a history of science-1-第34章

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0；000　stadia。　Some　commentators　have　thought　that　Aristotle　merely　referred　to　the　area　of　the　inhabited　portion　of　the　earth　and　not　to　the　circumference　of　the　earth　itself；　but　his　words　seem　doubtfully　susceptible　of　this　interpretation；　and　if　he　meant；　as　his　words　seem　to　imply；　that　philosophers　of　his　day　had　a　tolerably　precise　idea　of　the　globe；　we　must　assume　that　this　idea　was　based　upon　some　sort　of　measurement。　The　recorded　size；　400；000　stadia；　is　a　sufficient　approximation　to　the　truth　to　suggest　something　more　than　a　mere　unsupported　guess。　Now；　since　Aristotle　died　more　than　fifty　years　before　Eratosthenes　was　born；　his　report　as　to　the　alleged　size　of　the　earth　certainly　has　a　suggestiveness　that　cannot　be　overlooked；　but　it　arouses　speculations　without　giving　an　inkling　as　to　their　solution。　If　Eratosthenes　had　a　precursor　as　an　earth…measurer；　no　hint　or　rumor　has　come　down　to　us　that　would　enable　us　to　guess　who　that　precursor　may　have　been。　His　personality　is　as　deeply　enveloped　in　the　mists　of　the　past　as　are　the　personalities　of　the　great　prehistoric　discoverers。　For　the　purpose　of　the　historian；　Eratosthenes　must　stand　as　the　inventor　of　the　method　with　which　his　name　is　associated；　and　as　the　first　man　of　whom　we　can　say　with　certainty　that　he　measured　the　size　of　the　earth。　Right　worthily；　then；　had　the　Alexandrian　philosopher　won　his　proud　title　of　〃surveyor　of　the　world。〃

HIPPARCHUS；　〃THE　LOVER　OF　TRUTH〃　Eratosthenes　outlived　most　of　his　great　contemporaries。　He　saw　the　turning　of　that　first　and　greatest　century　of　Alexandrian　science；　the　third　century　before　our　era。　He　died　in　the　year　196　B。C。；　having；　it　is　said；　starved　himself　to　death　to　escape　the　miseries　of　blindness；to　the　measurer　of　shadows；　life　without　light　seemed　not　worth　the　living。　Eratosthenes　left　no　immediate　successor。　A　generation　later；　however；　another　great　figure　appeared　in　the　astronomical　world　in　the　person　of　Hipparchus；　a　man　who；　as　a　technical　observer；　had　perhaps　no　peer　in　the　ancient　world：　one　who　set　so　high　a　value　upon　accuracy　of　observation　as　to　earn　the　title　of　〃the　lover　of　truth。〃　Hipparchus　was　born　at　Nicaea；　in　Bithynia；　in　the　year　160　B。C。　His　life；　all　too　short　for　the　interests　of　science；　ended　in　the　year　125　B。C。　The　observations　of　the　great　astronomer　were　made　chiefly；　perhaps　entirely；　at　Rhodes。　A　misinterpretation　of　Ptolemy's　writings　led　to　the　idea　that　Hipparchus；　performed　his　chief　labors　in　Alexandria；　but　it　is　now　admitted　that　there　is　no　evidence　for　this。　Delambre　doubted；　and　most　subsequent　writers　follow　him　here；　whether　Hipparchus　ever　so　much　as　visited　Alexandria。　In　any　event　there　seems　to　be　no　question　that　Rhodes　may　claim　the　honor　of　being　the　chief　site　of　his　activities。　It　was　Hipparchus　whose　somewhat　equivocal　comment　on　the　work　of　Eratosthenes　we　have　already　noted。　No　counter…charge　in　kind　could　be　made　against　the　critic　himself；　he　was　an　astronomer　pure　and　simple。　His　gift　was　the　gift　of　accurate　observation　rather　than　the　gift　of　imagination。　No　scientific　progress　is　possible　without　scientific　guessing；　but　Hipparchus　belonged　to　that　class　of　observers　with　whom　hypothesis　is　held　rigidly　subservient　to　fact。　It　was　not　to　be　expected　that　his　mind　would　be　attracted　by　the　heliocentric　theory　of　Aristarchus。　He　used　the　facts　and　observations　gathered　by　his　great　predecessor　of　Samos；　but　he　declined　to　accept　his　theories。　For　him　the　world　was　central；　his　problem　was　to　explain；　if　he　could；　the　irregularities　of　motion　which　sun；　moon；　and　planets　showed　in　their　seeming　circuits　about　the　earth。　Hipparchus　had　the　gnomon　of　Eratosthenesdoubtless　in　a　perfected　formto　aid　him；　and　he　soon　proved　himself　a　master　in　its　use。　For　him；　as　we　have　said；　accuracy　was　everything；　this　was　the　one　element　that　led　to　all　his　great　successes。　Perhaps　his　greatest　feat　was　to　demonstrate　the　eccentricity　of　the　sun's　seeming　orbit。　We　of　to…day；　thanks　to　Keppler　and　his　followers；　know　that　the　earth　and　the　other　planetary　bodies　in　their　circuit　about　the　sun　describe　an　ellipse　and　not　a　circle。　But　in　the　day　of　Hipparchus；　though　the　ellipse　was　recognized　as　a　geometrical　figure　（it　had　been　described　and　named　along　with　the　parabola　and　hyperbola　by　Apollonius　of　Perga；　the　pupil　of　Euclid）；　yet　it　would　have　been　the　rankest　heresy　to　suggest　an　elliptical　course　for　any　heavenly　body。　A　metaphysical　theory；　as　propounded　perhaps　by　the　Pythagoreans　but　ardently　supported　by　Aristotle；　declared　that　the　circle　is　the　perfect　figure；　and　pronounced　it　inconceivable　that　the　motions　of　the　spheres　should　be　other　than　circular。　This　thought　dominated　the　mind　of　Hipparchus；　and　so　when　his　careful　measurements　led　him　to　the　discovery　that　the　northward　and　southward　journeyings　of　the　sun　did　not　divide　the　year　into　four　equal　parts；　there　was　nothing　open　to　him　but　to　either　assume　that　the　earth　does　not　lie　precisely　at　the　centre　of　the　sun's　circular　orbit　or　to　find　some　alternative　hypothesis。　In　point　of　fact；　the　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　poe