Tidal Locking

Tidal torque synchronizing a moon's rotation to its orbital period

Spin rate: 1.8ω_orb Lag angle δ: 8°

Torque: –

Timescale τ ∝ a⁶/(M²R⁵)

Phase: –

Tidal locking occurs when a moon's rotational period synchronizes with its orbital period. The planet raises a tidal bulge on the moon; friction causes the bulge to lead or lag the planet direction, creating a torque. The timescale τ ∝ a⁶Q/(M_p²R⁵) — the Moon locked to Earth in ~10⁹ years.