The easiest example we can give of bodies so contrasted is found in the case of the sun and the moon. Of course, as we have already seen, the sun is the splendid source of light which it scatters all around. Some of that light falls on our earth to give us the glories of the day; some of the sunbeams fall on the moon, and though the moon has itself no more light than earth or stones, yet when exposed to a torrent of sunbeams, she enjoys a day as we do. One side of her is brilliantly lighted; and this it is which renders our satellite visible.
Hence we explain the marked contrast between the sun and the moon. The whole of the sun is always bright; while half of the moon is always in darkness. When the bright side of the moon is turned directly towards us, then, no doubt, we see a complete circle, and we say the moon is full. On other occasions a portion only of the bright surface is directed to us, and thus are produced the beautiful crescents and semicircles and other phases of the moon.
Fig. 28.—To show that the Moon is lighted by Sunbeams.
A simple apparatus (illustrated in [Fig. 28]) will explain their various appearances. The large india-rubber ball there shown represents the moon, which I shall illuminate by a beam from the electric light. The side of the ball turned towards the light is glowing brilliantly, and from the right side of the room you see nearly the whole of the bright side. To you the moon is nearly full. From the centre of the room you see the moon like a semicircle, and from the left it appears a thin crescent of light. I alter the position of the ball with respect to the lamp, and now you see the phases are quite changed. To those on my left our mimic moon is now full; to those on my right the moon is almost new, or is visible with only a slender crescent. From the centre of the room the quarter is visible as before. We can also show the same series of changes by a little contrivance of [Figs. 29] and [30].
Thus every phase of the moon ([Fig. 31]), from the thinnest beautiful crescent of light that you can just see low in the west after sunset up to the splendor of the full moon, can be completely accounted for by the different aspects of a globe, of which one-half is brilliantly illuminated.
Fig. 29.
Fig. 30.
The Phases of the Moon.