Physics Laws & Theorems
Intermediate

Kepler's Third Law

The square of the orbital period is proportional to the cube of the semi-major axis.

Formula

T2∝a3T^2 \propto a^3

Variables

TPeriod
aSemi-major axis

Example

Neptune, 30× farther than Earth, takes 30^1.5 ≈ 165 years to orbit the Sun.

Did You Know?

Kepler's harmonic law ties a planet's year to its distance — the key Newton used to deduce gravity.

keplerorbitperiodlaw