Revue D'histoire Des Sciences
Science, Technology and Society
Beginning with the essential planetary phenomena to be explained, we examine the functions of the equant point and the eccentric deferent circle in Ptolemy's theory. We show that the necessity of the equant point becomes evident as soon as one attempts to save the lengths of the retrograde arcs, and the distances between them, particularly in the case of Mars. Finally, we propose a hypothesis concerning the line of investigation that led Ptolemy to discover the equant.
Evans, James C. 1984. "Fonction et origine probable du point équant de Ptolémée." Revue D'histoire Des Sciences 37(3-4): 193-213.