Portal:Solar System/Selected article/6
The Moon is Earth's only natural satellite. It orbits at an average distance of 384400 km (238900 mi), about 30 times the planet's diameter. Being tidally locked, it always faces Earth with its near side. This results in the lunar month of 29.5 days matching the lunar day. Exerting gravitational force on Earth, it and to a lesser extent the Sun are the main drivers of the tides.
The Moon is in geophysical terms a planetary-mass object or satellite planet. It has a mass that amounts to 1.2% of Earth's, and a diameter that is roughly one-quarter of Earth's or with 3,474 km (2,159 mi) about as wide as Australia. Within the Solar System it is the most massive and largest satellite in relation to its parent planet, the fifth most massive and largest moon overall, and more massive and larger than all known dwarf planets. Its surface gravity is about one sixth of Earth's, about half of that of Mars, and after Jupiter's moon Io the second highest among all Solar System moons. The body of the Moon is differentiated and terrestrial, with no significant hydrosphere, atmosphere, or magnetic field. It formed 4.51 billion years ago, not long after Earth's formation, out of the debris from a giant impact between Earth and a hypothesized Mars-sized body called Theia. (Full article...)