Kepler summarized planetary motion with three rules, or laws. Kepler's First Law, the Law of Ellipses: A planetary orbit is an ellipse with the Sun at one focus. Kepler's Second Law, the Law of Equal Areas: As a planet orbits the Sun, it sweeps out equal areas of the ellipse in equal times. This behavior means that the planet's orbital velocity varies with distance from the Sun. At perihelion, the planet is at maximum speed and at aphelion the planet crawls along at minimum speed. Kepler's Third Law, the Harmonic Law: Even though the orbital velocity of a planet changes constantly, one relationship does remain constant. The orbital period is directly related to the average distance between the planet and the Sun. This law implies that planetary orbital velocity decreases with increasing distance from the Sun. For instance, the orbital velocity of Mercury (47.9 km/s) is far greater than Pluto (4.7 km/s).