The Moon (sometimes called Luna to distinguish it from other moons) is the fifth largest moon in the solar system. It is airless, lifeless, and geologically inactive. The Moon is tidally locked with the Earth meaning that it rotates once for each orbit. To put this another way, the same side of the moon perpetually faces the earth. The Moon's gravitational pull is responsible for tides. It is the only other body in the solar system (besides
the earth) to ever have been visited by humans.