Kepler's third law states that the square of the orbital period of a planet is directly proportional to the cube of the semimajor axis of its orbit.

harmonic law

formulation date | 1619 (407 years ago) formulator | Johannes Kepler

T^2∝a^3

Agrees well with experiments for classical mechanics, but may require more sophisticated formulations such as special relativity, general relativity, or relativistic quantum mechanics for small scales, large speeds, or strong gravitational fields.

Kepler's laws | laws of physics