# Why does the face of the moon 'sync' with the earth? [duplicate]

We never see the dark side of the moon. It rotates so that the same side always faces us.

I heard two (dinner party) theories on this: 1. The moon was made by smashing out a chunk of the earth, and so somehow preserved some orbital rotation from the earth 2. The gravitational drag from the earth to the moon is lowest when the moon is in 'sync' to only show one face to the earth

I don't trust either of these.

My question is: "Why does the face of the moon 'sync' with the earth?"

• This is not a force, but radiation. Radiation also decays like $r^{-2}$, like gravitational forces (both is simple geometry). – Walter Nov 29 '14 at 17:35