Kepler’s second law of planetary motion, in astronomy and classical physics, one of three laws describing the motions of the planets in the solar system and which states that a radius vector joining any planet to the Sun sweeps out equal areas in equal lengths of time.

The validity of Kepler’s second law means a planet must have a higher than average velocity near perihelion (when it is closest to the Sun) and a lower than average velocity near aphelion (when it is farthest away from the Sun). The angular velocity must vary around the orbit in a similar way.

Kepler’s second law was derived by the German astronomer Johannes Kepler, whose analysis of the observations of the 16th-century Danish astronomer Tycho Brahe enabled him to announce his first two laws in the year 1609 and a third law nearly a decade later, in 1618. Kepler himself never numbered these laws or specially distinguished them from his other discoveries.

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Knowledge of this law proved crucial to Isaac Newton in 1684–85, when he formulated his famous law of gravitation between Earth and the Moon and between the Sun and the planets, postulated by him to have validity for all objects anywhere in the universe. Newton showed that the motion of bodies subject to central gravitational force need not always follow the elliptical orbits specified by the first law of Kepler but can take paths defined by other, open conic curves; the motion can be in parabolic or hyperbolic orbits, depending on the total energy of the body. Thus, an object of sufficient energy—e.g., a comet—can enter the solar system and leave again without returning. From Kepler’s second law, it may be observed further that the angular momentum of any planet about an axis through the Sun and perpendicular to the orbital plane is also unchanging. Kepler’s second law follows from the law of conservation of angular momentum.