什麼是蝕

當三個天體排成一線時，一個天體被另一個天體全部或區域性遮掩的天文現象。一種情形是：交蝕天體進入觀測者和被蝕天體之間，觀測者看到的被蝕天體全部或區域性的被交蝕天體遮掩，日蝕（solar eclipse）即屬此類。月球在繞地球執行的過程中，有時會走到日、地之間，這時若月球的影子掃過地球表面，就會發生月蝕（lunar eclipse）。由於日光不能穿透影子的中心部分（本影），對於本影內的地球上的觀測者，太陽圓面完全被月球遮蔽，稱為全蝕（total eclipse）；而對於位於月影的外圍部分（半影）的觀測者，太陽只有一部分被月球遮蔽，稱為偏蝕（partial eclipse）。蝕對古時的人來說是一種令人敬畏的現象，歷史上早有明載，有些居民（如巴比倫、馬雅和中國人）已習得準確地預知蝕發生的時間。全世界每年大約發生二～五次日蝕，可於地球上不同地點觀之，而日全蝕（total solar eclipse）則要好幾年才會出現一次。地球最接近太陽時即為月球和地球相隔最遠之際，此時月球的影子會全然落在太陽的範圍之內，外圍環繞著可見光環（即為日環蝕〔annular eclipse〕）。日全蝕具有重大科學研究價值，能提供更多有關色球（chromosphere）和日冕（corona）的知識，通常它們總是隱沒於光球（photosphere）的奪目光芒之中。月蝕的發生頻率和日蝕一般，月全蝕（total lunar eclipse）發生時，月球會呈古銅色，這是因為地球大氣把部分陽光（特別是紅光）折射入地球影錐的緣故。其他恆星或行星的交蝕現象（參閱eclipsing variable star）也可以提供科學家研究該天體的資料。亦請參閱Baily's beads。

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.