蚀
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蚀(eclipse)当三个天体排成一线时,一个天体被另一个天体全部或局部遮掩的天文现象。一种情形是:交蚀天体进入观测者和被蚀天体之间,观测者看到的被蚀天体全部或局部的被交蚀天体遮掩,日蚀(solareclipse)即属此类。月球在绕地球运行的过程中,有时会走到日、地之间,这时若月球的影子扫过地球表面,就会发生月蚀(lunareclipse)。由於日光不能穿透影子的中心部分(本影),对於本影内的地球上的观测者,太阳圆面完全被月球遮蔽,称为全蚀(totaleclipse);而对於位於月影的外围部分(半影)的观测者,太阳只有一部分被月球遮蔽,称为偏蚀(partialeclipse)。蚀对古时的人来说是一种令人敬畏的现象,历史上早有明载,有些居民(如巴比伦、马雅和中国人)已习得准确地预知蚀发生的时间。全世界每年大约发生二~五次日蚀,可於地球上不同地点观之,而日全蚀(totalsolareclipse)则要好几年才会出现一次。地球最接近太阳时即为月球和地球相隔最远之际,此时月球的影子会全然落在太阳的范围之内,外围环绕着可见光环(即为日环蚀〔annulareclipse〕)。日全蚀具有重大科学研究价值,能提供更多有关色球(chromosphere)和日冕(corona)的知识,通常它们总是隐没於光球(photosphere)的夺目光芒之中。月蚀的发生频率和日蚀一般,月全蚀(totallunareclipse)发生时,月球会呈古铜色,这是因为地球大气把部分阳光(特别是红光)折射入地球影锥的缘故。其他恒星或行星的交蚀现象(参阅eclipsingvariablestar)也可以提供科学家研究该天体的资料。亦请参阅Baily'sbeads。Englishversion:eclipseCompleteorpartialobscuringofonecelestialbodybyanotherwhenthreesuchobjectsbecomealigned.Inonetype,theeclipsingbodycomesbetweenanobserverandaluminoussource,appearingtocoverittotallyorpartly,aswhenthemooncomesbetweenearthandthesun(solareclipse).Asecondtypeoccurswhentheeclipsingbodycomesbetweentheluminoussourceandcastsashadowontheeclipsedobject,whichisdarkenedbyitsshadow,aswhenthemoonentersearth'sshadow(lunareclipse).Theshadowconsistsofthecentralumbra,intowhichnodirectsunlightpenetrates(totaleclipse),andthepenumbra,reachedbylightfromonlypartofthesun'sdisk(partialeclipse).Eclipsesareawe-inspiringandarechronicledintheoldestrecordsofhistory.Somecivilizations(e.g.,Babylonian,Maya,Chinese)learnedtopredicteclipsesaccurately.Solareclipsesvisiblefromdifferentpartsofearthoccurtwotofivetimesayear;onetotalsolareclipseoccursinmostyears.Whenearthisclosesttothesunandthemoonfarthestfromearth,themoon'sshadowmayfallentirelywithinthesun'sdisk,witharingofthediskvisiblearoundit(annulareclipse).Totalsolareclipseshaveincreasedknowledgeofthenatureofthechromosphereandcorona,usuallyinvisibleintheglareofthephotosphere.Lunareclipsesoccurjustasfrequentlyassolareclipses;duringtotallunareclipses,themoonmayappeardeepredfromsunlightrefractedthroughearth'satmosphere.Eclipsesofotherstarsandplanetsmayprovideinformationaboutthem(seeeclipsingvariablestar).SeealsoBaily'sbeads.