great circle is made up of a myriad of
starstoofainttoberesolvedwiththenakedeye.Totheancientobserver,itspresenceinspiredallkinds
ofspeculations,noneofthemverifiable.
Oneortwoofthetinypointsoflightmaydrawacloserscrutiny. Theyshinesteadily,incontrast
to the twinklingof the starsandtheyare amongthe brightestofthe star-like objects. Theremustbe
somereasonwhytheyaredifferent.Ifourobserverisgoingtowatchforafewhours,attentionwillbe
returnedtotheseobjects.
1.3 A few hours
1.3.1 Day
Theheavensareneverstatic. The slowly-movingshadowcastbyan uprightrodora boulderortree
reveals the Sun’s movementacross the sky. If observation is kept up throughoutthe day, the Sun is
Amonth 5
seentoriseabovetheeasternhorizon,climbuptheskyinacircleinclinedatsomeangletotheplane
definedbythehorizonandculminate,i.e.reachamaximumaltitudeabovethelinejoiningthenorth
tothesouthpoints,thendescendinamirrorimageofitsforenoonpathtosetonthewesternhorizon.
IftheMoonisseenduringthatday,itwillappeartoimitatetheSun’sbehaviourinrisingandsetting.
1.3.2 Night
Asdarknessfalls,thefirststarsbecomevisibleabovetheeasternhorizon. Withtheendingoftwilight
the fainter stars can be seen and, as the hours pass, the stellar groupsrise from the eastern horizon,
reachtheir maximumaltitudelike the Sun, thenset orbecomedim andinvisibleas daylightreturns.
The impression of being on a flat plane surmountedby a darkrevolvingbowl to which the stars are
attachedisstrong,especiallywhenitisseenthattherearemanystarsinaparticularregionofthesky
thatrevolve,neverrising,neversetting,aboutahuborpivot. Thesestarsaresaidtobecircumpolar.
Itis then clear thatthose otherstars thatrise andset do so simply becausetheir circularpathsabout
thispolearesobigthattheyintersectthehorizon.
TheMoonalsorevolvesacrossthisupturnedbowl.AlthoughtheMoonappearstohaveanangular
motionacrossthe skysimilar to thatof thestars, carefulobservationovera few hoursrevealsthatit
movesslightlyeastwardsrelativetothestarbackground.
Occasionally a brightobject, called a meteor, shootsacross the sky in a second, lookinglike a
fast-movingor ‘falling star’. It may be too that faintly luminous sheets are seen, hangingdown the
bowloftheheavenslikegreatcurtains. ThesearetheauroraeW1.1.
Ifourobserveriswatchingatanytime afterOctober4, 1957,it isquitelikelythatoneormore
faintspecksoflightwillbeseentocrossthesky,takingafewminutestodoso,theirpresencegiving
reminderthatman-madesatellitesarenowinorbitabouttheEarth.Indeed,oneofthelatestsatellites—
theInternationalSpaceStationW1.2—isexceedinglybright—asbrightasthebrightestplanetVenus—
andbearstestamenttothecontinualdevelopmentofmannedorbitinglaboratories.
1.4 A month
Themonthisthenextperiodofanysignificancetoourwatcher. Duringthistime,theideasaboutthe
heavensand their movementschange. It will be noted that after a few nightsthe first groupof stars
seen above the eastern horizon just after sunset is markedly higher at first sight, with other groups
underitbecomingthefirststarstoappear.Indeed,afteramonth,thefirstgroupisaboutthirtydegrees
abovetheeasternhorizonwhenthefirststarsareseenaftersunset.ItisthenapparentthattheSunmust
shiftitspositionagainstthestellarbackgroundastimepasses. Therateisslow(aboutonedegreeper
day—orabouttwoapparentsolardiameters)comparedwithitsdaily,ordiurnal,movementaboutthe
Earth.
The Sun is not the only object to move independently of the stellar patterns. A few nights’
observationsoftheMoon’spositionagainstthestars(itssiderealposition)showthatittoomovesbut
atamuchfasterrate,aboutthirteendegreesperday,sothatitisseentomakeonecompleterevolution
of the stellar background in twenty-seven and one-third days, returning to the same constellation it
occupiedat the beginningof the month. In addition, its shape changes. From a thin crescent, like a
reversed ‘C’, seen in the west just after sunset, it progresses to the phase we call first quarter about
seven dayslater. At this phase, the Moon’sterminatoris seen to be almost a straightline. Fourteen
daysafternewmoon,itisfullandatitsbrightest,appearingatitshighestintheskyaboutmidnight.
Seven days later it has dwindled to third quarter and rises before the Sun, a pale thin crescent once
more,amirrorimageofitsphasejustafternewmoon.Twenty-nineandone-halfdaysafternewmoon,
itisnewoncemore.
ItwasafairlyeasymatterfortheancientstoascertainthattheMoonwasnearertheEarththanthe
stars. FrequentlytheMoonwasseentoblotoutastar,occultingituntilitreappearedattheotheredge
6 Nakedeyeobservations
Figure1.1.Thechangeinlengthofashadowaccordingtothetimeofdayandthetimeofyear.
oftheMoon’sdisc. AndoccasionallytheMoonwaseclipsed,theEarthprogressivelyblockingoffthe
sunlightuntilthesatellite’sbrightnesshaddiminishedtoadull,copperyhue. Anevenmorealarming,
butrarer,occurrencetookplaceattimesduringdaylight: theMoonrevealeditsunseenpresencenear
theSunbyeclipsingthesolardisc,turningdayintonight,causingbirdstoseektheirnestsandcreating
superstitiousfearinthemindofprimitiveman.
Theobserverwhostudiesthenightskyforamonthorsoalsodiscoverssomethingnewaboutthe
oneortwostar-likeobjectsnotedthatdonottwinkle. Carefulmarkingoftheirpositionswithrespect
to neighbouringstarsshowsthattheytooare movingagainstthe stellar background. Theredoesnot
seemtobemuchsystem,however,aboutthesemovements. Inthecourseofamonth,onemaymove
inthedirectiontheMoontravelsin,whileasecondobject,inanotherpartofthesky,maymoveinthe
oppositedirection.Indeed,towardstheendofthismonth’sobservingsessions,eitherobjectmaycease
tomove,seemalmosttochangeitsmindandbegintoretraceitsstepsonthecelestialsphere. These
wanderers, or planets (‘planet’ is a Greek word meaning ‘wanderer’), are obviously of a different
naturefromthatofthefixed,twinklingstars.
1.5 A year
A year’spatientobserving,bydayandnight,providesthewatcherwithnewconcepts. Forexample,
theSun’sdailybehaviour,movingeasterlybitbybit,islinkedtotheseasonalchanges.
Eachday,formostobservers,theSunrises,increasesaltitudeuntilitculminatesonthemeridian
atapparentnoon,thenfallsdowntheskyuntilitsetsonthewesternhorizon. We haveseenthatthis
progresscanbestudiedbynotingthechangesindirectionandlengthoftheshadowcastbyavertical
rodstuckintheground(seefigure1.1).
As the days pass, the minimum daily length of shadow (at apparent noon) is seen to change,
becoming longest during winter and shortest during summer. This behaviour is also linked with
changes in the rising and setting directions of the Sun. Six months after the Sun has risen between
north and east and setting between north and west, it is rising between south and east and setting
Ayear 7
betweensouthandwest. Anothersixmonthshastopassbeforethesolarcycleiscompleted,withthe
Sunoncemorerisingbetweennorthandeastandsettingbetweennorthandwest.
All this could be explained by supposing that the Sun not only revolved with the stars on the
celestialsphereaboutthe Earthin oneday(itsdiurnalmovement)butthatitalso movedmuchmore
slowly along the path among the stars on the celestial sphere, making one revolution in one year,
returningtoitsoriginalpositionwithrespecttothestarsinthatperiodoftime. Wehavealreadyseen
thattheobserverwhonotesoveramonthwhatgroupofstarsisfirstvisibleabovetheeasternhorizon
aftersunsetwillhavealreadycometotheconclusionthattheSunmovesrelativetothestars. Nowit
isseenthatthereisaregularsecularprogressionrightroundthestellarbackgroundandthatwhenthe
Sunhasreturnedtoitsoriginalstellarposition,theseasonalcycleisalsocompleted.
The Sun’s stellar route was called the ecliptic by the ancients. The groups of stars intersected
by this path were called the housesof the Zodiac. The ecliptic is foundto be a greatcircle inclined
atabout231 degreestotheequator,thegreatcircleontheskycorrespondingtotheprojectionofthe
2
Earth’sequator,intersectingitattwopoints,thevernalandautumnalequinoxes,180degreesapart.
Itwasquitenatural,then, fortheancientstoworshiptheSun. Notonlydiditprovidelightand
warmth by day against the evils of the night but, in addition, its yearly progression was intimately
linkedtotheseasonsandsoalsotoseedtimeandharvest.Itwas,therefore,necessarytokeeptrackof
progresstouseitasaclockandacalendar.Tothisend,thescienceofsundial-makingbegan,ramifying
from simple obelisks that throw shadows on a fan of lines radiating from their bases, to extremely
ingeniousand complicatederectionsin stone and metal. Up to the 19thcentury, these constructions
rivalledmostpocket-watchesinaccuracyastimekeepers.
For calendrical purposes, lines of standing stones could be set up, pointing to the midsummer,
midwinterandequinoctialrisingandsettingpointsoftheSun. Inthe British Isles, therestill remain
hundredsofsuchsolarobservatories,witnessestoourforefathers’preoccupationwiththeSun-god.
Theobserverwhowatchesthenightskythroughoutayearcountsaboutthirteenrevolutionsofthe
stellarbackgroundbytheMooninthattime.Overthatperiodoftime,itisnotapparentthatanysimple
relationshipexistsbetweenthesiderealperiodofrevolutionoftheMoon,theperiodofitsphasesand
the year (the time it takes the Sun to performone complete circuit of the ecliptic). That knowledge
comesaftermuchmoreextendedobservation,certainlymeasuredindecades.
Itwouldbe noticed, however,thatthe Moon’ssiderealpath is verylittle inclinedto the ecliptic
(aboutfivedegrees)andifrecordswerekeptofthepointsoftheeclipticcrossedbytheMoon,itmight
be realizedthatthese pointswere slippingwestwardsata rate ofabouttwenty degreesperyear (see
figure1.2).
More information, too, would be acquired about the star-like objects that do not twinkle and
whichhavebeenfoundinthe courseofamonthto havea slowmovementwithrespecttothestellar
background.Theseplanets,liketheMoon,wouldneverbeseenmorethanafewdegreesfromtheplane
oftheecliptic,yetmonthaftermonththeywouldjourneythroughconstellationafterconstellation. In
thecaseofoneortwo,theirpathswouldincludenarrowloops,thoughonlyoneloopwouldbeobserved
foreachoftheseplanetsinthecourseoftheyear.
The year’s observations would not add much to the observer’s knowledge of the stars, except
to confirmthattheirpositionsand brightnessesrelativeto each otherdidnotalterand thateachstar,
unliketheSun,haditsownfixedrisingandsettingdirection,unlessitwascircumpolar. Itispossible,
however,thatinayear,theextra-carefulwatchermighthavecausetowonderiftheconclusionsabout
stars were withoutexceptionfor, byregularcomparisonof the brightnessof onestar with respectto
that of neighbouringones, it might be discovered that a few stars were variable in brightness. This
wascertainlyknowntotheArabianastronomersoftheMiddleAges. Theappearanceofanovamight
even be observed, i.e. a star appearingin a position where one had not been previously noted. This
occurrencemightwellleadto doubtaboutthe knowledgeofthe nowfamiliarconstellations—inany
eventitcouldbringaboutthedecisiontomakeastarmapforfutureuseifthephenomenonhappened
again. Itisalsopossiblethatinthecourseofayeartheobservermightseeacomet,astar-likeobject
8 Nakedeyeobservations
◦
Figure1.2.TheMoon’ssiderealpathcrossestheecliptictwiceeachmonthatanangleofabout5 .Forsuccessive
◦
lunationsthecrossingpointsmovewestward,coveringabout20 overayear. TheConstellationofLeoisshown
togiveanindicationofthescaleofthemovement.
withalongluminoustail. Thedevelopmentofthetailandthemovementofthecometheadcouldbe
detectedfromnighttonight.
Our observer by now must have come to tentative conclusions concerning the heavenly
phenomena studied and noted. The interpretations, and the use made of the world-picture, will be
constrainedby thecultureof the time. A manof Neolithictimesand a GreekofAthens’ goldenera
woulddevelopentirelydifferentcosmologiesfromidenticalobservations. Andahunterorfarmerhas
differentneeds,astronomicallyspeaking,fromasailor.
Chapter 2
Ancient world models
Firsttheorieswerenecessarilysimple.TheEarthwasaflatplanewithrivers,hills,seasandland,fixed,
eternal.Theheavenlybodiesrevolved,passingfromeasttowest.Butifthelandcontinuedindefinitely,
howcouldtheSunthatsetinthewestbethesameSunthatroseintheeastthenextmorning?Perhaps,
theBabyloniansreasoned,theEarthwasflatbutfinitewitha circleofoceanbeyondwhicha ringof
mountainssupportedtheheavens,thefirmament.Then,ifdoorswereprovidedinthebaseofthisgreat
solidhalf-sphereontheeasternandwesternsides,thecelestialbodieswouldbeabletoslipthroughthe
westerndoorsonsettingandbetransportedinsomemiraculouswaytotheeasttoreappearasordained.
TheBabylonianswereskilledastronomersthoughtheirworld-picturewasna¨ıve. Theyobserved
the positions of the Sun, Moon, planets and stars for many centuries with great accuracy. They
found that they could predict eclipses. Their observations were motivated by their belief that the
future of human beings could be predicted from celestial configurationsand events such as eclipses
or the appearanceof comets. Because of this, kings kept courtastrologers and the wealthy paid for
horoscopes. This belief in astrology, foundin all nations, should have withered away with alchemy
and the search for the philosopher’s stone but even today there are many who set great faith in this
pseudo-science.Itisperhapsneedlesstosaythatmodernastronomydemonstrateshowludicroussuch
beliefsare.
The Egyptians, astronomers almost as skilled as the Babylonians, had equally simple world-
pictures. They noticed that the yearly inundation of the Nile valley coincided with the days when
the star Sirius could be seen best in the morning twilight. This linking of celestial and earthly
eventsspurredontheirdevelopmentofastrologyandbroughtreligionintothe picture. TheSun-god
descendedatnight,passingbeneaththeEarthtovisitthedead.
Farming people were more interested in the solar cycle since it was linked with seed time and
harvest. SeafaringpeopleslikethePhoeniciansandtheMinoansusedtherisingandsettingdirections
ofthestarsasnavigationalaids.Itmaywellhavebeenasanaidtomemorythatthestarsweregrouped
inconstellations,embodyingmythscurrentatthattime.
As is to be expected, the ancient Chinese civilizations produced schools of astronomy and
cosmologicaltheories. SeriousChineseastronomyprobablybeganpriorto2000BCalthoughdetails
of events in that era are largely legendary. The story of the two Chinese astronomers, Ho and Hi,
executedforfailingtopredictaneclipseoftheSunin2137BCispossiblyapocryphalandmayrefer
to two astronomicalcollegesof a muchlater date destroyedin civil strife. Reliable historicaldetails
beginabout1000BC.Afarmingpeoplerequiredacalendarandsothelengthsofmonthandyearwere
quicklyascertained.Ayearof3651 dayswascertainlyusedby350BC.
4
By that date, the Chinese constellation figures, 122 in number and quite different from those
handeddowntousbytheGreeks,hadbeenmappedout,theSun’spath—theecliptic—beingdivided
into 12 regions. The size ofa regionwas notonlyconnectedwith the heavenlyarc inhabitedby the
Sun each month but also with the yearly journeyof the planet Jupiter. The other planetary motions
9
10 Ancientworldmodels
werealsostudied. Asinthewest,apseudo-scienceofastrologydevelopedfromsuchstudies. China
wasthecentreorhuboftheflatEarthwithheavenlyandhumaneventsincloseharmony:notonlydid
celestialeventsguideand controlmen, in particulartheEmperorandhis courtbutthe decisionsand
actionsofsuchpowerfulrulersinfluencedthestateofHeaven.
As mathematical knowledge grew and more accurate astronomical instruments for measuring
altitudes and angles were developed in succeeding centuries, the movements of the Sun, Moon
and planets were systematized in remarkably accurate tables for prediction purposes. Cometary
appearanceswerenoted, amongthemseveralapparitionsofHalley’scomet, andbythe 14thcentury
ADthestateofChineseastronomycomparedfavourablywiththatoftheArabsintheWest.
Invariousotherplaceswhereacivilizationhaddeveloped,astronomicalschoolsflourished. The
ravagesoftimeandbarbarismhavesadlydestroyedmostoftheworksofsuchschools,thoughhappily
some tracesremainto tellusof theheightsof thoughttheirpractitionersachieved. Forexample, we
shallseelaterhowingeniouswerethestepsmegalithicmantooktokeeptrackoftheSunandMoon.
ThisremarkablecivilizationflourishedinWesternEuropeinthethirdandsecondmillenniaBC.
Observations of eclipses were also recorded by early American Indians as, for example, by
Mayans. A sundial remaining in the ‘lost city’, Macchu Piccu, provides us with evidence that the
Incasof Peruused solar observationsto some purpose. The‘Puerta delSol’ atTiahuanaco,Bolivia,
tellsusofsolarobservationspriortotheIncas.
However, very few of the ideas and notions of astronomy and cosmology from any of these
civilizations have had an influence on the development of our understanding of the astronomical
Universe.OurstartingpointsfindtheiroriginsmainlyinancientGreece.
Acompletelynewdepartureinmankind’scontemplationandinterpretationoftheheavenscame
with the flowering of Greek civilization. Many of their thinkers had extraordinarilyoriginal minds,
were mentally courageous and devoted to rational thought. They were not afraid of questioning
cherishedbeliefsandoffollowingunsettling,disturbingtrainsofthought.
Many of them dismissed the ‘common-sense’ picture of solid, flat Earth and god-controlled
Heaven. They saw that a spherical Earth poised in space solved a lot of problems. Those stars and
planets not seen during the night were simply on the other side of the Earth. Stars were not seen
duringthedaybecausethedazzlingbrightSunblottedouttheirfeeblelight. TheMooncausedsolar
eclipses. Pythagoras,inthe6thcenturyBC,taughtthatthemovementsofalltheheavenlybodieswere
compoundedofoneormorecircularmovements.
In the next century, Philolaus, a follower of Pythagoras, suggested the bold idea that the Earth
wasnotthecentreoftheUniverseand,indeed,thatitmoved. AtthecentreoftheUniversetherewas
agiganticfire. AroundthisfirerevolvedtheEarth,Moon,Sunandplanetsinthatorder,incirclesof
varioussizes. HealsopostulatedabodycalledtheAnti-Earthtobringthetotalofmovingbodiesup
to the sacred numberof ten. This Anti-Earth revolved aboutthe centralfire within the Earth’s orbit
andwasneverseenfromtheEarthbecausetheEarthfacedoutwardstowardsthehomeofthegods—
Olympus—situatedbeyondthesphereofthefixedstars. PhilolausalsobelievedthattheSunwasnot
self-luminousbutshonebythelightitabsorbedfromOlympusandthecentralfire.
In contrastto this, Anaxagorastaughtthat the Sun was a mass of glowingmetalcomparablein
sizewithGreeceitself. Aristarchus,inthe3rdcenturyBC,agreedwithPhilolausthattheEarthmoved
andtaughtthatitrotatedonitsaxis,thusexplainingthediurnalmotionoftheheavens. Moreover,he
said,theSunisastarandtheEarthrevolvesroundit,allotherstarsbeingverymuchfartheraway.
Aristarchus, like Anaxagoras, had ideas about the relative sizes of Sun, Moon and Earth. The
Sun’sdiameterhad to be aboutseventimesthe diameterof the Earth, a figurefar removedfromthe
modernonebutembodyingtherightidea,namelythattheEarthismuchsmallerthantheSun.
Eratosthenes of Alexandria, living about 230 BC, used solar observations and a knowledge of
geometryand geographyto calculate the circumferenceof the Earth, obtaininga value within a few
percentoftoday’sacceptedfigure.
HeknewthatatthesummersolsticetheSunpassedthroughthezenithatSyeneinUpperEgypt,
