Muin

The Muin is an astronomical bouk that orbits planet Yird, bein Yird's anerly permanent naitural satellite. It is the fift-lairgest naitural satellite in the Solar Seestem, an the lairgest amang planetar satellites relative tae the size o the planet that it orbits (its primar). Follaein Jupiter's satellite Io, the Muin is seicond-densest satellite amang thae whase densities are kent.

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 a giant impact atween Yird an a Maurs-sized bouk cried Theia.

The Muin is in synchronous rotation wi Yird, ayeweys shawin the same face, wi its near side merked bi daurk volcanic maria that fill the spaces atween the bricht auncient crustal hielands an the prominent impact craters. As seen frae the Yird, it is the seicont-brichtest regularly veesible celestial object in Yird's sky, efter the Sun. Its surface is actually daurk, awtho compared tae the nicht sky it appears verra bricht, wi a reflectance juist slichtly heicher nor that o worn asphalt. Its gravitational influence produces the ocean tides, bouk tides, an the slicht lenthenin o the day.

The Muin's current orbital distance is ,Scott, Elaine. Our Moon: New Discoveries About Earth's Closest Companion. Houghton Mifflin Harcourt (2016) . page 7. or 1.28 licht-seiconts. This is aboot thirty times the diameter o Yird, wi its apparent size in the sky awmaist the same as that o the Sun, resultin in the Muin coverin the Sun nearly precisely in tot solar eclipse. This matchin o apparent veesual size will nae conteena in the faur futur, acause the Muin's distance frae Yird is slawly increasin.

The Soviet Union's Luna programme wis the first tae reach the Muin wi uncrewed spacecraft in 1959; the Unitit States' NASA Apollo program achieved the anerly crewed missions tae date, beginnin wi the first crewed lunar orbitin mission bi Apollo 8 in 1968, an sax crewed lunar laundins atween 1969 an 1972, wi the first bein Apollo 11. Thir missions returned lunar rocks that hae been uised tae develop a geological unnerstaundin o the Muin's oreegin, internal structur, an later history. Syne the Apollo 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 o phases 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 in leid, lunar based calendar seestems, airt, an meethologie.

Pheesical chairactereestics

Internal structur

 + Chemical composeetion o the lunar surface regolith (derived frae crustal rocks)CompoondFormulaComposeetion (wt %)
 MariaHielands
  seelicae
 SiO2
 45.4%
 45.5%
  alumina
 Al2O3
 14.9%
 24.0%
  lime
 CaO
 11.8%
 15.9%
  airn(II) oxideFeO
 14.1%
 5.9%
  magnesia
 MgO
 9.2%
 7.5%
  titanium dioxideTiO2
 3.9%
 0.6%
  sodium oxideNa2O
 0.6%
 0.6%
 Tot 99.9% 100.0%


The Muin is a differentiatit bouk: it haes a geochemically distinct crust, mantle, an core. The Muin haes a solit airn-rich inner core wi a radius o  an a fluid ooter core primarily made o liquid iron wi a radius o aboot . Aroond the core is a pairtially mowten boondary layer wi a radius o aboot . This structur is thocht tae hae developed throu the fractional creestallisation o a global magma ocean shortly efter the Muin's formation 4.5 billion year ago.
Creestallisation o this magma ocean wad hae creatit a mafic mantle frae the precipitation an sinkin o the minerals olivine, clinopyroxene, an orthopyroxene; efter aboot three-quarters o the magma ocean haed creestallised, lawer-density plagioclase meenerals coud form an fleet intae a crust atap. The feenal liquids tae crystallise wad hae been ineetially sandwiched atween the crust an mantle, wi a heich abundance o incompatible an heat-producin elements.
Conseestent wi this perspective, geochemical mappin made frae orbit suggests the crust o maistly anorthosite. The Muin 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.
The crust is on average aboot  thick.

The Muin is the seicont-densest satellite in the Solar Seestem, efter Io. Houiver, the inner core o the Muin is smaw, wi a radius o aboot  or less, aroond 20% o the radius o the Muin. Its composeetion is nae well defined, but is probably metallic airn alloyed wi a smaw amoont o sulfur an nickel; analyses o the Muin's time-variable rotation suggest that it is at least pairtly mowten.

Surface geology


The topografie o the Muin haes been meisurt wi laser altimetry an stereo eemage analysis. Its maist veesible topografic featur is the giant far-side Sooth Pole–Aitken basin, some  in diameter, the lairgest crater on the Muin an the seicont-lairgest confirmed impact crater in the Solar Seestem. At  deep, its fluir is the lawest pynt on the surface o the Muin. 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. Ither lairge impact basins, sic as Imbrium, Serenitatis, Crisium, Smythii, an Orientale, an aa possess regionally law elevations an elevatit rims. The faur side o the lunar surface is on average aboot  heicher nor that o the near side.

The discovery o faut scarp cliffs bi the Lunar Reconnaissance Orbiter suggest that the Muin haes scrinkit athin the past billion year, bi aboot 90 metres (300 ft). Seemilar scrinkage featurs exist on Mercur.

Volcanic featurs
thumb |Lunar nearside wi major maria an craters labeled |alt=Lunar nearside wi major maria an craters labeled

The daurk an relatively featurless lunar plains, clearly seen wi the nakit ee, are cried maria (Latiin for "seas"; seengular mare), as thay war ance believed tae be filled wi watter; thay are nou kent tae be vast solitifee'd puils o auncient basaltic lava. Awtho seemilar tae terrestrial basalts, lunar basalts hae mair airn an no minerals altered bi watter. The majority o thir lavas eruptit or flawed intae the depressions associatit wi impact basins. Several geologic provinces conteenin shield volcanoes an volcanic domes are foond within the near side "maria".

thumb |150px |left |Evidence of young lunar volcanism

Awmaist aw maria are on the near side o the Muin, an civer 31% o the surface o the near side, compared wi 2% o the faur side. This is thocht tae be due tae a concentration o heat-producin elements unner the crust on the near side, seen on geochemical maps obteened bi Lunar Prospector'''s gamma-ray spectrometer, that wad hae caused the unnerleein mantle tae heat up, pairtially melt, rise tae the surface an erupt. Maist o the Muin's mare basalts eruptit in the Imbrian period, 3.0–3.5 billion year aby, awtho some radiometrically datit samples are as auld as 4.2 billion year. Till recently, the youngest eruptions, datit bi crater coontin, appeared tae hae been anly 1.2 billion year aby. In 2006, a study o Ina, a tottie depression in Lacus 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. Muinquakes an releases o gas an aa indicate some continued lunar acteevity. In 2014 NASA annoonced "widespread evidence o young lunar volcanism" at 70 irregular mare patches identifee'd bi the Lunar 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. Juist prior tae this, evidence haes been presentit for 2–10 million years younger basaltic volcanism inside Lowell crater, 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.

The lichter-coloured regions o the Muin are cried terrae, or mair commonly highlands, acause thay are heicher nor maist maria. Thay hae been radiometrically datit tae haein formed 4.4 billion year aby, an mey represent plagioclase cumulates o the lunar magma ocean. In contrast tae Yird, na major lunar moontains are believed tae hae formed as a result o tectonic events.

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.

Impact craters

The ither major geologic process that haes affectit the Muin's surface is impact craterin, wi craters formed whan asteroids an comets collide wi the lunar surface. Thare are estimatit tae be aboot 300,000 craters wider nor  on the Muin's near side alane. The lunar geologic timescale is based on the maist prominent impact events, includin Nectaris, Imbrium, an Orientale, 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 regional stratigraphic horizon. 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. The radiometric ages o impact-meltit rocks collectit in the Apollo missions cluster atween 3.8 an 4.1 billion year auld: this haes been uised tae propone a Late Heavy Bombardment o impacts.

Blanketit on tap o the Muin's crust is a heichly comminutit (breuken intae ever smawer particles) an impact gairdened surface layer cried regolith, formed bi impact processes. The finer regolith, the lunar sile o seelicon dioxide gless, haes a textur resemblin snaw an a scent resemblin spent gunpouder. The regolith o aulder surfaces is generally thicker than for younger surfaces: it varies in thickness frae  in the hielands an  in the maria.
Beneath the finely comminuted regolith layer is the megaregolith, a layer o heichly fractured bedrock mony kilometres thick.

Comparison o heich-resolution eemages obteened bi the Lunar Reconnaissance Orbiter haes shawn a contemporary crater-production rate signeeficantly heicher nor previously estimatit. A seicontar craterin process caused bi distal ejecta is thocht tae churn the tap twa centimetres o regolith a hunder times mair quickly nor previous models suggestiton a timescale o 81,000 year.


Lunar swirls
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.

 Albedo an true colour o the Moon 
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, 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. 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.

Presence o watter
Liquid watter canna persist on the lunar surface. Whan exposed tae solar radiation, watter quickly decompones throu a process kent as photodissociation an is lost tae space. Houiver, syne the 1960s, scientists hae hypothesized that watter ice mey be depositit bi impactin comets or possibly produced bi the reaction o oxygen-rich lunar rocks, an hydrogen frae solar wind, leavin traces o watter that coud possibly survive in cauld, permanently shaidaed craters at aither pole on the Muin.  Computer simulations suggest that up tae  o the surface mey be in permanent shaidae. The presence o uisable quantities o watter on the Muin is an important factor in renderin lunar habitation as a cost-effective plan; the alternative o transportin watter frae Yird wad be prohibitively expensive.

In years syne, seegnaturs o watter hae been foond tae exeest on the lunar surface. In 1994, the bistatic radar experiment locatit on the Clementine spacecraft, indicatit the existence o smaw, frozen pockets o watter close tae the surface. Houiver, later radar observations bi Arecibo, suggest thir findings mey rather be rocks ejectit frae young impact craters. In 1998, the neutron spectrometer on the Lunar Prospector spacecraft shawed that heich concentrations o hydrogen are present in the first meter o deepth in the regolith near the polar regions. Volcanic lava beads, brocht back tae Yird abuird Apollo 15, shawed smaw amounts o watter in thair interior.

The 2008 Chandrayaan-1 spacecraft haes syne confirmed the existence o surface watter ice, uisin the on-buird Moon Mineralogy Mapper. The spectrometer observed absorption lines common tae hydroxyl, 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 1,000 ppm. In 2009, LCROSS sent a  impactor intae a permanently shaidaed polar crater, an detectit at least  o watter in a plume o ejectit material. Anither examination o the LCROSS data shawed the amoont o detectit watter tae be closer tae .

In Mey 2011, 615–1410 ppm watter in melt inclusions in lunar saumple 74220 wis reportit, the famous heich-titanium "orange gless sile" o volcanic oreegin collectit during the Apollo 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's upper 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.

Gravitational field
The gravitational field o the Muin haes been meisurt throu trackin the Doppler shift o radio signals emittit bi orbitin spacecraft. The main lunar gravity featurs are mascons, lairge positive gravitational anomalies associatit wi some o the giant impact basins, partly caused bi the dense mare basaltic lava flaws that fill thae basins. 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.

Magnetic field
The Muin haes a freemit magnetic field o aboot 1–100 nanoteslas, less nor ane-hundert that o Yird. It daes nae currently hae a global dipolar magnetic field an anly haes crustal magnetisation, probably acquired early in lunar history whan a dynamo wis still operatin. 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 the antipodes o the giant impact basins.

Atmosphere
The Muin haes an atmosphere sae tenuous as tae be nearly vacuum, wi a tot mass o less nor . The surface pressur o this smaw mass is aroond 3 × 10−15 atm (0.3 nPa); it varies wi the lunar day. Its soorces include ootgassin an sputterin, a product o the bombardment o lunar sile bi solar wind ions. Elements that hae been detectit include sodium an potassium, produced bi sputterin (an aa foond in the atmospheres o Mercur an Io); helium-4 an neon frae the solar wind; an argon-40, radon-222, an polonium-210, ootgassed efter thair creaution bi radioactive decay within the crust an mantle. The absence o sic neutral species (atoms or molecules) as oxygen, nitrogen, carbon, hydrogen an magnesium, that are present in the regolith, is nae unnerstuid. Watter vapour haes been detectit bi Chandrayaan-1 an foond tae vary wi latitude, wi a maximum at ~60–70 degrees; it is possibly generatit frae the sublimation o watter ice in the regolith. 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.

Dist
A permanent asymmetric muin dist clood exists aroond the Muin, creatit bi smaw particles frae comets. 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 bi LADEE'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 the Geminid, Quadrantid, Northren Taurid, an Omicron Centaurid meteor 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.

Saisons
The Muin's axial tilt wi respect tae the ecliptic is anly 1.5424°, 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. Frae eemages taken bi Clementine in 1994, it appears that fower moontainous regions on the rim o Peary Crater at the Muin's north pole mey remeen illuminatit for the entire lunar day, creautin peaks 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, an thir dark craters are extremely cauld: Lunar Reconnaissance Orbiter'' meisurt the lawest simmer temperaturs in craters at the soothren pole at  an juist  close tae the winter solstice in north polar Hermite Crater. This is the cauldest temperatur in the Solar Seestem iver meosurt bi a spacecraft, caulder oven than the surface o Pluto. 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.

 Relationship tae Yird

Orbit
thumb |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 27.3 days (its sidereal 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 same phase tae Yird, which is aboot 29.5 days (its synodic period). Unlik maist satellites o ither planets, the Muin orbits closer tae the ecliptic plane nor tae the planet's equatorial plane. The Muin's orbit is subtly perturbed bi the Sun an Yird in mony smaw, complex an interactin weys. For ensaumple, the plane o the Muin's orbit gradually rotates ance ivery 18.61Global influences of the 18.61 year nodal cycle and 8.85 year cycle of lunar perigee on high tidal levels, U. of Western Australia years, that affects ither aspects o lunar motion. Thir follae-on effects are mathematically describit bi Cassini's laws.

Relative size

The Muin is exceptionally lairge relative tae Yird: a quarter its diameter an 1/81 its mass. It is the lairgest muin in the Solar Seestem relative tae the size o its planet, tho Charon is lairger relative tae the dwarf planet Pluto, at 1/9 Pluto's mass. Yird an the Muin are nivertheless still conseederit a planet–satellite seestem, raither nor a dooble planet, acause thair barycentre, the common centre o mass, is locatit  (aboot a quarter o Yird's radius) aneath Yird's surface.

 Appearance frae Yird
thumb |left |Muin settin in wastren sky ower the High Desert in Californie

The Muin is in synchronous rotation: it rotates aboot its axis in aboot the same time it takes tae orbit Yird. This results in it ayeweys keepin nearly the same face turned taewart Yird. Houiver, due tae the effect o libration, 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 becam tidally locked in this orientation as a result o freectional effects associatit wi tidal deformations caused bi Yird. 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 the near side, an the opposite the faur 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. In 2016, planetary scientists, uisin data collectit on the muckle earlier Nasa Lunar 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.

The Muin haes an exceptionally law albedo, giein it a reflectance that is slichtly brichter nor that o worn asphalt. Despite this, it is the brichtest object in the sky efter the Sun. This is pairtly due tae the brichtness enhancement o the opposeetion effect; at quarter phase, the Muin is anly ane-tent as bricht, raither nor hauf as bricht, as at full muin.

Addeetionally, colour constancy in the visual 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 no limm daurkenin, due tae the reflective properties o lunar 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 the Muin illusion, first descrived in the 7t century BC. The full muin subtends an arc o aboot 0.52° (on average) in the sky, aboot the same apparent size as the Sun (see ).

The heichest altitude 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-year nodes cycle an aa haes an influence: whan the ascendin node o the lunar orbit is in the vernal equinox, the lunar declination 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.

The Muin is veesible for twa weeks ivery 27.3 days at the North an Sooth Pole. The Muin's licht is uised bi zooplankton in the Arctic whan the sun is ablo the horizon for months on end.

The distance atween the Muin an Yird varies frae aroond  tae  at perigees (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. Reportit as a "super muin", this closest pynt coincides within an oor o a full muin, an it wis 30% mair luminous than whan at its greatest distance due tae its angular diameter bein 14% greater, acause .    At lower levels, the human perception o reduced brightness as a percentage is providit bi the follaein formula:  

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. 

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, puir astronomical seein, or inadequate drawins. Houiver, ootgassin daes occasionally occur, an coud be responsible for a minor percentage o the reportit lunar transient phenomena. Recently, it haes been suggestit that aboot a  diameter region o the lunar surface wis modified bi a gas release event aboot a million years aby. 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 heich cirrostratus cloud, an smawer coronal rings whan the Muin is seen throu thin cloods.

The illuminatit area o the veesible sphere (degree o illumination) is gien bi , whaur  is the elongation (i.e., the angle atween Muin, the observer (on Yird) an the Sun).

Tidal effects

thumb |alt=Ower ane lunar month mair nor hauf o the Muin's surface can be seen frae Yird's surface. |The libration 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 in tidal 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 cried ocean tides. 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. 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 for ware an neap tides. 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, the inertia o watter's muivement, ocean basins that graw shallaer near laund, the sloshin o watter atween different ocean basins. 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 a torque in opposeetion tae the Yird's rotation. This "drains" angular momentum an rotational kinetic energy frae Yird's spin, slowing the Yird's rotation. That angular momentum, lost frae the Yird, is transferred tae the Muin in a process (confusingly kent as tidal 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 is increasin, an the Yird's spin is slawin in reaction. Measurements frae laser reflectors left in the Apollo missions (lunar rangin experiments) hae foond that the Muin's distance increases bi  per year (aboot the rate at that human fingernails growe).
Atomic clocks an aa shaw that Yird's day lenthens bi aboot 15 microseiconts ivery year, slawly increasin the rate at that UTC is adjuistit bi leap 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 wi Pluto an its muin Charon. Houiver, the Sun will become a reid giant engulfin the Yird-Muin seestem lang afore this occurrence.

In a lik manner, the lunar surface experiences tides o aroond  amplitude ower 27 days, wi twa components: a fixed ane due tae Yird, acause thay are in synchronous rotation, an a varyin component frae the Sun. The Yird-induced component arises frae libration, a result o the Muin's orbital eccentricity (if the Muin's orbit war perfectly circular, thare wad anerly be solar tides). 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. The cumulative effects o stress biggit up bi thir tidal forces produces muinquauks. 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 frae seismometers placed on the Muin bi Apollo astronauts frae 1969 throu 1972.

Eclipses

Eclipses can anerly occur whan the Sun, Yird, an Muin are aw in a straucht line (termed "syzygy"). Solar eclipses occur at new muin, whan the Muin is atween the Sun an Yird. In contrast, lunar eclipses occur at full 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 baith total (wi the Muin appearin lairger than the Sun) an annular (wi the Muin appearin smawer than the Sun) solar eclipses. In a tot eclipse, the Muin completely covers the disc o the Sun an the solar corona becomes vrrsible tae the nakit ee. Acause the distance atween the Muin an Yird is verra slawly increasin ower time, the angular diameter o the Muin is decreasin. An aa, as it evolves thewarts becomin a reid giant, the size o the Sun, an its apparent diameter in the sky, are slawly increasin. 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.

Acause the Muin's orbit aroond Yird is inclined bi aboot 5.145° (5° 9') tae the orbit 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. The periodicity an recurrence o eclipses o the Sun bi the Muin, an o the Muin bi Yird, is describit bi the saros, that haes a period o approximately 18 years.

Acause the Muin is continuously blockin oor view o a hauf-degree-wide circular area o the sky, the relatit phenomenon o occultation 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 the precession o the lunar orbit, ilk year different stars are occulted.

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Notes

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