Moon

This article is about Earths natural satellite. For moonsin general, see Natural satellite. For other uses, see Moon(disambiguation).

The Moon (Latin: Luna) is Earth's only natural satel-

The Moon, tinted reddish, during a lunar eclipse.

lite.[lower-alpha 5][lower-alpha 6][8] Although not the largest nat-ural satellite in the Solar System, it is, among the satellitesof major planets, the largest relative to the size of the ob-ject it orbits (its primary).[lower-alpha 7][lower-alpha 8] It is thesecond-densest satellite among those whose densities areknown (after Jupiter's satellite Io).The Moon is in synchronous rotation with Earth, al-ways showing the same face with its near side marked bydark volcanic maria that ll between the bright ancientcrustal highlands and the prominent impact craters. It isthe second-brightest regularly visible celestial object inEarths sky (after the Sun), as measured by illuminanceon the surface of Earth. Although it can appear a verybright white, its surface is actually dark, with a reectancejust slightly higher than that of worn asphalt. Its promi-nence in the sky and its regular cycle of phases have, sinceancient times, made the Moon an important cultural in-uence on language, calendars, art, and mythology. TheMoons gravitational inuence produces the ocean tidesand the slight lengthening of the day. The Moons cur-rent orbital distance is about thirty times the diameter ofEarth, causing it to have an apparent size in the sky al-most the same as that of the Sun. This allows the Moon tocover the Sun nearly precisely in total solar eclipse. Thismatching of apparent visual size is a coincidence. The

Moons linear distance from Earth is currently increasingat a rate of 3.820.07 cm per year, but this rate is notconstant.[9]

The Moon is thought to have formed nearly 4.5 billionyears ago, not long after Earth. Although there havebeen several hypotheses for its origin in the past, the cur-rent most widely accepted explanation is that the Moonformed from the debris left over after a giant impact be-tween Earth and a Mars-sized body.As of November 2014, the Moon is the only celestial bodyother than Earth on which humans have set foot. TheSoviet Union's Luna programme was the rst to reachthe Moon with unmanned spacecraft in 1959; the UnitedStates NASA Apollo program achieved the only mannedmissions to date, beginning with the rst manned lunarorbiting mission by Apollo 8 in 1968, and six mannedlunar landings between 1969 and 1972, with the rst be-ing Apollo 11. These missions returned over 380 kg oflunar rocks, which have been used to develop a geologicalunderstanding of the Moons origin, the formation of itsinternal structure, and its subsequent history.After the Apollo 17 mission in 1972, the Moon has beenvisited by only unmanned spacecraft. Of these, orbitalmissions have dominated: Since 2004, Japan, China, In-dia, the United States, and the European Space Agencyhave each sent lunar orbiters, which have contributedto conrming the discovery of lunar water ice in per-manently shadowed craters at the poles and bound intothe lunar regolith. The post-Apollo era has also seentwo rover missions: the nal Soviet Lunokhod missionin 1973, and Chinas ongoing Chang'e 3 mission, whichdeployed its Yutu rover on 14 December 2013.Future manned missions to the Moon have been planned,including government as well as privately funded eorts.The Moon remains, under the Outer Space Treaty, freeto all nations to explore for peaceful purposes.

1 Name and etymologyThe English proper name for Earths natural satellite isthe Moon.[10][11] The noun moon derives from moone(around 1380), which developed from mone (1135),which derives from Old English mna (dating from be-fore 725), which, like all Germanic language cognates,ultimately stems from Proto-Germanic *mnn.[12]

The principal modern English adjective pertaining to theMoon is lunar, derived from the Latin Luna. Another less

1

2 2 FORMATION

common adjective is selenic, derived from the AncientGreek Selene (), from which the prex seleno-"(as in selenography) is derived.[13][14]

2 FormationMain articles: Origin of the Moon and Giant impacthypothesis

Several mechanisms have been proposed for the Moonsformation 4.527 0.010 billion years ago,[lower-alpha 9]some 3050 million years after the origin of the SolarSystem.[15] Recent research presented by Rick Carlsonindicates a slightly lower age of between 4.40 and 4.45billion years.[16] [17] These mechanisms included the s-sion of the Moon from Earths crust through centrifugalforce[18] (which would require too great an initial spinof Earth),[19] the gravitational capture of a pre-formedMoon[20] (which would require an unfeasibly extendedatmosphere of Earth to dissipate the energy of the passingMoon),[19] and the co-formation of Earth and the Moontogether in the primordial accretion disk (which does notexplain the depletion of metals in the Moon).[19] Thesehypotheses also cannot account for the high angular mo-mentum of the EarthMoon system.[21]

The evolution of the Moon and a tour of the Moon.

The prevailing hypothesis today is that the EarthMoonsystem formed as a result of a giant impact, where a Mars-sized body (named Theia) collided with the newly formedproto-Earth, blasting material into orbit around it thataccreted to form the Moon.[22] This hypothesis perhapsbest explains the evidence, although not perfectly. Eigh-teen months prior to an October 1984 conference on lu-nar origins, Bill Hartmann, Roger Phillips, and Je Tay-lor challenged fellow lunar scientists: You have eighteenmonths. Go back to your Apollo data, go back to yourcomputer, do whatever you have to, but make up yourmind. Dont come to our conference unless you havesomething to say about the Moons birth. At the 1984conference at Kona, Hawaii, the giant impact hypothesisemerged as the most popular.

Before the conference, there were partisansof the three 'traditional' theories, plus a few

people who were starting to take the giant im-pact seriously, and there was a huge apatheticmiddle who didnt think the debate would everbe resolved. Afterward there were essentiallyonly two groups: the giant impact camp andthe agnostics."The Big Splat, or How Our Moon Cameto Be, Dana Mackenzie, John Wiley & Sons,Inc., Hoboken, New Jersey, 2003, pages 166-68.

Giant impacts are thought to have been common in theearly Solar System. Computer simulations modelling agiant impact are consistent with measurements of theangular momentum of the EarthMoon system and thesmall size of the lunar core. These simulations also showthat most of the Moon came from the impactor, notfrom the proto-Earth.[23] However, more-recent tests sug-gest more of the Moon coalesced from Earth and notthe impactor.[24][25][26] Meteorites show that other innerSolar System bodies such as Mars and Vesta have verydierent oxygen and tungsten isotopic compositions toEarth, whereas Earth and the Moon have nearly identicalisotopic compositions. Post-impact mixing of the vapor-ized material between the forming Earth and Moon couldhave equalized their isotopic compositions,[27] althoughthis is debated.[28]

The large amount of energy released in the giant impactevent and the subsequent re-accretion of material in Earthorbit would have melted the outer shell of Earth, forminga magma ocean.[29][30] The newly formed Moon wouldalso have had its own lunar magma ocean; estimates forits depth range from about 500 km to the entire radius ofthe Moon.[29]

Despite its accuracy in explaining many lines of evidence,there are still some diculties that are not fully explainedby the giant impact hypothesis, most of them involvingthe Moons composition.[31]

In 2001, a team at the Carnegie Institute of Washingtonreported the most precise measurement of the isotopicsignatures of lunar rocks.[32] To their surprise, the teamfound that the rocks from the Apollo program carriedan isotopic signature that was identical with rocks fromEarth, and were dierent from almost all other bodiesin the Solar System. Because most of the material thatwent into orbit to form the Moon was thought to comefrom Theia, this observation was unexpected. In 2007,researchers from the California Institute of Technologyannounced that there was less than a 1% chance that Theiaand Earth had identical isotopic signatures.[33] Publishedin 2012, an analysis of titanium isotopes in Apollo lunarsamples showed that the Moon has the same compositionas Earth,[34] which conicts with what is expected if theMoon formed far from Earths orbit or from Theia. Vari-ations on the giant impact hypothesis may explain thisdata.

3.2 Surface geology 3

3 Physical characteristics

3.1 Internal structure

Main article: Internal structure of the MoonThe Moon is a dierentiated body: it has a geochemically

Structure of the Moon

distinct crust, mantle, and core. The Moon has a solidiron-rich inner core with a radius of 240 kilometers anda uid outer core primarily made of liquid iron with aradius of roughly 300 kilometers. Around the core is apartially molten boundary layer with a radius of about500 kilometers.[36] This structure is thought to have de-veloped through the fractional crystallization of a globalmagma ocean shortly after the Moons formation 4.5 bil-lion years ago.[37] Crystallization of this magma oceanwould have created a mac mantle from the precipitationand sinking of the minerals olivine, clinopyroxene, andorthopyroxene; after about three-quarters of the magmaocean had crystallised, lower-density plagioclase mineralscould form and oat into a crust on top.[38] The nal liq-uids to crystallise would have been initially sandwichedbetween the crust and mantle, with a high abundanceof incompatible and heat-producing elements.[1] Consis-tent with this, geochemical mapping from orbit shows thecrust is mostly anorthosite,[7] and moon rock samples ofthe ood lavas erupted on the surface from partial melt-ing in the mantle conrm the mac mantle composition,which is more iron rich than that of Earth.[1] Geophysi-cal techniques suggest that the crust is on average ~50 kmthick.[1]

The Moon is the second densest satellite in the Solar Sys-tem after Io.[39] However, the inner core of the Moon issmall, with a radius of about 350 km or less;[1] this isonly ~20% the size of the Moon, in contrast to the ~50%of most other terrestrial bodies . Its composition is notwell constrained, but it is probably metallic iron alloyedwith a small amount of sulphur and nickel; analyses of the

Moons time-variable rotation indicate that it is at leastpartly molten.[40]

3.2 Surface geologyMain articles: Geology of the Moon and Moon rocksSee also: Topography of the Moon and List of featureson the MoonThe topography of the Moon has been measured with

Topography of the Moon

laser altimetry and stereo image analysis.[41] The mostvisible topographic feature is the giant far-side SouthPoleAitken basin, some 2,240 km in diameter, thelargest crater on the Moon and the largest known craterin the Solar System.[42][43] At 13 km deep, its oor is thelowest point on the surface of the Moon.[42][44] The high-est elevations on the surface of the Moon are located di-rectly to the northeast, and it has been suggested that thisarea might have been thickened by the oblique formationimpact of the South PoleAitken basin.[45] Other largeimpact basins, such as Imbrium, Serenitatis, Crisium,Smythii, and Orientale, also possess regionally low ele-vations and elevated rims.[42] The lunar far side is on av-erage about 1.9 km higher than the near side.[1]

3.2.1 Volcanic features

Main article: Lunar mareThe dark and relatively featureless lunar plains that can

clearly be seen with the naked eye are called maria (Latinfor seas"; singular mare), because they were believed byancient astronomers to be lled with water.[46] They arenow known to be vast solidied pools of ancient basalticlava. Although similar to terrestrial basalts, the marebasalts have much higher abundances of iron and arecompletely lacking in minerals altered by water.[47][48]The majority of these lavas erupted or owed into the de-pressions associated with impact basins. Several geologicprovinces containing shield volcanoes and volcanic domesare found within the near side maria.[49]

Maria are found almost exclusively on the near side of theMoon, covering 31% of the surface on the near side,[50]

4 3 PHYSICAL CHARACTERISTICS

MARE FRIGORISSea of cold

PLATO (crater)

MARE IMBRIUMSea of showers / rain

COPERNICUS (crater)

ARISTARCHUS (crater)

KEPLER (crater)

WEST (on the moon)EAST (in the sky)

OCEANUSPROCELLARUM

Ocean of storms

GRIMALDI (crater)

BYRGIUS (crater)MARE COGNITUM

Sea that has become known

MARE HUMORUMSea of moisture

NORTH

SOUTH

MARE SERENITATIS Sea of serenity

MARE TRANQUILLITATIS Sea of tranquility

MARE CRISIUM Sea of crises

MARE FECUNDITATIS Sea of fecundity/fertility

LANGRENUS (crater)

EAST (on the moon) WEST (in the sky)

MARE NECTARIS Sea of nectar

STEVINUS (crater)MARE VAPORUM Sea of vapours

MARE INSULARUM Sea of islands

MARE NUBIUM Sea of clouds

TYCHO (crater)

Lunar nearside with major maria and craters labeled

Evidence of young lunar volcanism.

compared with a few scattered patches on the far sidecovering only 2%.[51] This is thought to be due to aconcentration of heat-producing elements under the cruston the near side, seen on geochemical maps obtainedby Lunar Prospector's gamma-ray spectrometer, whichwould have caused the underlying mantle to heat up, par-tially melt, rise to the surface and erupt.[38][52][53] Most ofthe Moons mare basalts erupted during the Imbrian pe-riod, 3.03.5 billion years ago, although some radiometri-cally dated samples are as old as 4.2 billion years.[54] Theyoungest eruptions, dated by crater counting, appearedto have been only 1.2 billion years ago.[55] However, in2006 study of Ina, a tiny crater in Lacus Felicitatis, iden-tied jagged, relatively dust-free features that, due to thelack of erosion by infalling debris, appeared to be only 2million years old.[56] Moonquakes and releases of gas alsoindicate some continued lunar activity.[56] In 2014 NASAannounced widespread evidence of young lunar volcan-ism at 70 irregular mare patches identied by the LunarReconnaissance Orbiter, some less than 50 million yearsold, raising the possibility of a much warmer lunar mantle

than previously believed, at least on the near side wherethe deep crust is substantially warmer due to the greaterconcentration of radioactive elements.[57][58][59][60]

The lighter-coloured regions of the Moon are called ter-rae, or more commonly highlands, because they arehigher than most maria. They have been radiometri-cally dated to having formed 4.4 billion years ago, andmay represent plagioclase cumulates of the lunar magmaocean.[54][55] In contrast to Earth, no major lunar moun-tains are believed to have formed as a result of tectonicevents.[61]

The concentration of mare on the Near Side likely reectsthe substantially thicker crust of the highlands of the FarSide, which may have formed in a slow-velocity impactof a second terran moon a few tens of millions of yearsafter their formation.[62][63]

3.2.2 Impact craters

See also: List of craters on the MoonThe other major geologic process that has aected

Lunar crater Daedalus on the Moons far side

the Moons surface is impact cratering,[64] with cratersformed when asteroids and comets collide with the lu-nar surface. There are estimated to be roughly 300,000craters wider than 1 km on the Moons near side alone.[65]Some of these are named for scholars, scientists, artistsand explorers.[66] The lunar geologic timescale is basedon the most prominent impact events, including Nectaris,Imbrium, and Orientale, structures characterized by mul-tiple rings of uplifted material, typically hundreds tothousands of kilometres in diameter and associated witha broad apron of ejecta deposits that form a regionalstratigraphic horizon.[67] The lack of an atmosphere,weather and recent geological processes mean that manyof these craters are well-preserved. Although only a fewmulti-ring basins have been denitively dated, they are

3.3 Gravitational eld 5

useful for assigning relative ages. Because impact cratersaccumulate at a nearly constant rate, counting the num-ber of craters per unit area can be used to estimate theage of the surface.[67] The radiometric ages of impact-melted rocks collected during the Apollo missions clusterbetween 3.8 and 4.1 billion years old: this has been usedto propose a Late Heavy Bombardment of impacts.[68]

Blanketed on top of the Moons crust is a highlycomminuted (broken into ever smaller particles) andimpact gardened surface layer called regolith, formed byimpact processes. The ner regolith, the lunar soil ofsilicon dioxide glass, has a texture like snow and smelllike spent gunpowder.[69] The regolith of older surfacesis generally thicker than for younger surfaces: it varies inthickness from 1020 m in the highlands and 35 m in themaria.[70] Beneath the nely comminuted regolith layeris the megaregolith, a layer of highly fractured bedrockmany kilometres thick.[71]

3.2.3 Presence of water

Main article: Lunar water

Liquid water cannot persist on the lunar surface. Whenexposed to solar radiation, water quickly decomposesthrough a process known as photodissociation and is lostto space. However, since the 1960s, scientists havehypothesized that water ice may be deposited by im-pacting comets or possibly produced by the reaction ofoxygen-rich lunar rocks, and hydrogen from solar wind,leaving traces of water which could possibly survive incold, permanently shadowed craters at either pole onthe Moon.[72][73] Computer simulations suggest that upto 14,000 km2 of the surface may be in permanentshadow.[74] The presence of usable quantities of water onthe Moon is an important factor in rendering lunar habita-tion as a cost-eective plan; the alternative of transportingwater from Earth would be prohibitively expensive.[75]

In years since, signatures of water have been found to ex-ist on the lunar surface.[76] In 1994, the bistatic radar ex-periment located on the Clementine spacecraft, indicatedthe existence of small, frozen pockets of water close to thesurface. However, later radar observations by Arecibo,suggest these ndings may rather be rocks ejected fromyoung impact craters.[77] In 1998, the neutron spectrom-eter located on the Lunar Prospector spacecraft, indi-cated that high concentrations of hydrogen are presentin the rst meter of depth in the regolith near the polarregions.[78] In 2008, an analysis of volcanic lava beads,brought back to Earth aboard Apollo 15, showed smallamounts of water to exist in the interior of the beads.[79]

The 2008 Chandrayaan-1 spacecraft has since conrmedthe existence of surface water ice, using the on-boardMoon Mineralogy Mapper. The spectrometer observedabsorption lines common to hydroxyl, in reected sun-light, providing evidence of large quantities of water ice,

on the lunar surface. The spacecraft showed that con-centrations may possibly be as high as 1,000 ppm.[80] In2009, LCROSS sent a 2300 kg impactor into a perma-nently shadowed polar crater, and detected at least 100kg of water in a plume of ejected material.[81][82] Anotherexamination of the LCROSS data showed the amountof detected water, to be closer to 155 kilograms ( 12kg).[83]

In May 2011, Erik Hauri et al. reported[84] 6151410ppm water in melt inclusions in lunar sample 74220, thefamous high-titanium orange glass soil of volcanic ori-gin collected during the Apollo 17 mission in 1972. Theinclusions were formed during explosive eruptions on theMoon approximately 3.7 billion years ago. This concen-tration is comparable with that of magma in Earths uppermantle. Although of considerable selenological interest,Hauris announcement aords little comfort to would-belunar coloniststhe sample originated many kilometersbelow the surface, and the inclusions are so dicult toaccess that it took 39 years to nd them with a state-of-the-art ion microprobe instrument.

3.3 Gravitational eld

Main article: Gravity of the Moon

The gravitational eld of the Moon has been measuredthrough tracking the Doppler shift of radio signals emit-ted by orbiting spacecraft. The main lunar gravity fea-tures are mascons, large positive gravitational anomaliesassociated with some of the giant impact basins, partlycaused by the dense mare basaltic lava ows that ll thesebasins.[85][86] These anomalies greatly inuence the orbitof spacecraft about the Moon. There are some puzzles:lava ows by themselves cannot explain all of the grav-itational signature, and some mascons exist that are notlinked to mare volcanism.[87]

3.4 Magnetic eld

Main article: Magnetic eld of the Moon

The Moon has an external magnetic eld of about 1100 nanoteslas, less than one-hundredth that of Earth.It does not currently have a global dipolar magneticeld, as would be generated by a liquid metal coregeodynamo, and only has crustal magnetization, proba-bly acquired early in lunar history when a geodynamo wasstill operating.[88][89] Alternatively, some of the remnantmagnetization may be from transient magnetic elds gen-erated during large impact events, through the expansionof an impact-generated plasma cloud in the presence ofan ambient magnetic eldthis is supported by the ap-parent location of the largest crustal magnetizations nearthe antipodes of the giant impact basins.[90]

6 4 RELATIONSHIP TO EARTH

3.5 Atmosphere

At sunrise and sunset many Apollo crews saw glows and lightrays.[91]

Main article: Atmosphere of the Moon

The Moon has an atmosphere so tenuous as to be nearlyvacuum, with a total mass of less than 10 metric tons.[92]The surface pressure of this small mass is around 3 1015 atm (0.3 nPa); it varies with the lunar day. Itssources include outgassing and sputtering, the releaseof atoms from the bombardment of lunar soil by solarwind ions.[7][93] Elements that have been detected in-clude sodium and potassium, produced by sputtering,which are also found in the atmospheres of Mercury andIo; helium-4 from the solar wind; and argon-40, radon-222, and polonium-210, outgassed after their creationby radioactive decay within the crust and mantle.[94][95]The absence of such neutral species (atoms or molecules)as oxygen, nitrogen, carbon, hydrogen and magnesium,which are present in the regolith, is not understood.[94]Water vapour has been detected by Chandrayaan-1 andfound to vary with latitude, with a maximum at ~6070degrees; it is possibly generated from the sublimation ofwater ice in the regolith.[96] These gases can either returninto the regolith due to the Moons gravity or be lost tospace, either through solar radiation pressure or, if theyare ionized, by being swept away by the solar winds mag-netic eld.[94]

3.6 SeasonsThe Moons axial tilt with respect to the ecliptic is only1.5424,[97] much less than the 23.44 of Earth. Becauseof this, the Moons solar illumination varies much lesswith season, and topographical details play a crucial rolein seasonal eects.[98] From images taken by Clementinein 1994, it appears that four mountainous regions on therim of Peary crater at the Moons north pole may remainilluminated for the entire lunar day, creating peaks ofeternal light. No such regions exist at the south pole. Sim-ilarly, there are places that remain in permanent shadowat the bottoms of many polar craters,[74] and these darkcraters are extremely cold: Lunar Reconnaissance Orbitermeasured the lowest summer temperatures in craters atthe southern pole at 35 K (238 C)[99] and just 26 Kclose to the winter solstice in north polar Hermite Crater.This is the coldest temperature in the Solar System evermeasured by a spacecraft, colder even than the surface ofPluto.[98]

4 Relationship to Earth

Schematic of the EarthMoon system (without a consistent scale)

4.1 OrbitMain articles: Orbit of the Moon and Lunar theory

The Moon makes a complete orbit around Earthwith respect to the xed stars about once every 27.3days[lower-alpha 10] (its sidereal period). However, becauseEarth is moving in its orbit around the Sun at the sametime, it takes slightly longer for the Moon to show thesame phase to Earth, which is about 29.5 days[lower-alpha 11](its synodic period).[50] Unlike most satellites of otherplanets, the Moon orbits closer to the ecliptic plane thanto the planets equatorial plane. The Moons orbit is sub-tly perturbed by the Sun and Earth in many small, com-plex and interacting ways. For example, the plane of theMoons orbital motion gradually rotates, which aectsother aspects of lunar motion. These follow-on eectsare mathematically described by Cassinis laws.[100]

4.3 Appearance from Earth 7

Earth and Moon, showing their sizes and distance to scale. Theyellow bar represents a pulse of light traveling from Earth toMoon (approx. 400,000 km or 250,000 mi) in 1.26 seconds.

4.2 Relative sizeThe Moon is exceptionally large relative to Earth: a quar-ter the diameter of the planet and 1/81 its mass.[50] It isthe largest moon in the Solar System relative to the sizeof its planet, though Charon is larger relative to the dwarfplanet Pluto, at 1/9 Plutos mass.[101]

However, Earth and the Moon are still considered aplanetsatellite system, rather than a double-planet sys-tem, because their barycentre, the common centre ofmass, is located 1,700 km (about a quarter of Earths ra-dius) beneath Earths surface.[102]

4.3 Appearance from Earth

Moon setting in western sky over High Desert (California)

See also: Lunar phase, Earthshine and Observing theMoon

The Moon is in synchronous rotation: it rotates about itsaxis in about the same time it takes to orbit Earth. Thisresults in it nearly always keeping the same face turned to-wards Earth. The Moon used to rotate at a faster rate, butearly in its history, its rotation slowed and became tidallylocked in this orientation as a result of frictional eectsassociated with tidal deformations caused by Earth.[103]The side of the Moon that faces Earth is called the nearside, and the opposite side the far side. The far side isoften inaccurately called the dark side, but in fact, it isilluminated as often as the near side: once per lunar day,during the new moon phase we observe on Earth whenthe near side is dark.[104]

The Moon has an exceptionally low albedo, giving it a re-ectance that is slightly brighter than that of worn asphalt.Despite this, it is the brightest object in the sky after theSun.[50][lower-alpha 12] This is partly due to the brightnessenhancement of the opposition eect; at quarter phase,

the Moon is only one-tenth as bright, rather than half asbright, as at full moon.[105]

Additionally, colour constancy in the visual system recal-ibrates the relations between the colours of an object andits surroundings, and because the surrounding sky is com-paratively dark, the sunlit Moon is perceived as a brightobject. The edges of the full moon seem as bright as thecentre, with no limb darkening, due to the reective prop-erties of lunar soil, which reects more light back towardsthe Sun than in other directions. The Moon does appearlarger when close to the horizon, but this is a purely psy-chological eect, known as the Moon illusion, rst de-scribed in the 7th century BC.[106] The full moon subtendsan arc of about 0.52 (on average) in the sky, roughly thesame apparent size as the Sun (see eclipses).

The monthly changes of angle between the direction ofillumination by the Sun and viewing from Earth, and thephases of the Moon that result

The highest altitude of the Moon in the sky varies: al-though it has nearly the same limit as the Sun, it alters withthe lunar phase and with the season of the year, with thefull moon highest during winter. The 18.6-year nodes cy-cle also has an inuence: when the ascending node of thelunar orbit is in the vernal equinox, the lunar declinationcan go as far as 28 each month. This means the Mooncan go overhead at latitudes up to 28 from the equator,instead of only 18. The orientation of the Moons cres-cent also depends on the latitude of the observation site:close to the equator, an observer can see a smile-shapedcrescent Moon.[107]

The distance between the Moon and Earth varies fromaround 356,400 km to 406,700 km at the extremeperigees (closest) and apogees (farthest). On 19 March2011, it was closer to Earth when at full phase than it

8 4 RELATIONSHIP TO EARTH

has been since 1993, 14% closer than its farthest positionin apogee.[108] Reported as a "super moon", this closestpoint coincides within an hour of a full moon, and it was30% more luminous than when at its greatest distancedue to its angular diameter being 14% greater, because1:1421:30 .[109][110][111] At lower levels, the human per-ception of reduced brightness as a percentage is providedby the following formula:[112][113]

reduction perceived% = 100q

reduction actual%100

When the actual reduction is 1.00 / 1.30, or about 0.770,the perceived reduction is about 0.877, or 1.00 / 1.14.This gives a maximum perceived increase of 14% be-tween apogee and perigee moons of the same phase.[114]

There has been historical controversy over whether fea-tures on the Moons surface change over time. Today,many of these claims are thought to be illusory, result-ing from observation under dierent lighting conditions,poor astronomical seeing, or inadequate drawings. How-ever, outgassing does occasionally occur, and could beresponsible for a minor percentage of the reported lunartransient phenomena. Recently, it has been suggestedthat a roughly 3 km diameter region of the lunar surfacewas modied by a gas release event about a million yearsago.[115][116] The Moons appearance, like that of the Sun,can be aected by Earths atmosphere: common eectsare a 22 halo ring formed when the Moons light is re-fracted through the ice crystals of high cirrostratus cloud,and smaller coronal rings when the Moon is seen throughthin clouds.[117]

4.4 Tidal eects

Main articles: Tidal force, Tidal acceleration, Tide andTheory of tides

The tides on Earth are mostly generated by the gradientin intensity of the Moons gravitational pull from one sideof Earth to the other, the tidal forces. This forms twotidal bulges on Earth, which are most clearly seen in ele-vated sea level as ocean tides.[118] Because Earth spinsabout 27 times faster than the Moon moves around it,the bulges are dragged along with Earths surface fasterthan the Moon moves, rotating around Earth once a dayas it spins on its axis.[118] The ocean tides are magniedby other eects: frictional coupling of water to Earthsrotation through the ocean oors, the inertia of watersmovement, ocean basins that get shallower near land, andoscillations between dierent ocean basins.[119] The grav-itational attraction of the Sun on Earths oceans is almosthalf that of the Moon, and their gravitational interplay isresponsible for spring and neap tides.[118]

Gravitational coupling between the Moon and the bulgenearest the Moon acts as a torque on Earths rotation,draining angular momentum and rotational kinetic energyfrom Earths spin.[118][120] In turn, angular momentum is

The libration of the Moon over a single lunar month. Also visibleis the slight variation in the moons visual size from Earth.

added to the Moons orbit, accelerating it, which lifts theMoon into a higher orbit with a longer period. As a result,the distance between Earth and Moon is increasing, andEarths spin slowing down.[120] Measurements from lunarranging experiments with laser reectors left during theApollo missions have found that the Moons distance toEarth increases by 38 mm per year[121] (though this is only0.10 ppb/year of the radius of the Moons orbit). Atomicclocks also show that Earths day lengthens by about 15microseconds every year,[122] slowly increasing the rateat which UTC is adjusted by leap seconds. Left to runits course, this tidal drag would continue until the spin ofEarth and the orbital period of the Moon matched, cre-ating mutual tidal locking between the two, as is alreadycurrently the case with Pluto and its moon Charon. How-ever, the Sun will become a red giant long before that,engulng Earth.[123][124]

The lunar surface also experiences tides of amplitude ~10cm over 27 days, with two components: a xed one dueto Earth, because they are in synchronous rotation, and avarying component from the Sun.[120] The Earth-inducedcomponent arises from libration, a result of the Moonsorbital eccentricity; if the Moons orbit were perfectly cir-cular, there would only be solar tides.[120] Libration alsochanges the angle from which the Moon is seen, allow-ing about 59% of its surface to be seen from Earth (butonly half at any instant).[50] The cumulative eects ofstress built up by these tidal forces produces moonquakes.Moonquakes are much less common and weaker thanearthquakes, although they can last for up to an houra signicantly longer time than terrestrial earthquakesbecause of the absence of water to damp out the seismicvibrations. The existence of moonquakes was an unex-pected discovery from seismometers placed on the Moonby Apollo astronauts from 1969 through 1972.[125]

94.5 Eclipses

Main articles: Solar eclipse, Lunar eclipse and Eclipsecycle

The 1999 solar eclipse

The Moon passing in front of the Sun, from theSTEREO-B spacecraft.[126]From Earth, the Moon and Sun appear the same size.From a satellite in an Earth-trailing orbit, the Moon mayappear smaller than the Sun.

Eclipses can only occur when the Sun, Earth, and Moonare all in a straight line (termed "syzygy"). Solar eclipsesoccur at new moon, when the Moon is between the Sunand Earth. In contrast, lunar eclipses occur at full moon,when Earth is between the Sun and Moon. The appar-ent size of the Moon is roughly the same as that of theSun, with both being viewed at close to one-half a de-gree wide. The Sun is much larger than the Moon but itis the precise vastly greater distance that coincidentallygives it the same apparent size as the much closer andmuch smaller Moon from the perspective of Earth. Thevariations in apparent size, due to the non-circular orbits,are nearly the same as well, though occurring in dierentcycles. This makes possible both total (with the Moon ap-pearing larger than the Sun) and annular (with the Moonappearing smaller than the Sun) solar eclipses.[127] In atotal eclipse, the Moon completely covers the disc of theSun and the solar corona becomes visible to the nakedeye. Because the distance between the Moon and Earthis very slowly increasing over time,[118] the angular diam-eter of the Moon is decreasing. Also, as it evolves towardbecoming a red giant, the size of the Sun, and its appar-ent diameter in the sky, are slowly increasing.[lower-alpha 13]The combination of these two changes means that hun-dreds of millions of years ago, the Moon would alwayscompletely cover the Sun on solar eclipses, and no annulareclipses were possible. Likewise, hundreds of millions of

years in the future, the Moon will no longer cover the Suncompletely, and total solar eclipses will not occur.[128]

Because the Moons orbit around Earth is inclined byabout 5 to the orbit of Earth around the Sun, eclipsesdo not occur at every full and new moon. For an eclipseto occur, the Moon must be near the intersection of thetwo orbital planes.[129] The periodicity and recurrence ofeclipses of the Sun by the Moon, and of the Moon byEarth, is described by the saros, which has a period ofapproximately 18 years.[130]

As the Moon is continuously blocking our view of a half-degree-wide circular area of the sky,[lower-alpha 14][131] therelated phenomenon of occultation occurs when a brightstar or planet passes behind the Moon and is occulted:hidden from view. In this way, a solar eclipse is an oc-cultation of the Sun. Because the Moon is comparativelyclose to Earth, occultations of individual stars are not vis-ible everywhere on the planet, nor at the same time. Be-cause of the precession of the lunar orbit, each year dif-ferent stars are occulted.[132]

5 Study and explorationSee also: Robotic exploration of the Moon, List of pro-posed missions to the Moon, Colonization of the Moonand List of man-made objects on the Moon

Map of the Moon by Johannes Hevelius from his Selenographia(1647), the rst map to include the libration zones.

5.1 Early studies

Main articles: Exploration of the Moon: Early history,Selenography and Lunar theory

Understanding of the Moons cycles was an early devel-

10 5 STUDY AND EXPLORATION

opment of astronomy: by the 5th century BC, Babylonianastronomers had recorded the 18-year Saros cycle oflunar eclipses,[133] and Indian astronomers had describedthe Moons monthly elongation.[134] The Chinese as-tronomer Shi Shen (. 4th century BC) gave instruc-tions for predicting solar and lunar eclipses.[135] Later, thephysical form of the Moon and the cause of moonlightbecame understood. The ancient Greek philosopherAnaxagoras (d. 428 BC) reasoned that the Sun and Moonwere both giant spherical rocks, and that the latter re-ected the light of the former.[136][137] Although the Chi-nese of the Han Dynasty believed the Moon to be energyequated to qi, their 'radiating inuence' theory also recog-nized that the light of the Moon was merely a reection ofthe Sun, and Jing Fang (7837 BC) noted the sphericityof the Moon.[138] In 2nd century AD Lucian wrote a novelwhere the heroes travel to the Moon, which is inhabited.In 499 AD, the Indian astronomer Aryabhata mentionedin his Aryabhatiya that reected sunlight is the cause ofthe shining of the Moon.[139] The astronomer and physi-cist Alhazen (9651039) found that sunlight was not re-ected from the Moon like a mirror, but that light wasemitted from every part of the Moons sunlit surface in alldirections.[140] Shen Kuo (10311095) of the Song Dy-nasty created an allegory equating the waxing and wan-ing of the Moon to a round ball of reective silver that,when doused with white powder and viewed from theside, would appear to be a crescent.[141]

In Aristotles (384322 BC) description of the universe,the Moon marked the boundary between the spheres ofthe mutable elements (earth, water, air and re), and theimperishable stars of aether, an inuential philosophy thatwould dominate for centuries.[142] However, in the 2ndcentury BC, Seleucus of Seleucia correctly theorized thattides were due to the attraction of the Moon, and thattheir height depends on the Moons position relative tothe Sun.[143] In the same century, Aristarchus computedthe size and distance of the Moon from Earth, obtaininga value of about twenty times the radius of Earth for thedistance. These gures were greatly improved by Ptolemy(90168 AD): his values of a mean distance of 59 timesEarths radius and a diameter of 0.292 Earth diameterswere close to the correct values of about 60 and 0.273respectively.[144] Archimedes (287212 BC) designed aplanetarium that could calculate the motions of the Moonand other objects in the Solar System.[145]

During the Middle Ages, before the invention of the tele-scope, the Moon was increasingly recognised as a sphere,though many believed that it was perfectly smooth.[146]In 1609, Galileo Galilei drew one of the rst telescopicdrawings of the Moon in his book Sidereus Nuncius andnoted that it was not smooth but had mountains andcraters. Telescopic mapping of the Moon followed: laterin the 17th century, the eorts of Giovanni Battista Ric-cioli and Francesco Maria Grimaldi led to the system ofnaming of lunar features in use today. The more exact183436 Mappa Selenographica of Wilhelm Beer and

Johann Heinrich Mdler, and their associated 1837 bookDer Mond, the rst trigonometrically accurate study oflunar features, included the heights of more than a thou-sand mountains, and introduced the study of the Moonat accuracies possible in earthly geography.[147] Lunarcraters, rst noted by Galileo, were thought to be volcanicuntil the 1870s proposal of Richard Proctor that theywere formed by collisions.[50] This view gained support in1892 from the experimentation of geologist Grove KarlGilbert, and from comparative studies from 1920 to the1940s,[148] leading to the development of lunar stratigra-phy, which by the 1950s was becoming a new and grow-ing branch of astrogeology.[50]

5.2 First direct exploration: 19591976

5.2.1 Soviet missions

Soviet moonrover

Main articles: Luna program and Lunokhod programme

The Cold War-inspired Space Race between the SovietUnion and the U.S. led to an acceleration of interest inexploration of the Moon. Once launchers had the neces-sary capabilities, these nations sent unmanned probes onboth yby and impact/lander missions. Spacecraft fromthe Soviet Unions Luna program were the rst to accom-plish a number of goals: following three unnamed, failedmissions in 1958,[149] the rst man-made object to es-cape Earths gravity and pass near the Moon was Luna1; the rst man-made object to impact the lunar surfacewas Luna 2, and the rst photographs of the normally oc-cluded far side of the Moon were made by Luna 3, all in1959.The rst spacecraft to perform a successful lunar softlanding was Luna 9 and the rst unmanned vehicle to or-bit the Moon was Luna 10, both in 1966.[50] Rock and soilsamples were brought back to Earth by three Luna samplereturn missions (Luna 16 in 1970, Luna 20 in 1972, andLuna 24 in 1976), which returned 0.3 kg total.[150] Twopioneering robotic rovers landed on the Moon in 1970

5.3 Current era: 1990present 11

and 1973 as a part of Soviet Lunokhod programme.

5.2.2 United States missions

Main articles: Apollo program, Moon landing andProject HorizonThe United States launched unmanned probes to de-

Earth as viewed from Lunar orbit during the Apollo 8 mission(Christmas Eve, 1968).

velop an understanding of the lunar surface for an even-tual manned landing: the Jet Propulsion Laboratory'sSurveyor program landed its rst spacecraft four monthsafter Luna 9. NASA's manned Apollo program wasdeveloped in parallel; after a series of unmanned andmanned tests of the Apollo spacecraft in Earth orbit,and spurred on by a potential Soviet lunar ight, in 1968Apollo 8 made the rst crewed mission to lunar orbit. Thesubsequent landing of the rst humans on the Moon in1969 is seen by many as the culmination of the SpaceRace.[151]

Neil Armstrong became the rst person to walk on theMoon as the commander of the American mission Apollo11 by rst setting foot on the Moon at 02:56 UTC on21 July 1969.[152] The Apollo missions 11 to 17 (ex-cept Apollo 13, which aborted its planned lunar landing)returned 382 kg of lunar rock and soil in 2,196 sepa-rate samples.[153] The American Moon landing and re-turn was enabled by considerable technological advancesin the early 1960s, in domains such as ablation chemistry,software engineering and atmospheric re-entry technol-ogy, and by highly competent management of the enor-mous technical undertaking.[154][155]

Scientic instrument packages were installed on the lu-nar surface during all the Apollo landings. Long-lived instrument stations, including heat ow probes,seismometers, and magnetometers, were installed at theApollo 12, 14, 15, 16, and 17 landing sites. Direct

Neil Armstrong and the ag of the United States.

transmission of data to Earth concluded in late 1977 dueto budgetary considerations,[156][157] but as the stationslunar laser ranging corner-cube retroreector arrays arepassive instruments, they are still being used. Rangingto the stations is routinely performed from Earth-basedstations with an accuracy of a few centimetres, and datafrom this experiment are being used to place constraintson the size of the lunar core.[158]

According to national security documents declassied inJuly 2014, the United States government had planned inthe 1960s to construct a manned military outpost on themoon, which would have been home to a bombing systemtargeted at earthbound rivals. Part of Project Horizon,the plans also included conducting a lunar-based nucleartest, whose primary purpose would have been to demon-strate American military power to the world. However,the project was ultimately called o by Air Force leaders,who deemed it too perilous.[159]

5.3 Current era: 1990presentPost-Apollo and Luna, many more countries have be-come involved in direct exploration of the Moon. In1990, Japan became the third country to place a space-craft into lunar orbit with its Hiten spacecraft. The space-craft released a smaller probe, Hagoromo, in lunar or-bit, but the transmitter failed, preventing further scien-tic use of the mission.[160] In 1994, the U.S. sent thejoint Defense Department/NASA spacecraft Clementineto lunar orbit. This mission obtained the rst near-global topographic map of the Moon, and the rst globalmultispectral images of the lunar surface.[161] This wasfollowed in 1998 by the Lunar Prospector mission, whoseinstruments indicated the presence of excess hydrogen atthe lunar poles, which is likely to have been caused bythe presence of water ice in the upper few meters of the

12 7 LEGAL STATUS

regolith within permanently shadowed craters.[162]

The European spacecraft SMART-1, the second ion-propelled spacecraft, was in lunar orbit from 15 Novem-ber 2004 until its lunar impact on 3 September 2006, andmade the rst detailed survey of chemical elements on thelunar surface.[163]

China has pursued an ambitious program of lunar ex-ploration, beginning with Chang'e 1, which successfullyorbited the Moon from 5 November 2007 until its con-trolled lunar impact on 1 March 2009.[164] In its sixteen-month mission, it obtained a full image map of the Moon.China followed up this success with Chang'e 2 beginningin October 2010, which reached the Moon over twice asfast as Chang'e 1, mapped the Moon at a higher resolutionover an eight month period, then left lunar orbit in favor ofan extended stay at the EarthSun L2 Lagrangian point,before nally performing a yby of asteroid 4179 Toutatison 13 December 2012, and then heading o into deepspace. On 14 December 2013, Chang'e 3 improved uponits orbital mission predecessors by landing a lunar landeronto the Moons surface, which in turn deployed a lunarrover, named Yutu (Chinese: ; literally Jade Rabbit).In so doing, Chang'e 3 made the rst lunar soft landingsince Luna 24 in 1976, and the rst lunar rover missionsince Lunokhod 2 in 1973. China intends to launch an-other rover mission (Chang'e 4) in 2015, followed by asample return mission (Chang'e 5) in 2017.Between 4 October 2007 and 10 June 2009, the JapanAerospace Exploration Agency's Kaguya (Selene) mis-sion, a lunar orbiter tted with a high-denition videocamera, and two small radio-transmitter satellites, ob-tained lunar geophysics data and took the rst high-denition movies from beyond Earth orbit.[165][166] In-dias rst lunar mission, Chandrayaan I, orbited from 8November 2008 until loss of contact on 27 August 2009,creating a high resolution chemical, mineralogical andphoto-geological map of the lunar surface, and conrm-ing the presence of water molecules in lunar soil.[167] TheIndian Space Research Organisation planned to launchChandrayaan II in 2013, which would have included aRussian robotic lunar rover.[168][169] However, the failureof Russias Fobos-Grunt mission has delayed this project.The U.S. co-launched the Lunar Reconnaissance Orbiter(LRO) and the LCROSS impactor and follow-up observa-tion orbiter on 18 June 2009; LCROSS completed its mis-sion by making a planned and widely observed impact inthe crater Cabeus on 9 October 2009,[170] whereas LROis currently in operation, obtaining precise lunar altimetryand high-resolution imagery. In November 2011, theLRO passed over the Aristarchus crater, which spans40 kilometres and sinks more than 3.5 kilometres deep.The crater is one of the most visible ones from Earth.The Aristarchus plateau is one of the most geologicallydiverse places on the Moon: a mysterious raised atplateau, a giant rille carved by enormous outpourings oflava, elds of explosive volcanic ash, and all surroundedby massive ood basalts, said Mark Robinson, principal

investigator of the Lunar Reconnaissance Orbiter Cam-era at Arizona State University. NASA released photosof the crater on 25 December 2011.[171]

Two NASA GRAIL spacecraft begin orbiting the Moonaround 1 January 2012,[172] on a mission to learn moreabout the Moons internal structure. NASAs LADEEprobe, designed to study the lunar exosphere, achievedorbit on 6 October 2013.Other upcoming lunar missions include Russias Luna-Glob: an unmanned lander, set of seismometers, andan orbiter based on its failed Martian Fobos-Gruntmission.[173][174] Privately funded lunar exploration hasbeen promoted by the Google Lunar X Prize, announced13 September 2007, which oers US$20 million to any-one who can land a robotic rover on the Moon and meetother specied criteria.[175] Shackleton Energy Companyis building a program to establish operations on the southpole of the Moon to harvest water and supply theirPropellant Depots.[176]

NASA began to plan to resume manned missions follow-ing the call by U.S. President George W. Bush on 14 Jan-uary 2004 for a manned mission to the Moon by 2019and the construction of a lunar base by 2024.[177] TheConstellation program was funded and construction andtesting begun on a manned spacecraft and launch vehi-cle,[178] and design studies for a lunar base.[179] However,that program has been cancelled in favor of a mannedasteroid landing by 2025 and a manned Mars orbit by2035.[180] India has also expressed its hope to send amanned mission to the Moon by 2020.[181]

6 Astronomy from the MoonFor many years, the Moon has been recognized as anexcellent site for telescopes.[182] It is relatively nearby;astronomical seeing is not a concern; certain craters nearthe poles are permanently dark and cold, and thus espe-cially useful for infrared telescopes; and radio telescopeson the far side would be shielded from the radio chatter ofEarth.[183] The lunar soil, although it poses a problem forany moving parts of telescopes, can be mixed with carbonnanotubes and epoxies in the construction of mirrors upto 50 meters in diameter.[184] A lunar zenith telescope canbe made cheaply with ionic liquid.[185]

In April 1972, the Apollo 16 mission recorded variousastronomical photos and spectra in ultraviolet with the FarUltraviolet Camera/Spectrograph.[186]

7 Legal statusMain article: Space law

Although Luna landers scattered pennants of the Soviet

13

Union on the Moon, and U.S. ags were symbolicallyplanted at their landing sites by the Apollo astronauts,no nation currently claims ownership of any part of theMoons surface.[187] Russia and the U.S. are party to the1967 Outer Space Treaty,[188] which denes the Moonand all outer space as the "province of all mankind".[187]This treaty also restricts the use of the Moon to peace-ful purposes, explicitly banning military installations andweapons of mass destruction.[189] The 1979 Moon Agree-ment was created to restrict the exploitation of the Moonsresources by any single nation, but as of 2014, it has beensigned and ratied by only 16 nations. It has not beenratied by any state that engages in self-launched humanspace exploration or has plans to do so.[190] Although sev-eral individuals have made claims to the Moon in whole orin part, none of these are considered credible.[191][192][193]

8 In cultureFurther information: Moon in ction, Lunar calendar,Metonic cycle, Lunar deity, Lunar eect and Blue moonThe Moons regular phases make it a very convenient

Luna, the Moon, from a 1550 edition of Guido Bonatti's Liberastronomiae.

timepiece, and the periods of its waxing and waning formthe basis of many of the oldest calendars. Tally sticks,notched bones dating as far back as 2030,000 yearsago, are believed by some to mark the phases of theMoon.[194][195][196] The ~30-day month is an approxima-tion of the lunar cycle. The English noun month and itscognates in other Germanic languages stem from Proto-Germanic *mnth-, which is connected to the above-mentioned Proto-Germanic *mnn, indicating the us-age of a lunar calendar among the Germanic peoples(Germanic calendar) prior to the adoption of a solar cal-endar.[197] The PIE root of moon, *mh1nt, derives fromthe PIE verbal root *meh1-, to measure, indicat[ing]a functional conception of the moon, i.e. marker ofthe month (cf. the English words measure and men-strual),[198][199][200] and echoing the Moons importanceto many ancient cultures in measuring time (see Latinmensis and Ancient Greek (meis) or (mn),

meaning month).[201][202][203][204]

A crescentMoon and star (here the planet Venus) are a commonsymbol of Islam, appearing in ags like: (Turkey), (Algeria) and(Pakistan).

The Moon has been the subject of many works of art andliterature and the inspiration for countless others. It is amotif in the visual arts, the performing arts, poetry, proseand music. A 5,000-year-old rock carving at Knowth,Ireland, may represent the Moon, which would be the ear-liest depiction discovered.[205] The contrast between thebrighter highlands and the darker maria creates the pat-terns seen by dierent cultures as the Man in the Moon,the rabbit and the bualo, among others. In many prehis-toric and ancient cultures, the Moon was personied as adeity or other supernatural phenomenon, and astrologicalviews of the Moon continue to be propagated today.The Moon plays an important role in Islam; the Islamiccalendar is strictly lunar, and in many Muslim countriesthe months are determined by the visual sighting of thehilal, or earliest crescent moon, over the horizon.[206]The star and crescent, initially a symbol of the OttomanEmpire, has recently been adopted as a wider symbolfor the Muslim community. The splitting of the moon(Arabic: ) was a miracle attributed to theprophet Muhammad.[207]

The Moon has a long association with insanity and irra-tionality; the words lunacy and lunatic (popular shorten-ing loony) are derived from the Latin name for the Moon,Luna. Philosophers Aristotle and Pliny the Elder arguedthat the full moon induced insanity in susceptible individ-uals, believing that the brain, which is mostly water, mustbe aected by the Moon and its power over the tides,but the Moons gravity is too slight to aect any singleperson.[208] Even today, people insist that admissions topsychiatric hospitals, trac accidents, homicides or sui-cides increase during a full moon, although there is noscientic evidence to support such claims.[208]

9 See also

Former classication of planets

14 10 REFERENCES

Moon illusion Other moons of Earth 2006 RH120

10 References

10.1 Notes[1] Between 18.29 and 28.58 to Earths equator.[1]

s of t [lower-alpha 2] [lower-alpha 3] [lower-alpha 4] [lower-alpha 5][lower-alpha 6] [lower-alpha 7] [lower-alpha 9] [lower-alpha 10] [lower-alpha 11][lower-alpha 12] [lower-alpha 14] }{{efn|name=near-Earth aster-oids}}

10.2 Citations 15

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The Moon. Discovery 2008. BBC World Service. Bussey, B.; Spudis, P.D. (2004). The Clementine At-

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Cain, Fraser. Where does the Moon ComeFrom?". Universe Today. Retrieved 1 April 2008.(podcast and transcript)

Jolli, B. (2006). Wieczorek, M.; Shearer, C.;Neal, C., eds. New views of the Moon. Rev. Min-eral. Geochem. 60 (1) (Chantilly, Virginia: Min.Soc. Amer.). p. 721. doi:10.2138/rmg.2006.60.0.ISBN 0-939950-72-3. Retrieved 12 April 2007.

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