The rotation of the earth on its axis, as we know, causes the familiar daily apparent rising, passing, and setting of all the heavenly bodies. In this apparent motion the planets share as well as the sun, moon, and stars. But it is their movement _among_ the fixed stars, and not _with_ them, that distinguishes them as planets, and this it is necessary to know in order to keep track of them and be able to recognize them in their varying places and guises. For they sometimes shine in their greatest glory in one season, and sometimes in another, and at the recurrence of the same season they are sometimes in one part of the sky and sometimes in another, so that their ways of coming and going border almost on the mysterious, until one learns the manner of this apparent vagrancy. Happily, this knowledge is easily attained, and then the matter is simple enough. The apparent motions of the inferior planets, Mercury and Venus, always take place near the sun. Venus never wanders more than forty-eight degrees from it, and Mercury never more than twenty-eight. Most of the time they are much nearer than this. Since we cannot see either of them except when the sun is below the horizon, the consequence of their being always thus near to him is that they are in view for only a short time after the sun has set or before he has risen. If they are in the evening sky, and hence east of the sun, they soon follow him when he sinks below the western horizon. If they are west of the sun, and, consequently, are the first to rise in the morning, it is not long before his brilliant rays flood with light the eastern sky and blot the planets from our view. Venus can be seen sometimes for three hours at a time, Mercury for never more than one. Within this limited region of the sky they appear to journey evening by evening away from the sun, somewhat obliquely, but toward the zenith, until they have reached the end of their tether. Then they journey back and pass to the other side of the sun. There they climb their path toward the zenith, moving westward and, as we see them, obliquely upward. Morning by morning they get farther from the sun until their westward limit of freedom is reached, when they again draw in toward the sun, pass it, appear in the evening sky, and pull off up the sky toward the east again. Thus they swing from east to west of the sun, and back again, in unceasing repetition.