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

  title={On the function and the probable origin of Ptolemy's equant},
  author={James Evans},
  journal={American Journal of Physics},
  • James Evans
  • Published 1 December 1984
  • Physics
  • American Journal of Physics
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. 
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