Astronomy 104: Lecture 4 (Slide 7)

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Ptolemy's Almagest

The last truly great Greek Astronomer, Ptolemy 100 AD) was a loyal follower of the view that the Earth was the center of the cosmos. All of his work was based on a Geocentric Cosmogony, where everything in the solar system rotates around us. He wrote his model down in a 13 volume set called Almagest.

Deferents and Epicycles:

The Ptolemaic system explains the major motion of the planets with deferents and retrograde (backwards) motion with epicycles.

The diameters and rates of the deferents and epicycles were tweaked until they fit the observed motions of the planets. The answers turn out pretty well except:

The Moon's Epicycle required it to vary in its distance from Earth by a factor of 2! (Would you notice if the moon doubled in size every month?)
There is no underlying physical principle that can explain why an object would go around in an epicycle

Astronomers at the time knew of these shortcomings, BUT they just couldn't accept the fact that the Earth moves. Also, Almagest did predict the positions of the planets and moon well.

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