Luna is an astronomical body that orbits planet Earth, being its only permanent natural satellite. It is the fifth-largest natural satellite in Sol, and the largest among planetary satellites relative to the size of the planet that it orbits.
Luna is in synchronous rotation with Earth, always showing the same face, with its near side marked by dark volcanic maria that fill the spaces between the bright ancient crustal highlands and the prominent impact craters. It is the second-brightest regularly visible celestial object in Earth's sky, after its sun, as measured by illuminance on Earth's surface. Its surface is actually dark, although compared to the night sky it appears very bright, with a reflectance just slightly higher than that of worn asphalt.
The Moon's gravitational influence produces the ocean tides, body tides, and the slight lengthening of the day.
The 21st century was a time of rapid lunar development for Humanity as they further extended their grasp over Sol. Before the turn of the century, the United Nations built out extensive planetary defenses that serve as Earth's second line of defense in the event pirates happened to get past the UN Navy blockades.
In addition to military encampments, private and civilian property has continuously popped up all over the moon. Among these properties is the Lunar Research Post: the North American Union's central interstellar research facility.