Earth's Moon is the fifth largest of at least 168 known moons orbiting planets in the Solar System (Figure 1.73).
Figure 1.73. The Moon
moon radius: 1,079 miles (1,736 km)
orbital period: 27 days
average distance from Earth: 238,855 miles (383,300 km).
gravity: 1.622 m/s2
The Moon rotates ate the same rate that it revolves around the Earth (a synchronous rotation that keeps the same side of the Moon facing Earth).
The Moon lacks an atmosphere, and does not display any active geologic activity (such as earthquakes or volcanic eruptions). Like Earth, the Moon has a core, mantle, and a crust; geophysical data suggest the part of the Moon's core and mantle may be molten. The lack of atmosphere has helped to preserve geologic features that date back to early stages in the formation of the Solar System.
Most of what we have learned about the physical environment, composition, and origin of the Moon comes from the Apollo Missions (between 1961 and 1975) which culminated in a series of manned Moon landings between 1969 and 1972. Rock and lunar soil sample collected during those missions have helped resolving many questions and supporting theories about the origin of the Earth and Moon within the Solar System (discussed below).