Tidal locking occurs when the rotation period of a celestial object is equal to its orbital period. This means that one side of the object permanently faces the object it orbits. Often, the gravitational influence between the two objects is strong enough to create tidal forces, which can deform or even disrupt the shapes of the objects.

When a celestial body becomes locked with another, it always presents the same side to the object of its orbit. The moon exhibits this phenomenon with Earth. From Earth, we always see the same face of the moon, which is why the far side of the moon remained a complete mystery until spacecraft were able to reach it.

Mercury undergoes a more complex three-to-two spin-orbit resonance, which means that the planet turns three times on its axis for every two orbits of the sun, leading to a day-night cycle of almost 176 earth days.

Pluto and its large moon, Charon, rotate asynchronously because neither is spherical, and both have elongated shapes that wobble through space as they orbit, leading to a very complex dance.