Cosmic Order, the Movement of the Earth, and the Scientific Revolution
Chapter VII. The Postulate of Uniform Circular Motion of Celestial Spheres and Celestial Phenomena
← 126 | 127 → CHAPTER SEVEN
THE POSTULATE OF UNIFORM CIRCULAR MOTION OF CELESTIAL SPHERES AND CELESTIAL PHENOMENA
Copernicus’ terrestrial globe, ready to be launched on its planetary voyage, will perform its voyage pursuant to predetermined rules. When Copernicus elevates it to a position of a planet, the earth is able to move in the same way as other celestial bodies, that is, in uniform circular motion. Celestial motion is, also in Copernicus’ view, “uniform and circular, perpetuous, or compounded of several circular motions.”19
Circularity. In the Commentariolus, Copernicus derives the postulate of uniform circular celestial motion from the form or shape of a celestial body.20 He does the same in Chapter 4 of Book I of De revolutionibus by linking uniform circular motion to the sphere and its shape, which carries individual celestial bodies:
I shall now recall that the motion of the heavenly bodies is circular, since the motion appropriate to a Sphere is rotation in a circle. By this very act the sphere expresses its form in the simplest body, where neither beginning nor ← 127 | 128 → end can be found, nor can the one be distinguished from the other, while itself traverses the same points to return upon itself.21
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