THE MUSIC OF THE SPHERES.
History of the Planetary Systems from Tholes to Kepler. By J. L. E. Dreyer. (Cambridge University Press. 10s. 6d. net.)— The average man knows that Copernicus, something less than four centuries ago, was the first astronomer who perceived, or at least proved, the true relations of the earth to the starry host of heaven. But he has a very vague and imperfect notion of the earlier theories which undertook to explain the observed or apparent motions of the sun, moon, planets, and fixed stars. The expression "the music of the spheres" is almost the only trace which these cosmogonies have left upon our popular speech, just as astrology is best remembered by adjectives like "disastrous" or "jovial." We are apt to wonder why mankind was so belated in its discovery that the earth, instead of being the immovable centre of the universe, was merely an inconsiderable planet in attendance upon a third-rate star. One of the most difficult things is to put oneself into the place of an earlier thinker, and try to reason out the puzzling problems that are plain enough in the light of later knowledge. Dr. Dreyer, of Armagh Observatory, has undertaken this task for the astronomers from Thales to Kepler, and has given a most interesting account of their varying cosmogonies. He begins with the earliest cosmological ideas, such as those of the Egyptians, who thought that the earth was a. flat surface encircled by a great river, like the Nile, along which the sun was ferried in a boat, which every night went out of sight behind the lofty mountains of the North. He goes on to describe the "guesses at truth" of the Greek philosophers, some of whom made astonishingly happy shots at the real state of the case, and whose speculations culminated in that theory of concentric spheres, bearing sun, moon, planets, and fixed stars upon their transparent shells, to which Pythagoras attached the idea of heavenly harmony—the music of the spheres—which Ptolemy reduced to the form in which it was handed on, with little essential modification, to the age of Milton. As observation became more exact, new refinements had to be introduced into this theory, and it was of the complex structure of cycle and epicycle, deferent and eccentric, thus elaborated that Alfonso of Castillo said that "if he had been asked to help with the Creation, he would have done things more simply." Finally the structure, ingenious as it was, collapsed with its own weight, and the time was ripe for Copernicus to present a simpler and truer hypothesis. - Dr. Dreyees book is, in one sense, only a chapter in the eternally interesting history of human error. But it is well worth study for the sake of the skill that was lavished upon these early astro.. nominal theories, and it is indispensable to those who would understand literary allusions to the system of the universe, from Plato to Dante and Milton.
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