The Moon is the only natural satellite of Earth, and is the fifth largest satellite in the Solar System. The Moon does not rotate around its obliquity, so before humans photographed the whole moon they have only seen one side of it. The Moon has a diameter of 3,474 kilometers, which is approximately 1/4th of the Earth's. This body is the only other body than Earth that humans have walked on. Its gravity is 1/6th of Earth normal.
The Moon orbits the Earth at a barycenter 1,700 kilometers below the Earth's surface every 29.5 days. Being tidally locked to its primary, this makes only one side of the moon visible to Earth. It is believed that the Moon is slowly moving away from Earth, and if the two are not destroyed when the sun reaches the red giant stage, the orbital center of the two objects will be outside the Earth's surface. This would make the Moon a planet.
The prevailing theory on the Moon's formation was that an object the size of Mars named Theia hit the Earth at an oblique angle, which caused a large amount of rock and dust to spew out. This dust collected into the Moon since it was outside the Roche Limit.
- The Moon drifts away from the Earth a little bit each year due to tidal interactions.