|MadSci Network: Astronomy|
You asked a very intersting question that probably occurs to many people as they stare towards the moon night after night. Lets first go over a little background information. The moon travels around the earth once every 29 days. The moon, attracted by the Earth's gravitational pull, travels with the earth and circles the sun every 365 days. The moon rotates completely on its axis (the imaginary line through its north and south poles) only once during each trip around the earth.
When we look at the moon, we always see the same side. This is because the moon turns once on its axis in the same time that it circles the earth. This motion is called synchronous rotation. Unlike the earth, which rotates 365 times during each rotation around the sun, the moon only rotates once. This results is your observation.....only one side can ever be seen from a specific point on earth.
Try the links in the MadSci Library for more information on Astronomy.