Astronomers have sometimes fancied that they could catch a glimpse of one small world very near to the Sun, which they named VULCAN. But it is very doubtful whether there really is any such planet at all. If there is, he is almost lost in the glare of the Sun. The nearest to the Sun of which we know positively is named MERCURY. He is much smaller than our world, though larger than our Moon; and he whirls along at a dizzy speed. Outside the pathway of Mercury, like a large hoop round a little one, only at a good distance off, lies the oval-shaped pathway of VENUS. Though we often speak carelessly of this lovely world as “The Evening Star,” Venus is no star, but a planet like the Earth, shining only in the Sun’s light. And, although perhaps not really brighter in herself than all other worlds, Venus is by far the brightest in our sky. Mercury’s pathway lies too close to the Sun to give us often very good views of him. Besides, Mercury is not only much smaller than Venus, but much farther away from us. That is to say, Mercury at his nearest is farther off from us than Venus at her nearest. When Mercury happens to be between the Sun and us, while Venus happens to be far away on the other side of the Sun, just then, of course, Mercury for a little while is the closer to us of the two. If you have three hoops of different sizes you will be able to see quite easily how this comes about. Lay the hoops on the floor, one within another, and place a ball in the middle for the Sun. Then lay one marble, for the Earth, close to the outermost and largest hoop, and another marble for Mercury, close to the innermost and smallest hoop, on the _same side_ as the Earth-marble. Then put a marble, for Venus, close to the middle-sized hoop, still on the same side. So the three worlds are all together on the same side, as near as they ever can come one to another. And you will see that Mercury can never get so close to the Earth as Venus can. But now, leaving the Earth and Mercury alone, move the Venus-marble half-way round its hoop, to just the opposite side of the Sun. You will then understand how sometimes, for a little while, Venus may be actually much farther off than Mercury from our Earth. These worlds all travel on different pathways at different speeds, and the pathways are not of the same length. So the worlds never keep long side by side. For a little while they may journey in company; then one gets ahead and the other drops behind. By-and-by they are on opposite sides of the Sun; and then in time they draw near one to another again. Both Mercury and Venus have _phases_, or seeming changes of shape, like our Moon. They shine only on one side, that side which is towards the Sun; and sometimes we see only part of the bright side, not the whole of it. But the changes are too small at such a distance to be seen without a telescope.