Galileo, sunspots, and the motions of the earth - Redux

In the Third Day of the Dialogue Concerning the Two Chief World Systems (1632), Galileo presents an argument for the motion of the earth based on the annual motion of sunspots. Presented in a rather obscure and seemingly unorganized manner, this demonstration has been the source of much confusion and debate. Two key writings are Arthur Koestler's attack on Galileo's integrity, based on his reading of the proof as a piece of sophistry, and A. Mark Smith's defense of Galileo, based on a comparison of the Ptolemaic and Copernican explanations of the phenomenon. This essay reexamines the arguments of Galileo and others, uncovering especially a key flaw: that they disregarded, omitted, or trivialized the crucial role played by the precession of the equinoxes.