Who established the fundamental ideas of modern celestial mechanics based upon observations?

Explanation

Kepler was primarily a theoretician, whereas Galileo was primarily an observer. The principal similarity is that both made major contributions to the establishment of the heliocentric model of the solar system - Galileo by making discoveries which were much easier to understand in a heliocentric context, and Kepler by demonstrating that heliocentric (well, heliofocal) elliptical orbits described planetary motions much better than any other model.