a history of science-1-第9章

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rst　Thinite　dynasty；　citing　in　evidence　the　fact　that　the　legend　of　Osiris　explains　these　days　as　having　been　created　by　the　god　Thot　in　order　to　permit　Nuit　to　give　birth　to　all　her　children；　this　expedient　being　necessary　to　overcome　a　ban　which　had　been　pronounced　against　Nuit；　according　to　which　she　could　not　give　birth　to　children　on　any　day　of　the　year。　But；　of　course；　the　five　additional　days　do　not　suffice　fully　to　rectify　the　calendar。　There　remains　the　additional　quarter　of　a　day　to　be　accounted　for。　This；　of　course；　amounts　to　a　full　day　every　fourth　year。　We　shall　see　that　later　Alexandrian　science　hit　upon　the　expedient　of　adding　a　day　to　every　fourth　year；　an　expedient　which　the　Julian　calendar　adopted　and　which　still　gives　us　our　familiar　leap…year。　But；　unfortunately；　the　ancient　Egyptian　failed　to　recognize　the　need　of　this　additional　day；　or　if　he　did　recognize　it　he　failed　to　act　on　his　knowledge；　and　so　it　happened　that；　starting　somewhere　back　in　the　remote　past　with　a　new　year's　day　that　coincided　with　the　inundation　of　the　Nile；　there　was　a　constantly　shifting　maladjustment　of　calendar　and　seasons　as　time　went　on。　The　Egyptian　seasons；　it　should　be　explained；　were　three　in　number：　the　season　of　the　inundation；　the　season　of　the　seed…time；　and　the　season　of　the　harvest；　each　season　being；　of　course；　four　months　in　extent。　Originally；　as　just　mentioned；　the　season　of　the　inundations　began　and　coincided　with　the　actual　time　of　inundation。　The　more　precise　fixing　of　new　year's　day　was　accomplished　through　observation　of　the　time　of　the　so…called　heliacal　rising　of　the　dog…star；　Sirius；　which　bore　the　Egyptian　name　Sothis。　It　chances　that；　as　viewed　from　about　the　region　of　Heliopolis；　the　sun　at　the　time　of　the　summer　solstice　occupies　an　apparent　position　in　the　heavens　close　to　the　dog…star。　Now；　as　is　well　known；　the　Egyptians；　seeing　divinity　back　of　almost　every　phenomenon　of　nature；　very　naturally　paid　particular　reverence　to　so　obviously　influential　a　personage　as　the　sun…god。　In　particular　they　thought　it　fitting　to　do　homage　to　him　just　as　he　was　starting　out　on　his　tour　of　Egypt　in　the　morning；　and　that　they　might　know　the　precise　moment　of　his　coming；　the　Egyptian　astronomer　priests；　perched　on　the　hill…tops　near　their　temples；　were　wont　to　scan　the　eastern　horizon　with　reference　to　some　star　which　had　been　observed　to　precede　the　solar　luminary。　Of　course　the　precession　of　the　equinoxes；　due　to　that　axial　wobble　in　which　our　clumsy　earth　indulges；　would　change　the　apparent　position　of　the　fixed　stars　in　reference　to　the　sun；　so　that　the　same　star　could　not　do　service　as　heliacal　messenger　indefinitely；　but；　on　the　other　hand；　these　changes　are　so　slow　that　observations　by　many　generations　of　astronomers　would　be　required　to　detect　the　shifting。　It　is　believed　by　Lockyer；　though　the　evidence　is　not　quite　demonstrative；　that　the　astronomical　observations　of　the　Egyptians　date　back　to　a　period　when　Sothis；　the　dog…star；　was　not　in　close　association　with　the　sun　on　the　morning　of　the　summer　solstice。　Yet；　according　to　the　calculations　of　Biot；　the　heliacal　rising　of　Sothis　at　the　solstice　was　noted　as　early　as　the　year　3285　B。C。；　and　it　is　certain　that　this　star　continued　throughout　subsequent　centuries　to　keep　this　position　of　peculiar　prestige。　Hence　it　was　that　Sothis　came　to　be　associated　with　Isis；　one　of　the　most　important　divinities　of　Egypt；　and　that　the　day　in　which　Sothis　was　first　visible　in　the　morning　sky　marked　the　beginning　of　the　new　year；　that　day　coinciding；　as　already　noted；　with　the　summer　solstice　and　with　the　beginning　of　the　Nile　flow。　But　now　for　the　difficulties　introduced　by　that　unreckoned　quarter　of　a　day。　Obviously　with　a　calendar　of　365　days　only；　at　the　end　of　four　years；　the　calendar　year；　or　vague　year；　as　the　Egyptians　came　to　call　it；　had　gained　by　one　full　day　upon　the　actual　solar　year　that　is　to　say；　the　heliacal　rising　of　Sothis；　the　dog…　star；　would　not　occur　on　new　year's　day　of　the　faulty　calendar；　but　a　day　later。　And　with　each　succeeding　period　of　four　years　the　day　of　heliacal　rising；　which　marked　the　true　beginning　of　the　yearand　which　still；　of　course；　coincided　with　the　inundationwould　have　fallen　another　day　behind　the　calendar。　In　the　course　of　120　years　an　entire　month　would　be　lost；　and　in　480　years　so　great　would　become　the　shifting　that　the　seasons　would　be　altogether　misplaced；　the　actual　time　of　inundations　corresponding　with　what　the　calendar　registered　as　the　seed…time；　and　the　actual　seed…time　in　turn　corresponding　with　the　harvest…time　of　the　calendar。　At　first　thought　this　seems　very　awkward　and　confusing；　but　in　all　probability　the　effects　were　by　no　means　so　much　so　in　actual　practice。　We　need　go　no　farther　than　to　our　own　experience　to　know　that　the　names　of　seasons；　as　of　months　and　days；　come　to　have　in　the　minds　of　most　of　us　a　purely　conventional　significance。　Few　of　us　stop　to　give　a　thought　to　the　meaning　of　the　words　January；　February；　etc。；　except　as　they　connote　certain　climatic　conditions。　If；　then；　our　own　calendar　were　so　defective　that　in　the　course　of　120　years　the　month　of　February　had　shifted　back　to　occupy　the　position　of　the　original　January；　the　change　would　have　been　so　gradual；　covering　the　period　of　two　life…times　or　of　four　or　five　average　generations；　that　it　might　well　escape　general　observation。　Each　succeeding　generation　of　Egyptians；　then；　may　not　improbably　have　associated　the　names　of　the　seasons　with　the　contemporary　climatic　conditions；　troubling　themselves　little　with　the　thought　that　in　an　earlier　age　the　climatic　conditions　for　each　period　of　the　calendar　were　quite　different。　We　cannot　well　suppose；　however；　that　the　astronomer　priests　were　oblivious　to　the　true　state　of　things。　Upon　them　devolved　the　duty　of　predicting　the　time　of　the　Nile　flood；　a　duty　they　were　enabled　to　perform　without　difficulty　through　observation　of　the　rising　of　the　solstitial　sun　and　its　Sothic　messenger。　To　these　observers　it　must　finally　have　been　apparent　that　the　shifting　of　the　seasons　was　at　the　rate　of　one　day　in　four　years；　this　known；　it　required　no　great　mathematical　skill　to　compute　that　this　shifting　would　finally　effect　a　complete　circuit　of　the　calendar；　so　that　after　（4　X　365　=）　1460　years　the　first　day　of　the　calendar　year　would　again　coincide　with　the　heliacal　rising　of　Sothis　and　with　the　coming　of　the　Nile　flood。　In　other　words；　1461　vague　years　or　Egyptian　calendar　years　Of　365　days　each　correspond　to　1460　actual　solar　years　of　365　1/4　days　each。　This　period；　measured　thus　by　the　heliacal　rising　of　Sothis；　is　spoken　of　as　the　Sothic　cycle。　To　us　who　are　trained　from　childhood　to　understand　that　the　year　consists　of　（approximately）　365　1/4　days；　and　to　know　that　the　calendar　may　be　regulated　approximately　by　the　introduction　of　an　extra　day　every　fourth　year；　this　recognition　of　the　Sothic　cycle　seems　simple　enough。　Yet　if　the　average　man　of　us　will　reflect　how　little　he　knows；　of　his　own　knowledge；　of　the　exact　length　of　the　year；　it　will　soon　become　evident　that　the　appreciation　of　the　faults　of　the　calendar　and　the　knowledge　of　its　periodical　adjustment　constituted　a　relatively　high　development　of　scientific