A solar eclipse is caused by the fact that the moon, in her revolution round the earth, comes at certain periods between us and the sun, and obscures the light of the latter body either partially or totally. Owing to the fact that the plane of the orbit in which the moon revolves round the earth does not coincide with that in which the earth revolves round the sun, the eclipse is generally only partial, the moon not occupying the exact line between the centres of the sun and the earth. The dark body of the moon then appears to cut off a certain portion, larger or smaller, of the sun's light; but none of the extraordinary phenomena to be presently described are witnessed. Even during a partial eclipse, however, the observer may find considerable interest in watching the outline of the dark moon, as projected upon the bright background of the sun. It is frequently jagged or serrated, the projections indicating the existence, on the margin of the lunar globe, of lofty mountain ranges. [Illustration: FIG. 19.--ECLIPSES OF THE SUN AND MOON.] Occasionally the conditions are such that the moon comes centrally between the earth and the sun (Fig. 19), and then an eclipse occurs which may be either total or annular. The proportion between the respective distances from us of the sun and the moon is such that these two bodies, so vastly different in real bulk, are sensibly the same in apparent diameter, so that a very slight modification of the moon's distance is sufficient to reduce her diameter below that of the sun. The lunar orbit is not quite circular, but has a small eccentricity. It may therefore happen that an eclipse occurs when the moon is nearest the earth, at which point she will cover the sun's disc with a little to spare; or the eclipse may occur when she is furthest away from the earth, in which case the lunar diameter will appear less than that of the sun, and the eclipse will be only an annular one, and a bright ring or 'annulus' of sunlight will be seen surrounding the dark body of the moon at the time when the eclipse is central.