Kepler's third law of planetary motion states that the square of the orbital period T is proportional to

1. Kepler analysed Tycho Brahe's planetary data. 2. His third law: T² ∝ a³, where a is the semi-major axis of the elliptical orbit. 3. Equivalently: T² / a³ = 4 π² / (G M) is the same for all planets around the Sun. 4. For circular orbits, a is simply the orbital radius. _Source: syllabus Physics, Ch 7 "Gravitation", §7.5_