THE REVOLUTIONS BOOK I.

it brings them back original: "bis reducit"; completing the sentence from the previous page regarding the return of planets to their starting positions., in which several motions are understood. Since it is impossible that a simple celestial body should move irregularly in a single orbit. For that would have to happen either because of an inconstancy in the moving power original: "uirtutis mouentis"; the internal or external force believed to drive the spheres.—whether that power be an external force or part of its inner nature—or because of some unevenness in the revolving body. Since the mind recoils from both these ideas, and it is unworthy to suppose such a thing exists in those things which are established in the best order, it is reasonable to conclude that their uniform motions appear irregular to us. This happens either because the circles in which they revolve have different poles, or because the Earth is not at the center of the circles in which they move. To us, viewing them from the Earth, the passing of these stars appears irregular because of their unequal distances; those that are closer seem larger than those further away (as is demonstrated in the study of optics). Thus, even on equal sections of an orbit, the motions will appear unequal in equal amounts of time because of the varying distance from our line of sight. For this reason, I think it is necessary above all else that we carefully observe the relationship of the Earth to the heavens, lest while we wish to investigate the loftiest things, we remain ignorant of those closest to us, and by the same error attribute to the celestial bodies things that belong to the Earth.

Whether a circular motion belongs to the Earth, and concerning its location. Chapter V.

Now, since it has been demonstrated that the Earth also has the form of a globe, I think we must see whether a motion also follows from its form, and what place it holds in the universe; without these, it is impossible to find a certain explanation for the appearances in the heavens. Although authors generally agree that the Earth remains at rest in the center of the universe, so much so that they think the contrary view is unthinkable and even ridiculous, if we consider the matter more carefully, it will appear that this question is not yet settled, and therefore should by no means be disregarded. For every change in place that is seen is due either to the motion of the object being watched, or of the observer, or certainly to a differing movement of both. For between things moved equally in the same direction, motion is not perceived—I mean between the thing seen and the one seeing it. Now, the Earth is the place from which that celestial circuit is viewed and reproduced for our sight. If, therefore, some motion of the Earth...