In Newton's time, everything that was known about planetary motion could be summarized succinctly in three laws attributed to Johannes Kepler. The first law states that planets move around the sun on elliptical orbits. The second law states that a planet sweeps out equal areas in equal times. According to the third law, the square of the orbital period is proportional to the cube of the distance to the sun. These are purely empirical laws, however. They describe what happens without explaining why it happens.

Newton developed a mathematical formulation of gravity that explained both the motion of a falling apple and that of the planets. He showed that the gravitational force between any two objects is proportional to the product of their masses and inversely proportional to the square of the distance between them. When applied to the motion of a planet around the sun, this theory explained all three of Kepler's empirically derived laws.