Unit 2 : Activity 2 : Ptolemaic Model : Copernican Model : Brahe : Kepler : Galileo : Newton : Unit Exam

The Ptolemaic Model

Claudius Ptolemy lived in Rome around 100 AD. His model of the solar system and heavenly sphere was a refinement of previous models developed by Greek astronomers. Ptolemy’s major contribution, however, was that his model could so accurately explain the motions of heavenly bodies, it became the model for understanding the structure of the solar system. It is beyond the scope of this course to discuss all the complex social and historical implications of an Earth-centered versus a Sun-centered model of the solar system. But nearly all the early models, including Ptolemy’s version of the solar system, assumed that the Earth was the center of not only the solar system, but the entire universe.

The Ptolemaic model accounted for the apparent motions of the planets in a very direct way, by assuming that each planet moved on a small sphere or circle, called an epicycle, that moved on a larger sphere or circle, called a deferent. The stars, it was assumed, moved on a celestial sphere around the outside of the planetary spheres.

Ptolemy's fame comes partly from what he figured out, but his influence was largely because he wrote a great summary of everything known about astronomy. Ptolemy insisted that the job of the astronomer was to explain the motions of the wanderers using only uniform circular motion - the kind of motion that most gears and wheels show. To make the planets appear to speed up and slow down, three tricks were used. The epicycles we've just shown were the first trick. The second trick was to move the observer out of the center of the circle, putting us into an "eccentric" position. The third trick was called the equant and is illustrated here:

As an indication of exactly how good the Ptolemaic model is, modern planetariums are built using gears and motors that essentially reproduce the Ptolemaic model for the appearance of the sky as viewed from a stationary Earth. In the planetarium projector, motors and gears provide uniform motion of the heavenly bodies. One motor moves the planet projector around in a big circle, which in this case is the deferent, and another gear or motor takes the place of the epicycle.

A traditional planetarium projector

While the fact that we base planetarium projectors on the Ptolemaic model of the universe that was developed almost 2,000 years ago may seem impressive, a better test of the model is how long the model was accepted by society. In this case, the Ptolemaic model was not seriously challenged for over 1,300 years. When and why it finally needed to be replaced will be described in the next subunit.