When we recall that the Ptolemaic system of the universe was once taught side by side with the Copernican at Harvard and at Yale, we are impressed, not so much with the age of our universities, as with the youth of modern astronomy and with the extraordinary vitality of old ideas. That the Ptolemaic system in its fundamental principle was antiquated at the start, the older Greeks having had juster conceptions, does not lessen our wonder at its tenacity. But the fact helps us to understand why so much fossil error holds its ground in many astronomic text-books to-day. That stale intellectual bread is deemed better for the digestion of the young, is one reason why it often seems to them so dry. [Illustration: ORBITS OF THE INNER PLANETS.] Before entering upon the problem of the genesis and career of a world, it is essential to have acquaintance with the data upon which our deductions are to rest. To set forth, therefore, what is known of the several planets of our solar system, is a necessary preliminary to any understanding of how they came to be or whither they are tending; and as our knowledge has been vitally affected by modern discoveries about them, it is imperative that this exposition of the facts should be as near as possible abreast of the research itself. I shall, therefore, give the reader in this chapter a bird’s-eye view of the present state of planetary astronomy, which he will find almost a different part of speech from what it was thirty years ago. It is not so much in our knowledge of their paths as of their persons that our acquaintance with the planets has been improved. And this knowledge it is which has made possible our study of their evolution as worlds. Could we get a cosmic view of the solar system by leaving the world we live on for some suitable vantage-point in space, two attributes of it would impose themselves upon us—the general symmetry of the whole, and the impressively graded proportions of its particular parts.