a history of science-2-第13章

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heory　of　the　earth's　motion。　This　was　Tycho　Brahe；　one　of　the　greatest　observing　astronomers　of　any　age。　Tycho　Brahe　was　a　Dane；　born　at　Knudstrup　in　the　year　1546。　He　died　in　1601　at　Prague；　in　Bohemia。　During　a　considerable　portion　of　his　life　he　found　a　patron　in　Frederick；　King　of　Denmark；　who　assisted　him　to　build　a　splendid　observatory　on　the　Island　of　Huene。　On　the　death　of　his　patron　Tycho　moved　to　Germany；　where；　as　good　luck　would　have　it；　he　came　in　contact　with　the　youthful　Kepler；　and　thus；　no　doubt；　was　instrumental　in　stimulating　the　ambitions　of　one　who　in　later　years　was　to　be　known　as　a　far　greater　theorist　than　himself。　As　has　been　said；　Tycho　rejected　the　Copernican　theory　of　the　earth's　motion。　It　should　be　added；　however；　that　he　accepted　that　part　of　the　Copernican　theory　which　makes　the　sun　the　centre　of　all　the　planetary　motions；　the　earth　being　excepted。　He　thus　developed　a　system　of　his　own；　which　was　in　some　sort　a　compromise　between　the　Ptolemaic　and　the　Copernican　systems。　As　Tycho　conceived　it；　the　sun　revolves　about　the　earth；　carrying　with　it　the　planets…Mercury；　Venus；　Mars；　Jupiter；　and　Saturn；　which　planets　have　the　sun　and　not　the　earth　as　the　centre　of　their　orbits。　This　cosmical　scheme；　it　should　be　added；　may　be　made　to　explain　the　observed　motions　of　the　heavenly　bodies；　but　it　involves　a　much　more　complex　mechanism　than　is　postulated　by　the　Copernican　theory。　Various　explanations　have　been　offered　of　the　conservatism　which　held　the　great　Danish　astronomer　back　from　full　acceptance　of　the　relatively　simple　and；　as　we　now　know；　correct　Copernican　doctrine。　From　our　latter…day　point　of　view；　it　seems　so　much　more　natural　to　accept　than　to　reject　the　Copernican　system；　that　we　find　it　difficult　to　put　ourselves　in　the　place　of　a　sixteenth…century　observer。　Yet　if　we　recall　that　the　traditional　view；　having　warrant　of　acceptance　by　nearly　all　thinkers　of　every　age；　recorded　the　earth　as　a　fixed；　immovable　body；　we　shall　see　that　our　surprise　should　be　excited　rather　by　the　thinker　who　can　break　away　from　this　view　than　by　the　one　who　still　tends　to　cling　to　it。　Moreover；　it　is　useless　to　attempt　to　disguise　the　fact　that　something　more　than　a　mere　vague　tradition　was　supposed　to　support　the　idea　of　the　earth's　overshadowing　importance　in　the　cosmical　scheme。　The　sixteenth…century　mind　was　overmastered　by　the　tenets　of　ecclesiasticism；　and　it　was　a　dangerous　heresy　to　doubt　that　the　Hebrew　writings；　upon　which　ecclesiasticism　based　its　claim；　contained　the　last　word　regarding　matters　of　science。　But　the　writers　of　the　Hebrew　text　had　been　under　the　influence　of　that　Babylonian　conception　of　the　universe　which　accepted　the　earth　as　unqualifiedly　centralwhich；　indeed；　had　never　so　much　as　conceived　a　contradictory　hypothesis；　and　so　the　Western　world；　which　had　come　to　accept　these　writings　as　actually　supernatural　in　origin；　lay　under　the　spell　of　Oriental　ideas　of　a　pre…scientific　era。　In　our　own　day；　no　one　speaking　with　authority　thinks　of　these　Hebrew　writings　as　having　any　scientific　weight　whatever。　Their　interest　in　this　regard　is　purely　antiquarian；　hence　from　our　changed　point　of　view　it　seems　scarcely　credible　that　Tycho　Brahe　can　have　been　in　earnest　when　he　quotes　the　Hebrew　traditions　as　proof　that　the　sun　revolves　about　the　earth。　Yet　we　shall　see　that　for　almost　three　centuries　after　the　time　of　Tycho；　these　same　dreamings　continued　to　be　cited　in　opposition　to　those　scientific　advances　which　new　observations　made　necessary；　and　this　notwithstanding　the　fact　that　the　Oriental　phrasing　is；　for　the　most　part；　poetically　ambiguous　and　susceptible　of　shifting　interpretations；　as　the　criticism　of　successive　generations　has　amply　testified。　As　we　have　said；　Tycho　Brahe；　great　observer　as　he　was；　could　not　shake　himself　free　from　the　Oriental　incubus。　He　began　his　objections；　then；　to　the　Copernican　system　by　quoting　the　adverse　testimony　of　a　Hebrew　prophet　who　lived　more　than　a　thousand　years　B。C。　All　of　this　shows　sufficiently　that　Tycho　Brahe　was　not　a　great　theorist。　He　was　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