captured rotation

The Moon always shows the same side to the Earth. This is because its period of rotation on its axis is the same as its period of revolution around the Earth.

It is a phenomenon called captured rotation, and it the result of the Earth's gravitation pull on the Moon's uneven mass over many eons.

Most of the satellites in the solar system exhibit captured rotation, and Pluto's rotation has also been captured by its satellite Charon.

Inconstant Moon