Luna

Luna is the only natural satellite of Earth, homeworld of the UNSC. As of 2525, the moon has been colonized; the Luna OCS Academy is located there. Due to its still barren face, one can assume it has not been terraformed.

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 cut scene where the Master Chief returns to Earth onboard the Forerunner Ship, the moon is seen in orbit over Earth during the massive battle between UNSC and Covenant fleets. The moon can also be seen through a window in the multiplayer map, Midship.

Trivia

 * The moon would be, in that time, a regular Death Star. Being used to house troops and weaponry. Their might also be hidden MAC guns on the moon. This is only a theory though.