The Moon is Earth's satellite, and we can usually see it in the night sky. Other planets also have moons or "natural satellites". 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 just under 3,500 kilometres (2,200 mi) wide. That's over a quarter of the size of the Earth (about 12,600 kilometres (7,800 mi) wide). Because of this, the Earth and Moon together are sometimes called a binary or double planet system.

Phases
History of exploring the Moon
Characteristics
Origin of the Moon
Water on the Moon
Legal status
- United States missions
- Other missions
Legal status
Related pages
Images for kids
See also

Phases

The Moon being round, half of it is lit up by the sun. As it goes around (or orbits) the Earth, sometimes the side that people on Earth can see is all lit brightly. Other times only a small part of the side we see is lit. This is because the Moon does not send out its own light. People only see the parts that are being lit by sunlight. These different stages are called Phases of the Moon.

It takes the Moon about 29.53 days (29 days, 12 hours, 44 minutes) to complete the cycle, from big and bright to small and dim and back to big and bright. The phase when the Moon passes between the Earth and Sun is called the new moon. The next phase of the moon is called the "waxing crescent", followed by the "first quarter", "waxing gibbous", then to a full moon. A full Moon occurs when the moon and sun are on opposite sides of the Earth. As the Moon continues its orbit it becomes a "waning gibbous", "third quarter", "waning crescent", and finally back to a new moon. People used the moon to measure time. A month is approximately equal in time to a lunar cycle.

The phases of the Moon

The moon always shows the same side to Earth. Astronomers call this phenomenon tidal locking. This means that half of it can never be seen from Earth. The side facing away from Earth is called the far side or dark side of the Moon even though the sun does shine on it—we just never see it lit.

History of exploring the Moon

Buzz Aldrin standing on the moon in 1969

Before people stood on the Moon, the United States and the USSR sent robots to the Moon. These robots would orbit the Moon or land on its surface. The robots were the first man-made objects to touch the Moon.

Humans finally landed on the Moon on July 21, 1969. Astronauts Neil Armstrong and Buzz Aldrin landed their lunar ship (the Eagle) on the surface of the moon. Then, as half the world watched him on television, Armstrong climbed down the ladder of the Eagle and was the first human to touch the Moon as he said, "That's one small step for a man, one giant leap for mankind."

Even though their footprints were left on the moon a long time ago, it is likely that they are still there, as there is no wind or rain, making erosion extremely slow. The footprints do not get filled in or smoothed out.

More people landed on the moon between 1969 and 1972, when the last spaceship, Apollo 17 visited. Eugene Cernan of Apollo 17 was the last person to touch the moon.

Characteristics

Because it is smaller, the Moon has less gravity than Earth (only 1/6 of the amount on Earth). So if a person weighs 120 kg on Earth, the person would only weigh 20 kg on the moon. But even though the Moon's gravity is weaker than the Earth's gravity, it is still there. If person dropped a ball while standing on the moon, it would still fall down. However, it would fall much more slowly. A person who jumped as high as possible on the moon would jump higher than on Earth, but still fall back to the ground. Because the Moon has no atmosphere, there is no air resistance, so a feather will fall as fast as a hammer.

Without an atmosphere, the environment is not protected from heat or cold. Astronauts wore spacesuits, and carried oxygen to breathe. The suit weighed about as much as the astronaut. The Moon's gravity is weak, so it was not as heavy as on Earth.

In the Earth, the sky is blue because the blue rays of the sun bounce off the gases in the atmosphere, making it look like blue light is coming from the sky. But on the moon, because there is no atmosphere, the sky looks black, even in the daytime. There is no atmosphere to protect the moon from the rocks that fall from outer space, and these meteorites crash right into the moon and make wide, shallow holes called craters. The moon has thousands of them. Newer craters gradually wear away the older ones.

A blue moon is an additional full moon that appears in a subdivision of a year: either the third of four full moons in a season, or a second full moon in a month of the common calendar.

Origin of the Moon

The giant impact hypothesis is that the Moon was created out of the debris from a collision between the young Earth and a Mars-sized protoplanet. This is the favored scientific hypothesis for the formation of the Moon.

Water on the Moon

In 2009 NASA said that they had found a lot of water on the moon. The water is not liquid but is in the form of hydrates and hydroxides. Liquid water cannot exist on the Moon because photodissociation quickly breaks down the molecules. However, from the image NASA received, there is a history of water existence.

Legal status

During the Cold War, the United States Army thought about making a military post on the Moon, able to attack targets on Earth. They also considered conducting a nuclear weapon test on the Moon. The United States Air Force had similar plans. However, both plans were brushed-off as NASA moved from a military to a civilian-based agency.

Even though the Soviet Union left remains on the Moon, and the United States left a few flags, no country has control over the Moon. The U.S. and Soviet Union both signed the Outer Space Treaty, which calls the Moon and all of outer space the "province of all mankind". This treaty also banned all use of the military of the Moon, including nuclear weapons tests and military bases.

United States missions

The small blue-white semicircle of Earth, almost glowing with color in the blackness of space, rising over the limb of the desolate, cratered surface of the Moon.

Earthrise (Apollo 8, 1968, taken by William Anders)

Moon rock (Apollo 17, 1972)

Following President John F. Kennedy's 1961 commitment to a manned 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 manned missions.

The manned 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 is seen by many as the highest point of the Space Race.

Neil Armstrong official

Neil Armstrong became the first person to walk on the Moon as the commander of the American mission Apollo 11 by first setting foot on the Moon at 02:56 UTC on 21 July 1969. An estimated 500 million people worldwide watched the transmission by the Apollo TV camera, the largest television audience for a live broadcast at that time.

The Apollo missions 11 to 17 (except Apollo 13) returned 380.05 kilograms (837.87 lb) of lunar rock and soil in 2,196 separate samples. The American Moon landing and return was enabled by considerable technological advances in the early 1960s, in domains such as ablation chemistry, software engineering, and atmospheric re-entry technology, and by highly competent management of the enormous technical undertaking.

Scientific instrument packages were installed on the lunar surface during all the Apollo landings. Long-lived instrument stations, including heat flow probes, seismometers, and magnetometers, were installed at the Apollo 12, 14, 15, 16, and 17 landing sites. Direct transmission of data to Earth concluded in late 1977 because of budgetary considerations.

Other missions

The Moon

Near side of the Moon

Far side of the Moon

Lunar north pole

Lunar south pole

After the first Moon race there were years of near quietude but starting in the 1990s, many more countries have become involved in direct exploration of the Moon. In 1990, Japan became the third country to place a spacecraft into lunar orbit with its Hiten spacecraft. The spacecraft released a smaller probe, Hagoromo, in lunar orbit, but the transmitter failed, preventing further scientific use of the mission.

In 1994, the U.S. sent the joint Defense Department/NASA spacecraft Clementine to lunar orbit. This mission obtained the first near-global topographic map of the Moon, and the first global multispectral images of the lunar surface. This was followed in 1998 by the Lunar Prospector mission, whose instruments indicated the presence of excess hydrogen at the lunar poles, which is likely to have been caused by the presence of water ice in the upper few meters of the regolith within permanently shadowed craters.

India, Japan, China, the United States, and the European Space Agency each sent lunar orbiters, and especially ISRO's Chandrayaan-1 has contributed to confirming the discovery of lunar water ice in permanently shadowed craters at the poles and bound into the lunar regolith.

The European spacecraft SMART-1, the second ion-propelled spacecraft, was in lunar orbit from 15 November 2004 until its lunar impact on 3 September 2006, and made the first detailed survey of chemical elements on the lunar surface.

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. China intends to launch another rover mission (Chang'e 4) before 2020. The Moon remains, under the Outer Space Treaty, free to all nations to explore for peaceful purposes.

On 28 February 2018, SpaceX, Vodafone, Nokia and Audi announced a collaboration to install a 4G wireless communication network on the Moon, with the aim of streaming live footage on the surface to Earth.

Legal status

Apollo 15 flag

Although Luna landers scattered pennants of the Soviet Union on the Moon, and U.S. flags were symbolically planted at their landing sites by the Apollo astronauts, no nation claims ownership of any part of the Moon's surface. Russia, China, and the U.S. are party to the 1967 Outer Space Treaty, which defines the Moon and all outer space as the "province of all mankind".

This treaty also restricts the use of the Moon to peaceful purposes, explicitly banning military installations and weapons of mass destruction. The 1979 Moon Agreement was created to restrict the exploitation of the Moon's resources by any single nation, but as of November 2016, it has been signed and ratified by only 18 nations, none of which engages in self-launched human space exploration or has plans to do so. Although several individuals have made claims to the Moon in whole or in part, none of these are considered credible.

Images for kids

Size comparison of the main moons of the Solar System with Earth to scale. Nineteen moons are large enough to be round, several having subsurface oceans and one, Titan, having a considerable atmosphere.
The thin lunar atmosphere is visible on the Moon's surface at sunrise and sunset with the Lunar Horizon Glow and lunar twilight rays, like Earth's crepuscular rays. This Apollo 17 sketch depicts the glow and rays among the general zodiacal light.
Topography of the Moon
The largest mare, the main dark region of the near side, is Oceanus Procellarum, with smaller mare, such as Imbrium and Serenitatis, that sit within its ring. Left of the centerline is Procellarum proper.
Lunar crater Daedalus on the Moon's far side
Lunar Reconnaissance Orbiter Wide Angle Camera image of the lunar swirl Reiner Gamma
The geologic map of the Moon at 1-2.5M scale by Chinese Academy of Sciences. See the original file for higher resolution.
DSCOVR satellite sees the Moon passing in front of Earth
Comparison between the Moon on the left, rotating tidally locked (correct), and with the Moon on the right, without rotation (incorrect).
Libration, the slight variation in the Moon's apparent size and viewing angle over a single lunar month as viewed from Earth's north.
The changing apparent colour of the Moon, filtered by Earth's atmosphere.
Day moon, the moon is visible during daylight almost every day.
The Moon, tinted reddish, during a lunar eclipse
Galileo's sketches of the Moon from the ground-breaking Sidereus Nuncius, publishing among other findings the first descriptions of the Moons topography.
Map of the Moon by Johannes Hevelius from his Selenographia (1647), the first map to include the libration zones
First view in history of the far side of the Moon, taken by Luna 3, 7 October 1959
The first image from the lunar surface, photographed by Luna 9 in 1966.
A replica of Lunokhod 1, which reached the Moon becoming the first remote controlled rover on an extraterrestrial surface (1970)
Earthrise, the first colour image of Earth taken by a human from the Moon, during Apollo 8 (1968) the first time a crewed spacecraft left Earth orbit and reached another astronomical body.
Neil Armstrong, the first human on the Moon, working at the Lunar Module Eagle, a first lunar base, during Apollo 11 (1969), the first Moon landing
One of the first moon rocks (Lunar basalt 70017, Apollo 17, 1972), which were collected during the Apollo missions.
NASA's Moon Mineralogy Mapper equipment on India's Chandrayaan-1 for the first time discovered in 2008 water-rich minerals (light blue), shown in blue around a small crater from which it was ejected.
Remains of human activity, Apollo 17's Lunar Surface Experiments Package.
A photo of the reflector of the Lunar Laser Ranging Experiment of Apollo 11, still in use.
A Mooncake, given to people at the Harvest Moon Festival, the second most important celebration of the Chinese Lunar Calendar, after Chinese New Year.
Scale model of the Earth–Moon system: Sizes and distances are to scale.