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His determination of the planetary orbits was accomplished by comparing his own observations with those of former astronomers, especially with those of Timochares on Venus. To Ptolemy we are also indebted for a work on Geography used in European schools as late as the fifteenth century.
He does not appear to have made observations himself in Alexandria, but he uses those of Aristyllus and Timochares of that place. Indeed, his great discovery of the precession of the equinoxes was essentially founded on the discussion of the Alexandrian observations on Spica Virginis made by Timochares.
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