Kepler's Laws of Planetary Motion are three mathematical statements, formulated by German astronomer Johannes Kepler in the early 17th century, describing the motion of planets around the Sun. The laws state that (1) planets move in elliptical orbits with the Sun at one focus; (2) a line segment joining a planet and the Sun sweeps out equal areas during equal intervals of time; and (3) the square of a planet's orbital period is proportional to the cube of the semi-major axis of its orbit. The laws were derived primarily from observational data on the motion of Mars and were first published in *Astronomia Nova* (1609) and *Harmonice Mundi* (1619). For background on the data and reasoning that led to the laws, see kepler-laws-history.
The laws were formulated within the broader Heliocentrism framework, which itself was a contested model at the time relative to geocentric alternatives; see heliocentrism-viewpoint and Solar System Center - Heliocentrism Geocentrism Debate.
Kepler's three laws are treated in modern physics as accurate descriptions of two-body orbital motion under Newtonian gravitation, later shown by Isaac Newton to follow mathematically from the inverse-square law of gravitation and the laws of motion. They remain foundational in celestial mechanics, astrodynamics, and orbital mechanics education, and are used, with relativistic corrections in cases of strong gravitational fields or high precision (e.g., the orbit of Mercury), in calculating satellite trajectories, planetary orbits, and exoplanet detection.
The laws are understood as an idealization: they strictly apply to a two-body system and are modified in practice by perturbations from additional bodies, non-spherical mass distributions, and relativistic effects. Historical and philosophical discussion continues regarding the interpretive significance of the laws within the broader transition from geocentric to heliocentric astronomy, and regarding how the laws relate to competing models of planetary motion proposed by contemporaries such as Tycho Brahe; see kepler-laws-geocentric-retrograde-viewpoint.
There is broad scientific consensus, arrived at independently across observational astronomy, theoretical physics, and spaceflight engineering, that Kepler's laws accurately describe two-body orbital motion to the precision achievable before relativistic corrections become necessary, and that they are derivable from Newtonian gravitation. See kepler-laws-newtonian-derivation-consensus.