**Ptolemaic Epicycle Machine**

**What it shows:** Long before the time of Copernicus, the Greek astronomer Claudius Ptolemy created a model of all the planets' observed celestial motions. The model involved combinations of perfect circles rotating with uniform speed. Ptolemy explained the apparent "looping motion" of the planets by placing the center of one rotating circle, called the *epicycle*, which carried the planet, on another rotating circle, called the *deferent*, so that together the motions of the two circles produced the observed looping motion of the planet. Moreover, the...