This model has the planets orbiting the sun, but the sun orbiting the earth, and was intended to be like a compromise between the church and Copernicus' model. The planets orbited on elliptical paths, with the sun at the focus. The planets orbited a segment of equal area in equal amounts of time, which meant when they were closer, they moved faster. T 2 is proportional to R 3 . Newton's law of universal gravitation gave a mathematical, and theoretical basis for kepler's model.
Gravity provides the centripetal force necessary for kepler's third law.
To keep an object moving also required a force. This lead to the conclusion that if an object rolled down a slope and along a flat surface, it should go on to infinity, as it would never reach it's original height. This was also important for the fact that he was using experiment, rather than theoretical reasoning based on untestable assumptions. Newton's Pricipia Mathematica Philosopha Naturalis was written in latin, very formal, with geometric derivations of laws, and corollaries. Contained definitions used in his laws, which connected and quantified the motions of all objects in the universe.