any one of three laws governing planetary motion: each planet revolves in an ellipse, with the sun at one focus; the line connecting a planet to the sun sweeps out equal areas in equal periods of time (law of areas); or the square of the period of revolution of each planet is proportional to the cube of the semimajor axis of the planet's orbit (harmonic law).

three laws of planetary motion published by Johannes Kepler between 1609 and 1619. The first states that the orbit of a planet describes an ellipse with the sun at one focus. The second states that, during one orbit, the straight line joining the sun and a planet sweeps out equal areas in equal times. The third states that the squares of the periods of any two planets are proportional to the cubes of their orbital major axes