Home / Topics / Gravitation & Orbits
Tidal locking
Advanced
Tidal friction makes rotation match orbit, as with our Moon.
Tidal friction slows a body's rotation until its rotation period equals its orbital period, so it shows the same face. The Moon is locked to Earth.
Pluto and Charon are mutually locked.
Takes millions to billions of years.
In more detail
Tidal locking occurs when a moon's rotation period equals its orbital period, so it always shows the same face to its planet. Tidal friction slowly drives this state.
Real-world uses
- The Moon is tidally locked to Earth, which is why we always see the same side.
- Many moons and exoplanets are thought to be tidally locked.
- The Moon always shows the same face to Earth because tidal friction has locked its spin to its orbit.
Open the interactive lesson, with simulation and quiz →
Frequently asked questions
Does the Moon rotate at all?
Yes, it rotates once per orbit, which is why its near side always faces Earth.
Why does tidal locking happen?
Tidal bulges raised by the planet slowly slow the moon's spin until it matches its orbit.
Will Earth's day become locked to the Moon?
Eventually, tidal friction is slowing Earth's rotation, but the process takes billions of years and the Sun may change first.
Related topics in Gravitation & Orbits
Orbital transfers · Gravity assist · Tides · Roche limit · Lagrange points · Two-body problem