Asked by Dan Hampton

Tidal locking is the name given to the situation when an object’s orbital period matches its rotational period. A great example of this is our own Moon. The moon takes 28 days to go around the Earth and 28 days to rotate once around it’s axis. This results in the same face of the Moon always facing the Earth. We see other examples of this in our solar system and universe.

An extreme example is the case of Pluto and Charon. Charon is such a large satellite compared to Pluto that they are tidally locked together. This means that Pluto only sees one face of Charon and vice versa. It is as if a rod connects two points on their surface. This results in a bizarre phenomenon where the moon Charon would always be in the same place in Pluto’s night sky. Tidal locking can have an effect on the system. In the Earth-Moon system the Earth’s rotation is actually slowing. It is altered by a tiny amount but something that can be noticed in fossils that are millions of years old.

Answered by Josh Barker from the National Space Centre