The Evolution of Worlds

by Percival Lowell

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.

Round a great central globular mass, the Sun, far exceeding in size any of his attendants, circle a series of bodies at distances from him quite vast, compared with their dimensions. These, his principal planets, are in their turn centres to satellite systems of like character, but on a correspondingly reduced scale. All of them travel substantially in one plane, a fact giving the system thus seen in its entirety a remarkably level appearance, as of an ideal surface passing through the centre of the Sun. Departing somewhat from this general uniformity in their directions of motion, and also deviating more from circularity in their paths, some much smaller bodies, a certain distance out, dart now up now down across it at different angles and from all the points of the compass, agreeing with the others only in having the centre of the Sun their seemingly never attained goal of endeavor. These bodies are the asteroids. Surrounding the whole, and even penetrating within its orderly precincts, a third class would be visible which might be described for size as cosmic dust, and for display as heavenly pyrotechnics. Coming from all parts of space indifferently they would seem to seek the Sun in almost straight lines, bow to him in circuit, and then depart whence they came. For in such long ellipses do they journey that these seem to be parabolas. These visitants are the comets and their associates the meteor-streams.

Although for purposes of discrimination we have labelled the several classes apart, an essential fact about the whole company is to be noted: that no hard and fast line can be drawn separating the several constituents from one another. In size the members of the one class merge insensibly into the other. Some of the planets are hardly larger than some of the satellites; some of the satellites than some of the asteroids; some of the asteroids than comets and shooting-stars. In path, too, we find every gradation from almost perfect circularity like the orbits of Io and Europa to the very threshold of where one step more would cease to leave the body a member of the Sun’s family by turning its ellipse into an hyperbola. Finally, in inclination we have every angle of departure from orthodox platitude to unconforming uprightness. This point, that heavenly bodies, like terrestrial ones, show all possible grades of indistinction, is kin to that specific generalization by which Darwin revolutionized zoölogy a generation ago. It is as fundamental to planets as to plants. For it shows that the whole solar system is evolutionarily one.

A second point to be noticed in passing is that undue inclination and excessive eccentricity go together. The bodies that have their paths least circular have them, as a rule, the most atilt. And with these two qualities goes lack of size. It is the smallest bodies that deviate most from the general consensus of the system. With so much by way of generic preface, the pregnancy of which will become apparent as we proceed, we come now to particular consideration of its members in turn.

Nearest to the Sun of all the planets comes Mercury. So close is he to that luminary, and so far within the orbit of the earth, that he is not a very common object to the unaided eye. Copernicus is said never to have seen him, owing, doubtless, to the mists of the Vistula. By knowing when to look, however, he may be seen for a few days early in the spring in the west after sunset, or before sunrise in the east in autumn. He is then conspicuous, being about as bright as Capella, for which star or Arcturus he is easily mistaken by one not familiar with the constellations.

His mean distance from the Sun is thirty-six million miles, but so eccentric is his orbit, the most so of any of the principal planets, that he is at times half as far off again as at others. Even his orbital behavior is the least understood of any in the solar system. His orbit swings round at a rate which so far has defied analysis. It may be a case of reflected perturbation, one, that is, of which the indirect effect from another body becomes more perceptible than would be the direct effect on the body itself. As yet it baffles geometers.

As to his person, our ignorance until lately was profound. It is only recently that such fundamental facts about him as his size, his mass, and his density have been reached with any approach to precision. This was because he so closely hugs the Sun that observations upon his full, or nearly full, disk had never been attempted. When I say that his volume was not known to within a third of its amount, his mass not closer than one-half, while his received density was nearly double what we now have reason to suppose the fact, some idea of the depth of our nescience may be imagined. This, of course, did not prevent text-books from confidently misinstructing youth, or Nautical Almanacs from misguiding computers with figures that thus almost achieved immortality, so long had they passed current in spite of lacking that perfection which is usually assigned as its warrant.