Orbital Period Calculator (Kepler's Third Law)

Kepler's Third Law says that the square of an orbit's period is proportional to the cube of its semi-major axis. For objects orbiting the Sun, this simplifies to: T² = a³, where T is the period in Earth years and a is the semi-major axis in astronomical units (AU). Enter the average distance below.

How Kepler's Third Law Works

Johannes Kepler discovered in 1619 that for any object orbiting the Sun, T² = a³ where T is in years and a is in AU. This single equation predicts the orbital period of every planet, asteroid, and comet in our solar system from one measurement: its average distance from the Sun. Newton later showed why — the law follows directly from gravity and the geometry of ellipses.

Real Solar System Periods