Moon

The Moon is Earth's only proper natural satellite. It is one quarter the diameter of Earth making it the largest natural satellite in the Solar System relative to the size of its planet. It is the fifth largest satellite in the Solar System and is larger than any dwarf planet. The Moon orbits Earth at an average lunar distance of 384,400 km (238,900 mi), or 1.28 light-seconds. Its gravitational influence produces Earth's tides and slightly lengthens Earth's day. It is considered a planetary-mass moon and a differentiated rocky body; its surface gravity is about one-sixth of Earth's and it lacks any significant atmosphere, hydrosphere, or magnetic field. Jupiter's moon Io is the only satellite in the Solar System known to have a higher surface gravity and density.