Luna

Luna is a small moon orbiting Sol III. About 4 billion years ago, it is believed that a rogue planet named Theia or Orpheus, about the size of Sol IV is said to have hit the Earth, literaly 'peeling' off its crust. This debris gathered around Earth and eventually coalesced, becoming the planet's first and only natural satellite. While there are other theories as to Luna's creation, this scenario, known as the giant impact hypothesis is considered the more viable explanation.

Luna has a diameter at its equator of 3,476 kilometers, or 2,160 miles. From the poles, this diameter is 3,472 kilometers. Luna completes an orbit around Earth every 27.3 days. Luna is commonly referred to as the Moon, but is also known as Selene, Greek for moon as well as Luna, which is derived from Latin.

Wikipedia's entry on the Moon