The Moon is Earth’s only natural satellite and the fifth-largest moon in the Solar System. At roughly 1/4 Earth’s diameter and 1/81 Earth’s mass, it is unusually large relative to its planet — a ratio unmatched among the eight primary planets.

The Moon almost certainly formed about 4.51 billion years ago from debris ejected when a Mars-sized body (“Theia”) collided with the young Earth. This “giant impact” hypothesis is supported by the Moon’s bulk composition, its iron-poor interior, and the nearly identical oxygen isotope ratios of lunar and terrestrial rocks.

The Moon is tidally locked — it rotates on its axis in exactly the time it takes to orbit Earth, so the same “near side” always faces us. Its surface is divided between the dark, lava-filled lowland “maria” (Latin for seas) and the bright, heavily cratered “highlands.” Water ice has been confirmed in permanently shadowed craters at both poles by missions including LCROSS, Chandrayaan-1, and LRO.

The Moon is the only world beyond Earth upon which humans have walked. Six Apollo missions landed astronauts there between July 1969 (Apollo 11) and December 1972 (Apollo 17), returning 382 kg of samples. NASA’s Artemis program aims to return humans to the Moon in the mid-2020s, while China’s CNSA, India’s ISRO, Japan’s JAXA, Russia’s Roscosmos and multiple commercial operators are running robotic missions and planning future bases.