Kepler's Laws of Planetary Motion: Observation, Measurement & Evidence

Kepler's three laws describe the geometry and timing of planetary orbits: planets move on ellipses with the Sun at one focus, sweep out equal areas in equal times, and obey a relationship between orbital period and orbital size. Kepler's laws are used to estimate orbital periods, infer stellar masses from orbiting companions, schedule spacecraft encounters, and interpret exoplanet data.