Kepler's laws

Three fundamental statements about planetary motion, derived empirically by Johannes Kepler (1571-1630) on the basis of detailed observations of the planets made by Tycho Brahe (1546-1601): 1. The orbit of each planet is an ellipse with the Sun at one of the foci. 2. Each planet orbits the Sun such that the radius vector connecting the planet and the Sun sweeps out equal areas in equal times. 3. The squares of the sidereal periods of any two planets are proportional to the cubes of their mean distances from the Sun. The first two were published in 1609 in Astronomia Nova and the third in 1619 in Harmonice mundi. The physical basis for the laws was not understood until Isaac Newton (1642-1727) formulated his law of gravity.