Page 16 - GALIET PHYSICS BLOSSOMS I+

CLAUDIUS PTOLEMY’S EPICYCLE THEORY
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Ptolemy, around 140 ad, constructs the most complete geocentric model of all time. It explained really well the observed paths of the sun, the moon, and the five planets then known (Mercury, Venus, Mars, Jupiter, and Saturn). However, Ptolemy’s model was too intricate and complex for it required no less than 80 separate circles. In hindsight, its complexity shows a flawed theory, for simplicity is a sign of truth. Moderns recognize that Ptolemy erred in assuming a geocentric universe and in persisting with the theory of uniform circular motion. So ingrained in him was Plato’s philosophical dogma of uniform circular motion. Ptolemy missed Aristarchus of Samos observation that the Earth revolved around the sun and that it rotated on its axis once each day (a heliocentric system). Sadly, Grecians had dismissed it for Aristotle’s influence had been too strong. 5
Figure 3.6
Figure 2.7 The planets’ deferents moved on spheres within the celestial sphere that held the stars and all interior spheres. The planetary and solar spheres had their own additional motions, which made them move relative to the stars. In this drawing, we see Ptolemy’s eccentric and equant model at work. In order for Ptolemy to account for changes in velocity of the apparent planetary motions, he displaces the earth- centered deferent from:
5 Summarized from Chaisson, McMillan. Astronomy Today. 7th Edition. 36. 6 From Dr. Stamp’s slides on Greek Astronomy.
7 Chaisson, McMillan. Astronomy Today. 7th Edition. 37.
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