Once in a while the tiny body of Mercury is seen to creep, as a little black dot, across the face of the Sun: though this is only visible in a telescope. Then we have a “Transit of Mercury.” In the last chapter you were told what is meant by a Transit. If Mercury were as near to us as our Moon is, he would hide the Sun from us in his transit just as the Moon does in an eclipse--only more fully, because Mercury is bigger than our Moon. In a Transit, as in an Eclipse, there is no real drawing together of Sun and Planet. Be very clear in your mind about this. Mercury glides _between_ our Earth and the Sun, but he is just as far as usual from the Earth on one side and from the Sun on the other side. If you are gazing at a church-tower a great many miles away, and a bird near at hand flies between, hiding for a moment that tower from you, the bird may be said to “eclipse” the tower. But he does not go nearer to the church. He only moves into the straight line between you and the tower. And if, instead of this, a bird some distance off flies between--then you have a “transit.” The more distant bird cannot hide the church-tower, but you see his little body pass across it, as a dark spot. A transit of Mercury is not common. For though Mercury often passes between the Earth and the Sun he is not often _exactly_ between. His oval pathway is not quite on the same level as the Earth’s pathway. So he is usually a little too high or a little too low for us to see him against the Sun. Leaving Mercury behind we come next to the pathway of the planet VENUS. Mercury was a good deal smaller than our Earth; but Venus is almost the same size. Instead of being, like Mercury, only some thirty-six millions of miles away from the Sun, Venus is about sixty-five millions of miles off. She journeys round him at a rate of some twenty-two miles each second, and her year lasts about seven months and a half of Earth-time. In make she is not quite so solid and heavy as our Earth. Venus in her journey round the Sun, as our Moon travels round the Earth, is now believed to turn on her axis so very slowly that the same side is always towards the Sun, and the other side is always turned away. If this really is so, one half of Venus has an endless day, and the other half an endless night. The same state of things may possibly be also true of Mercury. Both these worlds are believed to have air around them, and Venus seems to be enwrapped in thick clouds. This helps to explain the great brilliancy of Venus. Nothing lights up so well in sunshine as masses of cloud, though of course we on Earth more generally see the dark _undersides_ of clouds. So far as we know, Venus is a lonely world. She seems to have no little moon-friend to journey with her in the sky.