Assumptions behind Ptolemy’s Proof of the Earth’s Centrality

Abstract

In his Almagest, Ptolemy provides an astronomical proof to establish the Earth’s centrality. In this paper, I offer a complete survey of this proof, present the main features of Copernican and Galilean heliocentric responses, and elucidate the meaning of some concepts involved in it. I argue that historical analysis allows us to state that the success of Ptolemy’s proof depended on some assumptions that were hidden in the meaning of some fundamental astronomical concepts, an issue that historians of astronomy usually neglect.