Why does the Earth only see one side of the Moon? (I know about sychronous orbit, I'm just asking why)
The Moon rotates exactly once per revolution of the Earth which is the reason why it shows only one face at the Earth. If you face a pole and move around the pole showing your face at the pole all the time, you will notice that you have rotated once per revolution around the pole.
Why has this happened to the Moon? It is due to tidal forces of the Earth. You know that the Moon's tidal forces causes the high and low tides on the Earth. In the same way, the Earth exerts tidal forces on the Moon which are more powerful as the Earth is more massive than the Moon. It turns out that these forces exert torques on the rotating system and tends to slow its rotation till it finally shows the same face towards the other body. Hence, it is the effect of tidal forces of the Earth on the Moon that have caused the Moon to show only one face to the Earth.
Similarly, the tidal forces of the Sun on Mercury have slowed down its rotational period to once in 59 days. The Moon's tidal forces will have the same effect on Earth, so that some day in future (billions of years hence), the Earth may show the same face to the Moon.
This page was last updated on July 18, 2015.