Abstract
The reading selection included below is comprised of the first several chapters of Book I of the Almagest, translated from Greek by Aaron Jensen. Ptolemy begins in Chap. 1 by explaining the difference between theoretical and practical philosophy. (Where does Ptolemy situate the study of astronomy?) Next, after explaining the outline of the Almagest in Chap. 2, Ptolemy offers a summary of his theory of the heavens and the earth. Thus, Chaps. 3–8 provide a framework for understanding the Almagest as a whole. He argues against those whowould claim that the earth itself is in motion. This, after all, was an ancient opinion espoused by the Pythagoreans but later rejected by Aristotle in his book On the Heavens. The astute reader of the Almagest will recognize many of Aristotle’s arguments and proofs in these chapters. (Can you identify them?)
The Ptolemaic worldview came under renewed and vigorous attack by thinkers during the sixteenth and seventeenth centuries. Indeed understanding the Almagest is essential in understanding the astronomicalworks of Copernicus, Kepler and Galileo, who wrote in reaction to the geocentric worldview, which Ptolemy had so clearly and meticulously laid out in the Almagest.
Only mathematics can provide sure and unshakeable knowledge to its devotees.
—Claudius Ptolemy
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Notes
- 1.
The German cartographer Martin Waldseemüller makes extensive use of Ptolemy’s astronomical and geographical work in his Introduction to Cosmography. See Chap. 9 in the present volume.
- 2.
The case of sphæra recta refers to observations made by a person standing on the equator of the earth, so that the axis of the celestial sphere is at a right angle to the zenith (the point directly overhead). The case of sphæra obliqua, on the other hand, refers to observations made by a person standing at any other latitude on the earth’s surface, so that the axis of the celestial sphere is at an oblique angle to the zenith. See Ex. 5.1, below.—[{K.K.]
- 3.
Medieval astronomers referred to the great circle passing through both the poles of the equator and the poles of the ecliptic as the equinoctial colure. See, for example, Martin Waldseemüller’s Introduction to Cosmography, included in Chap. 9 of the present volume.—[K.K.]
- 4.
The equinoctial colure is fixed to the rotating celestial sphere, while the meridian is not. Thus, the meridian, which is a great circle passing through both the poles of the equator and an observer’s zenith, overlaps the equinoctial colure once each day. See Chap. 3 of Waldseemüller’s Introduction to Cosmography, which is included in Chap. 9 of the present volume.—[K.K.]
- 5.
For the sake of clarity, I have omitted the several intervening spheres which, according to Ptolemy, govern the motion of the planets which lie between the sun and the sphere of fixed stars. For more information, see the Introduction to Chap. 4 of the present volume.
- 6.
During cold winter observing sessions, I have used Hot Hands Air-Activated Warmers by Kobayashi, Dalton, Georgia.
- 7.
Such as Sinnott, R. W., Sky and Telescope’s Pocket Sky Atlas, New Track Media, LLC, Cambridge, Massachusetts, 2006.
- 8.
I have been very happy with the Luminos Astronomy Companion for iOS by Wobbleworks LLC.
- 9.
This is of course the principle of operation behind old-fashioned analog clocks, which use the angular position of pointers to relay time. Newer digital clocks, which use only digits or other symbols, conceal this connection between time and astronomy, for the sake of simplicity.
- 10.
German astronomer Johannes Bayer systematically assigned names to over a thousand of the brightest stars. He assigned each a three-letter combination (indicating its parent constellation) preceded by a lower-case greek letter (indicating its relative magnitude, or brightness, among the other stars in its constellation). For example, Betelgeuse is given the Bayer designation α-Ori, since it is the brightest star in the constellation Orion. See, for example, Swerdlow, N. M., A Star Catalogue Used by Johannes Bayer, Journal for the History of Astronomy, 17, 189, 1986.
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Kuehn, K. (2015). The World of Ptolemy. 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_5
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