The Heliocentric Theory

Diagram taken from: http://www.astro.ucla.edu/~wright/solar_system.html

On the left is a figure representing the heliocentric model, while the graph on the right shows Kepler's Third Law.

Several people found fault with the Ptolemaic theory. Nicolaus Copernicus was convinced that the Earth was also a planet. Since he categorized Earth as a planet, he also believed that Earth was rotating like the other planets. Copernicus then made a new model with the Sun in the middle. The Earth, moon and the other planets were believed to move around the Sun in a counter-clockwise motion. This new theory was known as heliocentric or Sun-centered theory. This theory is also known by many as the Copernican System.

In the heliocentric theory the distance of the planets from the sun was determined by the size of the retrograde loops. For example, a small retrograde loop signifies that the planet is far away from the Sun. On the other hand, a large retrograde loop would signify that the planet is close to the Sun. A relation between the orbital period and orbital radius is known by the size of the orbit of a planet. This was discovered by Kepler and termed his third law. More information is available on the Kepler's Laws."