How did Newton's approach to understanding planetary motions differ from that of his predecessors?

Answer 1

Kepler used observation data by Brahe to find how planets moved. This is "A posteriori" reasoning (finding the theory after the facts were obtained).

Newton used his theories of motion and gravitation to explain the orbital motions. This is "A priori" reasoning (having a theory and using it to explain facts).

Kepler was a mystic as well as a scientist, and so was Newton. But they attacked the same problem from two different directions. Kepler searched for and found mathematical relationships inside the data. Newton used mathematical relationships he already had to verify the data's accuracy.

Answer 2

Whereas his predecessors, like Kepler, approached planetary motion from astronomical observations, Newton approached it from a theoretical, mathematical description of gravity. His equations described the orbits of the planets, as well as the motion of other objects but he derived them from first principles using gravitational theory.

Answer 3

That the Sun was in the center, not Earth as the Church had said for many years.