| Chapter 1: | The New Astronomy |
Figure 1.4. The bounded heliocentric model of Nicholas Copernicus, from De Revolutionibus (1543).
in the sky to the objects themselves. To Copernicus, on the other hand, retrograde motion was a matter of relative motion—an appearance resulting from the orbital motion of a planet relative to an observer on Earth who is also in motion.2 In effect, the five epicycles that were required to explain retrograde motion in the Ptolemaic scheme fall victim to the single proposition that the Earth orbits the Sun. The aesthetic appeal of the Copernican solution lies in its economy of suppositions, which accords with the precepts of Giles of Rome (c.1247–1316) and William of Occam (c.1284–1349), whose instrument of logic (“Occam's razor”) states that hypotheses are not to be multiplied without necessity—that is, that the preferred theory is the simplest one that explains the facts.
Concerning the fact that the planets shine, Copernicus criticized his geocentric predecessors who “do not admit that…planets have a certain