Question

What is an epicycle? Describe how it is important in Ptolemy’s explanation of the retrograde motion of the planets.

Answer 1

EPICYCLE - A circle in which a planet moves and which has a center that is itself carried around at the same time on the circumference of a large circle.

Ptolemy's model of the solar system was geocentric, where the sun , moon , planets and stars all orbit around the earth in perfectly circular orbits .

The problem with perfectperfectly circular orbit around the earth is that they do not explain the occasional backward motion or retrograde motion of the planets .

The greeks insisted that the motion of the planets be perfectly circular . Ptolemy modeled the planets making small circles around a point that orbited the earth . These smaller circles were called epicycles and they allowed the planets to move backward relative to the background stars .