ECLIPSES or THE SUN. These are caused, as we have stated, by the interposition of the moon between the earth and through which a greater or less portion of the sun is necessarily hid from view. By a process similar to that used in ascertaining the length of the earth's shadow, it can he shown that the greatest value of the length of the moon's shadow' is about 60 semi-diameters of the earth: at the same time, we know that the least distance of the 1110(111 from the earth is about 56 semi-diameters. ft follows that when a conjunction of the sun and moon happens at a time when the length of the shadow and the distance of the moon from the earth are, or are nearly. equal to the value, above stated, the moon's shadow extends to the earth and beyond it. Should the shadow in these cir cumstances fall Upon the earth, there will be a total eclipse of the sun in all places within it or mer which it DIOVP,7. If 1, (Fig. 3) be the moon. T the earth, and abl. the moon's shadow cast by the sun, there will Ice a total eclipse of the sun at every point that is completely within the portion al, of the earth's surface. Again, the smallest valne of the length of the moon's shadow may be shown to be about 53 semi-diam eters: of the earth, and the greatest distance of the moon from the earth is about G4 semi diameters. Suppose the moon interposed be tween the earth and sun when these values concur, it is clear that the moon's shadow will fall short of the earth. In this ease, the sun cannot be altogether hid from any point of the earth's surface; lad this case, or one approxi mate to it, is that in which there will occur an annular eclipse. In Fig. -I, suppose 0 to be the apex of the shadow which falls short of the earth, and conceive the cone of the shadow pro duced earthward beyond 0 into a second cone Ord; then from every point within the section ed of the earth's surface. the moon will be seen projected as a black disk on the middle of the disk of the sun, the portion nnobseured form ing a ring or annulus of light. While in the two eases just described the eclipse is total or regular at places within ab (Fig. 31 or cd 1 Fig. 4), respectively. it will be partial at other places: the moon will appear projected against a portion of the sun's disk, making a cir cular indentation. To ascertain the places
at which the eclipse will be partial, we have merely to form the came of the penumbra of the moon's shadow in the manner explained in treating of lunar eclipses: at all places on the earth's surface within that cone there will be a partial eclipse, A simple calculation shows it hat is the observed fact, that the cone of time penumbra is not nearly large enough to em braee the whole of the face of the earth di rected to the sun; in other words. solar eclipses are not universal, like those of the moon, i.e. they are not seen from all places that have tire sun above their horizon at the time of the eclipse, wide)] is the reason that though they are of more frequent occurrence than lunar eclipses, the latter are more frequently seen by the public. and therefore eonmionly supposed to occur more frequently. There are certain appearances. at tending an eclipse of the sun, when it is total, that are very remarkable. The darkening of the orb of day, more particularly when it is unlooked for. is calculated to impress a spectator with vague terror: even when expected, it tills the mind with awe, as a demonstration of the forces and motions of the mechanism of the universe. The sudden darkness, too, is impressive from its strangeness as much as from occurring by day: it resembles neither the darkness of night nor the gloom of twilight. Stars and planets appear, and all animals are dismayed by the gloomy as pect of nature.
There is one important phenomenon attending total eclipses of the sun, which is always seen and the cause of which cannot be said to Ice as yet fully understood. As long as the total eclipse lasts, there appears round the sun and moon a luminous corolla (q.v.). while at its base, and projecting beyond the dark edge of the moon, appear very brilliant prominences, gen erally of a red color. These are found to be constant attendants on eclipses, and methods have been invented for rendering them visible at any time without the interposition of the moon. (See SUN.) The spectroscope shows that they eon,ist mainly of hydrogen gas in an incandescent state. The prominences are sometimes seen to shoot up like flames, in wild fantastic shapes, with incredible velocity, and to the height of hundreds of thousands of miles.