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the Copernican System

earth, motion, motions, copernicus, notion, planets, circular, move and stars

COPERNICAN SYSTEM, THE, is that which represents the sun to be at rest in the center, and the earth and planets to move round it in ellipses; in other words, it is that which we now know, on unquestionable evidence, to be the true system of the world. It got its name from Copernicus, but, in point of fact, it may be described as being a growth to which he was only one of many contributors. The merit of having first formed the general notion of the system seems to be due to Pythagoras; Copernicus has the credit of having, after the lapse of centuries, again drawn the attention of philosophers to it, and of having greatly increased the probability of its truth by his calculations and arguments; for the rest, the glory of having matured its idea belongs to Kepler, Galileo, and others, and to our own Newton, through the discovery of the law of gravitation, demonstrated its truth effectually. Many who have been used to reverence the name of Copernicus in connection with this system, would lie surprised to find, on perusing his work Dc Revolutionibus Orbium, how much of error, unsound reasoning, and happy conjecture combined to secure for him in all times the association of the system with his name.

De Revolutionibus Orbium, dedicated to pope Paul III., consists of six books, in which Copernicus undertook to demonstrate his whole system. The character of the reasoning which then passed for demonstration, must be borne in mind in judging of the author's procedure in establishing his various positions. It was then thought a sufficient demon stration of a phenomenon to make a supposition, on which its occurrence would be intelligible, without attempting to bring the supposition itself, by an induction of facts, within the truth of nature; many abstract propositions, too, which would now appear to be simply silly, were at that time universally admitted to be of great weight in scien tific arguments.

Illustrations of both of these peculiarities may be gleaned from the first of the six books of De Revolutionibus. It contains the following propositions: 1. That the universe is spherical. This is established by such argilments, as that the sphere is the most perfect figure, etc. 2. That the earth is spherical, which flows from thin sanme kind of consider ations. 3. That the earth and sea make one globe. 4. That the motions of all the heavenly bodies must be uniform and circular, or compounded of uniform and circular motions. Here, again, we meet with singular reasons. A 'ample body must move cir• cularly, and nothing but circular motion could give periodicity to phenomena. 5. That, supposing the distance of the stars to be immerse, there is no reason why the earth should not have a motion round its axis as well as a motion in its orbit. 6. That the sphere of the stars is immensely distant. The proof is fanciful, and shows he had no notion of a universe of stars pervading space. 7 and 8. The ancients were wrong in

placing the earth at the center of the universe. The arguments under Ibis head are as imaginary as those which they were designed to refute. The falling of a body to the earth he deduces from the assumption that it is only given to wholes to move circularly, while it is of the nature of parts, separated from their wholes, to move in right lines. That there must be a centrum mundi, an entity unknown to modern science, is admitted, the question being as to its position. 9. It is possible for the earth to have several motions. 10. He establishes the order of the planets, and draws a diagram of the system much as it is now represented. It may be observed that, following the old systems, such as the Ptolemaic, he lays down a sphere for the fixed stars. (See FIRMAMENT.) It is clear, also, that he had no idea of the motions of the planets other than that they were such as would be caused by their being fixed hi immense crystal spheres revolving round the sun.

The most brilliant and valuable part of the De Revolutionibus is that in which he explained, for the first time, the variations of the seasons, the precession of the equinoxes, and the stations and retrogradations of the planets. In general, his explanations are right, and perfect as to the general nature of the causes of the phenomena. But Coper nicus had neither mathematical nor mechanical knowledge sufficient to enable him to explain more than the mean motions of the solar system. To account for irregularities, he was obliged to introduce a system of epicycles entirely resembling that of Ptolemy, See PTOLEMAIC Sys-mt. This arose from the false notion of his times, that all motions must be compounded of circular ones, with the application of which idea, and with the invention of convenient epicycles, the greater part of the De Rerolutionitnes is occupied. It may further be added, to rectify the vulgar notion regarding the relation of Copernicus to the system of the heavens, that he had no answer to offer to the mechanical objections to his system. Most of them, indeed, were such as could not possibly be met in the then state of mechanical knowledge. One of the commonest was that against the axial motion of the earth, that it was inconsistent with the fact of bodies falling to the points of the earth directly beneath the points from which they are dropped; for this lie had no answer, nor could he have, the laws of motion being not yet discovered. Such being the state of the case, the reader will consider whether, when Copernicus wrote that he held the doctrine of the earth's motion as a mere hypothesis, and not as absolutely in fact true, it is more likely that he made a concession to the religious prejudices of his times, or to difficulties surrounding his hypothesis, which he could well appreciate though not overcome.