Three laws of planetary motion formulated in 1609 and 1619 by German mathematician and astronomer Johannes Kepler: (1) the orbit of each planet is an ellipse with the Sun at one of the foci; (2) the radius vector of each planet sweeps out equal areas in equal times; (3) the squares of the periods of the planets are proportional to the cubes of their mean distances from the Sun.

Kepler derived the laws after exhaustive analysis of numerous observations of the planets, especially Mars, made by Danish astronomer Tycho Brahe without telescopic aid. British physicist and mathematician Isaac Newton later showed that Kepler's laws were a consequence of the theory of universal gravitation.