The Muin is thocht tae hae formed aboot 4.51 billion year ago,nae lang efter Yird. The maist widely acceptit explanation is that the Muin formed frae the debris left ower efter agiant impact atween Yird an aMaurs-sized bouk criedTheia.
The Muin is insynchronous rotation wi Yird, ayeweys shawin the same face, wi itsnear side merked bi daurk volcanicmaria that fill the spaces atween the bricht auncient crustal hielands an the prominentimpact craters. As seen frae the Yird, it is the seicont-brichtest regularly veesiblecelestial object in Yird's sky, efter the Sun. Its surface is actually daurk, awtho compared tae the nicht sky it appears verra bricht, wi areflectance juist slichtly heicher nor that o wornasphalt. Its gravitational influence produces theocean tides,bouk tides, an theslicht lenthenin o the day.
The Muin's currentorbital distance is384,400km (238,900mi),[10][11] or 1.28 licht-seiconts. This is aboot thirty times the diameter o Yird, wi itsapparent size in the sky awmaist the same as that o the Sun, resultin in the Muin coverin the Sun nearly precisely in totsolar eclipse. This matchin o apparent veesual size will nae conteena in the faur futur, acause the Muin's distance frae Yird is slawly increasin.
TheSoviet Union'sLuna programme wis the first tae reach the Muin wiuncrewed spacecraft in 1959; the Unitit States'NASAApollo program achieved the anerly crewed missions tae date, beginnin wi the first crewed lunar orbitin mission biApollo 8 in 1968, an sax crewed lunar laundins atween 1969 an 1972, wi the first beinApollo 11. Thir missions returnedlunar rocks that hae been uised tae develop ageological unnerstaundin o the Muin's oreegin,internal structur, anlater history. Syne theApollo 17 mission in 1972, the Muin haes been veesitit anerly bi uncrewed spacecraft.
Athin human cultur, baith the Muin's naitural prominence in the yirdly sky, an its regular cycle ophases as seen frae the Yird hae providit cultural references an influences for human societies an culturs syne time immemorial. Sic cultural influences can be foond inleid,lunar based calendar seestems,airt, anmeethologie.
The Muin is adifferentiatit bouk: it haes ageochemically distinctcrust,mantle, ancore. The Muin haes a solit airn-rich inner core wi a radius o240km (150mi) an a fluid ooter core primarily made o liquid iron wi a radius o aboot300km (190mi). Aroond the core is a pairtially mowten boondary layer wi a radius o aboot500km (310mi).[13][14] This structur is thocht tae hae developed throu thefractional creestallisation o a globalmagma ocean shortly efter the Muin's formation 4.5billion year ago.[15]Creestallisation o this magma ocean wad hae creatit amafic mantle frae theprecipitation an sinkin o the mineralsolivine,clinopyroxene, anorthopyroxene; efter aboot three-quarters o the magma ocean haed creestallised, lawer-densityplagioclase meenerals coud form an fleet intae a crust atap.[16] The feenal liquids tae crystallise wad hae been ineetially sandwiched atween the crust an mantle, wi a heich abundance oincompatible an heat-producin elements.[1]Conseestent wi this perspective, geochemical mappin made frae orbit suggests the crust o maistlyanorthosite.[9] TheMuin rock samples o the fluid lavas that erupted ontae the surface frae pairtial meltin in the mantle confirm the mafic mantle composeetion, that is mair airn rich nor that o Yird.[1]The crust is on average aboot50km (31mi) thick.[1]
The Muin is the seicont-densest satellite in the Solar Seestem, efterIo.[17] Houiver, the inner core o the Muin is smaw, wi a radius o aboot350km (220mi) or less,[1] aroond 20% o the radius o the Muin. Its composeetion is nae well defined, but is probably metallic airn alloyed wi a smaw amoont osulfur an nickel; analyses o the Muin's time-variable rotation suggest that it is at least pairtly mowten.[18]
Thetopografie o the Muin haes been meisurt wilaser altimetry anstereo eemage analysis.[19] Its maist veesibletopografic featur is the giant far-sideSooth Pole–Aitken basin, some2,240km (1,390mi) in diameter, the lairgest crater on the Muin an the seicont-lairgest confirmed impactcrater in the Solar Seestem.[20][21] At13km (8.1mi) deep, its fluir is the lawest pynt on the surface o the Muin.[20][22] The heichest elevations o the Muin's surface are locatit directly tae the northeast, an it haes been suggestit micht hae been thickened bi the oblique formation impact o the Sooth Pole–Aitken basin.[23] Ither lairge impact basins, sic asImbrium,Serenitatis,Crisium,Smythii, anOrientale, an aa possess regionally law elevations an elevatit rims.[20] The faur side o the lunar surface is on average aboot1.9km (1.2mi) heicher nor that o the near side.[1]
The discovery ofaut scarp cliffs bi theLunar Reconnaissance Orbiter suggest that the Muin haes scrinkit athin the past billion year, bi aboot 90 metres (300ft).[24] Seemilar scrinkage featurs exist onMercur.
The daurk an relatively featurless lunar plains, clearly seen wi the nakit ee, are criedmaria (Latiin for "seas"; seengularmare), as thay war ance believed tae be filled wi watter;[25] thay are nou kent tae be vast solitifee'd puils o auncientbasaltic lava. Awtho seemilar tae terrestrial basalts, lunar basalts hae mair airn an no minerals altered bi watter.[26][27] The majority o thir lavas eruptit or flawed intae the depressions associatit wiimpact basins. Severalgeologic provinces conteeninshield volcanoes an volcanicdomes are foond within the near side "maria".[28]
Awmaist aw maria are on the near side o the Muin, an civer 31% o the surface o the near side,[29] compared wi 2% o the faur side.[30] This is thocht tae be due tae aconcentration o heat-producin elements unner the crust on the near side, seen on geochemical maps obteened biLunar Prospector's gamma-ray spectrometer, that wad hae caused the unnerleein mantle tae heat up, pairtially melt, rise tae the surface an erupt.[16][31][32] Maist o the Muin'smare basalts eruptit in the Imbrian period, 3.0–3.5billion year aby, awtho some radiometrically datit samples are as auld as 4.2billion year.[33] Till recently, the youngest eruptions, datit bicrater coontin, appeared tae hae been anly 1.2billion year aby.[34] In 2006, a study oIna, a tottie depression inLacus Felicitatis, foond jagged, relatively dust-free featurs that, due tae the lack o erosion bi infawin debris, appeared tae be anly 2 million year auld.[35]Muinquakes an releases o gas an aa indicate some continued lunar acteevity.[35] In 2014 NASA annoonced "widespread evidence o young lunar volcanism" at 70irregular mare patches identifee'd bi theLunar Reconnaissance Orbiter, some less nor 50 million year auld. This raises the possibeelity o a muckle wairmer lunar mantle nor previously believed, at least on the near side whaur the deep crust is substantially waurmer due tae the greater concentration o radioactive elements.[36][37][38][39] Juist prior tae this, evidence haes been presentit for 2–10 million years younger basaltic volcanism inside Lowell crater,[40][41] Orientale basin, locatit in the transeetion zone atween the near an faur sides o the Muin. An ineetially hetter mantle and/or local enrichment o heat-producin elements in the mantle coud be responsible for prolanged acteevities an aa on the faur side in the Orientale basin.[42][43]
The lichter-coloured regions o the Muin are criedterrae, or mair commonlyhighlands, acause thay are heicher nor maist maria. Thay hae been radiometrically datit tae haein formed 4.4billion year aby, an mey representplagioclasecumulates o thelunar magma ocean.[33][34] In contrast tae Yird, na major lunar moontains are believed tae hae formed as a result o tectonic events.[44]
The concentration o maria on the Near Side likely reflects the substantially thicker crust o the helands o the Faur Side, that mey hae formed in a slaw-velocity impact o a seicont muin o Yird a few tens o millions o years efter thair formation.[45][46]
The ither major geologic process that haes affectit the Muin's surface isimpact craterin,[47] wi craters formed whan asteroids an comets collide wi the lunar surface. Thare are estimatit tae be aboot 300,000 craters wider nor1km (0.6mi) on the Muin's near side alane.[48] Thelunar geologic timescale is based on the maist prominent impact events, includinNectaris,Imbrium, anOrientale, structurs chairacterised bi multiple raings o upliftit material, atween hunders an thoosands o kilometres in diameter an associatit wi a braid apron o ejecta deposits that form a regionalstratigraphic horizon.[49] The lack o an atmosphere, wather an recent geological processes mean that mony o thir craters are well-preserved. Awtho anly a few multi-raing basins hae been definitively datit, thay are uisefu for assignin relative ages. Acause impact craters accumulate at a nearly constant rate, coontin the nummer o craters per unit area can be uised tae estimate the age o the surface.[49] The radiometric ages o impact-meltit rocks collectit in theApollo missions cluster atween 3.8 an 4.1billion year auld: this haes been uised tae propone aLate Heavy Bombardment o impacts.[50]
Blanketit on tap o the Muin's crust is a heichlycomminutit (breuken intae ever smawer particles) animpact gairdened surface layer criedregolith, formed bi impact processes. The finer regolith, thelunar sile oseelicon dioxide gless, haes a textur resemblin snaw an a scent resemblin spentgunpouder.[51] The regolith o aulder surfaces is generally thicker than for younger surfaces: it varies in thickness frae10–20km (6.2–12.4mi) in the hielands an3–5km (1.9–3.1mi) in the maria.[52]Beneath the finely comminuted regolith layer is themegaregolith, a layer o heichly fractured bedrock mony kilometres thick.[53]
Comparison o heich-resolution eemages obteened bi theLunar Reconnaissance Orbiter haes shawn a contemporary crater-production rate signeeficantly heicher nor previously estimatit. A seicontar craterin process caused bidistal ejecta is thocht tae churn the tap twa centimetres o regolith a hunder times mair quickly nor previous models suggestit–on a timescale o 81,000 year.[54][55]
Lunar swirls are enigmatic featurs foond across the Muin's surface, which are chairacterised bi a heich albedo, appearin optically immatur (i.e. the optical chairactereestics o a relatively young regolith), an eften displayin a sinuous shape. Thair curvilinear shape is eften accentuated bi law albedo regions that wind atween the bricht swirls.
The sicht o the Moon is shapit bi its reflectin propertie an the composition o its surface. The Moon’s albedo is aboot 0.12[56], compairable tae worn asphalt. Sae, altho it appears bricht in the nicht sky, it is realie relativel dark. This low albedo comes fae the lunar regolith, a layer o fine rock fragments formit bi meteorite impacts. Licht scatterin bi the regolith maks the phenomenon kent as the opposition surge, makin the Moon appear muckle brichter at fu Moon than at quarter phases.
The Moon’s true colour, wi nae atmospheric effects, is a mutit bruunish-gray[57]. This comes fae silicate minerals in the regolith, wi darker basaltic plains (maria) an lighter feldspar-rich highlands. The maria, formit bi auld volcanic flows, hae mair iron an titanium, giein them a darker tone. Frae Earth, the Moon’s colour can be altert bi the atmosphere, appearin red durin lunar eclipses or occasionallie blue due tae volcanic particles.
The Moon alsa shows retro-reflection, scatterin licht back til its source an creatin a uniform brichtness across its disc wi nae significant limb darkenin. Its apparent size an brichtness vary due tae its elliptical orbit, appearin up tae 30% brichter an 14% lairger at perigee compairit tae apogee, a phenomenon kent as a "Supermoon".
At times, the Moon can appear red or blue. It may appear red durin a lunar eclipse, because o the red spectrum o the Sun's licht bein refracted on the Moon bi Earth’s atmosphere. Because o this red colour, lunar eclipses are soms ca’d blood moons. The Moon can alsa seem red whan it appears at low angles an thru a thick atmosphere.
The Moon may appear blue dependin on the presence o certain particles in the air, sic as volcanic particles, in which case it is ca’d a blue moon.
Sin the words "red moon" an "blue moon" can alsa be used tae refer tae specific fu moons o the year, they dae nae alwais refer tae the presence o red or blue moonlicht.
Liquid watter canna persist on the lunar surface. Whan exposed tae solar radiation, watter quickly decompones throu a process kent asphotodissociation an is lost tae space. Houiver, syne the 1960s, scientists hae hypothesized that watter ice mey be depositit bi impactincomets or possibly produced bi the reaction o oxygen-rich lunar rocks, an hydrogen fraesolar wind, leavin traces o watter that coud possibly survive in cauld, permanently shaidaed craters at aither pole on the Muin.[58][59] Computer simulations suggest that up tae14,000km2 (5,400sqmi) o the surface mey be in permanent shaidae.[60] The presence o uisable quantities o watter on the Muin is an important factor in renderinlunar habitation as a cost-effective plan; the alternative o transportin watter frae Yird wad be prohibitively expensive.[61]
In years syne, seegnaturs o watter hae been foond tae exeest on the lunar surface.[62] In 1994, thebistatic radar experiment locatit on theClementine spacecraft, indicatit the existence o smaw, frozen pockets o watter close tae the surface. Houiver, later radar observations biArecibo, suggest thir findings mey rather be rocks ejectit frae young impact craters.[63] In 1998, theneutron spectrometer on theLunar Prospector spacecraft shawed that heich concentrations o hydrogen are present in the first meter o deepth in the regolith near the polar regions.[64] Volcanic lava beads, brocht back tae Yird abuird Apollo 15, shawed smaw amounts o watter in thair interior.[65]
The 2008Chandrayaan-1 spacecraft haes syne confirmed the existence o surface watter ice, uisin the on-buirdMoon Mineralogy Mapper. The spectrometer observed absorption lines common taehydroxyl, in reflected sunlicht, providin evidence o lairge quantities o watter ice, on the lunar surface. The spacecraft shawed that concentrations mey possibly be as heich as 1,000ppm.[66] In 2009,LCROSS sent a2,300kg (5,100lb) impactor intae a permanently shaidaed polar crater, an detectit at least100kg (220lb) o watter in a plume o ejectit material.[67][68] Anither examination o the LCROSS data shawed the amoont o detectit watter tae be closer tae155±12kg (342±26lb).[69]
In Mey 2011, 615–1410 ppm watter inmelt inclusions in lunar saumple 74220 wis reportit,[70] the famous heich-titanium "orange gless sile" o volcanic oreegin collectit during theApollo 17 mission in 1972. The inclusions war formed in explosive eruptions on the Muin approximately 3.7 billion year aby. This concentration is comparable wi that o magma in Yird'supper mantle. Awtho o considerable selenological interest, this announcement affords little comfort tae wad-be lunar colonists—the sample oreeginatit mony kilometres ablo the surface, an the inclusions are sae difficult tae access that it teuk 39 year tae find them wi a state-o-the-airt ion microprobe instrument.
The gravitational field o the Muin haes been meisurt throu trackin theDoppler shift o radio signals emittit bi orbitin spacecraft. The main lunar gravity featurs aremascons, lairge positive gravitational anomalies associatit wi some o the giantimpact basins, partly caused bi the dense mare basaltic lava flaws that fill thae basins.[71][72] The anomalies greatly influence the orbit o spacecraft aboot the Muin. Thare are some puzzles: lava flaws bi themsels canna expleen aw o the gravitational signatur, an some mascons exist that are nae linked tae mare volcanism.[73]
The Muin haes a freemitmagnetic field o aboot 1–100nanoteslas, less nor ane-hundertthat o Yird. It daes nae currently hae a globaldipolar magnetic field an anly haes crustal magnetisation, probably acquired early in lunar history whan a dynamo wis still operatin.[74][75] Alternatively, some o the remnant magnetization mey be frae transient magnetic fields generatit in lairge impact events throu the expansion o an impact-generatit plasma clood in the presence o an ambient magnetic field. This is supportit bi the apparent location o the lairgest crustal magnetisations near theantipodes o the giant impact basins.[76]
Sketch bi the Apollo 17 astronauts. The lunar atmosphere wis later studied biLADEE.[77][78]
The Muin haes an atmosphere sae tenuous as tae be nearlyvacuum, wi a tot mass o less nor10 metric ton (9.8 long ton; 11 short ton).[79] The surface pressur o this smaw mass is aroond 3 × 10−15atm (0.3nPa); it varies wi the lunar day. Its soorces includeootgassin ansputterin, a product o the bombardment o lunar sile bisolar wind ions.[9][80] Elements that hae been detectit includesodium anpotassium, produced bi sputterin (an aa foond in the atmospheres oMercur anIo);helium-4 anneon[81] frae the solar wind; anargon-40,radon-222, anpolonium-210, ootgassed efter thair creaution biradioactive decay within the crust an mantle.[82][83] The absence o sic neutral species (atoms or molecules) asoxygen,nitrogen,carbon,hydrogen anmagnesium, that are present in theregolith, is nae unnerstuid.[82] Watter vapour haes been detectit biChandrayaan-1 an foond tae vary wi latitude, wi a maximum at ~60–70degrees; it is possibly generatit frae thesublimation o watter ice in the regolith.[84] Thir gases aither return intae the regolith due tae the Muin's gravity or are lost tae space, aither throu solar radiation pressur or, if thay are ionized, bi bein swept awey bi the solar wind's magnetic field.[82]
A permanent asymmetricmuin dist clood exists aroond the Muin, creatit bi smaw particles fraecomets. Estimates are 5 tons o comet pairticles strike the Muin's surface ilk 24 oors. The pairticles strike the Muin's surface ejectin muin dist abuin the Muin. The dist stays abuin the Muin approximately 10 meenits, takkin 5 minutes tae rise, an 5 minutes tae faw. On average, 120 kilogrammes o dist are present abuin the Muin, rising tae 100 kilometers abuin the surface. The dist meisurments war made biLADEE's Lunar Dust EXperiment (LDEX), atween 20 an 100 kilometres abuin the surface, in a sax-month period. LDEX detectit an average o ane 0.3 micrometer muin dist pairticle ilk meenit. Dist pairticle coonts peaked during theGeminid,Quadrantid,Northren Taurid, anOmicron Centauridmeteor shawers, whan the Yird, an Muin, pass throu comet debris. The clood is asymmetric, mair dense near the boondary atween the Muin's dayside an nichtside.[85][86]
The Muin'saxial tilt wi respect tae theecliptic is anly 1.5424°,[87] muckle less nor the 23.44° o Yird. Acause o this, the Muin's solar illumination varies muckle less wi saison, an topografical details play a crucial role in saisonal effects.[88] Frae eemages taken biClementine in 1994, it appears that fower moontainous regions on the rim oPeary Crater at the Muin's north pole mey remeen illuminatit for the entirelunar day, creautinpeaks o eternal licht. No sic regions exist at the sooth pole. Seemilarly, thare are places that remeen in permanent shaidae at the bottoms o mony polar craters,[60] an thir dark craters are extremely cauld:Lunar Reconnaissance Orbiter meisurt the lawest simmer temperaturs in craters at the soothren pole at35K (−238°C; −397°F)[89] an juist26K (−247°C; −413°F) close tae the winter solstice in north polarHermite Crater. This is the cauldest temperatur in the Solar Seestem iver meosurt bi a spacecraft, caulder oven than the surface oPluto.[88] Average temperaturs o the Muin's surface are reportit, but temperaturs o different areas will vary greatly dependin upon whether thay are in sunlicht or shaidae.[90]
Yird–Muin seestem (schematic)DSCOVR satellite sees the Muin passin in front o Yird
The Muin maks a complete orbit aroond Yird wi respect tae the fixed starns aboot ance ivery 27.3days[f] (itssidereal period). Houiver, acause Yird is muivin in its orbit aroond the Sun at the same time, it takes slichtly langer for the Muin tae shaw the samephase tae Yird, which is aboot 29.5days[g] (itssynodic period).[29] Unlik maist satellites o ither planets, the Muin orbits closer tae theecliptic plane nor tae the planet'sequatorial plane. The Muin's orbit is subtlyperturbed bi the Sun an Yird in mony smaw, complex an interactin weys. For ensaumple, the plane o the Muin's orbitgradually rotates ance ivery 18.61[91] years, that affects ither aspects o lunar motion. Thir follae-on effects are mathematically describit biCassini's laws.[92]
The Muin is exceptionally lairge relative tae Yird: a quarter its diameter an 1/81 its mass.[29] It is the lairgest muin in the Solar Seestem relative tae the size o its planet,[h] thoCharon is lairger relative tae the dwarf planetPluto, at 1/9 Pluto's mass.[i][93] Yird an the Muin are nivertheless still conseederit a planet–satellite seestem, raither nor adooble planet, acause thairbarycentre, the common centre o mass, is locatit1,700km (1,100mi) (aboot a quarter o Yird's radius) aneath Yird's surface.[94]
Muin settin in wastren sky ower theHigh Desert in Californie
The Muin is insynchronous rotation: it rotates aboot its axis in aboot the same time it takes taeorbit Yird. This results in it ayeweys keepin nearly the same face turned taewart Yird. Houiver, due tae the effect olibration, aboot 59% o the Muin's surface can actually be seen frae Yird.
The Muin uised tae rotate at a fester rate, but early in its history, its rotation slawed an becamtidally locked in this orientation as a result ofreectional effects associatit witidal deformations caused bi Yird.[95] Wi time, the energy o rotation o the Muin on its axis wis dissipatit as heat, till thare wis no rotation o the Muin relative tae the Yird. The side o the Muin that faces Yird is cried thenear side, an the opposite thefaur side. The far side is eften inaccurately cried the "daurk side", but it is in fact illuminatit as eften as the near side: ance per lunar day, in the new muin phase we observe on Yird whan the near side is daurk.[96] In 2016, planetary scientists, uisin data collectit on the muckle earlier NasaLunar Prospector mission, foond twa hydrogen-rich areas on opposite sides o the Muin, probably in the form o watter ice. It is speculatit that thir patches war the poles o the Muin billions o years aby, afore it wis tidally locked tae Yird.[97]
The Muin haes an exceptionally lawalbedo, giein it a reflectance that is slichtly brichter nor that o worn asphalt. Despite this, it is the brichtest object in the sky efter theSun.[29][j] This is pairtly due tae the brichtness enhancement o theopposeetion effect; at quarter phase, the Muin is anly ane-tent as bricht, raither nor hauf as bricht, as at full muin.[98]
Addeetionally,colour constancy in thevisual seestem recalibrates the relations atween the colours o an object an its surroondins, an acause the surroondin sky is comparatively daurk, the sunlit Muin is perceived as a bricht object. The edges o the full muin seem as bricht as the centre, wi nolimm daurkenin, due tae thereflective properties olunar sile, that reflects mair licht back taewart the Sun nor in ither directions. The Muin daes appear lairger whan close tae the horizon, but this is a purely psychological effect, kent as theMuin illusion, first descrived in the 7t century BC.[99] The full muin subtends an arc o aboot 0.52° (on average) in the sky, aboot the same apparent size as the Sun (see§Eclipses).
The heichestaltitude o the Muin in the sky varies wi the lunar phase an the saison o the year. The full muin is heichest in winter. The 18.61-yearnodes cycle an aa haes an influence: whan theascendin node o the lunar orbit is in thevernal equinox, the lunardeclination can gae as far as 28° ilk month. This means the Muin can gang owerheid at latitudes up tae 28° frae the equator, insteid o anerly 18°. The orientation o the Muin's crescent an aa depends on the latitude o the observation steid: close tae the equator, an observer can see a smile-shaped crescent muin.[100]
The Muin is veesible for twa weeks ivery 27.3 days at theNorth anSooth Pole. The Muin's licht is uised bizooplankton in the Arctic whan the sun is ablo the horizon for months on end.[101]
The 14 November 2016supermuin wis356,511 kilometre (221,526mi) awey[102] frae the centre of Earth, the closest occurrence syne 26 Januar 1948. It will nae be closer until 25 November 2034.[103]
The distance atween the Muin an Yird varies frae aroond356,400km (221,500mi) tae406,700km (252,700mi) atperigees (closest) an apogees (faurthest), respectively. On 14 November 2016, it wis closer tae Yird whan at full phase than it haes been syne 1948, 14% closer than its faurthest poseetion in apogee.[104] Reportit as a "super muin", this closest pynt coincides within an oor o afull muin, an it wis 30% mair luminous than whan at its greatest distance due tae its angular diameter bein 14% greater, acause.[105][106][107] At lower levels, the human perception o reduced brightness as a percentage is providit bi the follaein formula:[108][109]
Whan the actual reduction is 1.00 / 1.30, or about 0.770, the perceived reduction is aboot 0.877, or 1.00 / 1.14. This gies a maximum perceived increase o 14% atween apogee an perigee muins o the same phase.[110]
Thare haes been historical controversy ower whether featurs on the Muin's surface chynge ower time. The day, mony o thir claims are thocht tae be illusory, resultin frae observation unner different lichtin condeetions, puirastronomical seein, or inadequate drawins. Houiver,ootgassin daes occasionally occur, an coud be responsible for a minor percentage o the reportitlunar transient phenomena. Recently, it haes been suggestit that aboot a3km (1.9mi) diameter region o the lunar surface wis modified bi a gas release event aboot a million years aby.[111][112] The Muin's appearance, lik that o the Sun, can be affectit bi Yird's atmosphere: common effects are a 22°halo ring formed whan the Muin's licht is refractit throu the ice creestals o heichcirrostratus cloud, an smawercoronal rings whan the Muin is seen throu thin cloods.[113]
The monthly cheenges o angle between the direction o illumination bi the Sun an viewin frae Yird, an thephases o the Muin that result as viewed frae the northren hemisphere. Yird–Muin distance is nae tae scale.
The illuminatit area o the veesible sphere (degree o illumination) is gien bi, whaur is theelongation (i.e., the angle atween Muin, the observer (on Yird) an the Sun).
Thelibration o the Muin ower a single lunar month. An aa veesible is the slicht variation in the Muin's veesual size frae Yird.
The gravitational attraction that masses hae for ane anither decreases inversely wi the square o the distance o thae masses frae ilk ither. As a result, the slichtly greater attraction that the Muin haes for the side o Yird closest tae the Muin, as compared tae the pairt o the Yird opposite the Muin, results intidal forces. Tidal forces affect baith the Yird's crust an oceans.
The maist obvious effect o tidal forces is tae cause twa bulges in the Yird's oceans, ane on the side facin the Muin an the ither on the side opposite. This results in elevatit sea levels criedocean tides.[114] As the Yird spins on its axis, ane o the ocean bulges (heich tide) is held in place "unner" the Muin, while anither sic tide is opposite. As a result, thare are twa heich tides, an twa law tides in aboot 24 oors.[114] Syne the Muin is orbitin the Yird in the same direction o the Yird's rotation, the heich tides occur aboot ivery 12 oors an 25 minutes; the 25 minutes is due tae the Muin's time tae orbit the Yird. The Sun haes the same tidal effect on the Yird, but its forces o attraction are anerly 40% that o the Muin's; the Sun's an Muin's interplay is responsible forware an neap tides.[114] If the Yird war a watter warld (ane wi no continents) it wad produce a tide o anly ane meter, an that tide wad be verra predictable, but the ocean tides are greatly modified bi ither effects: the freectional cooplin o watter tae Yird's rotation throu the ocean fluirs, theinertia o watter's muivement, ocean basins that graw shallaer near laund, the sloshin o watter atween different ocean basins.[115] As a result, the timing o the tides at maist pynts on the Yird is a product o observations that are explained, incidentally, bi theory.
While gravitation causes acceleration an movement o the Yird's fluid oceans, gravitational cooplin atween the Muin an Yird's solit bouk is maistly elastic an plastic. The result is a forder tidal effect o the Muin on the Yird that causes a bulge o the solit portion o the Yird nearest the Muin that acts as atorque in opposeetion tae the Yird's rotation. This "drains"angular momentum an rotationalkinetic energy frae Yird's spin, slowing the Yird's rotation.[114][116] That angular momentum, lost frae the Yird, is transferred tae the Muin in a process (confusingly kent astidal acceleration), that lifts the Muin intae a heicher orbit an results in its lawer orbital speed aboot the Yird. Sicweys the distance atween Yird an Muin isincreasin, an the Yird's spin is slawin in reaction.[116] Measurements frae laser reflectors left in the Apollo missions (lunar rangin experiments) hae foond that the Muin's distance increases bi38mm (1.5in) per year[117] (aboot the rate at that human fingernails growe).[118]Atomic clocks an aa shaw that Yird's day lenthens bi aboot 15microseiconts ivery year,[119] slawly increasin the rate at thatUTC is adjuistit bileap seiconts.Left tae run its coorse, this tidal drag wad continue till the spin o Yird an the orbital period o the Muin matched, creautin mutual tidal lockin atween the twa. As a result, the Muin wad be suspendit in the sky ower ane meridian, as is awreidy currently the case wiPluto an its muinCharon. Houiver, the Sun will become areid giant engulfin the Yird-Muin seestem lang afore this occurrence.[120][121]
In a lik manner, the lunar surface experiences tides o aroond10cm (4in) amplitude ower 27days, wi twa components: a fixed ane due tae Yird, acause thay are insynchronous rotation, an a varyin component frae the Sun.[116] The Yird-induced component arises fraelibration, a result o the Muin's orbital eccentricity (if the Muin's orbit war perfectly circular, thare wad anerly be solar tides).[116] Libration an aa cheenges the angle frae that the Muin is seen, allouin a tot o aboot 59% o its surface tae be seen frae Yird ower time.[29] The cumulative effects o stress biggit up bi thir tidal forces producesmuinquauks. Muinquakes are muckle less common an waiker nor are yirdquauks, awtho muin quauks can last for up tae an oor—a signeeficantly langer time than terrestrial quauks—acause o the absence o watter tae damp oot the seismic vibrations. The existence o muinquauks wis an unexpectit discovery fraeseismometers placed on the Muin biApolloastronauts frae 1969 throu 1972.[122]
Frae Yird, the Muin an the Sun appear the same size, as seen in the1999 solar eclipse (left), whauras frae theSTEREO-B spacecraft in an Yird-trailin orbit, the Muin appears mukle smawer than the Sun (richt).[123]
Eclipses can anerly occur whan the Sun, Yird, an Muin are aw in a straucht line (termed "syzygy").Solar eclipses occur atnew muin, whan the Muin is atween the Sun an Yird. In contrast,lunar eclipses occur atfull muin, whan Yird is atween the Sun an Muin. The apparent size o the Muin is aboot the same as that o the Sun, wi baith bein viewed at close tae ane-hauf a degree wide. The Sun is muckle lairger nor the Muin but it is the precise vastly greater distance that gies it the same apparent size as the muckle closer an much smawer Muin frae the perspective o Yird. The variations in apparent size, due tae the non-circular orbits, are nearly the same as well, tho occurrin in different cycles. This maks possible baithtotal (wi the Muin appearin lairger than the Sun) anannular (wi the Muin appearin smawer than the Sun) solar eclipses.[124] In a tot eclipse, the Muin completely covers the disc o the Sun an the solarcorona becomes vrrsible tae thenakit ee. Acause the distance atween the Muin an Yird is verra slawly increasin ower time,[114] the angular diameter o the Muin is decreasin. An aa, as it evolves thewarts becomin areid giant, the size o the Sun, an its apparent diameter in the sky, are slawly increasin.[k] The combination o thir twa cheenges means that hunders o millions o years ago, the Muin wad alyeweys completely civer the Sun on solar eclipses, an no annular eclipses war possible. Likwise, hunders o millions o years in the futur, the Muin will no langer civer the Sun completely, an tot solar eclipses will nae occur.[125]
Acause the Muin's orbit aroond Yird is inclined bi aboot 5.145° (5° 9') tae theorbit o Yird aroond the Sun, eclipses dae nae occur at ivery full an new muin. For an eclipse tae occur, the Muin must be near the intersection o the twa orbital planes.[126] The periodicity an recurrence o eclipses o the Sun bi the Muin, an o the Muin bi Yird, is describit bi thesaros, that haes a period o approximately 18years.[127]
Acause the Muin is continuously blockin oor view o a hauf-degree-wide circular area o the sky,[l][128] the relatit phenomenon ooccultation occurs whan a bricht starn or planet passes ahint the Muin an is occultit: hidden frae view. In this wey, a solar eclipse is an occultation o the Sun. Because the Muin is comparatively close tae Yird, occultations o individual starns are nae veesible iverywhaur on the planet, nor at the same time. Acause o theprecession o the lunar orbit, ilk year different stars are occulted.[129]
↑Thare are a nummer onear-Yird asteroids, includin3753 Cruithne, that areco-orbital wi Yird: thair orbits bring them close tae Yird for periods o time but then alter in the lang term (Morais et al, 2002). Thir arequasi-satellites– thay are not muins as thay dae nae orbit Yird. For mair information, seeIther muins o Yird.
↑Themaximum vailyie is gien based on scalin o the brichtness frae the vailyie o −12.74 gien for an equator tae Muin-centre distance o 378000km in the NASA factsheet reference tae the meenimum Yird–Muin distance gien thare, efter the latter is correctit for Yird's equatorial radius o 6378km, giein 350600km. Themeenimum vailyie (for a distantnew muin) is based on a seemilar scalin uisin the maximum Yird–Muin distance o 407000km (gien in the factsheet) an bi calculatin the brichtness o theyirdshine ontae sic a new muin. The brichtness o the yirdshine is[ Yirdalbedo ×(Yird radius / Radius oMuin's orbit)2] relative tae the direct solar illumination that occurs for a full muin. (Earth albedo = 0.367;Yird radius = (polar radius× equatorialradius)½ = 6 367 km.)
↑The range o angular size vailyies gien are based on semple scalin o the follaein vailyies gien in the fact sheet reference: at an Yird-equator tae Muin-centre distance o 378000km, theangular size is 1896arcseiconts. The same fact sheet gies extreme Yird–Muin distances o 407000km an 357000km. For the maximum angular size, the meenimum distance haes ae be correctit for Yird's equatorial radius o 6378km, giein 350600km.
↑Lucey et al. (2006) gie107 particles cm−3 bi day an105 pairticles cm−3 bi nicht. Alang wi equatorial surface temperaturs o 390K bi day an 100K bi nicht, theideal gas law yields the pressurs gien in the infobox (roondit tae the nearestorder o magnitude): 10−7Pa bi day an 10−10Pa bi nicht.
↑Mair accurately, the Muin's mean sidereal period (fixed starn tae fixed starn) is 27.321661days(27 d 07 o 43 meen 11.5 s), an its mean tropical orbital period (frae equinox tae equinox) is 27.321582days(27 d 07 o 43 meen 04.7 s) (Explanatory Supplement to the Astronomical Ephemeris, 1961, at p.107).
↑Mair accurately, the Muin's mean synodic period (atween mean solar conjunctions) is 29.530589days(29 d 12 h 44 min 02.9 s) (Explanatory Supplement to the Astronomical Ephemeris, 1961, at p.107).
↑Thare is no strang correlation atween the sizes o planets an the sizes o thair satellites. Lairger planets tend tae hae mair satellites, baith lairge an smaw, than smawer planets.
↑Wi 27% the diameter an 60% the density o Yird, the Muin haes 1.23% o the mass o Yird. The muinCharon is lairger relative tae its primarPluto, but Pluto is nou conseedert tae be adwarf planet.
↑The Sun'sapparent magnitude is −26.7, while the full muin's apparent magnitude is −12.7.
↑See graph inSun#Life phases. At present, the diameter o the Sun is increasin at a rate o aboot five percent per billion years. This is verra seemilar tae the rate at that the apparent angular diameter o the Muin is decreasin as it recedes frae Yird.
↑On average, the Muin covers an area o0.21078 square degrees on the nicht sky.
↑Spudis, Paul D.; Cook, A.; Robinson, M.; Bussey, B.; Fessler, B. (Januar 1998). "Topography of the South Polar Region from Clementine Stereo Imaging".Workshop on New Views of the Moon: Integrated Remotely Sensed, Geophysical, and Sample Datasets: 69.Bibcode:1998nvmi.conf...69S.
↑Schultz, P. H. (Mairch 1997). "Forming the south-pole Aitken basin – The extreme games".Conference Paper, 28th Annual Lunar and Planetary Science Conference.28: 1259.Bibcode:1997LPI....28.1259S.
↑Gupta, R.P.; Srivastava, N.; Tiwari, R.K. (2014). "Evidences of relatively new volcanic flows on the Moon".Current Science.107 (3): 454–460.
↑Whitten, J.; et al. (2011). "Lunar mare deposits associated with the Orientale impact basin: New insights into mineralogy, history, mode of emplacement, and relation to Orientale Basin evolution from Moon Mineralogy Mapper (M3) data from Chandrayaan-1".Journal of Geophysical Research.116: E00G09.Bibcode:2011JGRE..116.0G09W.doi:10.1029/2010JE003736.
↑Cho, Y.; et al. (2012). "Young mare volcanism in the Orientale region contemporary with the Procellarum KREEP Terrane (PKT) volcanism peak period 2 b. y. ago".Geophysical Research Letters.39: L11203.
↑Munsell, K. (4 December 2006)."Majestic Mountains".Solar System Exploration.NASA. Archived fraethe original on 17 September 2008. Retrieved12 Apryle 2007.
↑Globus, Ruth (1977). "Chapter 5, Appendix J: Impact Upon Lunar Atmosphere". In Richard D. Johnson & Charles Holbrow (ed.).Space Settlements: A Design Study.NASA. Archived fraethe original on 31 Mey 2010. Retrieved17 Mairch 2010.
↑Walker, John (Mey 1997)."Inconstant Moon".Earth and Moon Viewer. Fourth paragraph of "How Bright the Moonlight":Fourmilab. Archived fraethe original on 14 December 2013. Retrieved23 Januar 2014.14% [...] due to the logarithmic response of the human eye.
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