When you watch the rising sun, remember that, though it seems actually to climb the sky, and to mount higher and higher as the day goes on; and then, when it is setting, to go slowly down, down, behind the far away hills or the shining waves--it is all seeming. Just as, when you are going along in a fast train, the fields and trees and sheep all seem to be in motion, flying past you; yet you know that _you_ are moving as the train moves, and flying past _them_; so it is not the sun moving across the sky which makes day and night, but these changes are caused by the movement of our earth, as she spins round upon herself like a great top. You remember that Galileo was accused of denying the truth of the word of God, because the Bible speaks in many places of the sun _"arising"_ and _"going down."_ His accusers forgot that God does not teach us astronomy, but speaks in His word of things as they appear to our eyes. We have seen that our earth, with her faithful companion the moon, is not only the traveller round the sun; he is the great centre, and around him all the moving-stars, or planets, travel in their varied paths. But the moon has a little journey of her own to take besides this long one, for she travels round the earth, and takes nearly thirty days on her way. You know that the moon is always changing; you can never see it for two or three nights quite the same, but it seems each night a little smaller or a little larger than when you last saw it. When you looked out of the windows the other night, just before you went to bed, it was a very young moon indeed that you saw--not more than two days old, as we say in reckoning the moon's age. How small and thin it was--just like a curving rim of pale light upon the dark sky; but as you watch this crescent--or growing--moon, you will see it constantly getting larger and brighter, until from being half-moon it has become full-moon, for it faces the sun, and is bright all over that part which is turned towards you. When we speak of the "face of the moon," we mean that side which is always turned towards us. But why does "the gentle moon" always turn the same face to us? Astronomers tell us that it is because she also turns slowly round on her own axis while she is travelling round the earth. _How_ this is, I don't think I can explain to you: but it is true that we can see only one side of the moon, that side which catches the sunlight, and that hardly anything is known about the other side.