Kepler's Laws

Kepler's Laws, three laws concerning the motions of planets formulated by the German astronomer Johannes Kepler early in the 17th century. See Solar System.

Kepler based his laws on planetary data collected by the Danish astronomer Tycho Brahe, to whom he was an assistant. The proposals broke with a centuries-old belief that the planets moved in circular orbits. This was a feature of the Ptolemaic system, advanced by the Alexandrian astronomer Ptolemy in the 2nd century ad, and of the Copernican system, put forward by the Polish astronomer Nicolaus Copernicus, in the 16th century. According to Kepler's first law, the planets orbit the Sun in elliptical paths, with the Sun at one focus of the ellipse. The second law states that the areas described by the straight line joining the centre of the planet and the centre of the Sun are equal for equal time intervals; that is, the closer a planet comes to the Sun, the more rapidly it moves. Kepler's third law states that the ratio of the cube of a planet's mean distance, d, from the Sun to the square of its orbital period, t, is a constant—that is, d³/t² is the same for all planets.

These laws played an important part in the work of the 17th-century English astronomer, mathematician, and physicist Sir Isaac Newton, and are important for the understanding of the orbital paths of the Moon and of artificial satellites. See also Mechanics.