Jones, Alexander (Author)
Duke, Dennis W. (Author)
Centaurus: International Magazine of the History of Mathematics, Science, and Technology
Volume: 47
Pages: 226--235
Ptolemy provides two explanations of the origin of his highly precise planetary mean motions in the Almagest, asserting in one set of passages that they were obtained directly from analysis of pairs of observations widely spaced in time, but in another passage that they were derived from period relations expressed as corrections to the well-known Babylonian Goal-Year periods. We show that the latter account is true. Moreover, while some of these period relations may have themselves been calibrated through the observations that Ptolemy cites, those for Mercury and Saturn can be shown to have had a different origin.
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