Copernicus' Hypothesis

After I had addressed myself to this very difficult and almost insoluble problem, the suggestion at length came to me how it could be solved with fewer and much simpler constructions than were formally used, if some assumptions (which are called axioms) were granted me. They follow in this order. There is no one center of all the celestial circles or spheres. The center of the earth is not the center of the universe, but only of gravity and of the lunar sphere. All the spheres revolve about...