Page:Popular Astronomy - Airy - 1881.djvu/81

Rh astronomers of antiquity had got a rough notion of the distances of some of the celestial bodies. But one will do for our present purpose. The moon is a long way off. There are phenomena observed frequently, in the interpretation of which there can be no mistake, namely eclipses of the moon. We see that the moon, in her motion through the stars, dips into something which obscures her. There cannot be a doubt that it is the shadow of the earth. The moon goes into this shadow on one side, and comes out of it on the other side. The time which the moon occupies in passing through this shadow is, roughly speaking, four hours. The moon, then, is at such a distance that in passing through the shadow of an object as big as the earth, she is occupied only four hours. The moon, therefore, in her course describes the breadth of the earth in four hours; in one day she describes six times the breadth; and as thirty days is a rough measure for the time of her revolution, she describes in one revolution 180 times the breadth of the earth, and therefore the whole circumference of the moon's orbit is something about 180 times the breadth of the earth, and the diameter of the moon's orbit is about 60 times the breadth of the earth. Therefore the moon is distant from us by about 30 times the earth's breadth.

But there are other facts founded on observation. The sun is further off than the moon. There are phenomena called eclipses of the sun. We know that these correspond to times when the moon apparently approaches the sun; they are undoubtedly caused by the moon passing in front of the sun. Again, the stars are further off than the moon; because the moon passes in front of the stars, 2