American Journal Of Physics
Science, Technology and Society
We examine the roles of the equant and eccentric deferent circles on Ptolemy’s planetary theory. The necessity of these features of the model is demonstrated empirically by a study of the retrograde arcs of Mars. Finally, these results are used, together with available historical evidence, to suggest the line of investigation that probably led Ptolemy to his discovery of the equant.
Evans, James C.. 1984. "On the function and the probable origin of Ptolemy's equant." American Journal Of Physics 52(12): 1080-1089.