Ptolemy’s Picture

Why didn’t the world’s astronomers just toss out Aristotle’s geocentric model of the solar system?
There are at least three reasons.

• First, and most important, the geocentric picture appeals to common sense. On a given day, as we watch the sun and the stars rise and set, we do not feel like we are in motion. There are, for example, no great winds whipping at us as one might expect if the earth were tearing through space.
• Second, human beings are egocentric creatures. We tend to see ourselves as being at the center of things. Extend this egocentric tendency into the heavens, and you have an explanation for our species’ geocentric tendency. If, individually, we feel ourselves at the center of things, we also feel this collectively, as a planet.
• The final reason? Because Aristotle was Aristotle. His opinions were regarded with awe for centuries. Few dared—few even thought—to question his teachings. Instead, generations of European astronomers wrestled with Aristotle’s model in an effort to show how it was actually correct.

The most impressive of these wrestling matches was fought by Claudius Ptolemaeus, a Greek astronomer better known as Ptolemy, active in Alexandria during C.E. 127–145. Drawing on the work of the Greek astronomer Hipparchus (who died after 127 B.C.E.), Ptolemy developed the geocentric model into an impressively complex system of “deferents,” large circular orbits centered on the earth, and “epicycles,” small circles whose centers traveled around the circumferences of the deferents.

Some 80 deferents and epicycles came into play, along with several other highly complex geometric arrangements, which allowed Ptolemy to account for many of the perplexing planetary motions actually observed.