Luna

Luna, often referred to as the Moon, is the only natural satellite of Earth. As of 2525, the Luna OCS Academy located there. Due to its still barren face, one can assume it has not been terraformed, probably a result of Luna's low gravity, and lack of atmosphere.

In Halo 2, the moon can be seen from a variety of points. During The First Battle of Earth, the moon is visible from Cairo Station. In the final cutscene where the Master Chief returns to Earth onboard the Forerunner Dreadnought, the moon is seen in orbit over Earth during the space battle between the UNSC and Covenant fleets. The moon can also be seen through a window in the multiplayer maps Midship and Elongation.

"A face" (easter egg) is visible on the Lunar surface from the Halo 3 multiplayer map Standoff.

Trivia

 * The "Man in the Moon" is what appears to be a face on Luna's surface. This is so because of the way its craters are positioned.