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The Philosophical Justification for the Equant in Ptolemy’s Almagest

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In this paper I offer a close reading of Ptolemy’s philosophical defense of the equant in Almagest 9.2. I identify the challenge to the equant that his defense is supposed to meet, characterizing it as a dispute concerning the origin and authority of the astronomer’s first principles (ἀρχαί). I argue that the equant could be taken to violate a principle fundamental to the Almagest’s astronomical project, namely, that the heavenly bodies move only in uniform circular motions. I show that Ptolemy is not unaware of this potential objection, and explore two ways in which he seeks to fend it off.

Affiliations: 1: Department of the Classics, Harvard University 204 Boylston Hall, Harvard Yard, Cambridge ma 02138USA


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