# Ptolemy of Alexandria

Wrote a major book on astronomy, which dominated astronomy (in the "West") for more than a thousand years.

Many of his ideas came from previous astronomers.

The main ideas:

• Earth is at the center.
• Each planet moves on a small circle ("epicycle")
• The center of the small circle moves on a big circle ("deferent")

To see the relation of the model to predictions, try this exercise:
Suppose that three different astronomers, Ptommy, Pterry, and Ptammy have three different models for the path of planet X:

• Ptommy: X moves on an epicycle of radius 1 AU with a deferent of radius 5 AU.
• Pterry: X moves on an epicycle of radius 2 AU with a deferent of radius 5 AU.
• Ptammy: X moves on an epicycle of radius 2 AU with a deferent of radius 10 AU.
We consult the oracle at Delphi, who says (in her usual cryptic manner) that two of these astronomers have models that agree with data, while one has not actually looked at the sky and has a model that will not agree with the data. Which two astronomers' models could agree with data on the motion of planet X on the sky?

Oops... Actually Ptolemy's model is a little more complicated:

• Center of the deferent is not at Earth exactly.
• The angular speed of the center of the epicycle is constant as viewed from yet another point, the "equant."
This came with quantitative predictions.

Editorial comment:
• These ideas are often ridiculed today, especially by physicists. ("Too complicated")
• From a physics conference in Geneva, 1991: "So you see that by adding new symmetry groups, the Extended Technicolor Model is able to escape from some of the difficulties, but the original simplicity of the Technicolor Model has been sacrificed. Personally, I think it has too many epicycles."
• But this picture ruled for over a thousand years because it is very nearly right and makes good quantitative predictions.
• We have seen by finding planet orbits relative to the earth that Ptolemy was essentially right.
• There is no way to get the sizes of the orbits relative to earth right in this system if you don't have a way to measure distances.
• Actually, the sun centered orbits aren't exactly circles and the speed of a planet in its orbit is not quite constant, so the Ptolemaic system had some difficulties and needed to be a bit complicated.
• The only problem is that the earth is at the center.

ASTR 121 Home
Davison E. Soper, Institute of Theoretical Science, University of Oregon, Eugene OR 97403 USA soper@bovine.uoregon.edu