# Dictionary

## Kepler’s laws

Basic laws governing the orbital motions of planets around the sun. First law: Each planetary orbit is an ellipse, with the sun in one of its focus points. Second law: If you connect the planet and the sun by an imaginary line then, in equal time intervals, the line will sweep over equally large areas, independent on where the planet is on its orbit. Third law: dividing the square of a planet’s orbital period by the third power of it’s average distance from the sun gives the same value for all planets in the solar system; written as a formula: period²/(average distance to the sun)³ = const.

Kepler’s laws follow directly from the laws of classical mechanics and Newton’s law of gravity. However, they are only valid approximately – the gravitational pull of the planets on each other, as well as the fact that, ultimately, gravity is governed not by Newton’s laws, but by general relativity (see relativistic perihelion shift) lead to small deviations from perfectly elliptic orbits.