Great question! Interestingly, it takes the moon 27.322 days to make one full rotation around the earth. So why 27.322 days and not 30 day? Or 10 days? Well, it comes down to a combination of four key things: (1) the distance from the Earth to the moon (384,400 km), (2) the size (specifically the mass) of the moon and the size of the earth, (3) inertia (movement), and (4) gravity. The force of gravity keeps the moon locked in orbit around the earth, as nothing exists in space to stop it the inertia (also called the movement) of the moon causes it to keep on rotating around the earth. Because the moon is 384,400 km away from earth (or nearly the distance of 30 Earths!!) it takes it some time to make the circular journey around the earth; however, with a mean orbital speed of 1,023 meters per second, the moon is actually zipping around the earth at a pretty incredible rate! Finally, the relative sizes of the earth and the moon create a unique relationship between these two bodies which causes the moon to be locked in rotation together so that the same side of the moon always faces the earth.