a history of science-1-第47章

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e　sun　（reversing　the　point　of　view　in　accordance　with　modern　discoveries）　does　lie　at　one　focus　of　the　earth's　elliptical　orbit；　and　therefore　away　from　the　physical　centre　of　that　orbit；　in　other　words；　the　observations　of　Hipparchus　were　absolutely　accurate。　He　was　quite　correct　in　finding　that　the　sun　spends　more　time　on　one　side　of　the　equator　than　on　the　other。　When；　therefore；　he　estimated　the　relative　distance　of　the　earth　from　the　geometrical　centre　of　the　sun's　supposed　circular　orbit；　and　spoke　of　this　as　the　measure　of　the　sun's　eccentricity；　he　propounded　a　theory　in　which　true　data　of　observation　were　curiously　mingled　with　a　positively　inverted　theory。　That　the　theory　of　Hipparchus　was　absolutely　consistent　with　all　the　facts　of　this　particular　observation　is　the　best　evidence　that　could　be　given　of　the　difficulties　that　stood　in　the　way　of　a　true　explanation　of　the　mechanism　of　the　heavens。　But　it　is　not　merely　the　sun　which　was　observed　to　vary　in　the　speed　of　its　orbital　progress；　the　moon　and　the　planets　also　show　curious　accelerations　and　retardations　of　motion。　The　moon　in　particular　received　most　careful　attention　from　Hipparchus。　Dominated　by　his　conception　of　the　perfect　spheres；　he　could　find　but　one　explanation　of　the　anomalous　motions　which　he　observed；　and　this　was　to　assume　that　the　various　heavenly　bodies　do　not　fly　on　in　an　unvarying　arc　in　their　circuit　about　the　earth；　but　describe　minor　circles　as　they　go　which　can　be　likened　to　nothing　so　tangibly　as　to　a　light　attached　to　the　rim　of　a　wagon…wheel　in　motion。　If　such　an　invisible　wheel　be　imagined　as　carrying　the　sun；　for　example；　on　its　rim；　while　its　invisible　hub　follows　unswervingly　the　circle　of　the　sun's　mean　orbit　（this　wheel；　be　it　understood；　lying　in　the　plane　of　the　orbit；　not　at　right…　angles　to　it）；　then　it　must　be　obvious　that　while　the　hub　remains　always　at　the　same　distance　from　the　earth；　the　circling　rim　will　carry　the　sun　nearer　the　earth；　then　farther　away；　and　that　while　it　is　traversing　that　portion　of　the　are　which　brings　it　towards　the　earth；　the　actual　forward　progress　of　the　sun　will　be　retarded　notwithstanding　the　uniform　motion　of　the　hub；　just　as　it　will　be　accelerated　in　the　opposite　arc。　Now；　if　we　suppose　our　sun…bearing　wheel　to　turn　so　slowly　that　the　sun　revolves　but　once　about　its　imaginary　hub　while　the　wheel　itself　is　making　the　entire　circuit　of　the　orbit；　we　shall　have　accounted　for　the　observed　fact　that　the　sun　passes　more　quickly　through　one…half　of　the　orbit　than　through　the　other。　Moreover；　if　we　can　visualize　the　process　and　imagine　the　sun　to　have　left　a　visible　line　of　fire　behind　him　throughout　the　course；　we　shall　see　that　in　reality　the　two　circular　motions　involved　have　really　resulted　in　producing　an　elliptical　orbit。　The　idea　is　perhaps　made　clearer　if　we　picture　the　actual　progress　of　the　lantern　attached　to　the　rim　of　an　ordinary　cart…wheel。　When　the　cart　is　drawn　forward　the　lantern　is　made　to　revolve　in　a　circle　as　regards　the　hub　of　the　wheel；　but　since　that　hub　is　constantly　going　forward；　the　actual　path　described　by　the　lantern　is　not　a　circle　at　all　but　a　waving　line。　It　is　precisely　the　same　with　the　imagined　course　of　the　sun　in　its　orbit；　only　that　we　view　these　lines　just　as　we　should　view　the　lantern　on　the　wheel　if　we　looked　at　it　from　directly　above　and　not　from　the　side。　The　proof　that　the　sun　is　describing　this　waving　line；　and　therefore　must　be　considered　as　attached　to　an　imaginary　wheel；　is　furnished；　as　it　seemed　to　Hipparchus；　by　the　observed　fact　of　the　sun's　varying　speed。　That　is　one　way　of　looking　at　the　matter。　It　is　an　hypothesis　that　explains　the　observed　factsafter　a　fashion；　and　indeed　a　very　remarkable　fashion。　The　idea　of　such　an　explanation　did　not　originate　with　Hipparchus。　The　germs　of　the　thought　were　as　old　as　the　Pythagorean　doctrine　that　the　earth　revolves　about　a　centre　that　we　cannot　see。　Eudoxus　gave　the　conception　greater　tangibility；　and　may　be　considered　as　the　father　of　this　doctrine　of　wheelsepicycles；　as　they　came　to　be　called。　Two　centuries　before　the　time　of　Hipparchus　he　conceived　a　doctrine　of　spheres　which　Aristotle　found　most　interesting；　and　which　served　to　explain；　along　the　lines　we　have　just　followed；　the　observed　motions　of　the　heavenly　bodies。　Calippus；　the　reformer　of　the　calendar；　is　said　to　have　carried　an　account　of　this　theory　to　Aristotle。　As　new　irregularities　of　motion　of　the　sun；　moon；　and　planetary　bodies　were　pointed　out；　new　epicycles　were　invented。　There　is　no　limit　to　the　number　of　imaginary　circles　that　may　be　inscribed　about　an　imaginary　centre；　and　if　we　conceive　each　one　of　these　circles　to　have　a　proper　motion　of　its　own；　and　each　one　to　carry　the　sun　in　the　line　of　that　motion；　except　as　it　is　diverted　by　the　other　motionsif　we　can　visualize　this　complex　mingling　of　wheelswe　shall　certainly　be　able　to　imagine　the　heavenly　body　which　lies　at　the　juncture　of　all　the　rims；　as　being　carried　forward　in　as　erratic　and　wobbly　a　manner　as　could　be　desired。　In　other　words；　the　theory　of　epicycles　will　account　for　all　the　facts　of　the　observed　motions　of　all　the　heavenly　bodies；　but　in　so　doing　it　fills　the　universe　with　a　most　bewildering　network　of　intersecting　circles。　Even　in　the　time　of　Calippus　fifty…five　of　these　spheres　were　computed。　We　may　well　believe　that　the　clear…seeing　Aristarchus　would　look　askance　at　such　a　complex　system　of　imaginary　machinery。　But　Hipparchus；　pre…eminently　an　observer　rather　than　a　theorizer；　seems　to　have　been　content　to　accept　the　theory　of　epicycles　as　he　found　it；　though　his　studies　added　to　its　complexities；　and　Hipparchus　was　the　dominant　scientific　personality　of　his　century。　What　he　believed　became　as　a　law　to　his　immediate　successors。　His　tenets　were　accepted　as　final　by　their　great　popularizer；　Ptolemy；　three　centuries　later；　and　so　the　heliocentric　theory　of　Aristarchus　passed　under　a　cloud　almost　at　the　hour　of　its　dawning；　there　to　remain　obscured　and　forgotten　for　the　long　lapse　of　centuries。　A　thousand　pities　that　the　greatest　observing　astronomer　of　antiquity　could　not；　like　one　of　his　great　precursors；　have　approached　astronomy　from　the　stand…point　of　geography　and　poetry。　Had　he　done　so；　perhaps　he　might　have　reflected；　like　Aristarchus　before　him；　that　it　seems　absurd　for　our　earth　to　hold　the　giant　sun　in　thraldom；　then　perhaps　his　imagination　would　have　reached　out　to　the　heliocentric　doctrine；　and　the　cobweb　hypothesis　of　epicycles；　with　that　yet　more　intangible　figment　of　the　perfect　circle；　might　have　been　wiped　away。　But　it　was　not　to　be。　With　Aristarchus　the　scientific　imagination　had　reached　its　highest　flight；　but　with　Hipparchus　it　was　beginning　to　settle　back　into　regions　of　foggier　atmosphere　and　narrower　horizons。　For　what；　after　all；　does　it　matter　that　Hipparchus　should　go　on　to　measure　the　precise　length　of　the　year　and　the　apparent　size　of　the　moon's　disk；　that　he　should　make　a　chart　of　the　heavens　showing　the　place　of　1080　stars；　even　that　he　should　discover　the　precession　of　the　equinox；what；　after　all；　is　the　significance　of　these　details　as　against　the　all…essential　fact　that　the　greatest　scientific　authority　of　his　centurythe　one　truly　heroic　scientific　figure　of　his　epochshould　have　lent　all　the　forces　of　his　commanding　influence　to　the　old；　false　theory　of　cosmology；　when　the　true　theory　had　been　propounded　and　when　he；　perhaps；　was　the　only　man　in　the　world　who　might　have　substantiated　and　vitalized　that　theory？　It　is　easy　to　overestimate　the　influence　of　any　single　man；　and；　contrariwise；　to　underestimate　the　power　of　the　Zeitgeist。　But　when　we　reflect　that　the　doctrines　of　Hipparchus；　as　promulgated　by　Ptolemy；　became；　as　it　were；　the　last　word　of　astronomical　science　for　both　the　Eastern　and　Western　worlds；　and　so　continued　after　a　thousand　years；　it　is　perhaps　not　too　much　to　say　that　Hipparchus；　〃the　lover　of　truth；〃　missed　one　of　the　greatest　opportunities　for　the　promulgation　of　truth　ever　vouchsafed　to　a　devotee　of　pure　science。　But　all　this；　of　course；　detracts　nothing　from　the　merits　of　Hipparchus　as　an　observing　astronomer。　A　few　words　more　must　be　said　as　to　his　specific　discoveries　in　this　field。　According　to　his　measurement；　the　tropic