The Moon is Earth’s only natural satellite, a rocky world about 384,400 kilometers away on average whose gravity, history and surface preserve clues to the early solar system.
Size, distance and orbit
The Moon has a radius of about 1,740 kilometers—less than one-third Earth’s width—and orbits at an average distance of about 384,400 kilometers. Its orbit is not a perfect circle, so the actual distance changes over the month.
The Moon rotates once in roughly the same time it takes to orbit Earth. This synchronous rotation is why nearly the same lunar hemisphere faces Earth, although libration lets us see slightly more than half of the surface over time.
Why the Moon has phases
Half of the Moon is always illuminated by the Sun except during an eclipse. The phases occur because our viewing angle changes as the Moon moves around Earth. A new moon is near the Sun’s direction in the sky; a full moon is on the opposite side of Earth from the Sun.
The far side is not permanently dark. It receives sunlight just like the near side, but it usually faces away from Earth.
A surface shaped by impacts and volcanism
Bright highlands and dark basaltic plains called maria dominate the lunar surface. The maria formed when ancient lava filled large impact basins. With no thick atmosphere or active weather, impact craters can remain visible for billions of years.
Moon rocks returned by Apollo and studied by later missions help scientists reconstruct both lunar history and the bombardment history of the early inner solar system.
Water and the lunar poles
The Moon is extremely dry compared with Earth, but water molecules and water ice have been detected in several forms. Permanently shadowed polar craters can remain cold enough to preserve ice delivered or produced over long periods.
That polar water is scientifically important and is one reason modern lunar missions focus on the South Pole. Whether and how it could be used as an exploration resource is an engineering and policy question still being studied.
How the Moon affects Earth
The Moon’s gravity is the largest astronomical driver of Earth’s ocean tides. Tidal interactions also slowly transfer angular momentum, causing the Moon to recede from Earth by a few centimeters per year and gradually slowing Earth’s rotation over geological time.
The Moon also stabilizes important aspects of Earth’s rotational dynamics, while eclipses provide striking demonstrations of the geometry of the Sun-Earth-Moon system.
