a history of science-2-第14章

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ever　theorizing；　but　such　was　the　peculiar　quality　of　his　mind　that　his　theories　never　satisfied　him　for　long　unless　he　could　put　them　to　the　test　of　observation。　Thanks　to　this　happy　combination　of　qualities；　Kepler　became　the　discoverer　of　three　famous　laws　of　planetary　motion　which　lie　at　the　very　foundation　of　modern　astronomy；　and　which　were　to　be　largely　instrumental　in　guiding　Newton　to　his　still　greater　generalization。　These　laws　of　planetary　motion　were　vastly　important　as　corroborating　the　Copernican　theory　of　the　universe；　though　their　position　in　this　regard　was　not　immediately　recognized　by　contemporary　thinkers。　Let　us　examine　with　some　detail　into　their　discovery；　meantime　catching　a　glimpse　of　the　life　history　of　the　remarkable　man　whose　name　they　bear。

JOHANN　KEPLER　AND　THE　LAWS　OF　PLANETARY　MOTION　Johann　Kepler　was　born　the　27th　of　December；　1571；　in　the　little　town　of　Weil；　in　Wurtemburg。　He　was　a　weak；　sickly　child；　further　enfeebled　by　a　severe　attack　of　small…pox。　It　would　seem　paradoxical　to　assert　that　the　parents　of　such　a　genius　were　mismated；　but　their　home　was　not　a　happy　one；　the　mother　being　of　a　nervous　temperament；　which　perhaps　in　some　measure　accounted　for　the　genius　of　the　child。　The　father　led　the　life　of　a　soldier；　and　finally　perished　in　the　campaign　against　the　Turks。　Young　Kepler's　studies　were　directed　with　an　eye　to　the　ministry。　After　a　preliminary　training　he　attended　the　university　at　Tubingen；　where　he　came　under　the　influence　of　the　celebrated　Maestlin　and　became　his　life…long　friend。　Curiously　enough；　it　is　recorded　that　at　first　Kepler　had　no　taste　for　astronomy　or　for　mathematics。　But　the　doors　of　the　ministry　being　presently　barred　to　him；　he　turned　with　enthusiasm　to　the　study　of　astronomy；　being　from　the　first　an　ardent　advocate　of　the　Copernican　system。　His　teacher；　Maestlin；　accepted　the　same　doctrine；　though　he　was　obliged；　for　theological　reasons；　to　teach　the　Ptolemaic　system；　as　also　to　oppose　the　Gregorian　reform　of　the　calendar。　The　Gregorian　calendar；　it　should　be　explained；　is　so　called　because　it　was　instituted　by　Pope　Gregory　XIII。；　who　put　it　into　effect　in　the　year　1582；　up　to　which　time　the　so…called　Julian　calendar；　as　introduced　by　Julius　Caesar；　had　been　everywhere　accepted　in　Christendom。　This　Julian　calendar；　as　we　have　seen；　was　a　great　improvement　on　preceding　ones；　but　still　lacked　something　of　perfection　inasmuch　as　its　theoretical　day　differed　appreciably　from　the　actual　day。　In　the　course　of　fifteen　hundred　years；　since　the　time　of　Caesar；　this　defect　amounted　to　a　discrepancy　of　about　eleven　days。　Pope　Gregory　proposed　to　correct　this　by　omitting　ten　days　from　the　calendar；　which　was　done　in　September；　1582。　To　prevent　similar　inaccuracies　in　the　future；　the　Gregorian　calendar　provided　that　once　in　four　centuries　the　additional　day　to　make　a　leap…year　should　be　omitted；　the　date　selected　for　such　omission　being　the　last　year　of　every　fourth　century。　Thus　the　years　1500；　1900；　and　2300；　A。D。；　would　not　be　leap…years。　By　this　arrangement　an　approximate　rectification　of　the　calendar　was　effected；　though　even　this　does　not　make　it　absolutely　exact。　Such　a　rectification　as　this　was　obviously　desirable；　but　there　was　really　no　necessity　for　the　omission　of　the　ten　days　from　the　calendar。　The　equinoctial　day　had　shifted　so　that　in　the　year　1582　it　fell　on　the　10th　of　March　and　September。　There　was　no　reason　why　it　should　not　have　remained　there。　It　would　greatly　have　simplified　the　task　of　future　historians　had　Gregory　contented　himself　with　providing　for　the　future　stability　of　the　calendar　without　making　the　needless　shift　in　question。　We　are　so　accustomed　to　think　of　the　21st　of　March　and　21st　of　September　as　the　natural　periods　of　the　equinox；　that　we　are　likely　to　forget　that　these　are　purely　arbitrary　dates　for　which　the　10th　might　have　been　substituted　without　any　inconvenience　or　inconsistency。　But　the　opposition　to　the　new　calendar；　to　which　reference　has　been　made；　was　not　based　on　any　such　considerations　as　these。　It　was　due；　largely　at　any　rate；　to　the　fact　that　Germany　at　this　time　was　under　sway　of　the　Lutheran　revolt　against　the　papacy。　So　effective　was　the　opposition　that　the　Gregorian　calendar　did　not　come　into　vogue　in　Germany　until　the　year　1699。　It　may　be　added　that　England；　under　stress　of　the　same　manner　of　prejudice；　held　out　against　the　new　reckoning　until　the　year　1751；　while　Russia　does　not　accept　it　even　now。　As　the　Protestant　leaders　thus　opposed　the　papal　attitude　in　a　matter　of　so　practical　a　character　as　the　calendar；　it　might　perhaps　have　been　expected　that　the　Lutherans　would　have　had　a　leaning　towards　the　Copernican　theory　of　the　universe；　since　this　theory　was　opposed　by　the　papacy。　Such；　however；　was　not　the　case。　Luther　himself　pointed　out　with　great　strenuousness；　as　a　final　and　demonstrative　argument；　the　fact　that　Joshua　commanded　the　sun　and　not　the　earth　to　stand　still；　and　his　followers　were　quite　as　intolerant　towards　the　new　teaching　as　were　their　ultramontane　opponents。　Kepler　himself　was；　at　various　times；　to　feel　the　restraint　of　ecclesiastical　opposition；　though　he　was　never　subjected　to　direct　persecution；　as　was　his　friend　and　contemporary；　Galileo。　At　the　very　outset　of　Kepler's　career　there　was；　indeed；　question　as　to　the　publication　of　a　work　he　had　written；　because　that　work　took　for　granted　the　truth　of　the　Copernican　doctrine。　This　work　appeared；　however；　in　the　year　1596。　It　bore　the　title　Mysterium　Cosmographium；　and　it　attempted　to　explain　the　positions　of　the　various　planetary　bodies。　Copernicus　had　devoted　much　time　to　observation　of　the　planets　with　reference　to　measuring　their　distance；　and　his　efforts　had　been　attended　with　considerable　success。　He　did　not；　indeed；　know　the　actual　distance　of　the　sun；　and；　therefore；　was　quite　unable　to　fix　the　distance　of　any　planet；　but；　on　the　other　hand；　he　determined　the　relative　distance　of　all　the　planets　then　known；　as　measured　in　terms　of　the　sun's　distance；　with　remarkable　accuracy。　With　these　measurements　as　a　guide；　Kepler　was　led　to　a　very　fanciful　theory；　according　to　which　the　orbits　of　the　five　principal　planets　sustain　a　peculiar　relation　to　the　five　regular　solids　of　geometry。　His　theory　was　this：　〃Around　the　orbit　of　the　earth　describe　a　dodecahedronthe　circle　comprising　it　will　be　that　of　Mars；　around　Mars　describe　a　tetrahedronthe　circle　comprising　it　will　be　that　of　Jupiter；　around　Jupiter　describe　a　cubethe　circle　comprising　it　will　be　that　of　Saturn；　now　within　the　earth's　orbit　inscribe　an　icosahedronthe　inscribed　circle　will　be　that　of　Venus；　in　the　orbit　of　Venus　inscribe　an　octahedron　the　circle　inscribed　will　be　that　of　Mercury。〃＇3＇　Though　this　arrangement　was　a　fanciful　one；　which　no　one　would　now　recall　had　not　the　theorizer　obtained　subsequent　fame　on　more　substantial　grounds；　yet　it　evidenced　a　philosophical　spirit　on　the　part　of　the　astronomer　which；　misdirected　as　it　was　in　this　instance；　promised　well　for　the　future。　Tycho　Brahe；　to　whom　a　copy　of　the　work　was　sent；　had　the　acumen　to　recognize　it　as　a　work　of　genius。　He　summoned　the　young　astronomer　to　be　his　assistant　at　Prague；　and　no　doubt　the　association　thus　begun　was　instrumental　in　determining　the　character　of　Kepler's　future　work。　It　was　precisely　the　training　in　minute　observation　that　could　avail　most　for　a　mind　which；　like　Kepler's；　tended　instinctively　to　the　formulation　of　theories。　When　Tycho　Brahe　died；　in　1601；　Kepler　became　his　successor。　In　due　time　he　secured　access　to　all　the　unpublished　observations　of　his　great　predecessor；　and　these　were　of　inestimable　value　to　him　in　the　progress　of　his　own　studies。　Kepler　was　not　only　an　ardent　worker　and　an　enthusiastic　theorizer；　but　he　was　an　indefatigable　writer；　and　it　pleased　him　to　take　the　public　fully　into　his　confidence；　not　merely　as　to　his　successes；　but　as　to　his　failures。　Thus　his　works　elaborate　false　theories　as　well　as　correct　ones；　and　detail　the　observations　through　which　the　incorrect　guesses　were　refuted　by　their　originator。　Some　of　these　accounts　are　highly　interesting；　but　they　must　not　detain　us　here。　For　our　present　purpose　it　must　suffice　to　point　out　the　three　important　theories；　which；　as　cul