The transits of Venus are rare. Two come near together, separated by only eight years, and then for more than one hundred years the little dark body of Venus is never seen from earth to glide over the sun’s photosphere. There was a transit of Venus in the year 1761, and another in the year 1769. There was a transit of Venus in 1874, and another in 1882. At the last transits it was found that the sun, instead of being ninety-five millions of miles away, as astronomers thought, was only ninety-three millions of miles away. The reason why these transits happen so seldom, is that the orbits of Mercury and Venus lie in rather a different plane or level from the earth’s orbit. So, like the moon, though often passing between us and the sun, they generally go just a little higher or just a little lower than his bright face. Mercury and Venus show phases like the moon, although they do not circle round the earth as the moon does. These “phases,” or changes of shape, are probably never visible except through a telescope. It will be easier to think about the phases of Venus alone, than to consider both together. Her orbit lies within the earth’s orbit, and the earth and Venus travel round the sun--as do all the planets--in the same direction. But as Venus’s pathway is shorter than ours, and as her speed is greater, she is much the quickest about her yearly journey, and she overtakes us again and again at different points of our orbit in turn. [Illustration: PHASES OF VENUS.] At one time she comes between us and the sun. That is her nearest position to us, and she is then only about twenty-five millions of miles distant. A beautiful sight she would be, but unfortunately her bright side is entirely turned away, and only her dark side is turned towards us. So then she is “new Venus,” and is invisible. At another time she is completely beyond the sun, and at her farthest position away from us. Her shining is quite lost in the sun’s rays coming between. And though we get a good view of her as “full Venus,” at a little to one side or the other, yet so great is her distance--as much as one hundred and fifty-seven millions of miles--that her size and brightness are very much lessened. Between these two nearest and farthest points, she occupies two middle distances, one on each side of the sun. Then, like the moon at her “quarters,” she turns to us only half of her bright side. But this is the best view of Venus that we have, as a brilliant, untwinkling, starlike form,--the Evening Star of ancients and of poets. Between these four leading positions Venus is always traveling gradually from one to another--always either waxing or waning in size and in brightness. Mercury passes through the same seeming changes.