Kepler's Third Law says that for any two bodies orbiting the same central mass, the square of their periods divided by the cube of their semi-major axes is the same constant.
Kepler's Third Law says that for any two bodies orbiting the same central mass, the square of their periods divided by the cube of their semi-major axes is the same constant. That lets you compare two orbits — like two planets around the Sun — without even knowing the central mass.
Earth orbits the Sun with T1 = 1 year at a1 = 1 AU. Mars orbits at a2 = 1.524 AU. Leaving T2 blank: T2 = 1 × (1.524/1)^1.5 ≈ 1.88 years — matching Mars's real orbital period.
The square of a planet's orbital period is proportional to the cube of its orbital semi-major axis: T² ∝ a³.
Given either orbital period or distance, it solves for the other using the proportional relationship, often calibrated to Earth's orbit as a reference.
Once astronomers measure a planet's orbital period, this law lets them calculate its distance from its star without directly measuring it.