On the function and the probable origin of Ptolemy’s equant

James Evans, University of Puget Sound


We examine the roles of the equant point and the eccentric deferent circle in 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.