- Mean RADIUS: 1738.1 km
- MASS: 0.0123 (Earth=1)
- DENSITY: 3350 (kg/m3)
- GRAVITY: 0.165 (Earth=1)
- ORBIT PERIOD: 27.3217 (Earth days)
- ROTATION PERIOD: 1.00 (Earth days)
- SEMIMAJOR AXIS OF ORBIT: 0.3844 X 106 km
- ECCENTRICITY OF ORBIT: 0.0549
Because of its size and rocky composition, the moon has also been called a terrestrial planet along with Mercury, Venus, Earth, and Mars. It has no atmosphere, but there is water ice in some deep craters. The moon is the only extra-planetary body that a human has visited. Besides the sun, the moon is the brightest object in the sky and very easily seen with the naked eye. Using a telescope, you can easily map out many of the Lunar features.
The asymmetric nature of the gravitational interaction between the Earth and the Moon causes the Moon to rotate synchronously. In other words, it is locked in phase with its orbit so that the same side is always facing toward the Earth.
Although there were several theories concerning the origin of the Moon (co-accretion {Created at the same time as earth}, fission {split off from Earth}, and capture {caught by Earth's gravity}), after the Apollo missions, it is now widely believed that a very large object (Mars size or larger) collided with the Earth, knocking loose a chunk, which became the Moon.
