Environment, Space

What is tidal locking?

Why the Moon is always facing the same side to Earth? The answer is tidal coupling or tidal locking and it is this post’s subject.

Why tidal locking happens?

tidal locking
The Moon spins exactly a rotation when it does a complete turn around Earth. Source: Quora.

Moon elliptical orbit, inclination between orbital planes of Earth and Moon and tilted axes of both, make possible observe more than half of lunar surface. Including the natural satellite’s north and south poles. However, 41% of lunar surface remains hidden for an Earth observer.

Moon elliptical orbit.
Source: verestrellas.
Earth Moon system
Source: Wikimedia.
moon oscillation
The Moon apparent oscillation is due to elliptical orbit and to rotation axis tilt. Source: imgur.

How it started?

Moon formation
The most accepted theory for Moon formation claims that an object on the size of Mars (Theia) collided with Earth at 4.5 billions years ago. Due to this collision, was formed a disk of debris and dust. These pieces attracted to each other due to gravity and formed the Moon. Source: ExtremeTech.

At the time of formation, Moon and Earth rotated much faster than today. Both exert gravitational forces on each other, causing tides on Earth and a small stretching on Moon.

forces that cause tidal locking
Earth’s rotation and gravitational forces between tidal bulges (dark blue) and the Moon, produce torques that reduce rotational speed of two bodies. This phenomenon is called tidal friction. Which also makes the Moon expands it’s orbit and move away from Earth, to conserve angular momentum. Source: Wikimedia.

For having lower mass, Moon obtained tidal locking first. If the Moon rotated slower than Earth, tidal forces would have accelerated the rotation to achieve tidal locking. 

Tidal locking between Pluto and Charon.
Many natural satellites from other planets are also locked in relation to its respective planets. The dwarf planet Pluto (orbit in red) and it’s biggest moon Charon (orbit in green) are locked to each other. This system’s center of mass is outside Pluto. Source: itpedia.

50 billion years from now, Earth will be locked and will have the same side to the Moon. Unless both are swallowed by the red giant the Sun will become in 5 billion years.

How long it takes to obtain tidal locking?

This is the simplified equation to calculate estimated time t, in years, to a moon achieve tidal locking in relation to its planet. This equation can only be used for spherical moons.

t\approx 6\frac{a^{6}R\mu }{m_{s}m_{p}^{2}}\cdot 10^{10}


  • m_{p} is the mass of primary body, or planet.
  • m_{s} is the mass of secondary body, or moon.
  • a is distance between two bodies.
  • R is satellite’s radius.
  • \mu is natural satellite’s rigidity, in N/m^{2}. Depends on moon’s composition, if it’s rocky, made of ice, etc.
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