What is The purpose of describing planetary orbits in terms of epicycles and deferents?

Answer 1

The purpose of describing planetary orbits in terms of epicycles and deferents was to explain the apparent motion of the planets. In the geocentric model of the universe, the Earth was at the center, and the planets orbited around it. However, the planets did not move in perfect circles. They would sometimes appear to slow down or even reverse direction. This was called retrograde motion.

The epicycle and deferent model was an attempt to explain retrograde motion. In this model, the planet was thought to move in a small circle called an epicycle, which in turn moved around a larger circle called a deferent. The center of the epicycle was not at the center of the deferent, but was offset from it. This offset caused the planet to appear to slow down or even reverse direction as it moved around the deferent.

Answer 2

If your fundamental philosophy of astronomy is that all motion MUST ALWAYS BE circular, then careful observation of the heavens is going to cause your some heartburn. Because the celestial objects - the Sun, the Moon, the planets - don't quite appear to move in circular paths, even though Aristotle says they must.

(Side note: Aristotle may have been the wrongest man to ever live. His philosophy was laughable, his mathematics foolish, and his astronomy arrogantly wrong. His medical teachings may have caused more human deaths than any other source throughout human history. NEVER listen to Aristotle.)

Ahem. As I was saying, the more closely you observe the heavens, the more things DON'T move in circular paths, although Aristotle says that they must. Since Aristotle must always be right, then there must be some other CIRCULAR movements imposed on the obvious circular movements so that combined movement approximates what we really see.

Picture a planet - Mars, for example - moving in a circular path around the Earth. Except that it DOESN'T move that way. OK, put Mars on a little circle moving around its center, and the center of the circle moving in a circular path around the Earth. OK, now we're closer. These little circles were called "epicycles". They were all imaginary concepts to preserve the circular-ness of movement.

We know now that all celestial movement is elliptical, not circular, and that the Sun is (approximately) at the center of the solar system. So all the stuff about epicycles is unneeded complication. But you have to kill Aristotle first.