On the Revolutions of the Heavenly Spheres

REVOLUTIONS BOOK I.

...times brings it back, in which multiple motions are understood. Since it is impossible that a simple celestial body should move irregularly in a single orbit. For such a thing would have to happen either because of an inconsistency in the moving power original: "uirtutis mouẽtis" — the internal or external force that keeps a planet in motion.—whether that force be an external addition or part of its inner nature—or because of some defect in the body being revolved. Since the human intellect shrinks from both of these ideas, and it is unworthy to imagine such a flaw in things that are established in the most perfect order, it is reasonable to conclude that their uniform motions only appear irregular to us. This happens either because the poles of their circles are different, or because the Earth is not located in the center of the circles in which they revolve. To us, looking from the Earth, the passing of these stars appears to vary because of their unequal distances: those closer to us appear larger than those further away (as is demonstrated in the study of optics). Thus, even though the stars travel equal distances along their orbits, they will appear to move at different speeds during equal amounts of time because of the varying distance from our eyes.

This presupposition serves all the previous axioms.

For this reason, I think it is necessary above all else that we carefully observe the relationship of the Earth to the heavens. Otherwise, while we wish to investigate the highest things, we might remain ignorant of the things closest to us, and by that same error, attribute to the heavenly bodies things that actually belong to the Earth.

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

A decorative initial letter 'I' showing a scholar or authority figure seated in a room, representing the start of a new section of the argument.
Now, because it has been demonstrated that the Earth also has the shape of a globe, I believe we must see whether its motion also follows its shape, and what place it occupies in the universe. Without these facts, it is impossible to find a reliable explanation for the phenomena seen in the heavens. Although authors generally agree that the Earth rests at the center of the world original: "medio mundi" — in the context of the 16th century, "mundus" referred to the entire universe., so that they think it unthinkable and even ridiculous to hold the opposite view, if we consider the matter more carefully, it will appear that this question is not yet settled, and therefore should by no means be dismissed.

That the Earth moves.

For every perceived change in place is due to the motion of the object seen, or of the observer, or certainly to an unequal movement of both. For when two things move equally in the same direction, no motion is perceived—I mean between the thing seen and the one seeing it. Now, the Earth is the place from which that heavenly circuit is observed and reproduced for our sight. Therefore, if any motion is attributed to the Earth...

# As with sailors, for whom the land and everything they see appear to move.