Question

which idea did ptolemys model use to explain why the planets appeared to move backward as they moved in their orbits?
the planets spun in epicycles.
the planets rotated on their axes.
the planets revolved around the sun.
the planets were made of different materials.

Explanation:

Ptolemy's geocentric model used the concept of epicycles. Planets were thought to move in small - circular paths (epicycles) while the center of the epicycle moved in a larger circle around the Earth. This explained the apparent retrograde motion of planets.

Answer:

The planets spun in epicycles.