Ever Exacting Theoretical Refinements

With hindsight we sometimes tend to take for granted the brilliance of perceptual breakthroughs. Newton's insightful realization that the force inducing a fruit to fall from a tree was the same as that causing the moon to orbit the earth is an example. A logical continuance was the extrapolation that this force was universal and explained similar phenomenon throughout the universe.

Prior to the realization of Newton's ideas about gravitation Kepler had formulated mathematical descriptions of planetary orbits. Although Kepler did so outside a gravitational framework, his orbital depictions were accurate for solar system models involving planets orbiting the sun. This is due to the fact that in relation to the sun planetary masses are by comparison quite minute. So minute in fact, that the center of the two masses in a planet/sun model lies within the sun itself. Because of this Kepler's third law was a very close approximation of actual planet orbits. Just as Einstein's Theory of General Relativity allowed for more exacting gravitational predictions than Newton's theory, Newton's gravitational theory allowed for more exacting orbital predictions than Kepler.

For two equal celestial bodies the center of mass would lie between them. They would orbit around their common center of mass. While Kepler's laws were a great historic milestone Newton's work furthered our understanding of an underlying force and the manner by which it acted.

The linked webpage Sir Isaac Newton: The Universal Law of Gravitation provides a more complete explanation of Newton's gravitational concepts.