Nicolaus Copernicus: On the Revolutions of the Celestial Spheres

Explanation and Analysis of the Document

This excerpt, from book I, argues that the universe and earth are spherical and that the sun, not the earth, is the center of the solar system. The conclusion that the universe is spherical can be reached for numerous reasons, all deriving from the fact that the sphere is a “perfect” form. Accordingly, the universe is spherical because the sun, the moon, and the rest of the planets are spherical. The earth, too, is spherical, “because it presses from all sides upon its center.” In support of his view, Copernicus notes observations of the stars that can be made as the observer approaches the poles or travels in an east-west direction. He adds the commonplace observation that a person can see farther from the top of a ship's mast than from the deck because of the earth's curvature.

Copernicus goes on to argue that the earth is just another planet in the solar system and universe. He notes that “the great majority of authors of course agree that the earth stands still in the center of the universe, and consider it inconceivable and ridiculous to suppose the opposite.” He goes on: “From this supposition follows another question of no less importance, concerning the place of the earth, although it has been accepted and believed by almost all, that the earth occupies the middle of the universe.” Copernicus, however, maintains that the sun and the stars do not revolve around the earth but rather that the earth rotates in a twenty-four-hour day, which makes it appear that the stars revolve around the earth. Thus, he says, the “movement” of the heavens is an illusion, caused by the movement of the observer on earth, which, in turn, is moving through space. Again, he turns to a commonsense observation: “And why will we not recognize that the appearance of a daily revolution belongs to the heavens, but the actuality to the earth; and that the relation is similar to that of which one says: 'We run out of the harbor, the lands and cities retreat from us.'”

Copernicus notes that the earth's spherical shape cannot always be understood because of elevations and depressions, but these do not erase the fact that overall the earth is spherical. He devotes considerable attention to the motions and orbit of the planets, often contrasting his ideas with Aristotle's. The excerpt implies that the size of the universe is vast and staggering but also that the universe is finite in size. Within this universe, the earthly world is not different from the heavenly one; the earth is subject to the same laws of motion as bodies in the celestial sphere and the universe as a whole. Copernicus concludes his argument by saying: “So it appears from all these considerations that the movement of the earth is more probable than its fixity, especially in regard to the daily revolution, which is most peculiar to the earth.” The Copernican revolution was under way.