Eclipses, in Astronomy, are special phenomena which occur when the sun, earth, and moon are in a line. When the earth is between the moon and the sun, the moon lies in the shadow of the earth, and so suffers temporary obscuration; a lunar eclipse then takes place. When the moon passes between the earth and the sun, the latter is at certain places on the earth obscured by the dark body of the moon, and a solar eclipse takes place. The comparative rarity of these occurrences, and the grandeur of the effects produced, have combined to impress the ancients and uneducated people of later ages with a deep sense of their supernatural origin.
(a) Lunar Eclipses. The shadow cast by the earth is conical, and may be shown to extend about 1,000,000 miles from its surface. At a distance of a quarter of a million miles away the width of this shadow is about 6,000 miles; and if the moon passes into it at that approximate distance from the earth, its disc of 2,000 miles diameter may be partially or totally obscured. The moon and sun may be on opposite sides of the earth, and yet the former not in shadow. This is due to the fact that the moon's orbit round the earth is not exactly in the same plane as that of the earth's orbit round the sun. If it were so, we should have total eclipses at every full moon; but since the two planes are inclined to each other at an angle of 5° 9', eclipses will occur when the moon is at or near its nodes or positions of coincidence with the plane of the ecliptic. Partial eclipses are produced when only a portion of the moon passes into shadow; annular eclipses such as are sometimes observed in the case of the sun cannot occur with the moon.
(b) Solar- Eclipses. The shadow cast by the moon is also conical, and extends over a slightly varying distance of about a quarter of a million miles from the moon's surface. This being the approximate distance of the moon from the earth, it is seen that when the moon is between the earth and the sun the shadow may reach the earth. The extreme limit of the shadow may range from 23,000 miles short of the earth, in which case an entire eclipse of the sun is impossible, to 15,000 miles beyond the earth. In the latter case a circular shadow will be projected on the surface of the globe, travelling onwards slowly in the direction of the motion of the moon. Within this shadow or umbra the body of the sun cannot be observed, and a, total eclipse prevails. A circular region exists round this shadow, in which only part of the sun is visible; this region is therefore partly in shadow, and is called the penumbra. Outside the penumbra the whole sun may be viewed; the moon's shadow is not nearly large enough to render a solar eclipse co-existent over all parts of the earth's face towards the sun.
When the moon's shadow falls short of the earth there still exists a shaded circeelar region on the globe where the black disc of the moon obscures the central portion of the sun. But the disc-has not a sufficiently large angular diameter to hide the whole face, and a ring of light still remains. This is an annular eclipse; it takes place when the moon is at a great distance from the earth compared with the sun's distance. It is evident that solar eclipses resemble lunar ones in occurring only when the moon lies nearly in the ecliptic. The general appearances of the sun's surroundings during an eclipse are mentioned in the article Sun. Four is the usual number of eclipses of sun and moon in a year; they cannot be less than two or more than seven.