While other planets like Jupiter have a dozen moons, the earth has only one. It is approximately 1/4 the diameter of the earth and completes a revolution around the earth every 28 days. The moon always keeps the same side turned toward us. The dark spots on the moon face are called mare which is Latin for "seas". However, they contain no water. The mare are places where the rock was formed more recently than in lighter areas. The moon has thousands of craters from meteor impacts because, unlike the earth, it has no atmosphere to absorb the energy of incoming meteors.