concept

Spin-orbit resonance

A simple whole-number relationship between how many times a body spins and how many times it orbits. Tidal locking is the 1:1 case (one spin per orbit). But other states exist: Mercury is trapped in a 3:2 resonance - exactly three spins for every two trips around the Sun - because its orbit is so eccentric. It is the exception that explains why large round moons settle into 1:1.