Abstract
At the outset of Book I of his, Ptolemy provides an overview of his world-view, describing the size, shape, position and motion of the heavens and the earth. Now in Book III, Ptolemy focuses his attention on the motion of the sun. In the reading selection that follows, he considers how the length of the year is determined. This is no simple problem. First, one must decide how the year is to be defined. How does Ptolemy do it? Is the motion of the sun uniform, or does its speed vary from year to year, or perhaps even from day to day? Next, one must find and interpret the ancient historical documents which record, to varying degrees of reliability, the time at which significant astronomical events—such as solstices and equinoxes—were observed.
We think it is in general fitting to demonstrate the phenomena through the simplest hypotheses of all—as long as there doesg not appear to be anything from the observations worth mentioning which conflicts with this supposition.
—Claudius Ptolemy
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Notes
- 1.
For a history of such measurements, even up until the seventeenth century, see Heilbron, J. L., The Sun in the Church, Harvard University Press, Cambridge, Massachusetts, 1999.
- 2.
See Toomer, G. J. (Ed.), Ptolemy’s Almagest, Princeton University Press, Princeton, NJ, 1998.
- 3.
To be clear, in the astronomical calendar, the year -1 corresponds to 2 B.C. and year 1 to 1 A.D.
- 4.
… as opposed to a purely mathematical perspective. See Ptolemy’s distinction between physics, mathematics and theology in his Preface of Book I of the Almagest, found in Chap. 5 of the present volume.—[K.K.].
- 5.
The tables of the mean motion of the sun have been omitted from this volume for brevity. —[{K.K.].
- 6.
The eccentric model for the sun’s motion, which Ptolemy inherited from Hipparchus, is described in Chap. 3 of Book III of the Almagest.
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Kuehn, K. (2015). Measuring the Tropical Year. In: A Student's Guide Through the Great Physics Texts. Undergraduate Lecture Notes in Physics. Springer, New York, NY. https://doi.org/10.1007/978-1-4939-1360-2_6
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DOI: https://doi.org/10.1007/978-1-4939-1360-2_6
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