a history of science-2-第11部分

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　essentially　an　observer；　but　in　this　regard　he　won　a　secure　place　in　the　very　first　rank。　Indeed；　he　was　easily　the　greatest　observing　astronomer　since　Hipparchus；　between　whom　and　himself　there　were　many　points　of　resemblance。　Hipparchus；　it　will　be　recalled；　rejected　the　Aristarchian　conception　of　the　universe　just　as　Tycho　rejected　the　conception　of　Copernicus。　But　if　Tycho　propounded　no　great　generalizations；　the　list　of　specific　advances　due　to　him　is　a　long　one；　and　some　of　these　were　to　prove　important　aids　in　the　hands　of　later　workers　to　the　secure　demonstration　of　the　Copernican　idea。　One　of　his　most　important　series　of　studies　had　to　do　with　comets。　Regarding　these　bodies　there　had　been　the　greatest　uncertainty　in　the　minds　of　astronomers。　The　greatest　variety　of　opinions　regarding　them　prevailed；　they　were　thought　on　the　one　hand　to　be　divine　messengers；　and　on　the　other　to　be　merely　igneous　phenomena　of　the　earth's　atmosphere。　Tycho　Brahe　declared　that　a　comet　which　he　observed　in　the　year　1577　had　no　parallax；　proving　its　extreme　distance。　The　observed　course　of　the　comet　intersected　the　planetary　orbits；　which　fact　gave　a　quietus　to　the　long…mooted　question　as　to　whether　the　Ptolemaic　spheres　were　transparent　solids　or　merely　imaginary；　since　the　comet　was　seen　to　intersect　these　alleged　spheres；　it　was　obvious　that　they　could　not　be　the　solid　substance　that　they　were　commonly　imagined　to　be；　and　this　fact　in　itself　went　far　towards　discrediting　the　Ptolemaic　system。　It　should　be　recalled；　however；　that　this　supposition　of　tangible　spheres　for　the　various　planetary　and　stellar　orbits　was　a　mediaeval　interpretation　of　Ptolemy's　theory　rather　than　an　interpretation　of　Ptolemy　himself；　there　being　nothing　to　show　that　the　Alexandrian　astronomer　regarded　his　cycles　and　epicycles　as　other　than　theoretical。　An　interesting　practical　discovery　made　by　Tycho　was　his　method　of　determining　the　latitude　of　a　place　by　means　of　two　observations　made　at　an　interval　of　twelve　hours。　Hitherto　it　had　been　necessary　to　observe　the　sun's　angle　on　the　equinoctial　days；　a　period　of　six　months　being　therefore　required。　Tycho　measured　the　angle　of　elevation　of　some　star　situated　near　the　pole；　when　on　the　meridian；　and　then；　twelve　hours　later；　measured　the　angle　of　elevation　of　the　same　star　when　it　again　came　to　the　meridian　at　the　opposite　point　of　its　apparent　circle　about　the　polestar。　Half　the　sum　of　these　angles　gives　the　latitude　of　the　place　of　observation。　As　illustrating　the　accuracy　of　Tycho's　observations；　it　may　be　noted　that　he　rediscovered　a　third　inequality　of　the　moon's　motion　at　its　variation；　he；　in　common　with　other　European　astronomers；　being　then　quite　unaware　that　this　inequality　had　been　observed　by　an　Arabian　astronomer。　Tycho　proved　also　that　the　angle　of　inclination　of　the　moon's　orbit　to　the　ecliptic　is　subject　to　slight　variation。　The　very　brilliant　new　star　which　shone　forth　suddenly　in　the　constellation　of　Cassiopeia　in　the　year　1572；　was　made　the　object　of　special　studies　by　Tycho；　who　proved　that　the　star　had　no　sensible　parallax　and　consequently　was　far　beyond　the　planetary　regions。　The　appearance　of　a　new　star　was　a　phenomenon　not　unknown　to　the　ancients；　since　Pliny　records　that　Hipparchus　was　led　by　such　an　appearance　to　make　his　catalogue　of　the　fixed　stars。　But　the　phenomenon　is　sufficiently　uncommon　to　attract　unusual　attention。　A　similar　phenomenon　occurred　in　the　year　1604；　when　the　new　starin　this　case　appearing　in　the　constellation　of　Serpentariuswas　explained　by　Kepler　as　probably　proceeding　from　a　vast　combustion。　This　explanationin　which　Kepler　is　said　to　have　followed。　Tychois　fully　in　accord　with　the　most　recent　theories　on　the　subject；　as　we　shall　see　in　due　course。　It　is　surprising　to　hear　Tycho　credited　with　so　startling　a　theory；　but；　on　the　other　hand；　such　an　explanation　is　precisely　what　should　be　expected　from　the　other　astronomer　named。　For　Johann　Kepler；　or；　as　he　was　originally　named；　Johann　von　Kappel；　was　one　of　the　most　speculative　astronomers　of　any　age。　He　was　forever　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