Despite centuries of study through increasingly powerful telescopes, the Moon really only began to reveal its secrets during the Apollo landings in the 1960s and 1970s.
Scientists studying rocks returned by the astronauts believe the Moon formed about 4.6 billion years ago when a planet-sized object smashed into the early Earth and sprayed molten rock into orbit around the battered planet. It is thought these orbiting fragments slowly came together to form the Moon.
Photo: The Moon taken by the Apollo 17 astronauts (NASA)
Earth's satellite had a turbulent birth.
Apollo 12 finds hardy bacteria on an old Moon probe.
The Apollo 12 astronauts located Surveyor 3, a 1967 unmanned Moon probe, and returned a piece of it to Earth. Scientists wanted to know what effect 33 months on the Moon had had on the probe. Inside the spacecraft's camera they found droplets from a sneeze accidentally sealed into the instrument by one of its builders. The bacteria in the droplets "came back to life" once they were returned to the right conditions. This showed how hardy life can be.
A Russian probe sends back the first pictures taken from the Moon's surface.
Luna 9, the first Moon probe to achieve a soft landing and send back photos of the lunar surface, caught the West by surprise in 1966. The Russian spacecraft's data transmissions were intercepted by Jodrell Bank Observatory, and the pictures were published in a British newspaper before they were published in Russia.
Apollo 8 orbits the Moon as NASA prepares for a manned landing.
Apollo 8 orbited the Moon on Christmas Eve in 1968 and temporarily lost contact with mission controllers when the spacecraft went behind the Moon. This mission was one of the final test flights before the 1969 Apollo 11 lunar landing.
Patrick Moore and his guest tackle a common Moon hoax claim.
Playing devil's advocate, Sir Patrick Moore asks space imaging expert Douglas Arnold about a common claim made by people who say the Moon landings were faked.
Why does the Moon, which has no tectonic plates, shake?
Scientists were surprised to find that there are quakes on the Moon. It has none of the features such as tectonic plates that cause earthquakes.
The Moon is the only natural satellite of the Earth[d] and the fifth largest moon in the Solar System. It is the largest natural satellite of a planet in the Solar System relative to the size of its primary,[e] having 27% the diameter and 60% the density of Earth, resulting in 1⁄81 (1.23%) its mass. Among satellites with known densities, the Moon is the second densest, after Io, a satellite of Jupiter.
The Moon is in synchronous rotation with Earth, always showing the same face with its near side marked by dark volcanic maria that fill between the bright ancient crustal highlands and the prominent impact craters. It is the most luminous object in the sky after the Sun. Although it appears a very bright white, its surface is actually dark, with a reflectance just slightly higher than that of worn asphalt. Its prominence in the sky and its regular cycle of phases have, since ancient times, made the Moon an important cultural influence on language, calendars, art and mythology. The Moon's gravitational influence produces the ocean tides and the minute lengthening of the day. The Moon's current orbital distance, about thirty times the diameter of Earth, causes it to appear almost the same size in the sky as the Sun, allowing it to cover the Sun nearly precisely in total solar eclipses. This matching of apparent visual size is a coincidence. The Moon's linear distance from Earth is currently increasing at a rate of 3.82±0.07 cm per year, but this rate is not constant.
The Moon is thought to have formed nearly 4.5 billion years ago, not long after Earth. Although there have been several hypotheses for its origin in the past, the current most widely accepted explanation is that the Moon formed from the debris left over after a giant impact between Earth and a Mars-sized body.
The Moon is the only celestial body other than Earth on which humans have set foot. The Soviet Union's Luna programme was the first to reach the Moon with unmanned spacecraft in 1959; the United States' NASA Apollo program achieved the only manned missions to date, beginning with the first manned lunar orbiting mission by Apollo 8 in 1968, and six manned lunar landings between 1969 and 1972, with the first being Apollo 11. These missions returned over 380 kg of lunar rocks, which have been used to develop a geological understanding of the Moon's origins, the formation of its internal structure, and its subsequent history.
After the Apollo 17 mission in 1972, the Moon has been visited only by unmanned spacecraft. Of these, orbital missions have dominated: Since 2004, Japan, China, India, the United States, and the European Space Agency have each sent lunar orbiters, which have contributed to confirming the discovery of lunar water ice in permanently shadowed craters at the poles and bound into the lunar regolith. The post-Apollo era has also seen two rover missions: the final Soviet Lunokhod mission in 1973, and China's ongoing Chang'e 3 mission, which deployed its Yutu rover on 14 December 2013.
Future manned missions to the Moon have been planned, including government as well as privately funded efforts. The Moon remains, under the Outer Space Treaty, free to all nations to explore for peaceful purposes.