COPERNICAN SYSTEM. Tl:e system which represents the sun to be at rest and the earth and planets to move round it; in other words, that which we now know, on unquestion able evidence, to be the true system of astronomy. ( See PTOLEMATC SYSTEM.) It has its name from Copernicus, but, in point of fact, it may lie 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 in creased the probability of its truth by his cal culations and arguments: for the rest, the glory of having natured the idea belongs to Kepler, Galileo, and Newton, who, through the discovery of the law of gravitation, finally demonstrated its truth effectually. Many who have been used to reverence the name of Copernicus in connec tion with this system would he surprised to find, on perusing his work, Dc Rerolutionibus 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.
Dc Revolutionibus Orbium, dedicated to Pope Paul III., consists of six books, in which Coper nicus 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 establish ing his various positions. It was then thought a sufficient demonstration 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 univer6ally admitted to be of great weight in scientific arguments.
Illustrations of both of these peculiarities may be gleaned from the first of the six books of De Rerolation-ilms. It contains the following propo sitions: (1) That the universe is spherical. This is established by such arguments as that the sphere is the most perfect figure, etc. (2) That the earth is spherical, which flows from the same kind of considerations. (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 simple body must move circularly, and nothing but circular motion could give periodieity to phenomena. (5) That, supposing
the distance of the stars to be immense, there is no reason why the earth should not have a mo tion round its axis as well as a motion in its orbit. (6) That the sphere of the stars is immensely distant. (7 and S) The ancients were wrong in placing the earth at the centre of the universe. The arguments under this head are as imaginary as those which they were designed to refute. The falling of a body to the earth is deduced 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 straight lines. That there must be a (Cut•u• mundi, an entity not recognized by mod ern science, is admitted. the question being as to its position. (9) It is possible for the earth to have several motions. (10) Copernicus 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.
The most brilliant and valuable part of the De Rcruiutionibus 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. Ihit Copernicus had neither mathematical inn mechanical knowledge sufficient to enable Inin to explain more than the mean motions of the solar system. To account for irregularities, he was obliged to introduce a system of epicycles en tirely resembling that of Ptolemy. (See PTOLE MAIC SYSTEM.) This arose front the false no tion of his time that. all motions must be com pounded of circular ones, with the application of whieh idea, and with the invention of convenient. epicycles, the greater part of the Dc Revolutioni bus 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 he met by the mechanical knowledge of the time. 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 di rectly beneath the points from which they are dropped; for this he had no answer, nor could be have, the laws of motion being not yet dis covered.