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Figure 1.2
Figure 1.1.3
The sun moved around a large circle. (the deferent).
The deferent explained direct motion.
Planets moved around mini circular orbits (the epicycle)
The epicycle explains retrograde motion.
As time and observation evolved, this Model was improved to account for new observations. The greatest astronomer of antiquity, Hipparchus, innovated Apollonius’ epicycle theory. He had to slightly displace the earth from the center of the deferents.
Hipparchus’s Model
Figure 24
2 This figure and eplanations were drawn from Chaisson McMillan’s Astronomy Today. 7th Edition. 36, and from Dr. Stamp’s lecture notes and slides on Greek Astronomy.
3 This interactive figure was drawn from http://mathdl.maa.org/mathDL/46/?pa=content&sa=viewDocument&nodeId=3120&pf=1.
4 From Dr. Stamp’s slides on Greek Astronomy.
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With two separate circular orbits, this basic model was able to show the retrograde motions of planets quite well, and account for changes in brightness because it showed too how the planet’s distance varied from Earth. If the planet in the epicycle is closer to the earth, it is brighter; if it is furthest from the earth, it is dimmer. In the end, it predicted things quite well for its times.
