The
Moon is
Earth'sEarth is the third planet from the Sun. It is the fifth largest of the eight planets in the solar system, and the largest of the terrestrial planets in the Solar System in terms of diameter, mass and density...
only
natural satelliteA natural satellite or moon is a celestial body that orbits a planet or smaller body, which is called the primary. Technically, the term natural satellite could refer to a planet orbiting a star, or a dwarf galaxy orbiting a major galaxy, but it is normally synonymous with moon and used to identify...
and the fifth largest satellite in the
Solar SystemThe Solar System consists of the Sun and those celestial objects bound to it by gravity, all of which formed from the collapse of a giant molecular cloud approximately 4.6 billion years ago...
. The average centre-to-centre distance from the Earth to the Moon is , about thirty times the diameter of the Earth. The common centre of mass of the system (the barycentre) is located at about —a quarter the Earth's
radiusIn classical geometry, a radius of a circle or sphere is any line segment from its center to its perimeter. By extension, the radius of a circle or sphere is the length of any such segment, which is half the diameter....
—beneath the surface of the Earth. The Moon makes a complete
orbitIn physics, an orbit is the gravitationally curved path of one object around a point or another body, for example the gravitational orbit of a planet around a star....
around the Earth every 27.3 days (the
orbital periodThe orbital period is the time taken for a given object to make one complete orbit about another object.When mentioned without further qualification in astronomy this refers to the sidereal period of an astronomical object, which is calculated with respect to the stars.There are several kinds of...
), and the periodic variations in the geometry of the Earth–Moon–
SunThe Sun is the star at the center of the Solar System. The Earth and other matter orbit the Sun, which by itself accounts for about 99.86% of the Solar System's mass....
system are responsible for the
phases of the MoonA lunar phase or phase of the moon refers to the appearance of the illuminated portion of the Moon as seen by an observer, usually on Earth. The lunar phases vary cyclically as the Moon orbits the Earth, according to the changing relative positions of the Earth, Moon and Sun...
, which repeat every 29.5 days (the synodic period).
The Moon's diameter is , a little more than a quarter of that of the Earth. Thus, the Moon's surface area is less than a tenth of the Earth (about a quarter of Earth's land area, approximately as large as
RussiaRussia , officially known as both Russia and the Russian Federation , is a country in northern Eurasia . It is a semi-presidential republic, comprising 83 federal subjects...
,
CanadaCanada is a country occupying most of northern North America, extending from the Atlantic Ocean in the east to the Pacific Ocean in the west and northward into the Arctic Ocean...
, and the
United StatesThe United States of America is a federal constitutional republic comprising fifty states and a federal district...
combined), and its volume is about 2 percent that of Earth. The pull of
gravityGravitation is a natural phenomenon by which objects with mass attract one another. In everyday life, gravitation is most commonly thought of as the agency which lends weight to objects with mass. Gravitation causes dispersed matter to coalesce, thus accounting for the existence of the Earth, the...
at its surface is about 17 percent of that at the Earth's surface.
The Moon is the only
celestial bodyAstronomical objects are significant physical entities, associations or structures which current science has confirmed to exist in outer space. This does not necessarily mean that more current science will not disprove their existence. Some astronomical objects, such as Themis and Neith are, in...
on which human beings have made a manned
landingA moon landing is arrival of a spacecraft on the surface of the Moon. This includes both manned and unmanned missions. The first human-made object to reach the surface of the Moon was the Soviet Union's Luna 2 mission on September 13, 1959...
. While the
Soviet UnionThe Union of Soviet Socialist Republics was a constitutionally socialist state that existed in Eurasia from 1922 to 1991. The name is a translation of the , tr. Soyuz Sovetskikh Sotsialisticheskikh Respublik, abbreviated СССР, SSSR. The common short name is Soviet Union, from , Sovetskiy Soyuz...
's
Luna programmeThe Luna programme , occasionally called Lunik or Lunnik, was a series of robotic spacecraft missions sent to the Moon by the Soviet Union between 1959 and 1976. Fifteen were successful, each designed as either an orbiter or lander, and accomplished many firsts in space exploration...
was the first to reach the Moon with unmanned
spacecraftA spacecraft is a craft or machine designed for spaceflight. On a sub-orbital spaceflight, a spacecraft enters space then returns to the Earth. For an orbital spaceflight, a spacecraft enters a closed orbit around the planetary body. Spacecraft used for human spaceflight carry people on board as...
, the
NASAThe National Aeronautics and Space Administration is an agency of the United States government, responsible for the nation's public space program. NASA was established by the National Aeronautics and Space Act on July 29, 1958, replacing its predecessor, the National Advisory Committee for...
Apollo program achieved the only manned missions to date, beginning with the first manned lunar mission by
Apollo 8Apollo 8 was the first human spaceflight mission to achieve a velocity sufficient to allow escape from the gravitational field of planet Earth; the first to be captured by and escape from the gravitational field of another celestial body; and the first crewed voyage to return to planet Earth from...
in 1968, and six manned lunar landings between 1969 and 1972 – the first being
Apollo 11The Apollo 11 mission was the first human spaceflight to land on the Moon. Launched on July 16, 1969, it carried Mission Commander Neil Alden Armstrong, Command Module Pilot Michael Collins, and Lunar Module Pilot Edwin Eugene 'Buzz' Aldrin, Jr...
in 1969. Human exploration of the Moon temporarily ceased with the conclusion of the Apollo program, although a few robotic landers and orbiters have been sent to the Moon since that time. The U.S. has committed to return to the Moon by 2018.
Name and etymology
The proper
EnglishEnglish is a West Germanic language that developed in England during the Anglo-Saxon era. As a result of the military, economic, scientific, political, and cultural influence of the British Empire during the 18th, 19th, and early 20th centuries, and of the United States since the mid 20th century,...
name for Earth's natural satellite is, simply, the Moon (capitalized).
Moon is a
Germanic wordThe Germanic languages are a group of related languages that constitute a branch of the Indo-European language family. The common ancestor of all the languages in this branch is Proto-Germanic, spoken in approximately the mid-1st millennium BC in Iron Age northern Europe...
, related to the Latin (month). It is ultimately a derivative of the
Proto-Indo-EuropeanA pie is a baked dish which is usually made of a pastry dough shell that covers or completely contains a filling of various sweet or savoury ingredients...
root
me-, also represented in
measure (
timeTime is a component of the measuring system used to sequence events, to compare the durations of events and the intervals between them, and to quantify the motions of objects...
), with reminders of its importance in measuring time in words derived from it like
MondayMonday is the day of the week between Sunday and Tuesday.-Etymology:The English noun Monday derived sometime before 1200 from monedæi, which itself developed from Old English mōnandæg and mōndæg , which is cognate to other Germanic languages, including Old Frisian mōnadeig, Middle Low German and...
,
monthThe month is a unit of time, used with calendars, which is approximately as long as some natural period related to the motion of the Moon; month and Moon are cognates. The traditional concept arose with the cycle of moon phases; such months are synodic months and last approximately 29.53 days...
and
menstrualThe menstrual cycle is a cycle of physiological changes that occurs in fertile females. Overt menstruation occurs primarily in humans and close evolutionary relatives such as chimpanzees...
. The related adjective is
lunar, as well as an adjectival prefix
seleno- and suffix
-selene (from
selēnē, , the
Ancient GreekAncient Greek is the historical stage in the development of the Greek language spanning across the Archaic , Classical , and Hellenistic periods of ancient Greece and the ancient world. It is predated in the 2nd millennium BC by Mycenaean Greek...
word for the Moon). In English, the word
moon exclusively meant "the Moon" until 1665, when it was extended to refer to the recently discovered
natural satelliteA natural satellite or moon is a celestial body that orbits a planet or smaller body, which is called the primary. Technically, the term natural satellite could refer to a planet orbiting a star, or a dwarf galaxy orbiting a major galaxy, but it is normally synonymous with moon and used to identify...
s of other planets. Subsequently, these objects were given distinct names in order to avoid confusion. The Moon is occasionally referred to by its Latin name , primarily in science fiction.
Two sides of the Moon
The Moon is in
synchronous rotationIn astronomy, synchronous rotation is a planetological term describing a body orbiting another, where the orbiting body takes as long to rotate on its axis as it does to make one orbit; and therefore always keeps the same hemisphere pointed at the body it is orbiting...
, which means it rotates about its axis in about the same time it takes to
orbitIn physics, an orbit is the gravitationally curved path of one object around a point or another body, for example the gravitational orbit of a planet around a star....
the Earth. This results in it keeping nearly the same face turned towards the Earth at all times. The Moon used to rotate at a faster rate, but early in its history, its rotation slowed and became
lockedTidal locking occurs when the gravitational gradient makes one side of an astronomical body always face another; for example, one side of the Earth's Moon always faces the Earth. A tidally locked body takes just as long to rotate around its own axis as it does to revolve around its partner. This...
in this orientation as a result of
frictionFriction is the force resisting the relative lateral motion of solid surfaces, fluid layers, or material elements in contact. It is usually subdivided into several varieties:...
al effects associated with
tidalThe tidal force is a secondary effect of the force of gravity and is responsible for the tides. It arises because the gravitational force exerted on one body by a second body is not constant across its diameter...
deformations caused by the Earth.
Small variations (
librationIn astronomy libration refers to the various orbital conditions which make it possible to see more than 50% of the moon's surface over time, even though the front of the Moon is tidally locked to always face towards Earth...
) in the angle from which the Moon is seen allow about 59% of its surface to be seen from the Earth (but only half at any instant).
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Near side of the MoonThe near side of the Moon is the lunar hemisphere that is permanently turned towards the Earth, whereas the opposite side is the far side of the Moon. Only one side of the Moon is visible from Earth because the Moon rotates about its spin axis at the same rate that the Moon orbits the Earth, a...
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Far side of the Moon The far side of the Moon is the lunar hemisphere that is permanently turned away from the Earth. The far hemisphere was first photographed by the Soviet Luna 3 probe in 1959, and was first directly observed by human eyes when the Apollo 8 mission orbited the Moon in 1968. The rugged terrain is...
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The side of the Moon that faces Earth is called the
near sideThe near side of the Moon is the lunar hemisphere that is permanently turned towards the Earth, whereas the opposite side is the far side of the Moon. Only one side of the Moon is visible from Earth because the Moon rotates about its spin axis at the same rate that the Moon orbits the Earth, a...
, and the opposite side the
far sideThe far side of the Moon is the lunar hemisphere that is permanently turned away from the Earth. The far hemisphere was first photographed by the Soviet Luna 3 probe in 1959, and was first directly observed by human eyes when the Apollo 8 mission orbited the Moon in 1968. The rugged terrain is...
. The far side is often inaccurately called the "dark side," but in fact, it is illuminated exactly as often as the near side: once per lunar day, during the new Moon phase we observe on Earth when the near side is dark. The far side of the Moon was first photographed by the Soviet probe
Luna 3The Soviet space probe Luna 3 was the third spacecraft sent successfully to the Moon, and it was an early feat in the human exploration of outer space...
in 1959. One distinguishing feature of the far side is its almost complete lack of
mariaThe lunar maria are large, dark, basaltic plains on Earth's Moon, formed by ancient volcanic eruptions. They were dubbed maria, Latin for "seas", by early astronomers who mistook them for actual seas. They are less reflective than the "highlands" as a result of their iron-rich compositions, and...
.
Maria
The dark and relatively featureless lunar plains which can clearly be seen with the naked eye are called
mariaThe lunar maria are large, dark, basaltic plains on Earth's Moon, formed by ancient volcanic eruptions. They were dubbed maria, Latin for "seas", by early astronomers who mistook them for actual seas. They are less reflective than the "highlands" as a result of their iron-rich compositions, and...
(singular
mare), Latin for seas, since they were believed by ancient
astronomerAn astronomer is a scientist who studies celestial bodies such as planets, stars, and galaxies.Historically, astronomy was more concerned with the classification and description of phenomena in the sky, while astrophysics attempted to explain these phenomena and the differences between them using...
s to be filled with water. These are now known to be vast solidified pools of ancient
basaltBasalt is a common extrusive volcanic rock. It is usually grey to black and fine-grained due to rapid cooling of lava at the surface of a planet. It may be porphyritic containing larger crystals in a fine matrix, or vesicular, or frothy scoria. Unweathered basalt is black or grey.On Earth, most...
ic lava. The majority of these lavas erupted or flowed into the depressions associated with
impact basinsIn the broadest sense, the term impact crater can be applied to any depression, natural or manmade, resulting from the high velocity impact of a projectile with a larger body...
that formed by the collisions of meteors and comets with the lunar surface. (
Oceanus ProcellarumOceanus Procellarum , Latin for "Ocean of Storms", is a vast lunar mare on the western edge of the near side of Earth's Moon. Its name derives from the old superstition that its appearance during the second quarter heralded bad weather...
is a major exception in that it does not correspond to a known impact basin). Maria are found almost exclusively on the near side of the Moon, with the far side having only a few scattered patches covering about 2% of its surface, compared with about 31% on the near side. The most likely explanation for this difference is related to a higher concentration of heat-producing elements on the near-side hemisphere, as has been demonstrated by geochemical maps obtained from the
Lunar ProspectorThe Lunar Prospector mission was the third selected by NASA for full development and construction as part of the Discovery Program. At a cost of $62.8 million, the 19-month mission was designed for a low polar orbit investigation of the Moon, including mapping of surface composition and possible...
gamma-ray spectrometer. Several provinces containing
shield volcanoA shield volcano is a large volcano with shallow-sloping sides. Shield volcanoes are formed from fluid lava that can travel long distances across slight inclines, resulting in their relatively flat, broad profile...
es and volcanic
domeA lunar dome is a type of shield volcano that is found on the surface of the Earth's Moon. They are typically formed by highly viscous, possibly silica-rich lava, erupting from localized vents followed by relatively slow cooling. Lunar domes are wide, rounded, circular features with a gentle slope...
s are found within the near side maria.
Terrae
The lighter-colored regions of the Moon are called
terrae, or more commonly just
highlands, since they are higher than most maria. Several prominent mountain ranges on the near side are found along the periphery of the giant
impact basinsIn the broadest sense, the term impact crater can be applied to any depression, natural or manmade, resulting from the high velocity impact of a projectile with a larger body...
, many of which have been filled by mare basalt. These are hypothesized to be the surviving remnants of the impact basin's outer rims. In contrast to the Earth, no major lunar mountains are believed to have formed as a result of tectonic events.
From images taken by the
Clementine missionClementine was a joint space project between the Ballistic Missile Defense Organization and NASA...
in 1994, it appears that four mountainous regions on the rim of the 73 km-wide
Peary craterPeary is the closest large lunar impact crater to the lunar north pole. At this latitude the crater interior receives little sunlight, and parts of the southern floor remain permanently cloaked in shadow. From the Earth the crater appears on the northern lunar limb, and is seen from the side.The...
at the Moon's north pole remain illuminated for the entire lunar day. These
peaks of eternal lightPeak of Eternal Light describes a point on a body within the Solar System which is eternally bathed in sunlight. This is due to both the bodies' rotation and the point's altitude...
are possible because of the Moon's extremely small axial tilt to the
ecliptic planeThe plane of the ecliptic is the plane of the Earth's orbit around the Sun. It is the primary reference plane when describing the position of bodies in the Solar System, with celestial latitude being measured relative to the ecliptic plane. In the course of a year, the Sun's apparent path through...
. No similar regions of eternal light were found at the south pole, although the rim of
Shackleton craterShackleton, named after Ernest Shackleton, a noted explorer of the Antarctic, is an impact crater that lies at the south pole of the Moon. The peaks along the crater's rim are exposed to almost continual sunlight, while the interior is perpetually in shadow...
is illuminated for about 80% of the lunar day. Other consequences of the Moon's small axial tilt are regions that remain in permanent shadow at the bottoms of many polar craters.
Impact craters
The surface of Earth's Moon is marked by
impact craterIn the broadest sense, the term impact crater can be applied to any depression, natural or manmade, resulting from the high velocity impact of a projectile with a larger body...
s which form when asteroids and comets collide with the lunar surface. There are about half a million craters with diameters greater than 1 km on the Moon. Since impact craters accumulate at a nearly constant rate, the number of craters per unit area superposed on a geologic unit can be used to estimate the age of the surface (see
crater countingCrater counting refers to a method for estimating the age of a planet's surface. The method is based upon the hypothesis that a new surface forms with zero impact craters, and that impact craters accumulate at some known, roughly constant, rate...
). The lack of an atmosphere, weather and recent geological processes ensures that many of these craters have remained relatively well preserved in comparison to those on Earth.
The largest crater on the Moon, which also has the distinction of being one of the largest known craters in the Solar System, is the
South Pole-Aitken basinThe South Pole-Aitken basin is an impact crater on Earth's Moon. Roughly 2500 kilometers in diameter and 13 kilometers deep, it is the second largest known impact crater in the entire Solar System, the largest being the one on Mars' northern hemisphere which is approximately four times as big. An...
. It is on the far side, between the South Pole and equator, and is some 2,240 km in diameter and 13 km in depth. Prominent impact basins on the near side include
ImbriumMare Imbrium, Latin for "Sea of Showers" or "Sea of Rains", is a vast lunar mare filling a basin on Earth's Moon. Mare Imbrium was created when lava flooded the giant crater formed when a very large object hit the Moon long ago...
,
SerenitatisMare Serenitatis is a lunar mare that sits just to the east of Mare Imbrium on the Moon.It is located within the Serenitatis basin, which is of the Nectarian epoch. The material surrounding the mare is of the Lower Imbrian epoch, while the mare material is of the Upper Imbrian epoch...
,
CrisiumMare Crisium is a lunar mare located in the Moon's Crisium basin, just northeast of Mare Tranquillitatis. This basin is of the Pre-Imbrian period, 4.55 to 3.85 billion years ago. This mare is 376 miles in diameter, and 176,000 km
2 in area. It has a very flat floor, with a ring of...
, and
NectarisThe Sea of Nectar is a small lunar mare or sea located between the Sea of Tranquillity and the Sea of Fecundity . Montes Pyrenaeus borders the mare to the west and the large crater near the south center of the mare is known as Rosse...
.
Regolith
Blanketed atop the Moon's crust is a highly
comminutedComminution is one of the four main groups of mechanical processing and describes the movement of the particle size distribution into a range of finer particle sizes ....
(broken into ever smaller particles) and "impact gardened" surface layer called
regolithRegolith is a layer of loose, heterogeneous material covering solid rock. The term is a combination of two Greek words: Rhegos , which means blanket, and Lithos , which means rock. It includes dust, soil, broken rock, and other related materials and is present on Earth, the Moon, some asteroids,...
. Since the regolith forms by impact processes, the regolith of older surfaces is generally thicker than for younger surfaces. In particular, it has been estimated that the regolith varies in thickness from about 3–5 m in the maria, and by about 10–20 m in the highlands. Beneath the finely comminuted regolith layer is what is generally referred to as the
megaregolith. This layer is much thicker (on the order of tens of kilometres) and comprises highly fractured bedrock.
Astronauts have reported that the dust from the surface felt like snow and smelled like spent
gunpowderGunpowder, also called black powder, is a mixture of sulfur, charcoal, and potassium nitrate. It burns rapidly, producing volumes of hot solids and gases which can be used as a propellant in firearms and as a pyrotechnic composition in fireworks. The term gunpowder also refers broadly to any...
. The dust is mostly made of
silicon dioxideThe chemical compound silicon dioxide, also known as silica , is an oxide of silicon with a chemical formula of ' and has been known for its hardness since antiquity. Silica is most commonly found in nature as sand or quartz, as well as in the cell walls of diatoms...
glass (SiO
2), most likely created from the meteors that have crashed into the Moon's surface. It also contains
calciumCalcium is the chemical element with the symbol Ca and atomic number 20. It has an atomic mass of 40.078 amu. Calcium is a soft grey alkaline earth metal, and is the fifth most abundant element by mass in the Earth's crust...
and
magnesiumMagnesium is a chemical element with the symbol Mg, atomic number 12 and common oxidation number +2. It is an alkaline earth metal and the eighth most abundant element in the earth's crust by mass, although ninth in the Universe as a whole...
.
Presence of water
The continuous bombardment of the Moon by
cometA comet is a Small Solar System Body that has coma and is bigger than a meteoroid. When close enough to the Sun, a comet exhibits a visible coma , and sometimes a tail, both because of the effects of solar radiation upon the comet's nucleus...
s and
meteoroidA meteoroid is a sand to boulder-sized particle of debris in the Solar System. The visible path of a meteoroid that enters Earth's atmosphere is called a meteor. If a meteoroid reaches the ground, it is then called a meteorite. Many meteors are part of a meteor shower...
s has most likely added small amounts of water to the lunar surface. If so, sunlight would split much of this water into its constituent elements of hydrogen and oxygen, both of which would ordinarily escape into space over time, because of the Moon's weak gravity. However, because of the slightness of the axial tilt of the Moon's spin axis to the ecliptic plane—only 1.5°—some deep craters near the poles never receive direct light from the Sun and are thus in permanent shadow (see
Shackleton craterShackleton, named after Ernest Shackleton, a noted explorer of the Antarctic, is an impact crater that lies at the south pole of the Moon. The peaks along the crater's rim are exposed to almost continual sunlight, while the interior is perpetually in shadow...
). Water molecules that ended up in these craters could be stable for long periods of time.
Clementine has mapped craters at the lunar south pole that are shadowed in this way, and computer simulations suggest that up to 14,000 km² might be in permanent shadow. Results from the
Clementine missionClementine was a joint space project between the Ballistic Missile Defense Organization and NASA...
bistatic radar experiment are consistent with small, frozen pockets of water close to the surface, and data from the
Lunar ProspectorThe Lunar Prospector mission was the third selected by NASA for full development and construction as part of the Discovery Program. At a cost of $62.8 million, the 19-month mission was designed for a low polar orbit investigation of the Moon, including mapping of surface composition and possible...
neutron spectrometer indicate that anomalously high concentrations of hydrogen are present in the upper metre of the regolith near the polar regions. Estimate for the quantity of water on the Moon is 32 ounces per one ton of top layer of Moon's surface.
Water ice can be mined and then split into its constituent hydrogen and oxygen atoms by means of nuclear generators or electric power stations equipped with solar panels. The presence of usable quantities of water on the Moon is an important factor in rendering lunar habitation cost-effective, since transporting water from Earth would be prohibitively expensive. However, recent observations made with the
AreciboThe Arecibo Observatory is a radio telescope located close to the city of Arecibo in Puerto Rico. It is operated by Cornell University under cooperative agreement with the National Science Foundation. The observatory works as the National Astronomy and Ionosphere Center although both names are...
planetary radar suggest that some of the near-polar Clementine radar data that were previously interpreted as being indicative of water ice might instead be a result of rocks ejected from young impact craters. The question of how much water there is on the Moon has not been resolved.
In July 2008, small amounts of water were found in the interior of volcanic pearls from the Moon (brought to Earth in 1971 by the Apollo 15 astronauts).
On September 24, 2009, the
Indian Space Research OrganisationThe Indian Space Research Organisation is the primary body for space research under the control of the government of India, and one of the leading space research organisations in the world. It was established in its modern form in 1969 as a result of coordinated efforts initiated earlier...
(ISRO) reported that their first lunar mission, Chandrayaan-1 using
NASAThe National Aeronautics and Space Administration is an agency of the United States government, responsible for the nation's public space program. NASA was established by the National Aeronautics and Space Act on July 29, 1958, replacing its predecessor, the National Advisory Committee for...
's
Moon Mineralogy MapperThe Moon Mineralogy Mapper is one of two instruments that NASA contributed to India's first mission to the Moon, Chandrayaan-1, launched October 22, 2008...
, found evidence of large quantities of water on the Moon's surface, and that water is still presently being formed. The instrument observed an absorption line in the
spectrumA material's absorption spectrum shows the fraction of incident electromagnetic radiation absorbed by the material over a range of frequencies. Atoms, for example, have absorption lines at wavelengths corresponding to the differences between the energy levels of its atomic orbitals. Each chemical...
of sunlight reflected from the Moon, indicating that light of a particular wavelength (around 2.8
micronMicron can refer to:*Micron, an obsolete name for micrometre, a unit of length in the metric system: one millionth of a metre, formerly abbreviated with the Greek letter μ...
s) is being absorbed more readily than other nearby wavelengths. The position and shape of the line indicate the absorption is due to water. A nearby line also revealed the presence of the closely-related molecule
hydroxylIn chemistry, hydroxyl is composed of molecules consisting of an oxygen atom and a hydrogen atom connected by a covalent bond . The neutral form is a hydroxyl radical. The hydroxyl anion is called hydroxide; it is a diatomic ion with a charge of negative one...
, which consists of an oxygen atom with a single hydrogen atom. The exact abundance of water was not determined, but the team believed it could be as high as 1,000 parts per million in the top layer of Lunar soil.
Internal structure
The Moon is a
differentiatedIn planetary science, planetary differentiation is the process of separating out different constituents of a planetary body as a consequence of their physical or chemical behaviour, whereby the body evolves into compositionally distinct layers; the denser materials of a planet sink to the center,...
body, being composed of a geochemically distinct
crustIn geology, a crust is the outermost solid shell of a rocky planet or moon, which is chemically distinct from the underlying mantle. The crusts of Earth, our Moon, Mercury, Venus, Mars, Io, and other planetary bodies have been generated largely by igneous processes, and these crusts are richer in...
,
mantleThe mantle is a part of an astronomical object. The interior of the Earth, similar to the other terrestrial planets, is chemically divided into layers. The mantle is a highly viscous layer between the crust and the outer core. Earth's mantle is about 2,970 km thick rocky shell that...
, and
coreThe planetary core consists of the innermost layer of a planet.The core may be a solid or a liquid layer, as is Mercury's, while the cores of Mars and Venus are thought to be completely solid as they lack an internally generated magnetic field...
. This structure is hypothesized to have resulted from the
fractional crystallizationFractional crystallization is one of the most important geochemical and physical processes operating within the Earth's crust and mantle. Fractional crystallization is the removal and segregation from a melt of mineral precipitates; except in special cases, removal of the crystals changes the...
of a
magma oceanAccording to the giant impact hypothesis a large amount of energy was liberated in the formation of the Moon and it is predicted that as a result a large portion of the Moon was once completely molten, forming a lunar magma ocean...
shortly after its formation, at about 4.4 billion years ago. The energy required to melt the outer portion of the Moon is commonly attributed to a
giant impactThe giant impact hypothesis is the currently favored scientific hypothesis for the formation of the Moon, which is thought to have formed as a result of a collision between the young Earth and a Mars-sized body that is sometimes called Theia for the mythical Greek Titan who ruled the Sun...
event that is postulated to have formed the Earth-Moon system, and the subsequent reaccretion of material in Earth orbit. Crystallization of this magma ocean would have given rise to a
maficMafic is an adjective describing a silicate mineral or rock that is rich in magnesium and iron; the term was derived by contracting "magnesium" and "ferric". Most mafic minerals are dark in color and the relative density is greater than 3. Common rock-forming mafic minerals include olivine,...
mantle and a
plagioclasePlagioclase is a very important series of tectosilicate minerals within the feldspar family. Rather than referring to a particular mineral with a specific chemical composition, plagioclase is a solid solution series, more properly known as the plagioclase feldspar series...
-rich crust (see
Origin and geologic evolution below).
Geochemical mapping from orbit implies that the crust of the Moon is largely
anorthositicAnorthosite is a phaneritic, intrusive igneous rock characterized by a predominance of plagioclase feldspar , and a minimal mafic component...
in composition, consistent with the magma ocean hypothesis. In terms of elements, the crust is composed primarily of
oxygenOxygen Oxygen Oxygen (acid, literally "sharp", from the taste of acids) and -γενής (-genēs) (producer, literally begetter) is the element with atomic number 8 and represented by the symbol O...
(41% to 46% by mass),
siliconSilicon is the most common metalloid. It is a chemical element, which has the symbol Si and atomic number 14. A tetravalent metalloid, silicon is less reactive than its chemical analog carbon...
(21%),
magnesiumMagnesium is a chemical element with the symbol Mg, atomic number 12 and common oxidation number +2. It is an alkaline earth metal and the eighth most abundant element in the earth's crust by mass, although ninth in the Universe as a whole...
(6%),
ironIron is a metallic chemical element with the symbol Fe and atomic number 26. Iron is a group 8 and period 4 element and is therefore classified as a transition metal. Iron and iron alloys are by far the most common metals and the most common ferromagnetic materials in everyday use...
(13%),
calciumCalcium is the chemical element with the symbol Ca and atomic number 20. It has an atomic mass of 40.078 amu. Calcium is a soft grey alkaline earth metal, and is the fifth most abundant element by mass in the Earth's crust...
(8%), and
aluminiumAluminium or aluminum is a silvery white and ductile member of the boron group of chemical elements. It has the symbol Al; its atomic number is 13. It is not soluble in water under normal circumstances....
(7%). Based on geophysical techniques, its thickness is estimated to be on average about 50 km.
Partial melting within the mantle of the Moon gave rise to the eruption of mare basalts on the lunar surface. Analyses of these basalts indicate that the mantle is composed predominantly of the minerals
olivineThe mineral olivine is a magnesium iron silicate with the formula
2SiO
4...
, orthopyroxene and clinopyroxene, and that the lunar mantle is more iron rich than that of the Earth. Some lunar basalts contain high abundances of
titaniumTitanium is a chemical element with the symbol Ti and atomic number 22. Sometimes called the “space age metal”, it has a low density and is a strong, lustrous, corrosion-resistant transition metal with a silver color.Titanium can be alloyed with iron, aluminium, vanadium, molybdenum, among other...
(present in the mineral
ilmeniteIlmenite is a weakly magnetic titanium-iron oxide mineral which is iron-black or steel-gray. It is a crystalline iron titanium oxide . It crystallizes in the trigonal system, and it has the same crystal structure as corundum and hematite....
), suggesting that the mantle is highly heterogeneous in composition. Moonquakes have been found to occur deep within the mantle of the Moon about a thousand kilometres below the surface. These occur with monthly periodicities and are related to tidal stresses caused by the eccentric orbit of the Moon about the Earth.
The Moon has a mean density of 3 346.4 kg/m³, making it the second densest moon in the Solar System after
IoIo is the innermost of the four Galilean moons of the planet Jupiter and, with a diameter of 3,642 kilometres, the fourth-largest moon in the Solar System. It was named after Io, a priestess of Hera who became one of the lovers of Zeus.With over 400 active volcanoes, Io is the most...
. Nevertheless, several lines of evidence imply that the core of the Moon is small, with a radius of about 350 km or less. This corresponds to only about 20% the size of the Moon, in contrast to about 50% as is the case for most other terrestrial bodies. The composition of the lunar core is not well constrained, but most believe that it is composed of metallic iron alloyed with a small amount of
sulfurSulfur or sulphur is the chemical element that has the atomic number 16. It is denoted with the symbol S. It is an abundant, multivalent non-metal. Sulfur, in its native form, is a yellow crystalline solid. In nature, it can be found as the pure element and as sulfide and sulfate minerals...
and
nickelNickel is a chemical element, with the chemical symbol Ni and atomic number 28. It is a silvery-white lustrous metal with a slight golden tinge. It is one of the four ferromagnetic elements at about room temperature, other three being iron, cobalt and gadolinium...
. Analyses of the Moon's time-variable rotation indicate that the core is at least partly molten.
Topography
The
topographyTopography is the study of Earth's surface shape and features or those ofplanets, moons, and asteroids...
of the Moon has been measured by the methods of laser altimetry and stereo image analysis, most recently from data obtained during the
Clementine missionClementine was a joint space project between the Ballistic Missile Defense Organization and NASA...
. The most visible topographic feature is the giant far side
South Pole-Aitken basinThe South Pole-Aitken basin is an impact crater on Earth's Moon. Roughly 2500 kilometers in diameter and 13 kilometers deep, it is the second largest known impact crater in the entire Solar System, the largest being the one on Mars' northern hemisphere which is approximately four times as big. An...
, which possesses the lowest elevations of the Moon. The highest elevations are found just to the north-east of this basin, and it has been suggested that this area might represent thick
ejectaEjecta can mean:*In volcanology, particles that came out of a volcanic vent, traveled through the air or under water, and fell back on the ground surface or on the ocean floor...
deposits that were emplaced during an oblique South Pole-Aitken basin impact event. Other large impact basins, such as
ImbriumMare Imbrium, Latin for "Sea of Showers" or "Sea of Rains", is a vast lunar mare filling a basin on Earth's Moon. Mare Imbrium was created when lava flooded the giant crater formed when a very large object hit the Moon long ago...
,
SerenitatisMare Serenitatis is a lunar mare that sits just to the east of Mare Imbrium on the Moon.It is located within the Serenitatis basin, which is of the Nectarian epoch. The material surrounding the mare is of the Lower Imbrian epoch, while the mare material is of the Upper Imbrian epoch...
,
CrisiumMare Crisium is a lunar mare located in the Moon's Crisium basin, just northeast of Mare Tranquillitatis. This basin is of the Pre-Imbrian period, 4.55 to 3.85 billion years ago. This mare is 376 miles in diameter, and 176,000 km
2 in area. It has a very flat floor, with a ring of...
,
SmythiiMare Smythii is a lunar mare located along the equator on the easternmost edge of the lunar near side. The Smythii basin where the mare is located is of the Pre-Nectarian epoch, while the surrounding features are of the Nectarian system...
, and
OrientaleMare Orientale is one of the most striking large scale lunar features, resembling a target ring bull's-eye. Located on the extreme western edge of the lunar nearside, this impact basin is difficult to see from an Earthbound perspective....
, also possess regionally low elevations and elevated rims. Another distinguishing feature of the Moon's shape is that the elevations are on average about 1.9 km higher on the far side than the near side.
Gravity field
The gravitational field of the Moon has been determined through tracking of radio signals emitted by orbiting spacecraft. The principle used depends on the
Doppler effectThe Doppler effect , named after Austrian physicist Christian Doppler who proposed it in 1842, is the change in frequency of a wave for an observer moving relative to the source of the wave. It is commonly heard when a vehicle sounding a siren or horn approaches, passes, and recedes from an observer...
, whereby the spacecraft acceleration in the line-of-sight direction can be determined by means of small shifts in frequency of the radio signal, and the distance from the spacecraft to a station on Earth. However, because of the Moon's
synchronous rotationIn astronomy, synchronous rotation is a planetological term describing a body orbiting another, where the orbiting body takes as long to rotate on its axis as it does to make one orbit; and therefore always keeps the same hemisphere pointed at the body it is orbiting...
it is not possible to track spacecraft much over the limbs of the Moon, and the farside gravity field is thus only poorly characterised.
The major characteristic of the Moon's gravitational field is the presence of mascons, which are large positive gravitational anomalies associated with some of the giant
impact basinsIn the broadest sense, the term impact crater can be applied to any depression, natural or manmade, resulting from the high velocity impact of a projectile with a larger body...
. These anomalies greatly influence the orbit of spacecraft about the Moon, and an accurate gravitational model is necessary in the planning of both manned and unmanned missions. The mascons are in part due to the presence of dense mare basaltic lava flows that fill some of the impact basins. However, lava flows by themselves can not explain the entirety of the gravitational signature, and uplift of the crust-mantle interface is required as well. Based on
Lunar ProspectorThe Lunar Prospector mission was the third selected by NASA for full development and construction as part of the Discovery Program. At a cost of $62.8 million, the 19-month mission was designed for a low polar orbit investigation of the Moon, including mapping of surface composition and possible...
gravitational models, it has been suggested that some mascons exist that do not show evidence for mare basaltic volcanism. The huge expanse of mare basaltic volcanism associated with
Oceanus ProcellarumOceanus Procellarum , Latin for "Ocean of Storms", is a vast lunar mare on the western edge of the near side of Earth's Moon. Its name derives from the old superstition that its appearance during the second quarter heralded bad weather...
does not possess a positive gravitational anomaly.
Magnetic field
The Moon has an external
magnetic fieldMagnetic fields surround magnetic materials and electric currents and are detected by the force they exert on other magnetic materials and moving electric charges...
of the order of one to a hundred
nanoteslaThe tesla is the SI derived unit of magnetic field B . One tesla is equal to one weber per square meter, and it was defined in 1960 in honor of the Serbian-American inventor, physicist, and electrical engineer Nikola Tesla...
—less than one hundredth that of the
EarthEarth's magnetic field is approximately a magnetic dipole, with the magnetic field S pole near the Earth's geographic north pole and the other magnetic field N pole near the Earth's geographic south pole...
, which is 30–60 microtesla. Other major differences are that the Moon does not currently have a
dipolarIn physics, there are two kinds of dipoles:*An electric dipole is a separation of positive and negative charges. The simplest example of this is a pair of electric charges of equal magnitude but opposite sign, separated by some, usually small, distance. A permanent electric dipole is called an...
magnetic field (as would be generated by a geodynamo in its core), and the magnetizations that are present are almost entirely crustal in origin. One hypothesis holds that the crustal magnetizations were acquired early in lunar history when a geodynamo was still operating. The small size of the lunar core, however, is a potential obstacle to this theory. Alternatively, it is possible that on an airless body such as the Moon, transient magnetic fields could be generated during large impact events. In support of this, it has been noted that the largest crustal magnetizations appear to be located near the
antipodesIn geography, the antipodes of any place on Earth is the point on the Earth's surface which is diametrically opposite to it...
of the giant impact basins. It has been proposed that such a phenomenon could result from the free expansion of an impact generated plasma cloud around the Moon in the presence of an ambient magnetic field.
Atmosphere
The Moon has an atmosphere so thin as to be almost negligible, with a total atmospheric mass of less than 10
4 kg. The effective surface pressure of this small mass is around 3
atmThe standard atmosphere is an international reference pressure defined as 101,325 Pa and formerly used as unit of pressure . For practical purposes it has been replaced by the bar which is 100,000 Pa...
. This pressure varies, of course, with the diurnal moon cycle. One source of its atmosphere is
outgassingOutgassing is the slow release of a gas that was trapped, frozen, absorbed or adsorbed in some material...
—the release of gases such as
radonRadon is a chemical element with symbol Rn and atomic number 86. It is a radioactive, colorless, odorless, tasteless noble gas, occurring naturally as the decay product of radium. It is one of the heaviest substances that remains a gas under normal conditions and is considered to be a health...
that originate by
radioactive decayRadioactive decay is the process in which an unstable atomic nucleus spontaneously loses energy by emitting ionizing particles and radiation. This decay, or loss of energy, results in an atom of one type, called the parent nuclide transforming to an atom of a different type, named the daughter...
processes within the crust and mantle. Another important source is generated through the process of
sputteringSputtering is a process whereby atoms are ejected from a solid target material due to bombardment of the target by energetic ions. It is commonly used for thin-film deposition, etching and analytical techniques .- Physics of sputtering :...
, which involves the bombardment of micrometeorites, solar wind ions, electrons, and sunlight. Gases that are released by sputtering can either reimplant into the
regolithRegolith is a layer of loose, heterogeneous material covering solid rock. The term is a combination of two Greek words: Rhegos , which means blanket, and Lithos , which means rock. It includes dust, soil, broken rock, and other related materials and is present on Earth, the Moon, some asteroids,...
as a result of the Moon's gravity, or can be lost to space either by solar radiation pressure or by being swept away by the solar wind magnetic field if they are ionised. The elements
sodiumSodium is a metallic element with a symbol Na and atomic number 11. It is a soft, silvery-white, highly reactive metal and is a member of the alkali metals within "group 1"...
(Na) and
potassiumPotassium is the chemical element with the symbol K , atomic number 19, and atomic mass 39.0983. Potassium was first isolated from potash...
(K) have been detected using earth-based spectroscopic methods, whereas the element
radonRadon is a chemical element with symbol Rn and atomic number 86. It is a radioactive, colorless, odorless, tasteless noble gas, occurring naturally as the decay product of radium. It is one of the heaviest substances that remains a gas under normal conditions and is considered to be a health...
–222 (
222Rn) and polonium-210 (
210Po) have been inferred from data obtained from the
Lunar ProspectorThe Lunar Prospector mission was the third selected by NASA for full development and construction as part of the Discovery Program. At a cost of $62.8 million, the 19-month mission was designed for a low polar orbit investigation of the Moon, including mapping of surface composition and possible...
alpha particleAlpha particles consist of two protons and two neutrons bound together into a particle identical to a helium nucleus; hence, it can be written as or . They have a net spin of zero, and normally a total energy of about 5 MeV...
spectrometer.
ArgonArgon is a chemical element designated by the symbol Ar. Argon has atomic number 18 and is the third element in group 18 of the periodic table . Argon is present in the Earth's atmosphere at 0.94%. Terrestrially, it is the most abundant and most frequently used of the noble gases...
–40 (
40Ar),
helium-4Helium-4 is a non-radioactive and light isotope of helium. It is by far the most abundant of the two naturally occurring isotopes of helium, making up about 99.99986% of the helium on earth. Its nucleus is the same as an alpha particle, consisting of two protons and two neutrons. The total spin of...
(
4He),
oxygenOxygen Oxygen Oxygen (acid, literally "sharp", from the taste of acids) and -γενής (-genēs) (producer, literally begetter) is the element with atomic number 8 and represented by the symbol O...
(O
2) and/or
methaneMethane is a chemical compound with the chemical formula . It is the simplest alkane, and the principal component of natural gas. Methane's bond angles are 109.5 degrees. Burning methane in the presence of oxygen produces carbon dioxide and water. The relative abundance of methane and its clean...
(CH
4),
nitrogenNitrogen is a chemical element that has the symbol N and atomic number 7 and atomic mass 14.00674 u. Elemental nitrogen is a colorless, odorless, tasteless and mostly inert diatomic gas at standard conditions, constituting 78% by volume of Earth's atmosphere.Many industrially important...
(N
2) and/or
carbon monoxideCarbon monoxide is a colorless, odorless and tasteless gas, yet very toxic to humans. It consists of one carbon atom and one oxygen atom, connected by a covalent double bond and a dative covalent bond...
(CO), and
carbon dioxideCarbon dioxide is a chemical compound composed of two oxygen atoms covalently bonded to a single carbon atom. It is a gas at standard temperature and pressure and exists in Earth's atmosphere in this state...
(CO
2) were detected by in-situ detectors placed by the Apollo astronauts.
Surface temperature
During the lunar day, the surface temperature averages 107 °C, and during the lunar night, it averages −153 °C.
Formation
Several mechanisms have been suggested for the Moon's formation. The formation of the Moon is hypothesized to have occurred 4.527 ± 0.010 billion years ago, about 30–50 million years after the origin of the Solar System.
Fission hypothesis : Early speculation proposed that the Moon broke off from the Earth's crust because of
centrifugal forceIn classical mechanics, centrifugal force is an outward force associated with curved motion, that is, rotation about some center...
s, leaving a basin presumed to be the
Pacific OceanThe Pacific Ocean is the largest of the Earth's oceanic divisions. Its name is derived from the Latin name Tepre Pacificum, "peaceful sea", bestowed upon it by the Portuguese explorer Ferdinand Magellan. It extends from the Arctic in the north to Antarctica in the south, bounded by Asia and...
behind as a scar. This idea, however, would require too great an initial spin of the Earth and also would have resulted in the Moon's orbit following Earth's
equatorial planeIn astronomy and navigation, the celestial sphere is an imaginary sphere of arbitrarily large radius, concentric with the Earth and rotating upon the same axis. All objects in the sky can be thought of as projected upon the celestial sphere. Projected upward from Earth's equator and poles are the...
rather than its current path.
Capture hypothesis : Other speculation has centered on the Moon being formed elsewhere and subsequently being captured by Earth's gravity. However, the conditions conjectured necessary for such a mechanism to work, such as an
extended atmosphere of the EarthThe Earth's atmosphere is a layer of gases surrounding the planet Earth that is retained by Earth's gravity. The atmosphere protects life on Earth by absorbing ultraviolet solar radiation, warming the surface through heat retention , and reducing temperature extremes between day and night...
in order to
dissipateIn physics, dissipation embodies the concept of a dynamical system where important mechanical modes, such as waves or oscillations, lose energy over time, typically due to the action of friction or turbulence. The lost energy is converted into heat, raising the temperature of the system...
the energy of the passing Moon, are improbable.
Co-formation hypothesis : The co-formation hypothesis proposes that the Earth and the Moon formed together at the same time and place from the primordial accretion disk. The Moon would have formed from material surrounding the proto-Earth, similar to the formation of the planets around the Sun. Some suggest that this hypothesis fails to adequately explain the depletion of metallic iron in the Moon.
- A major deficiency in all these hypotheses is that they cannot readily account for the high angular momentum
Angular momentum is a quantity that is useful in describing the rotational state of a physical system. For a rigid body rotating around an axis of symmetry , the angular momentum can be expressed as the product of the body's moment of inertia and its angular velocity...
of the Earth–Moon system.
Giant impact hypothesis
- The prevailing hypothesis today is that the Earth–Moon system formed as a result of a giant impact
The giant impact hypothesis is the currently favored scientific hypothesis for the formation of the Moon, which is thought to have formed as a result of a collision between the young Earth and a Mars-sized body that is sometimes called Theia for the mythical Greek Titan who ruled the Sun...
. A MarsMars is the fourth planet from the Sun in the Solar System. The planet is named after Mars, the Roman god of war. It is also referred to as the "Red Planet" because of its reddish appearance, due to iron oxide prevalent on its surface....
-sized body (labelled "Theia") is hypothesized to have hit the proto-Earth, blasting sufficient material into orbit around the proto-Earth to form the Moon through accretion. As accretion is the process by which all planetary bodies are therorized to have formed, giant impacts are thought to have affected most if not all planets. Computer simulations modelling a giant impact are consistent with measurements of the angular momentumAngular momentum is a quantity that is useful in describing the rotational state of a physical system. For a rigid body rotating around an axis of symmetry , the angular momentum can be expressed as the product of the body's moment of inertia and its angular velocity...
of the Earth–Moon system, as well as the small size of the lunar core. Unresolved questions regarding this theory concern the determination of the relative sizes of the proto-Earth and Theia and of how much material from these two bodies formed the Moon. Recent oxygen isotope composition analysis of the Moon shows its oxygen isotope composition is more similar to the Earth's than this hypothesis would suggest.
Lunar magma ocean
As a result of the large amount of energy converted during both the giant impact event and the subsequent reaccretion of material in Earth orbit, it is commonly hypothesized that a large portion of the Moon was once initially molten. The molten outer portion of the Moon at this time is referred to as a
magma oceanAccording to the giant impact hypothesis a large amount of energy was liberated in the formation of the Moon and it is predicted that as a result a large portion of the Moon was once completely molten, forming a lunar magma ocean...
, and estimates for its depth range from about 500 km to the entire radius of the Moon.
As the magma ocean cooled, it fractionally crystallised and
differentiatedIn planetary science, planetary differentiation is the process of separating out different constituents of a planetary body as a consequence of their physical or chemical behaviour, whereby the body evolves into compositionally distinct layers; the denser materials of a planet sink to the center,...
, giving rise to a geochemically distinct crust and mantle. The mantle is inferred to have formed largely by the precipitation and sinking of the minerals
olivineThe mineral olivine is a magnesium iron silicate with the formula
2SiO
4...
, clinopyroxene, and orthopyroxene. After about three-quarters of magma ocean crystallisation was complete, the mineral
anorthiteAnorthite is a compositional variety of plagioclase feldspar. Plagioclase is an abundant mineral in the Earth's crust. The formula of pure anorthite is CaAl
2Si
2O
8.-Mineralogy :...
is inferred to have precipitated and floated to the surface because of its low density, forming the crust.
The final liquids to crystallise from the magma ocean would have been initially sandwiched between the crust and mantle, and would have contained a high abundance of incompatible and heat-producing elements. This geochemical component is referred to by the acronym
KREEPKREEP, an acronym built from the letters K , REE and P , is a geochemical component of some lunar impact melt breccia and basalt rocks...
, for
potassiumPotassium is the chemical element with the symbol K , atomic number 19, and atomic mass 39.0983. Potassium was first isolated from potash...
(K), rare earth elements (REE), and
phosphorusPhosphorus is the chemical element that has the symbol P and atomic number 15. A multivalent nonmetal of the nitrogen group, phosphorus is commonly found in inorganic phosphate rocks. Elemental phosphorus exists in two major forms - white phosphorus and red phosphorus...
(P), and appears to be concentrated within the Procellarum KREEP Terrane, which is a small geologic province that encompasses most of
Oceanus ProcellarumOceanus Procellarum , Latin for "Ocean of Storms", is a vast lunar mare on the western edge of the near side of Earth's Moon. Its name derives from the old superstition that its appearance during the second quarter heralded bad weather...
and
Mare ImbriumMare Imbrium, Latin for "Sea of Showers" or "Sea of Rains", is a vast lunar mare filling a basin on Earth's Moon. Mare Imbrium was created when lava flooded the giant crater formed when a very large object hit the Moon long ago...
on the near side of the Moon.
Geologic evolution
A large portion of the Moon's post–magma-ocean geologic evolution was dominated by impact cratering. The
lunar geologic timescaleThe lunar geological timescale divides the history of Earth's Moon into five generally recognized periods: the Copernican, Eratosthenian, Imbrian , Nectarian, and Pre-Nectarian...
is largely divided in time on the basis of prominent basin-forming impact events, such as
NectarisThe Nectarian Period of the lunar geologic timescale runs from 3920 million years ago to 3850 million years ago. It is the period during which the Nectaris Basin and other major basins were formed by large impact events...
,
ImbriumIn the lunar geologic timescale, the Early Imbrian epoch occurred between 3850 million years ago to about 3800 million years ago. It overlaps the end of the Late Heavy Bombardment of the inner solar system. The impact which created the huge Mare Imbrium basin occurred at the start of the epoch...
, and
OrientaleMare Orientale is one of the most striking large scale lunar features, resembling a target ring bull's-eye. Located on the extreme western edge of the lunar nearside, this impact basin is difficult to see from an Earthbound perspective....
. These impact structures are characterised by multiple rings of uplifted material, and are typically hundreds to thousands of kilometres in diameter. Each multi-ring basin is associated with a broad apron of ejecta deposits that forms a regional stratigraphic horizon. While only a few multi-ring basins have been definitively dated, they are useful for assigning relative ages on the basis of
stratigraphicStratigraphy, a branch of geology, studies rock layers and layering . It is primarily used in the study of sedimentary and layered volcanic rocks....
grounds. The continuous effects of impact cratering are responsible for forming the
regolithRegolith is a layer of loose, heterogeneous material covering solid rock. The term is a combination of two Greek words: Rhegos , which means blanket, and Lithos , which means rock. It includes dust, soil, broken rock, and other related materials and is present on Earth, the Moon, some asteroids,...
.
The other major geologic process that affected the Moon's surface was
mare volcanismThe lunar maria are large, dark, basaltic plains on Earth's Moon, formed by ancient volcanic eruptions. They were dubbed maria, Latin for "seas", by early astronomers who mistook them for actual seas. They are less reflective than the "highlands" as a result of their iron-rich compositions, and...
. The enhancement of heat-producing elements within the Procellarum KREEP Terrane is thought to have caused the underlying mantle to heat up, and eventually, to partially melt. A portion of these magmas rose to the surface and erupted, accounting for the high concentration of mare basalts on the near side of the Moon. Most of the Moon's
mare basaltsThe lunar maria are large, dark, basaltic plains on Earth's Moon, formed by ancient volcanic eruptions. They were dubbed maria, Latin for "seas", by early astronomers who mistook them for actual seas. They are less reflective than the "highlands" as a result of their iron-rich compositions, and...
erupted during the Imbrian period in this geologic province 3.0–3.5 billion years ago. Nevertheless, some dated samples are as old as 4.2 billion years, and the youngest eruptions, based on the method of
crater countingCrater counting refers to a method for estimating the age of a planet's surface. The method is based upon the hypothesis that a new surface forms with zero impact craters, and that impact craters accumulate at some known, roughly constant, rate...
, are hypothesized to have occurred only 1.2 billion years ago.
There has been controversy over whether features on the Moon's surface undergo changes over time. Some observers have claimed that craters either appeared or disappeared, or that other forms of transient phenomena had occurred. Today, many of these claims are thought to be illusory, resulting from observation under different lighting conditions, poor
astronomical seeingAstronomical seeing refers to the blurring and twinkling of astronomical objects such as stars caused by turbulence in the Earth's atmosphere. It is the equivalent of looking at the bottom of a lake on a windy day. The astronomical seeing conditions on a given night at a given location describe how...
, or the inadequacy of earlier drawings. Nevertheless, it is known that the phenomenon of
outgassingOutgassing is the slow release of a gas that was trapped, frozen, absorbed or adsorbed in some material...
does occasionally occur, and these events could be responsible for a minor percentage of the reported
lunar transient phenomenaA transient lunar phenomenon , or lunar transient phenomenon , is a short-lived light, color, or change in appearance on the lunar surface....
. Recently, it has been suggested that a roughly 3 km diameter region of the lunar surface was modified by a gas release event about a million years ago.
Moon rocks
Moon rocks fall into two main categories, based on whether they underlie the lunar highlands (terrae) or the maria. The lunar highlands rocks are composed of three suites: the
ferroan anorthosite suite, the
magnesian suite, and the
alkali suite (some consider the alkali suite to be a subset of the mg-suite). The ferroan anorthosite suite rocks are composed almost exclusively of the mineral
anorthiteAnorthite is a compositional variety of plagioclase feldspar. Plagioclase is an abundant mineral in the Earth's crust. The formula of pure anorthite is CaAl
2Si
2O
8.-Mineralogy :...
(a calic plagioclase feldspar), and are hypothesized to represent plagioclase flotation cumulates of the lunar magma ocean. The ferroan anorthosites have been dated using radiometric methods to have formed about 4.4 billion years ago.
The mg- and alkali-suite rocks are predominantly mafic plutonic rocks. Typical rocks are
duniteDunite is an igneous, plutonic rock, of ultramafic composition, with coarse-grained or phaneritic texture. The mineral assemblage is greater than 90% olivine, with minor amounts of other minerals such as pyroxene, chromite and pyrope. Dunite is the olivine-rich end-member of the peridotite group...
s,
troctoliteTroctolite is a mafic intrusive rock type. It consists essentially of variable amounts of olivine and calcic plagioclase along with variable minor pyroxene...
s,
gabbroGabbro refers to a large group of dark, coarse-grained, intrusive mafic igneous rocks chemically equivalent to basalt. The rocks are plutonic, formed when molten magma is trapped beneath the Earth's surface and cools into a crystalline mass....
s, alkali
anorthositeAnorthosite is a phaneritic, intrusive igneous rock characterized by a predominance of plagioclase feldspar , and a minimal mafic component...
s, and more rarely,
graniteGranite is a common and widely occurring type of intrusive, felsic, igneous rock. Granite has a medium to coarse texture, occasionally with some individual crystals larger than the groundmass forming a rock known as porphyry. Granites can be pink to dark gray or even black, depending on their...
. In contrast to the ferroan anorthosite suite, these rocks all have relatively high Mg/Fe ratios in their mafic minerals. In general, these rocks represent intrusions into the already-formed highlands crust (though a few rare samples appear to represent extrusive lavas), and they have been dated to have formed about 4.4–3.9 billion years ago. Many of these rocks have high abundances of, or are genetically related to, the geochemical component
KREEPKREEP, an acronym built from the letters K , REE and P , is a geochemical component of some lunar impact melt breccia and basalt rocks...
.
The lunar maria consist entirely of mare basalts. While similar to terrestrial basalts, they have much higher abundances of iron, are completely lacking in hydrous alteration products, and have a large range of titanium abundances.
Orbit and relationship to Earth
The Moon makes a complete orbit around the Earth with respect to the fixed stars about once every 27.3 days(its sidereal period). However, since the Earth is moving in its orbit about the Sun at the same time, it takes slightly longer for the Moon to show its same
phaseA lunar phase or phase of the moon refers to the appearance of the illuminated portion of the Moon as seen by an observer, usually on Earth. The lunar phases vary cyclically as the Moon orbits the Earth, according to the changing relative positions of the Earth, Moon and Sun...
to Earth, which is about 29.5 days (its synodic period). Unlike most satellites of other planets, the Moon orbits near the
eclipticThe ecliptic is the apparent path that the Sun traces out in the sky during the year, appearing to move eastwards on an imaginary spherical surface, the celestial sphere, relative to the fixed stars. More accurately, it is the intersection of the celestial sphere with the ecliptic plane, which is...
and not the Earth's equatorial plane. It is the largest moon in the solar system relative to the size of its planet. (
CharonCharon, discovered in 1978 at the United States Naval Observatory Flagstaff Station, is the largest satellite of the dwarf planet Pluto. Following the 2005 discovery of two other natural satellites of Pluto , Charon may also be referred to as Pluto I...
is larger relative to the
dwarf planetA dwarf planet, as defined by the International Astronomical Union , is a celestial body orbiting the Sun that is massive enough to be rounded by its own gravity but has not cleared its neighbouring region of planetesimals and is not a satellite. More explicitly, it has to have sufficient mass to...
PlutoPluto, formal designation 134340 Pluto, is the second-largest known dwarf planet in the Solar System and the tenth-largest body observed directly orbiting the Sun...
.) The
natural satelliteA natural satellite or moon is a celestial body that orbits a planet or smaller body, which is called the primary. Technically, the term natural satellite could refer to a planet orbiting a star, or a dwarf galaxy orbiting a major galaxy, but it is normally synonymous with moon and used to identify...
s orbiting other planets are called "moons", after Earth's Moon.
Most of the tidal effects seen on the Earth are caused by the Moon's gravitational pull, with the Sun making a somewhat smaller contribution. Tidal drag slows the Earth's rotation by about 0.002 seconds per day per century. As a result of the conservation of angular momentum, the slowing of Earth's rotation is accompanied by an increase of the mean Earth-Moon distance of about 3.8 m per century, or 3.8 cm per year.
The Moon is exceptionally large relative to the Earth, being a quarter the diameter of the planet and 1/81 its mass. However, the Earth and Moon are still commonly considered a planet-satellite system, rather than a double-planet system, since the common centre of mass of the system (the barycentre) is located about 1,700 km beneath the surface of the Earth, or about a quarter of the Earth's radius. The surface of the Moon is less than one-tenth that of the Earth, and only about a quarter the size of the Earth's land area (or about as large as Russia, Canada, and the U.S. combined).
The current obliquity of the Moon means that the Sun never rises above 1.85° at the poles. The axial tilt of the Moon has remained at its present orientation for the past two billion years, allowing the craters at the poles to remain in permanent shadow for that length of time. Prior to that point, the Moon had much larger values for its obliquity, possibly reaching angles as high as 77° for periods of several hundred thousand years.
In 1997, the asteroid
3753 Cruithne3753 Cruithne is an asteroid in orbit around the Sun in 1:1 orbital resonance with that of the Earth. It is a periodic inclusion planetoid orbiting the Sun in an apparent horseshoe orbit...
was found to have an unusual Earth-associated
horseshoe orbitA horseshoe orbit appears when a viewer on an orbiting body watches the movement of another orbiting body, whose orbit is skinnier , but has about the same period...
. However, astronomers do not consider it to be a second moon of Earth, and its orbit is not stable in the long term. Three other near-Earth asteroids,
54509 YORP54509 YORP is an Apollo Near-Earth Object discovered on August 3, 2000 by the Lincoln Laboratory Near-Earth Asteroid Research Team at Socorro. Measurements of the rotation rate of this object provided the first observational evidence of the YORP effect, hence the name of the asteroid...
,
(85770) 1998 UP1' is a near Earth, Aten asteroid orbiting at nearly a 1:1 resonance with Earth.-Orbit:With an orbital period of 364.48 d, is in a near perfect 1:1 orbital Resonance with Earth. Although their periods are almost identical, their orbits are very different; has a highly eccentric orbit and moves...
and
2002 AA29Asteroid ' is a near-Earth asteroid discovered in January 2002 by the LINEAR asteroid survey, measuring about 60 meters across....
, which exist in orbits similar to Cruithne's, have since been discovered.
Ocean tides
Earth's
ocean tidesTides are the rises and falls of sea level caused by the combined effect of rotation of the Earth and the gravitation of the Moon and the Sun. The tides occur with a period of approximately 12 and a half hours and are influenced by the shape of the near-shore bottom.Most coastal areas experience...
are initiated by the
tidal forceThe tidal force is a secondary effect of the force of gravity and is responsible for the tides. It arises because the gravitational force exerted on one body by a second body is not constant across its diameter...
(a gradient in intensity) of Moon's gravity and are magnified by a host of effects in Earth's oceans. The gravitational tidal force arises because the side of Earth facing the Moon (nearest it) is attracted more strongly by the Moon's gravity than is the center of the Earth and—even less so—the Earth's far side. The gravitational tide stretches the Earth's oceans into an ellipse with the Earth in the center. The effect takes the form of two
bulges—elevated sea level relative to the Earth; one nearest the Moon and one farthest from it. Since these two bulges rotate around the Earth once a day as it spins on its axis, ocean water is continuously rushing towards the ever-moving bulges. The effects of the two bulges and the massive ocean currents chasing them are magnified by an interplay of other effects; namely frictional coupling of water to Earth's rotation through the ocean floors, inertia of water's movement, ocean basins that get shallower near land, and oscillations between different ocean basins. The magnifying effect is a bit like water sloshing high up the sloped end of a bathtub after a relatively small disturbance of one's body in the deep part of the tub.
Gravitational coupling between the Moon and the ocean bulge nearest the Moon affects its orbit. The Earth rotates on its axis in the very same direction, and roughly 27 times faster, than the Moon orbits the Earth. Thus, frictional coupling between the sea floors and ocean waters, as well as water's
inertiaInertia is the resistance of any physical object, to a change in its state of motion. It is represented numerically by an object's mass. The principle of inertia is one of the fundamental principles of classical physics which are used to describe the motion of matter and how it is affected by...
, drags the peak of the near-Moon tidal bulge slightly forward of the imaginary line connecting the centers of the Earth and Moon. From the Moon's perspective, the center of mass of the near-Moon tidal bulge is perpetually slightly
ahead of the point about which it is orbiting. Precisely the opposite effect occurs with the bulge farthest from the Moon; it lags
behind the imaginary line. However it is 12,756 km farther away and has slightly less gravitational coupling to the Moon. Consequently, the Moon is constantly being gravitationally attracted forward in its orbit about the Earth. This gravitational coupling drains
kinetic energyThe kinetic energy of an object is the extra energy which it possesses due to its motion. It is defined as the work needed to accelerate a body of a given mass from rest to its current velocity. Having gained this energy during its acceleration, the body maintains this kinetic energy unless its...
and
angular momentumAngular momentum is a quantity that is useful in describing the rotational state of a physical system. For a rigid body rotating around an axis of symmetry , the angular momentum can be expressed as the product of the body's moment of inertia and its angular velocity...
from the Earth's rotation (see also,
DayA day is a unit of time equivalent to approximately 24 hours. It is not an SI unit but it is accepted for use with SI. The SI unit of time is the second....
and
Leap secondA leap second is a positive or negative one-second adjustment to the Coordinated Universal Time time scale that keeps it close to mean solar time. UTC, which is used as the basis for official time-of-day radio broadcasts for civil time, is maintained using extremely precise atomic clocks...
). In turn, angular momentum is added to the Moon's orbit, which lifts the Moon into a higher orbit with a longer period. The effect on the Moon's orbital radius is a small one, just 0.10
ppbParts-per notation is used, especially in science and engineering, to denote relative proportions in measured quantities; particularly in low-value proportions at the parts-per-million , parts-per-billion , and parts-per-trillion level...
/year, but results in a measurable 3.82 cm annual increase in the Earth-Moon distance. Cumulatively, this effect becomes ever more significant over time; since
astronauts first landed on the MoonThe Apollo 11 mission was the first human spaceflight to land on the Moon. Launched on July 16, 1969, it carried Mission Commander Neil Alden Armstrong, Command Module Pilot Michael Collins, and Lunar Module Pilot Edwin Eugene 'Buzz' Aldrin, Jr...
approximately years ago, it is metres farther away.
Eclipses
Eclipses can occur only when the Sun, Earth, and Moon are all in a straight line.
Solar eclipseA solar eclipse occurs when the moon passes between the Sun and the Earth so that the Sun is fully or partially covered. This can only happen during a new moon, when the Sun and Moon are in conjunction as seen from the Earth. At least two and up to five solar eclipses can occur each year on Earth,...
s occur near a
new MoonIn astronomical terminology, the phrase new moon is the lunar phase that occurs when the Moon, in its monthly orbital motion around Earth, lies between Earth and the Sun, and is therefore in conjunction with the Sun as seen from Earth...
, when the Moon is between the Sun and Earth. In contrast,
lunar eclipseA lunar eclipse is an eclipse which occurs whenever the moon passes behind the earth such that the earth blocks the sun’s rays from striking the moon. This can occur only when the Sun, Earth, and Moon are aligned exactly, or very closely so, with the Earth in the middle. Hence, there is always a...
s occur near a
full MoonFull moon is a lunar phase.Full Moon may also refer to:-In literature:*Full Moon , a novel by P. G. Wodehouse*Full Moon o Sagashite, a manga published in North America as Full Moon...
, when the Earth is between the Sun and Moon.
Because the Moon's orbit around the Earth is inclined by about 5° with respect to the
orbit of the Earth around the SunThe ecliptic is the apparent path that the Sun traces out in the sky during the year, appearing to move eastwards on an imaginary spherical surface, the celestial sphere, relative to the fixed stars. More accurately, it is the intersection of the celestial sphere with the ecliptic plane, which is...
, eclipses do not occur at every full and new Moon. For an eclipse to occur, the Moon must be near the intersection of the two orbital planes.
The periodicity and recurrence of eclipses of the Sun by the Moon, and of the Moon by the Earth, is described by the
saros cycleThe Saros cycle is an eclipse cycle with a period of about 18 years 11 days 8 hours that can be used to predict eclipses of the Sun and Moon...
, which has a period of approximately 6 585.3 days (18 years 11 days 8 hours).
The angular diameters of the Moon and the Sun as seen from Earth overlap in their variation, so that both
totalA total eclipse is an eclipse where either the Sun is entirely covered by the Moon, or the Earth's shadow entirely covers the Moon.Total Eclipse may also refer to:-Music:* Total Eclipse , a Goa trance music group...
and annular solar eclipses are possible. In a total eclipse, the Moon completely covers the disc of the Sun and the solar
coronaA corona is a type of plasma "atmosphere" of the Sun or other celestial body, extending millions of kilometres into space, most easily seen during a total solar eclipse, but also observable in a coronagraph...
becomes visible to the
naked eyeThe naked eye is a figure of speech referring to human visual perception that is unaided by enhancing equipment, such as a telescope or microscope. Vision corrected to normal acuity using corrective lenses is considered "naked"...
. Since the distance between the Moon and the Earth is very slightly increasing over time, the angular diameter of the Moon is decreasing. This means that hundreds of millions of years ago the Moon could always completely cover the Sun on solar eclipses so that no annular eclipses were possible. Likewise, about 600 million years from now (assuming that the angular diameter of the Sun will not change), the Moon will no longer cover the Sun completely and only annular eclipses will occur.
A phenomenon related to eclipse is
occultationAn occultation is an event that occurs when one object is hidden by another object that passes between it and the observer. The word is used in astronomy and can also be used in a general sense to describe when an object in the foreground occults objects in the background...
. The Moon is continuously blocking our view of the sky by a 1/2 degree-wide circular area. When a bright star or planet
passes behind the Moon it is
occulted or hidden from view. A solar eclipse is an occultation of the Sun. Because the Moon is close to Earth, occultations of individual stars are not visible everywhere, nor at the same time. Because of the precession of the lunar orbit, each year different stars are occulted.
Observation
During its brightest phase, at "full Moon", the Moon has an
apparent magnitudeThe apparent magnitude of a celestial body is a measure of its brightness as seen by an observer on Earth, normalized to the value it would have in the absence of the atmosphere...
of about −12.6. By comparison, the Sun has an apparent magnitude of −26.8. When the Moon is in a quarter phase, its brightness is not half of a full Moon, but only about a tenth. This is because the lunar surface is not a perfect
Lambertian reflectorIf a surface exhibits Lambertian reflectance, light falling on it is scattered such that the apparent brightness of the surface to an observer is the same regardless of the observer's angle of view. More technically, the surface luminance is isotropic...
. When the Moon is full the
opposition effectThe opposition effect is the brightening of a rough surface, or an object with many particles, when illuminated from directly behind the observer...
makes it appear brighter, but away from full there are shadows projected onto the surface which diminish the amount of reflected light.
On average, the Moon covers an area of 0.21078 square degrees on the night sky.
The Moon appears larger when close to the horizon. This is a purely psychological effect (see
Moon illusionThe Moon illusion is an optical illusion in which the Moon appears larger near the horizon than it does while higher up in the sky. This optical illusion also occurs with the sun and star constellations...
). It is actually about 1.5% smaller when the Moon is near the horizon than when it is high in the sky (because it is farther away by up to one Earth radius).
The Moon appears as a relatively bright object in the sky, in spite of its low
albedoThe albedo of an object is the extent to which it diffusely reflects light from light sources such as the Sun. It is therefore a more specific form of the term reflectivity. Albedo is defined as the ratio of diffusely reflected to incident electromagnetic radiation. It is a unitless measure...
. The Moon is about the poorest
reflectorA reflector can mean one of several things:Science* Reflector, a device that causes reflection * In luminaires and reflectors are used to intensify light in the desired direction....
in the
solar systemThe Solar System consists of the Sun and those celestial objects bound to it by gravity, all of which formed from the collapse of a giant molecular cloud approximately 4.6 billion years ago...
and reflects only about 7% of the light incident upon it (about the same proportion as is reflected by a lump of
coalCoal is a readily combustible black or brownish-black sedimentary rock normally occurring in rock strata in layers or veins called coal beds. The harder forms, such as anthracite coal, can be regarded as metamorphic rock because of later exposure to elevated temperature and pressure...
). However, the Moon is not a Lambertian scatterer and reflects more light back towards the Sun (albedo of 12%) than in other directions because of the spherical glass beads in the moondust. This increases the brightness of a full Moon. It also has the effect of making the edges of a full Moon seem about as bright as the centre. Besides this,
color constancyColor constancy is an example of subjective constancy and a feature of the human color perception system which ensures that the perceived color of objects remains relatively constant under varying illumination conditions. A green apple for instance looks green to us at midday, when the main...
in the
visual systemThe visual system is the part of the central nervous system which enables organisms to see.It interprets the information from visible light to build a representation of the world surrounding the body...
recalibrates the relations between the colours of an object and its surroundings, and since the surrounding sky is comparatively dark the sunlit Moon is perceived as a bright object.
The highest altitude of the Moon on a day varies and has nearly the same limits as the Sun. It also depends on the Earth season and lunar phase, with the full Moon being highest in winter. Moreover, the 18.6 year nodes cycle also has an influence, as when the ascending node of the lunar orbit is in the vernal equinox, the lunar declination can go as far as 28° each month (which happened most recently in 2006). This results that the Moon can go overhead on latitudes up to 28 degrees from the equator (e.g.
FloridaFlorida is a state located in the southeastern region of the United States, bordering Alabama to the northwest and Georgia to the north. It was the 27th state admitted to the United States...
,
Canary IslandsThe Canary Islands are a Spanish archipelago which, in turn, forms one of the Spanish Autonomous Communities and an Outermost Region of the European Union. The archipelago is located just off the northwest coast of mainland Africa, 100 km west of the disputed border between Morocco and the...
or in the southern hemisphere
BrisbaneBrisbane is the state capital of the Australian state of Queensland and is the largest city in that state. With an estimated population of approximately 2 million, it is also the third most populous city in Australia....
). Slightly more than 9 years later (next time in 2015) the declination reaches only 18° N or S each month.
The orientation of the Moon's crescent also depends on the latitude of the observation site. Close to the equator, an observer can see a
boat Moon.
Like the Sun, the Moon can give rise to atmospheric effects, including a 22°
haloA halo is an optical phenomenon produced by ice crystals creating colored or white arcs and spots in the sky. Many are near the sun or moon but others are elsewhere and even in the opposite part of the sky...
ring, and the smaller
coronal ringsIn meteorology, a corona is produced by the diffraction of light from either the Sun or the Moon by individual small water droplets of a cloud....
seen more often through thin clouds. For more information on how the Moon appears in Earth's sky, see
lunar phaseA lunar phase or phase of the moon refers to the appearance of the illuminated portion of the Moon as seen by an observer, usually on Earth. The lunar phases vary cyclically as the Moon orbits the Earth, according to the changing relative positions of the Earth, Moon and Sun...
.
Exploration
The first leap in lunar observation was prompted by the invention of the telescope.
Galileo GalileiGalileo Galilei was an Italian physicist, mathematician, astronomer, and philosopher who played a major role in the Scientific Revolution. His achievements include improvements to the telescope and consequent astronomical observations, and support for Copernicanism...
made good use of this new instrument and observed mountains and craters on the Moon's surface.
The
Cold WarThe Cold War was the continuing state of political conflict, military tension, and economic competition existing after World War II , primarily between the USSR and its satellite states, and the powers of the Western world, including the United States...
-inspired
space raceThe Space Race was an informal competition between the United States and the Soviet Union, as each side tried to match or better the other's accomplishments in exploring outer space...
between the Soviet Union and the U.S. led to an acceleration of interest in the Moon. Unmanned probes, both flyby and impact/lander missions, were sent almost as soon as launcher capabilities would allow. The Soviet Union's
Luna programThe Luna programme , occasionally called Lunik or Lunnik, was a series of robotic spacecraft missions sent to the Moon by the Soviet Union between 1959 and 1976. Fifteen were successful, each designed as either an orbiter or lander, and accomplished many firsts in space exploration...
was the first to reach the Moon with unmanned
spacecraftA spacecraft is a craft or machine designed for spaceflight. On a sub-orbital spaceflight, a spacecraft enters space then returns to the Earth. For an orbital spaceflight, a spacecraft enters a closed orbit around the planetary body. Spacecraft used for human spaceflight carry people on board as...
. The first man-made object to escape Earth's gravity and pass near the Moon was
Luna 1Luna 1 , first known as First Cosmic Ship, then known as Mechta was the first spacecraft to reach the vicinity of the Moon and the first of the Luna program of Soviet automatic interplanetary stations successfully launched in the direction of the Moon.While traveling through the outer Van Allen...
, the first man-made object to impact the lunar surface was
Luna 2Luna 2 was the second of the Soviet Union's Luna programme spacecraft launched in the direction of the Moon. It was the first spacecraft to reach the surface of the Moon. It crashed upon the lunar surface east of Mare Serenitatis near the craters Aristides, Archimedes, and Autolycus...
, and the first photographs of the normally occluded far side of the Moon were made by
Luna 3The Soviet space probe Luna 3 was the third spacecraft sent successfully to the Moon, and it was an early feat in the human exploration of outer space...
, all in 1959. The first spacecraft to perform a successful lunar soft landing was
Luna 9Luna 9 , was an unmanned space mission of the Soviet Union's Luna program. On February 3, 1966 the Luna 9 spacecraft was the first spacecraft to achieve a lunar soft landing and to transmit photographic data to Earth....
and the first unmanned vehicle to orbit the Moon was
Luna 10Luna 10 was an unmanned space mission of the Luna program, also called Lunik 10.The Luna 10 spacecraft was launched towards the Moon from an Earth orbiting platform on March 31, 1966. It was the first artificial satellite of the Moon...
, both in 1966. Moon samples have been brought back to Earth by three Luna missions (
Luna 16Luna 16 was an unmanned space mission of the Luna program, also called Lunnik 16 .Luna 16 was the first robotic probe to land on the Moon and return a sample to Earth...
,
20Luna 20 was an unmanned space mission of the Luna program, also called Lunik 20.Luna 20 was placed in an intermediate Earth parking orbit and from this orbit was sent towards the Moon. It entered lunar orbit on February 18, 1972...
, and
24Luna 24 was an unmanned space mission of the Luna program, also called Lunnik 24. The last of the Luna series of spacecraft, the mission of the Luna 24 probe was the third Soviet mission to retrieve Lunar soil samples .*On-orbit dry mass: 4800 kgThe probe landed in the area known as Mare Crisium...
) and the Apollo missions 11 to 17 (except
Apollo 13Apollo 13 was the third manned mission by NASA that was intended to land on the moon, but a mid-mission technical malfunction forced the lunar landing to be aborted. The crew members were commander James A. Lovell, command module pilot John L. "Jack" Swigert, and lunar module pilot Fred W....
, which aborted its planned lunar landing).
The landing of the first humans on the Moon in 1969 is seen by many as the culmination of the space race.
Neil ArmstrongNeil Alden Armstrong is an American aviator and a former astronaut, test pilot, university professor, and United States Naval Aviator. He was the first person to set foot on the Moon. His first spaceflight was aboard Gemini 8 in 1966, for which he was the command pilot...
became the first person to walk on the Moon as the commander of the American mission
Apollo 11The Apollo 11 mission was the first human spaceflight to land on the Moon. Launched on July 16, 1969, it carried Mission Commander Neil Alden Armstrong, Command Module Pilot Michael Collins, and Lunar Module Pilot Edwin Eugene 'Buzz' Aldrin, Jr...
by first setting foot on the Moon at 02:56 UTC on July 21, 1969. The American
Moon landingA moon landing is arrival of a spacecraft on the surface of the Moon. This includes both manned and unmanned missions. The first human-made object to reach the surface of the Moon was the Soviet Union's Luna 2 mission on September 13, 1959...
and return was enabled by considerable technological advances, in domains such as
ablationAblation means removal of material from the surface of an object by vaporization, chipping, or other erosive processes. The term occurs in space physics associated with atmospheric reentry, in glaciology, medicine, and passive fire protection.-Space physics:...
chemistry and atmospheric re-entry technology, in the early 1960s.
Scientific instrument packages were installed on the lunar surface during all of the
Apollo missions. Long-lived ALSEP stations (Apollo lunar surface experiment package) were installed at the
Apollo 12Apollo 12 was the sixth manned mission in the Apollo program and the second to land on the Moon. The mission was commanded by Charles "Pete" Conrad. It was launched on 14 November 1969, four months after Apollo 11. Pete Conrad and Alan L. Bean performed just over one day and seven hours of lunar...
,
14Apollo 14 was the eighth manned mission in the Apollo program and the third mission to land on the Moon. The nine-day mission was launched on January 31, 1971, with lunar touch down on February 5. The Lunar Module landed in the Fra Mauro formation; this had originally been the target of the...
,
15Apollo 15 was the ninth manned mission in the Apollo program and the fourth mission to land on the Moon. It was the first of what were termed "J missions", long duration stays on the Moon with a greater focus on science than had been possible on previous missions. The mission began on July 26,...
,
16Apollo 16 was the tenth manned mission in the Apollo program, the fifth mission to land on the Moon and the first to land in a highlands area. The mission was launched on April 16, 1972, and concluded on April 27. It was a J-class mission, featuring a Lunar Rover and it brought back 94.7 kg of...
, and
17Apollo 17 was the eleventh manned space mission in the NASA Apollo program. It was the first night launch of a U.S. human spaceflight and the sixth and final lunar landing mission of the Apollo program. The mission was launched at 12:33 a.m. EST on December 7, 1972, and concluded on December 19. It...
landing sites, whereas a temporary station referred to as EASEP (Early Apollo Scientific Experiments Package) was installed during the Apollo 11 mission. The ALSEP stations contained, among others, heat flow probes, seismometers, magnetometers, and corner-cube retroreflectors. Transmission of data to Earth was terminated on September 30, 1977 because of budgetary considerations. Since the
lunar laser rangingThe ongoing Lunar Laser Ranging Experiment measures the distance between the Earth and the Moon using laser ranging. Lasers on Earth are aimed at retroreflectors previously planted on the Moon and the time delay for the reflected light to return is determined. Since the speed of light is known with...
(LLR) corner-cube arrays are passive instruments, they are still being used. Ranging to the LLR stations is routinely performed from earth-based stations with an accuracy of a few centimetres, and data from this experiment are being used to place constraints on the size of the lunar core.
have now passed since Eugene Cernan and
Harrison SchmittHarrison Hagan "Jack" Schmitt is an American geologist, a former NASA astronaut, University Professor and a U.S. Senator for one term....
, as part of the mission
Apollo 17Apollo 17 was the eleventh manned space mission in the NASA Apollo program. It was the first night launch of a U.S. human spaceflight and the sixth and final lunar landing mission of the Apollo program. The mission was launched at 12:33 a.m. EST on December 7, 1972, and concluded on December 19. It...
, left the surface of the Moon on December 14, 1972 (Cernan being the last to enter the
LMThe Apollo Lunar Module was the lander portion of the Apollo spacecraft built for the US Apollo program by Grumman to achieve the transit from lunar orbit to the surface and back...
) and no one has set foot on it since.
From the mid-1960s to the mid-1970s, there were 65 instances of artificial objects reaching the Moon (both manned and robotic, with ten in 1971 alone), with the last being
Luna 24Luna 24 was an unmanned space mission of the Luna program, also called Lunnik 24. The last of the Luna series of spacecraft, the mission of the Luna 24 probe was the third Soviet mission to retrieve Lunar soil samples .*On-orbit dry mass: 4800 kgThe probe landed in the area known as Mare Crisium...
in 1976. Only 18 of these were controlled Moon landings, with nine completing a round trip from Earth and returning samples of Moon rocks. The Soviet Union then turned its primary attention to
VenusVenus is the second-closest planet to the Sun, orbiting it every 224.7 Earth days. The planet is named after Venus, the Roman goddess of love and beauty. After the Moon, it is the brightest natural object in the night sky, reaching an apparent magnitude of −4.6...
and
space stationA space station is an artificial structure designed for humans to live in outer space. To date, only low earth orbital stations have been implemented, otherwise known as orbital stations...
s, and the U.S. to
MarsMars is the fourth planet from the Sun in the Solar System. The planet is named after Mars, the Roman god of war. It is also referred to as the "Red Planet" because of its reddish appearance, due to iron oxide prevalent on its surface....
and beyond. In 1990, Japan orbited the Moon with the
HitenThe Hiten spacecraft , built by the Institute of Space and Astronautical Science of Japan, was launched on January 24, 1990. It was Japan's first lunar probe, the first robotic lunar probe since the Soviet Union's Luna 24 in 1976, and the first lunar probe launched by a country other than Soviet...
spacecraft, becoming the third country to place a spacecraft into lunar orbit. The spacecraft released a smaller probe,
Hagormo, in lunar orbit, but the transmitter failed, thereby preventing further scientific use of the mission.
In 1994, the U.S. finally returned to the Moon, robotically at least, sending the Joint Defense Department/NASA spacecraft
ClementineClementine was a joint space project between the Ballistic Missile Defense Organization and NASA...
. 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 by the
Lunar ProspectorThe Lunar Prospector mission was the third selected by NASA for full development and construction as part of the Discovery Program. At a cost of $62.8 million, the 19-month mission was designed for a low polar orbit investigation of the Moon, including mapping of surface composition and possible...
mission in 1998. The
neutronThe neutron is a subatomic particle with no net electric charge and a mass slightly larger than that of a proton.Neutron are usually found in atomic nuclei. The nuclei of most atoms consist of protons and neutrons, which are therefore collectively referred to as nucleons. The number of protons in a...
spectrometerA spectrometer is an instrument used to measure properties of light over a specific portion of the electromagnetic spectrum, typically used in spectroscopic analysis to identify materials. The variable measured is most often the light's intensity but could also, for instance, be the polarization...
on
Lunar Prospector 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. The European spacecraft
Smart 1 was launched September 27, 2003 and was in lunar orbit from November 15, 2004 to September 3, 2006.
On January 14, 2004, U.S. President
George W. BushGeorge Walker Bush was the 43rd President of the United States from 2001 to 2009 and the 46th Governor of Texas from 1995 to 2000....
called for a plan to resume manned missions to the Moon by 2020 (see
Vision for Space ExplorationThe Vision for Space Exploration is the United States space policy announced on January 14, 2004 by then-U.S. President George W. Bush. It is seen as a response to the Space Shuttle Columbia disaster, the state of human spaceflight at NASA, and a way to regain public enthusiasm for space...
). NASA is now planning for the construction of a permanent outpost at one of the lunar poles. The People's Republic of China has expressed ambitious plans for exploring the Moon and has started the
Chang'e programChang’e 1 was an unmanned Chinese lunar-orbiting spacecraft, part of the first phase of the Chinese Lunar Exploration Program. The spacecraft was named after the Chinese moon goddess, Chang'e....
for lunar exploration, successfully launching its first spacecraft, Chang'e-1, on October 24, 2007. Like NASA, China hopes to land people on the Moon by 2020. The U.S. launched the
Lunar Reconnaissance OrbiterThe Lunar Precursor Robotic Program is a program of robotic spacecraft missions which NASA will use to prepare for future human spaceflight missions to the Moon. Two LPRP missions, the Lunar Reconnaissance Orbiter and the Lunar Crater Observation and Sensing Satellite , were launched in June 2009...
and the
Lunar Crater Observation and Sensing Satellite on June 18, 2009 (the two missions were co-manifested). Russia also announced to resume its previously frozen project
Luna-GlobLuna-Glob is the name of a Moon-exploration program by the Russian Federal Space Agency based on plans dating back to 1997. Due to financial problems, however, the project was put on hold only to be revived a few years later. Initially scheduled for launch in 2012, the mission has been brought...
, consisting of an unmanned lander and orbiter, which is slated to land in 2012.
The
Google Lunar X PrizeThe Google Lunar X PRIZE, sometimes referred to as simply Moon 2.0, is a space competition organized by the X PRIZE Foundation, and sponsored by Google...
, announced September 13, 2007, hopes to boost and encourage privately funded lunar exploration. The
X Prize FoundationThe X PRIZE Foundation is a non-profit prize institute that designs and manages public competitions for the benefit of humanity.-Mission:The X PRIZE Foundation mission is to bring about “radical breakthroughs for the benefit of humanity” through incentivized competition...
is offering anyone US$20 million who can land a robotic rover on the Moon and meet other specified criteria.
On September 14, 2007 the
Japan Aerospace Exploration AgencyThe , or JAXA, is Japan's national aerospace agency. JAXA was formed on October 1, 2003, as an Independent Administrative Institution through the merger of three previously independent organizations...
launched
SELENESELENE , better known in Japan by its nickname , was the second Japanese lunar orbiter spacecraft. Produced by the Institute of Space and Astronautical Science and NASDA , the spacecraft was launched September 14, 2007...
, also known as Kaguya, a lunar orbiter which is fitted with a
high-definition cameraHigh-definition video or HD video refers to any video system of higher resolution than standard-definition video, and most commonly involves display resolutions of 1280×720 pixels or 1920×1080 pixels...
and two small satellites. The mission is expected to last one year.
On October 22, 2008 India successfully launched the
Chandrayaan IChandrayaan-1, was India's first unmanned lunar probe. It was launched by the Indian Space Research Organisation in October 2008, and operated until August 2009. The mission included a lunar orbiter and an impactor...
(a
SanskritSanskrit is a historical Indo-Aryan language, one of the liturgical languages of Hinduism and Buddhism, and one of the 22 official languages of India. It is also declared as a classical language by the government of India....
word literally meaning the 'Moon-craft') unmanned mission to the Moon and intends to launch several further unmanned missions. The country plans to launch
Chandrayaan II in 2010 or 2011, which is slated to include a robotic lunar rover. India also has expressed its hope for a manned mission to the Moon by 2020.
Human understanding
The Moon has been the subject of many works of art and literature and the inspiration for countless others. It is a motif in the visual arts, the performing arts, poetry, prose and music. A 5000-year-old rock carving at
KnowthKnowth is a Neolithic passage grave, an ancient monument of Brú na Bóinne in the valley of the River Boyne in Ireland.Knowth is the largest of all passage graves situated within the Brú na Bóinne complex. The site consists of one large mound and 17 smaller satellite tombs...
,
IrelandIreland is the third-largest island in Europe and the twentieth-largest island in the world. It lies to the north-west of continental Europe and is surrounded by hundreds of islands and islets. To the east of Ireland, separated by the Irish Sea, is the island of Great Britain...
may represent the Moon, which would be the earliest depiction discovered. In many prehistoric and ancient cultures, the Moon was thought to be a
deityIn mythology, a lunar deity is a god or goddess associated with or symbolizing the moon. These deities can have a variety of functions and traditions depending upon the culture, but they are often related to or an enemy of the solar deity. Lunar deities can be both male and female, and are usually...
or other
supernaturalThe term supernatural or supranatural pertains to an order of existence beyond the scientifically visible universe. Religious miracles are typically supernatural claims, as are spells and curses, divination, the belief that there is an afterlife for the dead, and innumerable others...
phenomenon, and
astrological viewsThe Moon is the earth's companion satellite, though some astronomers believe that it approaches being a planet in its own right. The Moon is large enough for its gravity to affect the Earth, stabilising its orbit and producing the regular ebb and flow of the tides. The Moon is also familiar to us...
of the Moon continue to be propagated today.
Among the first in the Western world to offer a scientific explanation for the Moon was the
GreekAncient Greece is the civilisation belonging to the period of Greek history lasting from the Greek Dark Ages ca. 1100 BC and the Dorian invasion, to 146 BC and the Roman conquest of Greece after the Battle of Corinth. It is generally considered to be the seminal culture which provided the...
philosopher
AnaxagorasAnaxagoras was a Pre-Socratic Greek philosopher. Born in Clazomenae in Asia Minor, Anaxagoras was the first philosopher to bring philosophy from Ionia to Athens. He attempted to give a scientific account of eclipses, meteors, rainbows, and the sun, which he described as a fiery mass larger than...
(d. 428 BC), who reasoned that the Sun and Moon were both giant spherical rocks, and that the latter reflected the light of the former. His atheistic view of the heavens was one cause for his imprisonment and eventual exile.
In
Aristotle'sAristotle was a Greek philosopher, a student of Plato and teacher of Alexander the Great. He wrote on many subjects, including physics, metaphysics, poetry, theater, music, logic, rhetoric, politics, government, ethics, biology, and zoology.Together with Plato and Socrates , Aristotle is one of...
(384–322 BC) description of the universe, the Moon marked the boundary between the spheres of the mutable elements (earth, water, air and fire), and the imperishable stars of
aetherAether originally was the personification of the "upper sky", space and heaven, in Greek mythology.The term aether, æther or ether may also refer to one of the following:...
. This separation was held to be part of physics for many centuries after.
Aristarchus* Aristarchus , on the moon* Aristarchus of Samos , Greek astronomer and mathematician* Aristarchus of Samothrace , Greek grammarian* Aristarchus of Tegea , Greek writer...
went a step further and
computedOn the Sizes and Distances [of the Sun and Moon] is the only extant work written by Aristarchus of Samos, an ancient Greek astronomer who lived circa 310 BC - 230 BC...
the distance from earth, together with its size, obtaining a value of 20 earth radius for the distance (the real value is 60. The earth radius was known since
EratosthenesEratosthenes of Cyrene was a Greek mathematician, elegiac poet, athlete, geographer, and astronomer. He made several discoveries and inventions including a system of latitude and longitude...
)
During the Warring States of China, astronomer
Shi ShenShi Shen was a Chinese astronomer and contemporary of Gan De born in the State of Wei, also known as the Master Shi Shen .-Observations:...
(fl. 4th century BC) gave instructions for predicting solar and lunar eclipses based on the relative positions of the Moon and Sun. Although the Chinese of the
Han DynastyThe Han Dynasty was the second imperial dynasty of China, preceded by the Qin Dynasty and succeeded by the Three Kingdoms . It was founded by the peasant rebel leader Liu Bang, known posthumously as Emperor Gaozu of Han. It was briefly interrupted by the Xin Dynasty of the former regent Wang Mang...
(202 BC–202 AD) believed the Moon to be energy equated to
qiIn traditional Chinese culture, qi is an active principle forming part of any living thing....
, their 'radiating influence' theory recognized that the light of the Moon was merely a reflection of the Sun (mentioned by Anaxagoras above). This was supported by mainstream thinkers such as
Jing FangJing Fang , born Li Fang , courtesy name Junming , was a Chinese music theorist, mathematician and astrologer born in present-day Puyang, Henan during the Han Dynasty . He was the first to notice how closely a succession of 53 just fifths approximates 31 octaves...
(78–37 BC) and
Zhang HengZhang Heng was an astronomer, mathematician, inventor, geographer, cartographer, artist, poet, statesman, and literary scholar from Nanyang, Henan, and lived during the Eastern Han Dynasty of China. He was educated in the capital cities of Luoyang and Chang'an, and began his career as a minor...
(78–139 AD), but it was also opposed by the influential philosopher
Wang ChongWang Chong , courtesy name Zhongren , was a Chinese philosopher during the Han Dynasty who developed a rational, secular, naturalistic, and mechanistic account of the world and of human beings. His main work was the Lùnhéng...
(27–97 AD). Jing Fang noted the sphericity of the Moon, while Zhang Heng accurately described a lunar eclipse and solar eclipse. These assertions were supported by
Shen KuoShen Kuo or Shen Gua , style name Cunzhong and pseudonym Mengqi Weng , was a polymathic Chinese scientist and statesman of the Song Dynasty...
(1031–1095) of the
Song DynastyThe Song Dynasty was a ruling dynasty in China between 960 and 1279; it succeeded the Five Dynasties and Ten Kingdoms Period, and was followed by the Yuan Dynasty. It was the first government in world history to issue banknotes or paper money, and the first Chinese government to establish a...
(960–1279) who created an allegory equating the waxing and waning of the Moon to a round ball of reflective silver that, when doused with white powder and viewed from the side, would appear to be a crescent. He also noted that the reason for the Sun and Moon not eclipsing every time their paths met was because of a small obliquity in their orbital paths.
By the
Middle AgesThe Middle Ages of European history is a period of European history covering roughly a millennium in the 5th century through 16th centuries. More specific starting and ending points are sometimes adopted by scholars to suit their respective specializations or current focus...
, before the invention of the telescope, more and more people began to recognise the Moon as a sphere, though they believed that it was "perfectly smooth". In 1609,
Galileo GalileiGalileo Galilei was an Italian physicist, mathematician, astronomer, and philosopher who played a major role in the Scientific Revolution. His achievements include improvements to the telescope and consequent astronomical observations, and support for Copernicanism...
drew one of the first telescopic drawings of the Moon in his book and noted that it was not smooth but had mountains and craters. Later in the 17th century,
Giovanni Battista RiccioliGiovanni Battista Riccioli , was an Italian astronomer and a Roman Catholic Priest. He was a Jesuit who entered the order in 1614. He was also the first person to measure the rate of acceleration of a freely falling body....
and
Francesco Maria GrimaldiFrancesco Maria Grimaldi, born April 2, 1618 in Bologna and dead on December 28, 1663 in Bologna, was an Italian Jesuit priest, mathematician and physicist who taught at the Jesuit college in Bologna....
drew a map of the Moon and gave many craters the names they still have today.
On maps, the dark parts of the Moon's surface were called
maria (singular
mare) or seas, and the light parts were called
terrae or continents.
The possibility that the Moon contains vegetation and is inhabited by selenites was seriously considered by major astronomers even into the first decades of the 19th century. The contrast between the brighter highlands and darker maria create the patterns seen by different cultures as the
Man in the MoonThe Man in the Moon is an imaginary figure resembling a human face, head or body, that observers from some cultural backgrounds typically perceive in the bright disc of the full moon...
, the
rabbitThe Moon rabbit, also called the Jade Rabbit, is a rabbit that lives on the moon in East Asian folklore. The legends about the moon rabbit are based on the traditional pareidolia that identifies the markings of the moon as a rabbit pounding in a mortar...
and the buffalo, among others.
In 1835, the
Great Moon Hoax"The Great Moon Hoax" was a series of six articles that were published in the New York Sun beginning on August 25, 1835, about the supposed discovery of life and even civilization on the Moon...
fooled some people into thinking that there were exotic animals living on the Moon. Almost at the same time however (during 1834–1836),
Wilhelm BeerWilhelm Wolff Beer was a banker and astronomer from Berlin, Prussia, and the brother of Giacomo Meyerbeer.- Astronomy :...
and Johann Heinrich Mädler were publishing their four-volume and the book in 1837, which firmly established the conclusion that the Moon has no bodies of water nor any appreciable atmosphere.
The far side of the Moon remained completely unknown until the
Luna 3The Soviet space probe Luna 3 was the third spacecraft sent successfully to the Moon, and it was an early feat in the human exploration of outer space...
probe was launched in 1959, and it was extensively mapped by the
Lunar Orbiter programThe Lunar Orbiter program was a series of five unmanned Lunar orbiter missions launched by the United States in 1966 through 1967 with the purpose of mapping the lunar surface before the Apollo landings. All five missions were successful, and 99% of the Moon was photographed with a resolution of...
in the 1960s.
Legal status
Although several pennants of the
Soviet UnionThe Union of Soviet Socialist Republics was a constitutionally socialist state that existed in Eurasia from 1922 to 1991. The name is a translation of the , tr. Soyuz Sovetskikh Sotsialisticheskikh Respublik, abbreviated СССР, SSSR. The common short name is Soviet Union, from , Sovetskiy Soyuz...
were scattered by
Luna 2Luna 2 was the second of the Soviet Union's Luna programme spacecraft launched in the direction of the Moon. It was the first spacecraft to reach the surface of the Moon. It crashed upon the lunar surface east of Mare Serenitatis near the craters Aristides, Archimedes, and Autolycus...
in 1959 and by later landing missions, and U.S. flags have been symbolically planted on the Moon, no nation currently claims ownership of any part of the Moon's surface. Russia and the U.S. are party to the
Outer Space TreatyThe Outer Space Treaty, formally known as the Treaty on Principles Governing the Activities of States in the Exploration and Use of Outer Space, including the Moon and Other Celestial Bodies, is a treaty that forms the basis of international space law...
, which places the Moon under the same jurisdiction as
international watersThe terms international waters or trans-boundary waters apply where any of the following types of bodies of water transcend international boundaries: oceans, large marine ecosystems, enclosed or semi-enclosed regional seas and estuaries, rivers, lakes, groundwater systems , and wetlands...
. This treaty also restricts the use of the Moon to peaceful purposes, explicitly banning military installations and
weapons of mass destructionThe term weapon of mass destruction is often used to describe a weapon that can kill large numbers of humans and/or cause great damage to man-made structures , natural structures , or the biosphere in general....
(including nuclear weapons).
A second treaty, the
Moon TreatyThe Agreement Governing the Activities of States on the Moon and Other Celestial Bodies, better known as the Moon Treaty or Moon Agreement, is an international treaty that turns jurisdiction of all heavenly bodies over to the international community...
, was proposed to restrict the exploitation of the Moon's resources by any single nation, but it has not been signed by any of the space-faring nations. Several individuals have made
claims to the MoonExtraterrestrial real estate is land on other planets or natural satellites or parts of space that is sold either through organizations or by individuals. Ownership of extraterrestrial real estate is not recognised by any authority...
in whole or in part, although none of these are generally considered credible.
See also
- 3753 Cruithne
3753 Cruithne is an asteroid in orbit around the Sun in 1:1 orbital resonance with that of the Earth. It is a periodic inclusion planetoid orbiting the Sun in an apparent horseshoe orbit...
- Apollo 11
The Apollo 11 mission was the first human spaceflight to land on the Moon. Launched on July 16, 1969, it carried Mission Commander Neil Alden Armstrong, Command Module Pilot Michael Collins, and Lunar Module Pilot Edwin Eugene 'Buzz' Aldrin, Jr...
– first Moon mission
- Blue moon
A blue moon is a full moon that is not timed to the regular monthly pattern. Most years have twelve full moons which occur approximately monthly, but in addition to those twelve full lunar cycles, each calendar year contains an excess of roughly eleven days. The extra days accumulate, so that every...
- Cassini's Laws
Cassini's laws provide a compact description of the motion of the Moon. They were established in 1693 by Giovanni Domenico Cassini, a prominent scientist of his time....
- Colonization of the Moon
The colonization of the Moon is the proposed establishment of permanent human communities on the Moon. Advocates of space exploration have seen settlement of the Moon as a logical step in the expansion of humanity beyond the Earth. National claims to the best locations on the moon may eventually...
- Extraterrestrial real estate
Extraterrestrial real estate is land on other planets or natural satellites or parts of space that is sold either through organizations or by individuals. Ownership of extraterrestrial real estate is not recognised by any authority...
- Google Lunar X Prize
The Google Lunar X PRIZE, sometimes referred to as simply Moon 2.0, is a space competition organized by the X PRIZE Foundation, and sponsored by Google...
- Late Heavy Bombardment
The Late Heavy Bombardment is a period of time approximately 3.8 to 4.1 billion years ago during which a large number of impact craters are believed to have formed on the Moon, and by inference on Earth, Mercury, Venus, and Mars as well...
- List of Apollo astronauts (includes list of people who have walked on the Moon)
- List of artificial objects on the Moon
- List of craters on the Moon
- List of features on the Moon
- List of maria on the Moon
- List of mountains on the Moon
- List of valleys on the Moon
- Lunar phase
A lunar phase or phase of the moon refers to the appearance of the illuminated portion of the Moon as seen by an observer, usually on Earth. The lunar phases vary cyclically as the Moon orbits the Earth, according to the changing relative positions of the Earth, Moon and Sun...
- Lunar space elevator
A lunar space elevator is a proposed cable running from the surface of the Moon into space.It is similar in concept to the better known Earth space elevator idea...
- Lunar theory
Lunar theory attempts to account for the motions of the moon. There are many irregularities in the moon's motion, and many attempts have been made over a long history to account for them. After centuries of being heavily problematic, the lunar motions are nowadays modelled to a very high degree...
- Month
The month is a unit of time, used with calendars, which is approximately as long as some natural period related to the motion of the Moon; month and Moon are cognates. The traditional concept arose with the cycle of moon phases; such months are synodic months and last approximately 29.53 days...
- Moon in art and literature
The Moon has been the subject of many works of art and literature and the inspiration for countless others. It is a motif in the visual arts, the performing arts, poetry, prose and music.-Literary:...
- Orbit of the Moon
- Selenography
Selenography is the study of the surface and physical features of the Moon. Historically, the principal concern of selenographists was the mapping and naming of the lunar maria, craters, mountain ranges, and other various features...
- Space weathering
Space weathering is a blanket term used for a number of processes that act on any body exposed to the harsh space environment. Airless bodies incur many weathering processes:* collisions of galactic cosmic rays and solar cosmic rays,* irradiation, implantation, and sputtering from solar wind...
External links
Images and maps
- Video of Earth rising from moon orbit by the camera of the JAXA (Japanese) satellite Kaguya (Selene)
Selene is the Titan goddess of the moon.In Greek mythology, Seléne was an archaic lunar deity and the daughter of the Titans Hyperion and Theia. In Roman mythology, the moon goddess is called Luna, Latin for "moon"....
. See press release (The EarthEarth is the third planet from the Sun. It is the fifth largest of the eight planets in the solar system, and the largest of the terrestrial planets in the Solar System in terms of diameter, mass and density...
only appears to rise because of the orbiting satellite. As seen from a stationary point on the Moon's surface the Earth doesn't move, because the Moon always shows the same sideTidal locking occurs when the gravitational gradient makes one side of an astronomical body always face another; for example, one side of the Earth's Moon always faces the Earth. A tidally locked body takes just as long to rotate around its own axis as it does to revolve around its partner. This...
to the Earth.)
- Japan Aerospace Exploration Agency (JAXA), numerous Selene satellite Moon images beginning in 2007.
Exploration
Moon phases See when the next new crescent moon is visible for any location. Updates every month. From the
United States Naval ObservatoryThe United States Naval Observatory is one of the oldest scientific agencies in the United States, with a primary mission to produce Positioning, Navigation, and Timing for the U.S. Navy and the U.S. Department of Defense...
partner
HM Nautical Almanac OfficeHer Majesty's Nautical Almanac Office , now part of the United Kingdom Hydrographic Office, was established in 1832 on the site of the Royal Greenwich Observatory , where the Nautical Almanac had been published since 1767....
sponsored web site
crescent moon watch
Others
Cartographic resources
Movies
- Movie of the Moon at National Oceanic and Atmospheric Administration