The moon

 The Moon is the only natural satellite of the Earth. It is 384 400 km away from the Earth and has a diameter of 3476 km. It takes about 27.32 days to go around the Earth. Due to tidal effects, the period of rotation of its axis coincides with the period of translation around the Earth, so the Moon always presents the same face to the Earth observer.

One of the most important hypotheses about the origin of the Moon explains that a very large body collided with the primitive Earth and from the material that was expelled into space our satellite was formed.