78
THE BEAUTY OF THE HEAVENS.
Gemini; when at e, it has still gone forward, and is in Taurus ; but from thispoint it appears to retrograde, and apparently moves back again through thesame constellations, among which it before seemed to move forward : for whenat f, it appears to have moved back to Gemini; at o, it appears to have movedback to Cancer ; when at h, it seems still to move back, for it has then nearlyreached Leo again, whence it began to move forward in the first instance.When it has again arrived at a, it appears again to be at the beginning of Leo;and when again it advances towards b, its apparent forward motion is repeated.The scale, k l, on the left hand of the scene, assists the eye in tracing thebackward and forward movements. The circle of globes, on the outer side ofthat on which the planet Mercury is placed, represents the planet Venus in herorbit, and, from her situation as regards the earth and sun, she presents thesame phenomena.
Scene No. LVI.— The Earth ’s Shadows.
It is known that every planet, both primary and secondary, receives itslight from the sun, and must, therefore, of necessity, cast its shadow towardsthat side which is opposite to the sun. Such shadows are, of course, merely aprivation of light in the space hidden from the sun by the planet, and arealways proportionate in their extent to the relative magnitudes of the sun andthe planet. If they were both of the same size, the form of the shadow cast bythe planet would be that of a cylinder, of the same diameter as the sun ; itslength unlimited. But we know that the sun is much larger than any of theplanets, and, therefore, their shadows must converge to a point.
Such is the condition of the earth. The scene shews the sun and the earth ;the sun being much larger than the earth, the shadow of the latter tends to apoint. The distance of this point from the earth is calculated to be about threetimes and a half the distance of the moon from the earth, or 840,000 miles. Inaddition to this shadow, which may be called the dark shadow of the earth,there is a slight shade which surrounds it, and spreads on all sides, beingsomewhat darker near the centre than at its edges ; this not being a perfect sha-dow, is termed the penumbra , from two Latin words, signifying, almost a shadow'.If an observer were placed within the dark shadow, the entire body of the sunwould be hidden from him ; but if he emerged from the dark shadow into thepenumbra, he would gradually see more and more of the sun, until he reachedits outer edge, where he would see the whole. On these circumstances dependthe phenomena called eclipses.