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These maria formed when large impacts on the far side of the Moon heated up low lying layers of its crust on the near side. The lunar surface is covered in lunar dust and marked by mountains, impact craters, their ejecta, ray-like streaks and, mostly on the near side of the Moon, by dark maria ('seas'), which are plains of cooled magma. 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.

The body of the Moon is differentiated and terrestrial, with no significant hydrosphere, atmosphere, or magnetic field. 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. 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. 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. The Moon is in geophysical terms a planetary-mass object or satellite planet. The Moon's gravitational pull – and to a lesser extent the Sun's – are the main drivers of the tides.

This results in the lunar day of 29.5 days matching the lunar month. The Moon always presents the same side to Earth, because gravitational pull has locked its rotation to the planet. It orbits at an average distance of 384 400 km ( 238 900 mi), about 30 times the planet's diameter. The Moon is Earth's only natural satellite.

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