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476 interesting yet fanciful speculation. It remained for a subsequent age to furnish proof of the truth of the Copernican system which could not be gainsaid or resisted.

At the beginning of the seventeenth century, the dicta of Aristotle, in regard to matters of science as well as philosophy, were still accepted, as they had been for many centuries preceding, as of infallible authority. In regard to the subject of our inquiry, he taught that bodies at the surface of the earth fell or tended to fall toward the center of the earth, not in virtue of any attraction of the earth, but in virtue of the fact that the center of the earth was the center of the material universe—that if the earth itself should be moved out of its place and then left free to move, it would return to its place by the same law of nature which controlled all terrestrial bodies. He taught moreover that celestial bodies were different in kind from bodies terrestrial—that whilst the latter were imperfect, corruptible and changeable, the former were perfect, (and therefore, according to his fancy, perfectly spherical in form) incorruptible, unchangeable and self-luminous. Being different in kind, he held that they were subject to entirely different physical laws; that whereas the motion of terrestrial bodies when free to move was rectilinear, by a necessity of their nature, the motion of celestial bodies was circular by a like necessity of their nature. His language on this point is worth quoting as an illustration of the contrast between the ancient and modern method of philosophizing in regard to natural phenomena. He says: "All simple motion must be rectilinear or circular; either to a center or from a center, each of which is rectilinear, or about a center. It is natural for two of the elements—earth and water—which are heavy, to tend to a center; two—air and fire—which are light, to tend from a center. As the motion of all terrestrial elements is therefore rectilinear, it seems reasonable that celestial bodies, which are of a different nature, should have only the other simple motion possible, namely, circular motion."

The year 1609 marks a new era in the history of astronomy. In this year two events occurred, independent, yet alike memorable as contributing to the overthrow of the theory in regard to the structure of the material universe which had previously prevailed and establishing the doctrine of Copernicus upon an immovable foundation. The invention of the telescope by Galileo, and the immediate discovery by means of it of the inequalities of the moon's surface, the phases of Venus, the satellites of Jupiter and the rings of Saturn, at once annihilated the fancies of Aristotle as to the perfectly spherical form of the planets, their self-luminosity, and their difference in kind from bodies terrestrial. The other memorable event referred to was the publication of Kepler's great work on 'The Motions of Mars,' in which, with much that was fanciful, two of the three laws of planetary motion were for