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（solar eclipse）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（lunar eclipse）。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（total eclipse）；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（partial eclipse）。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（total solar eclipse）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〔annular eclipse〕）。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（total lunar eclipse）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eclipsing variable star）也可以提供科学家研究该天体的资料。亦请参阅Baily's beads。

English version：

eclipse

Complete or partial obscuring of one celestial body by another when three such objects become aligned. In one type, the eclipsing body comes between an observer and a luminous source, appearing to cover it totally or partly, as when the moon comes between earth and the sun (solar eclipse). A second type occurs when the eclipsing body comes between the luminous source and casts a shadow on the eclipsed object, which is darkened by its shadow, as when the moon enters earth's shadow (lunar eclipse). The shadow consists of the central umbra, into which no direct sunlight penetrates (total eclipse), and the penumbra, reached by light from only part of the sun's disk (partial eclipse). Eclipses are awe-inspiring and are chronicled in the oldest records of history. Some civilizations (e.g., Babylonian, Maya, Chinese) learned to predict eclipses accurately. Solar eclipses visible from different parts of earth occur two to five times a year; one total solar eclipse occurs in most years. When earth is closest to the sun and the moon farthest from earth, the moon's shadow may fall entirely within the sun's disk, with a ring of the disk visible around it (annular eclipse). Total solar eclipses have increased knowledge of the nature of the chromosphere and corona, usually invisible in the glare of the photosphere. Lunar eclipses occur just as frequently as solar eclipses; during total lunar eclipses, the moon may appear deep red from sunlight refracted through earth's atmosphere. Eclipses of other stars and planets may provide information about them (see eclipsing variable star). See also Baily's beads.

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