Moon

**The** **Moon** is an astronomical body that orbits planet Earth, being Earth's only permanent natural satellite. It is the fifth-largest natural satellite in the Solar System, and the largest among planetary satellites relative to the size of the planet that it orbits (its primary). Following Jupiter's satellite Io, the Moon is second-densest satellite among those whose densities are known.

## Background

The Moon is thought to have formed about 4.51 billion years ago, not long after Earth. The 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 called Theia.

The Moon is in synchronous rotation with Earth, always showing the same face, with its near side marked by dark volcanic maria that fill the spaces between the bright ancient crustal highlands and the prominent impact craters.

As seen from the Earth, it is the second-brightest regularly visible celestial object in Earth's sky, after the Sun. Its surface is actually dark, although compared to the night sky it appears very bright, with a reflectance just slightly higher than that of worn asphalt. Its gravitational influence produces the ocean tides, body tides, and the slight lengthening of the day.

The Moon's average orbital distance at the present time is 384,402 km (238,856 mi), or 1.28 light-seconds. This is about thirty times the diameter of Earth, with its apparent size in the sky almost the same as that of the Sun (due to it being 400x farther **and** larger), resulting in the Moon covering the Sun nearly precisely in total solar eclipse. This matching of apparent visual size will not continue in the far future, because the Moon's distance from Earth is slowly increasing.

The Soviet Union's Luna programme was the first to reach the Moon with uncrewed spacecraft in 1959; the United States' NASA Apollo program achieved the only crewed missions to date, beginning with the first crewed lunar orbiting mission by Apollo 8 in 1968, and six crewed lunar landings between 1969 and 1972, with the first being Apollo 11. These missions returned lunar rocks which have been used to develop a geological understanding of the Moon's origin, internal structure, and later history. Since the Apollo 17 mission in 1972, the Moon has been visited only by uncrewed spacecraft.

Within human culture, both the Moon's natural prominence in the earthly sky, and its regular cycle of phases as seen from the Earth have provided cultural references and influences for human societies and cultures since time immemorial. Such cultural influences can be found in language, lunar based calendar systems, art, and mythology including the Greek goddess of the Moon Artemis and the Chinese goddess of the Moon Chang'e.

## Locations on the Moon

* The Sea of Tranquillity
* Moltke Crater

## Exploration of the Moon

The Cold War-inspired Space Race between the Soviet Union and the U.S. led to an acceleration of interest in exploration of the Moon.

### Soviet Missions

Spacecraft from the Soviet Union's *Luna program* were the first to accomplish a number of goals: following three unnamed, failed missions in 1958, the first human-made object to escape Earth's gravity and pass near the Moon was Luna 1; the first human-made object to impact the lunar surface was Luna 2, and the first photographs of the normally occluded far side of the Moon were made by Luna 3, all in 1959.

The first spacecraft to perform a successful lunar soft landing was Luna 9 and the first unstaffed vehicle to orbit the Moon was Luna 10, both in 1966. Rock and soil samples were brought back to Earth by three Luna sample return missions (Luna 16 in 1970, Luna 20 in 1972, and Luna 24 in 1976), which returned 0.3 kg total. Two pioneering robotic rovers landed on the Moon in 1970 and 1973 as a part of Soviet *Lunokhod program*.

### United States Missions

Following President John F. Kennedy's 1961 commitment to a staffed moon landing before the end of the decade, the United States, under NASA leadership, launched a series of unmanned probes to develop an understanding of the lunar surface in preparation for staffed missions: the Jet Propulsion Laboratory's *Ranger program* produced the first close-up pictures; the *Lunar Orbiter program* produced maps of the entire Moon; the *Surveyor program* landed its first spacecraft four months after Luna 9.

The *Apollo program* was developed in parallel; after a series of unmanned and manned tests of the Apollo spacecraft in Earth orbit, and spurred on by a potential Soviet lunar flight, in 1968 Apollo 8 made the first manned mission to lunar orbit. The subsequent landing of the first humans on the Moon in 1969 by Apollo 11 is seen by many as the culmination of the Space Race. The Apollo missions 11 to 17 (except Apollo 13, which aborted its planned lunar landing) returned 380.05 kilograms (837.87 lb) of lunar rock and soil in 2,196 separate samples.

## References

1. ↑ Wikipedia - Moon
2. ↑ Wikipedia - Luna program
3. ↑ Wikipedia - Lunokhod program
4. ↑ Wikipedia - Apollo program