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Wright State University CORE Scholar

Classics Ancient Fair Religion, Philosophy, and

2020

The According to Ptolemy

Benjamin Smith Wright State University - Main Campus, [email protected]

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Repository Citation Smith , B. (2020). The Cosmos According to Ptolemy. Dayton, Ohio.

This Poster is brought to you for free and open access by the Religion, Philosophy, and Classics at CORE Scholar. It has been accepted for inclusion in Classics Ancient Science Fair by an authorized administrator of CORE Scholar. For more information, please contact [email protected]. THE COSMOS ACCORDING TO PTOLEMY Benjamin C. Smith Wright State University Ancient Science Fair

INTRODUCTION AND ABSTRACT THE OF THE DEFERENTS, EPICYCLES, & THE PROBLEM In the second century AD, Ptolemaeus, the In The , Ptolemy begins by refuting all The Ptolemaic Model demonstrates how the Although the Ptolemaic Model would account for from , would provide a other possible positions of the Earth. heavenly bodies behave in a Geocentric Universe. most observed cosmological phenomena, the geometric conception of the cosmos with the ability predictions for the lunar motion during the 1st and The , moon, and revolve in small to predict planetary motion, in what would be known 3rd quarters yielded discrepancies. In Book V, his argument is that if the Earth is not the rotations around a center point, known as epicycles, as the Ptolemaic, or Geocentric, Model of the Ptolemy asserts that the lunar deferent circle must center, then the plane of the horizon would not which themselves revolve around the Earth in large Universe. Prior to Ptolemy, the notion in ancient not revolve around the Earth, but a point revolving bisect the stellar sphere. rotations in what are known as deferent circles. was that the cosmos was ”perfect”; the around the Earth in a smaller circle. heavenly bodies must move along the perfect curve (a circle) as the perfect shape (a sphere). In Book VII he furthers his argument: Yet, irregularities occur in observing the planets in "So it seems to me, at least, superfluous to seek the K concentric circles alone. Ptolemy’s answer, building causes of the motion towards the center, when once upon and , would present an the fact that the earth occupies the middle position eccentric system in which the heavenly bodies move in the universe and that all weights move towards it, /' I l in epicycles along deferent paths. Though is made so clear from the appearances themselves.” I I I I I I 'O modifications would be necessary, the Ptolemaic … and if the Earth had any natural motion I i T Model would maintain a geocentric cosmos and C (according to Aristotelian Dynamics) “it would very Jupit predict planetary movement for the next millennium. Sphere of quickly have fallen completely out of the universe the itself.” RETROGRADE MOTION

Figure 3 above illustrates the epicycles of the Sun, moon, and planets, along deferent circles. Further examination of the Ptolemaic Model would Figure 5 above illustrates the lunar epicycle in the yield modifications to the movement of the planets. Ptolemaic Model. The center of the lunar epicycle (K) (except for ). orbits a point (C), which itself orbits the Earth (T). The angular velocity at which K orbits C and C orbits Ptolemy finds that the heavenly bodies revolve T is equal (though in opposite directions). around the Earth, but it is NOT the center, rather near the central point. CONCLUSION Figure 3 below illustrates a (P) moving along an epicycle with the center (K) following an While the Ptolemaic Model is inaccurate in the post- eccentric circle with a center (C). However, K moves era, Ptolemy himself and his with respect to the (E), not C, or the Earth contributions to astronomy remain. His passion and (T). promotion of astronomy outlasts antiquity. According to Ptolemy, the purpose of astronomy is not in obtaining knowledge alone, whether theologically or mathematically, but to acknowledge and appreciate the “divine beauty” that ascends into

Equanl the atmosphere each evening. Nevertheless, civilizations would uphold Ptolemy’s eccentric Figure 2 above illustrates the Ptolemaic Model of the C~T _____r_ J -Yo universe, with Earth in the middle and the apparent epicycle system for over 1,300 years, until Nicolas Figure 1 above demonstrates apparent retrograde retrograde motion of the heavenly bodies around. Copernicus would heretically and correctly contrive motion in the Ptolemaic Model of the cosmos. a Heliocentric Model of the universe with elliptical orbits. While the Ptolemaic Model is a Geocentric Model, Retrograde motion is an apparent phenomenon in Ptolemy failure lies in the relationship between the REFERENCES AND ACKNOWLEDGEMENTS which planets appear to slow down and briefly movement of the planets and the Sun. Mercury and reverse in orbit around the Earth. remain points on a line between the Earth and SPONSOR: In actuality, the Earth is simply passing “slower” the position of the Sun. The lines between the outer *The equant is a point on the line TCA, through the Rebecca M. Edwards, Ph.D. planets with longer orbits around the Sun. planets – , , and Saturn – and the center center of the eccentric circle. Department of Religion, Philosophy, & Classics To account for retrograde motion and to preserve of their respective epicycles are constantly parallel The line between the Equant and the mean aux the premise for a geocentric cosmos, Ptolemy would to the line between the Earth and the Sun. See revolves with angular velocity around the Equant. construct a geometric system with eccentric motion Figure 3. SOURCES: The same line measures the movement of the planet and epicycles. Lloyd, G.E.R. Greek Science After Aristotle, W.W. along an epicycle. Norton & Company. 1973

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