The Moon is Earth's only proper natural satellite. At one-quarter the diameter of Earth, it is the largest natural satellite in the Solar System relative to the size of its planet, and the fifth largest satellite in the Solar System overall. Orbiting Earth at an average lunar distance of 384,400 km (238,900 mi), or about 30 times Earth's diameter, its gravitational influence is the main driver of Earth's tides and slightly lengthens Earth's day. The Moon is classified as a planetary-mass object and a differentiated rocky body, and lacks any significant atmosphere, hydrosphere, or magnetic field. Its surface gravity is about one-sixth of Earth's ; Jupiter's moon Io is the only satellite in the Solar System known to have a higher surface gravity and density.