Moon

The Moon is Earth's only natural satellite and our closest celestial neighbour. It orbits the Earth at an average distance of roughly 384,000 kilometres and is the largest moon relative to its planet in the solar system. Because it is so close, it is by far the brightest object in the night sky, and its gravity is the main cause of the ocean tides. It has no atmosphere, no liquid water on its surface and no life that we know of.

The Moon has shaped human culture for as long as people have looked up at the sky. It appears in calendars, myths and religions around the world, and it became the first and so far only other world that humans have visited.

How the Moon formed

The leading scientific explanation for the Moon's origin is the giant-impact hypothesis. It proposes that, early in Earth's history, a large body roughly the size of Mars struck the young Earth. The debris thrown into orbit by this collision gradually came together to form the Moon. This idea helps explain several features, including the Moon's relatively low density and the chemical similarities between lunar and terrestrial rocks. The exact details are still debated, but the giant-impact model is the most widely accepted.

Surface and interior

The Moon's surface shows two main kinds of terrain. The bright, heavily cratered areas are the highlands, which are among the oldest surfaces in the solar system. The darker patches, visible to the naked eye as the "face" or "seas" of the Moon, are plains of solidified lava called maria. The Moon is covered with craters formed by impacts over billions of years, since it lacks an atmosphere to burn up incoming objects and lacks weather to erode the scars. Beneath the surface, the Moon has a crust, a mantle and a small core. It has no active volcanoes today, though small moonquakes occur.

Phases and the lunar cycle

The Moon does not produce its own light; it shines by reflecting sunlight. As it orbits the Earth, the portion lit by the Sun as seen from Earth changes, producing the familiar phases: new moon, waxing crescent, first quarter, waxing gibbous, full moon, and then the waning phases back to new. The Moon takes about 29.5 days to complete this cycle of phases. The same side of the Moon always faces Earth because the Moon rotates on its axis in the same time it takes to orbit us, a state called synchronous rotation. The far side, sometimes misleadingly called the "dark side", receives just as much sunlight as the near side.

Eclipses

Because the Moon's orbit is slightly tilted, the Sun, Earth and Moon do not line up perfectly every month. When they do, eclipses occur. A solar eclipse happens when the Moon passes between the Sun and Earth and blocks the Sun, either partly or totally. A lunar eclipse happens when Earth passes between the Sun and Moon, casting its shadow on the Moon. Solar eclipses are visible only along a narrow path and last only minutes, while lunar eclipses can be seen from the whole night side of Earth and last longer.

Tides

The Moon's gravity pulls on Earth's oceans, causing the water to bulge slightly toward and away from the Moon. As Earth rotates through these bulges, coastal areas experience two high tides and two low tides in roughly a day. The Sun also affects the tides; when the Sun and Moon line up at new and full moon, their effects add up and produce higher "spring" tides, while at quarter phases they partly cancel to give lower "neap" tides.

Exploration

The Moon is the only world beyond Earth that people have walked on. Between 1969 and 1972, astronauts landed there several times and brought back hundreds of kilograms of rock and soil for study. Robotic spacecraft from many countries have since orbited and landed on the Moon, mapping its surface and studying its resources. Interest has grown again in recent years, with several nations planning new missions and discussing the possibility of a long-term human presence and resource use, including possible water ice in permanently shadowed craters near the poles.

Common misconceptions

One common error is that the Moon causes the seasons; seasons are caused by the tilt of Earth's axis. Another is that there is a "dark side" of the Moon that never sees sunlight, which is untrue. Some people think the Moon is only visible at night, but it is often visible in daylight. Finally, the idea that the Moon landing was faked is a persistent myth contradicted by extensive physical evidence, including samples, photographs and independent tracking.

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