Although it seemed clear that Kepler's equations of planetary motion were correct, there was, until 1687, no theory to explain why the planets moved the way they did. It took one of the most brilliant physicists of all time, Sir Isaac Newton, to come up with a physical description of why the planets moved according to Kepler's Laws.

In his Philiosophae Naturalis Principia Mathematica—more commonly known simply as Principia—Newton explained how the gravity of the sun determined the orbits of the planets, described the Law of Universal Gravitation, and invented differential calculus in the process!