Luna

Luna is the only natural satellite of Earth, homeworld of the UNSC. As of 2525, the moon appears to have been colonized; the Luna OCS Academy is located there and Captain Jacob Keyes lived, trained, and taught there for some time. Due to its 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.

Believed to be so in the forums of Bungie.net, an individual discovered a "good thought," they said that the Moon or (luna) was the main part of the mechanism that fires the installations. He goes on to say that they (the gigantic "doors" opening on the halo 3 teaser) are not the ark, but wrather the earth itself is, and that the "doors" are really an impulse being sent out. Like a wireless rc car, the "doors" would release a MASSIVE ammount of energy from within, which in turn the moon recieved and distributed it throughout the universe at sub-luminos speeds. Though that is all theory, it is a pleasent one.