Heliocentric Model
	Copernicus - 1453AD

1. The sun is at the center of the universe.
2. All the heavenly spheres revolve around the sun.
3. The distance from the earth to the sphere of stars is much greater than the distance from the earth to the sun.
4. The daily motion of the heavens relative to horizon results from the earth's motions about its axis.
5. The apparent motion of the sun relative to the stars results from the annual revolution of the earth around the sun.
6. The planets' retrograde motions occur from the motion of the earth relative to the other planets. Retrograde motion now becomes a natural result of planets' revolutions. No epicycles are needed.

Retrograde Motion and Varying Brightness of the Planets

The Copernican system by banishing the idea that the Earth was the center of the Solar System, immediately led to a simple explanation of both the varying brightness of the planets and retrograde motion:

1. The planets in such a system naturally vary in brightness because they are not always the same distance from the Earth.
2. The retrograde motion could be explained in terms of geometry and a faster motion for planets with smaller orbits, as illustrated in the following animation.