Despite what my wife thinks (and the Pink Floyd albums says), there is no such thing as a "Dark Side of the Moon". Granted, the face of the Moon facing away from the Sun is dark, but that face changes as the Moon orbits the Earth. A better description of the faces of the Moon would be to call them the Near Side of the Moon and the Far Side of the Moon. The near side is the face that always pointed to the Earth, and the far side is always pointed away. Why is this?
The reason why we should use near side and far side is because the Moon is in a 1:1 resonance with the Earth. Remember that the Sun-Mercury system has a 3:2 resonance, so for every two orbits around the Sun, Mercury rotates three times on its axis. For the Moon-Earth system, this means that for every one complete orbit around the Earth, the Moon rotates on its axis just one time. Because of this, the same side of the Moon is always facing Earth. It wasn't until 1959, when the Soviet Union's Luna 3 space probe photographed the far side. In 1968, it wasn't observed by human eyes for the first time during the Apollo 8 mission.
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