Luna

Luna or to more specific, the moon is the only natural satellite of Earth, homeworld of Humanity. As of 2525, the moon has been colonized with the Luna OCS Academy being 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 Space Battle between the UNSC and Covenant fleets. The moon can also be seen through a window in the multiplayer maps Midship and Elongation.

It can be seen in Halo 3 with a face on it in Standoff.