Moon facts for kids

Kids Encyclopedia Facts
Moon Moon symbol
Full moon
A full moon as seen from Earth's northern hemisphere
Names
Adjective lunar, selenic
Orbit
Shortest distance from what it orbits 363,104 km  (0.002 4 AU)
Longest distance from what it orbits 405,696 km  (0.002 7 AU)
Longest distance from the center of its orbital path
("semi-major axis")
384,399 km  (0.002 57 AU)
How long it takes to complete an orbit 27.321 582 d  (27 d 7 h 43.1 min)
How long an orbit seems to take
(from the central body)
29.530 589 d  (29 d 12 h 44 min 2.9 s)
Average speed 1.022 km/s
Angle above the reference plane
("inclination")
5.145° to the ecliptic
(between 18.29° and 28.58° to Earth's equator)
What it orbits Earth
Size and other qualities
Average radius 1,737.10 km  (0.273 Earths)
Distance around its equator 10,921 km (equatorial)
Surface area 3.793 × 107 km²  (0.074 Earths)
Volume 2.195 8 × 1010 k  (0.020 Earths)
Mass 7.347 7 × 1022 kg  (0.012 3 Earths)
Average density 3,346.4 kg/m³
Surface gravity 1.622 m/s² (0.165 4 g)
Escape velocity 2.38 km/s
Turning speed 4.627 m/s
Angle at which it turns
(in relation to its orbit)
1.542 4° (to ecliptic)
6.687° (to orbit plane)
How much light it reflects 0.12
Surface temp. Min. Avg. Max.
equator 100 K 220 K
85°N0.3em 70 K 130 K 230 K
Seeming brightness
("apparent magnitude")
−2.5 to −12.9
−12.74 (mean full moon)
Pressure 2.25  × 10-12 torr

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 kilometers (km) wide. That's over a quarter of the size of the Earth (about 12,600 km wide). Because of this, the Earth and Moon together are sometimes called a binary or double planet system.

Ancient and medieval studies

Moon-apollo17-schmitt boulder
Planetary geology, alternatively known as astrogeology or exogeology, is a planetary science discipline concerned with the geology of the celestial bodies such as the planets and their moons, asteroids, comets, and meteorites

Understanding of the Moon's cycles was an early development of astronomy: by the 5th century BC, Babylonian astronomers had recorded the 18-year Saros cycle of lunar eclipses.

In the 2nd century BC, Seleucus of Seleucia (a Hellenistic astronomer and philosopher) correctly theorized that tides were due to the attraction of the Moon, and that their height depends on the Moon's position relative to the Sun. In the same century, Aristarchus computed the size and distance of the Moon from Earth, obtaining a value of about twenty times the radius of Earth for the distance. Archimedes (287–212 BC) designed a planetarium that could calculate the motions of the Moon and other objects in the Solar System. During the Middle Ages, before the invention of the telescope, the Moon was increasingly recognised as a sphere, though many believed that it was "perfectly smooth".

In 1609, Galileo Galilei drew one of the first telescopic drawings of the Moon and noted that it was not smooth but had mountains and craters. Telescopic mapping of the Moon followed: later in the 17th century, the efforts of Giovanni Battista Riccioli and Francesco Maria Grimaldi led to the system of naming of lunar features in use today. Lunar craters, first noted by Galileo, were thought to be volcanic until the 1870s proposal of Richard Proctor that they were formed by collisions. This view gained support in 1892 from the experimentation of geologist Grove Karl Gilbert leading to the development of lunar stratigraphy, which by the 1950s was becoming a new and growing branch of astrogeology.

The Moon's regular phases make it a very convenient timepiece, and the periods of its waxing and waning form the basis of many of the oldest calendars. Tally sticks, notched bones dating as far back as 20–30,000 years ago, are believed by some to mark the phases of the Moon. The ~30-day month is an approximation of the lunar cycle.

Physical characteristics

Gravity

Apollo Crew Big Bertha
Astronauts of the Apollo 14 lunar landing mission show off some of the lunar rocks they collected on their mission in 1971

The gravity on the moon is one-sixth of the Earth's gravity. Our moon is about a quarter the size of the Earth, because it is smaller, the Moon has less gravity than Earth (only 1/6 of the amount on Earth). So if a person weighs 120kg on Earth, the person would only weigh 20kg 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. The Moon drifts away from Earth at the rate of four centimeters each year.

Atmosphere

ISS-51 Jack Fischer during spacesuit fit check
Jack Fischer during spacesuit fit check

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 but because the Moon's gravity is weak, 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. And because there is no atmosphere to protect the moon from the rocks that fall from outer space. 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.

Water on the Moon

Moon colony with rover
Artist's concept of a lunar base - Moon colony with rover

Liquid water cannot persist on the lunar surface. When exposed to solar radiation, water quickly decomposes through a process known as photodissociation and is lost to space. However, since the 1960s, scientists have thought that water ice may be deposited by impacting comets or possibly produced by the reaction of oxygen-rich lunar rocks, and hydrogen from solar wind, leaving traces of water which could possibly persist in cold, shadowed craters at either pole on the Moon.

Computer simulations suggest that up to 14,000 km2 (5,400 sq mi) of the surface may be in permanent shadow. The presence of usable quantities of water on the Moon is an important factor in rendering lunar habitation as a cost-effective plan; the alternative of transporting water from Earth would be almost too expensive to conceive doing.

Analysis of the findings of the Moon Mineralogy Mapper (M3) revealed in August 2018 for the first time "definitive evidence" for water-ice on the lunar surface. The data revealed the distinct reflective signatures of water-ice, as opposed to dust and other reflective substances. The ice deposits were found on the North and South poles, although it is more abundant in the South, where water is trapped in permanently shadowed craters and crevices, allowing it to persist as ice on the surface since they are shielded from the sun.

Geology

Moon-craters
Lunar crater Daedalus on the Moon's far side

It has a geochemically distinct crust, mantle, and core. The Moon has a solid iron-rich inner core with a radius possibly as small as 240 kilometres (150 mi) and a fluid outer core primarily made of liquid iron with a radius of roughly 300 kilometres (190 mi). Around the core is a partially molten boundary layer with a radius of about 500 kilometres (310 mi). The crust is on average about 50 kilometres (31 mi) thick.

The Moon is the second-densest satellite in the Solar System, after Io. However, the inner core of the Moon is small, around 20% of the radius of the Moon. Its composition is not well defined, but is probably metallic iron alloyed with a small amount of sulfur and nickel; analyses of the Moon's suggests that it is at least partly molten. Because the core is solid, the Moon does not have its own magnetic field and only has crustal magnetization.

Pasteur D crater Apollo 15
Pasteur D crater

The surface of the Moon is made of rocks and dust. The outer layer of the Moon is called the crust. The crust is about 70 km thick on the near side and 100 km thick on the far side. It is thinner under the maria and thicker under the highlands. There may be more maria on the near side because the crust is thinner. It was easier for lava to rise up to the surface. The lunar maria are large, dark, basaltic plains on Earth's Moon, formed by ancient volcanic eruptions.

The other major geologic process that has affected the Moon's surface is impact cratering, with craters formed when asteroids and comets collide with the lunar surface. There are estimated to be roughly 300,000 craters wider than 1 km (0.6 mi) on the Moon's near side alone. The ages of impact-melted rocks collected during the Apollo missions are between 3.8 and 4.1 billion years old. The largest one is called the South Pole-Aitken Basin and is roughly 2500 km across.

Lunar Regolith 70050 from Apollo 17 in National Museum of Natural History
Lunar Regolith from Apollo 17 in National Museum of Natural History

Blanketed on top of the Moon's crust is a highly comminuted (broken into ever smaller particles) and impact gardened surface layer called regolith, formed by impact processes. The finer regolith, the lunar soil of silicon dioxide glass, has a texture resembling snow and a scent resembling spent gunpowder.

The moon experiences quakes called moonquakes which are much less common and weaker than earthquakes, although moonquakes can last for up to an hour – significantly longer than terrestrial quakes – because of the absence of water to damp out the seismic vibrations. The existence of moonquakes was an unexpected discovery from seismometers placed on the Moon by Apollo astronauts from 1969 through 1972.

A permanent asymmetric moon dust cloud exists around the Moon, created by small particles from comets. Estimates are 5 tons of comet particles strike the Moon's surface each 24 hours. The particles strike the Moon's surface ejecting moon dust above the Moon. The dust stays above the Moon approximately 10 minutes, taking 5 minutes to rise, and 5 minutes to fall. On average, 120 kilograms of dust are present above the Moon, rising to 100 kilometers above the surface.

Phases

Phases of a Blood Moon (21829901591)
Phases of a Blood Moon

The Moon is lit up by the sun as it goes around (or orbits) the Earth. This means on Earth one can see the whole Moon and other times only small parts of it. 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. As the Moon passes between the Earth and Sun, this phase 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 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 dark side of the Moon even though the sun does shine on it—we just never see it lit.

Tidal effects

Maroochydore Beach at low tide, Maroochydore, Queensland 03
Beach at low tide

The slightly greater attraction that the Moon has for the side of Earth closest to the Moon, as compared to the part of the Earth opposite the Moon, results in tidal forces. Tidal forces affect both the Earth's crust and oceans.

The most obvious effect of tidal forces is to cause two bulges in the Earth's oceans, one on the side facing the Moon and the other on the side opposite. This results in elevated sea levels called ocean tides. As the Earth spins on its axis, one of the ocean bulges (high tide) is held in place "under" the Moon, while another such tide is opposite. As a result, there are two high tides, and two low tides in about 24 hours. Since the Moon is orbiting the Earth in the same direction of the Earth's rotation, the high tides occur about every 12 hours and 25 minutes.

Observation and exploration

Soviet missions

Soviet moonrover
Soviet moon-rover

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. Once launchers had the necessary capabilities, these nations sent unmanned probes on both flyby and impact/lander 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.

Sputnik asm
The Soviet Union achieved an early lead in the Space Race by launching the first artificial satellite Sputnik 1 1957

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 unmanned 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, which returned 0.3 kg total. Two pioneering robotic rovers landed on the Moon in 1970 and 1973 as a part of Soviet Lunokhod programme.

Luna 24 was the last Soviet/Russian mission to the Moon.

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.
Moon rock (Apollo 17, 1972)

During the late 1950s at the height of the Cold War, the United States Army conducted a classified feasibility study that proposed the construction of a manned military outpost on the Moon called Project Horizon with the potential to conduct a wide range of missions from scientific research to nuclear Earth bombardment. The study included the possibility of conducting a lunar-based nuclear test. The Air Force, which at the time was in competition with the Army for a leading role in the space program, developed its own similar plan called Lunex. However, both these proposals were ultimately passed over as the space program was largely transferred from the military to the civilian agency NASA.

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 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 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 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, rover, LM, Irwin
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.

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Moon Facts for Kids. Kiddle Encyclopedia.