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.
Moon craters puzzle pre-Apollo 11 scientists.
There were competing theories about the origins of Moon craters before the Apollo landings. Some researchers thought that craters were volcanic in origin, something we now know is false. Others correctly theorised that Moon craters are created by impacts.
Neil Armstrong makes a hair-raising landing on the Moon.
Project Apollo, the United States's manned Moon landing programme, culminated in 1969 with Apollo 11. Mission commander Neil Armstrong's famous words, "That's one small step for man, one giant leap for mankind", spoken as he stepped onto the lunar surface, were heard by an estimated 600 million television viewers. This clip contains some re-enactment footage.
America's manned space programme faces early tragedy.
Three American astronauts, Gus Grissom, Ed White and Roger Chaffee, were accidentally killed in a 1967 training exercise later named Apollo 1. The men died when fire broke out in their oxygen-filled space capsule.
NASA prepares for the ultimate goal of a man on the Moon.
The 1968 Apollo 8 spacecraft was the first manned probe to orbit the Moon. The astronauts on board were Frank Borman, James Lovell and William Anders.
Some say the Apollo programme was a waste of money.
With NASA calling for a return to manned Moon landings, some say the Apollo landings did not achieve much of substance and were a waste of money.
The Moon (Latin: Luna) is the Earth's only natural satellite.[e][f] Although not the largest natural satellite in the Solar System, it is, among the satellites of major planets, the largest relative to the size of the object it orbits (its primary) [g][h] and, after Jupiter's satellite Io, it is the second most dense satellite among those whose densities are known.
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 second-brightest regularly-visible celestial object in Earth's sky, after the Sun (measured by illuminance on the surface of the Earth). Although it can appear 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 slight lengthening of the day. The Moon's current orbital distance is about thirty times the diameter of Earth, causing it to have an apparent size in the sky almost the same as that of the Sun. This allows the Moon to cover the Sun nearly precisely in total solar eclipse. 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 currently 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 origin, the formation of its internal structure, and its subsequent history.
After the Apollo 17 mission in 1972, the Moon has been visited by only 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.