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{{Two other uses|Earth's natural satellite|moons in general|Natural satellite}}
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{{pp-semi-protected|small=yes}}
{{pp-move-indef}}
{{Use dmy dates|date=July 2012}}
{{Infobox planet
| name = Moon
| apsis = gee
| symbol = [[File:Moon symbol decrescent.svg|19px|Moon symbol]]
| image = [[File:FullMoon2010.jpg|280px|alt=Full moon in the darkness of the night sky. It is patterned with a mix of light-tone regions and darker, irregular blotches, and scattered with varying sizes of impact craters, circles surrounded by out-thrown rays of bright ejecta.|Full moon]]
| caption = [[Full moon]] as seen from Earth's [[northern hemisphere]]
| periapsis = {{val|363295|u=km}}<br>({{val|0.0024|ul=AU}})
| apoapsis = {{val|405503|u=km}} <br>({{val|0.0027|u=AU}})
| semimajor = {{val|384399|u=km}}<br> ({{val|0.00257|u=AU}})<ref name="W06"/>
| eccentricity = {{val|0.0549}}<ref name="W06"/>
| period = {{val|27.321582|ul=d}} {{nowrap|(27&thinsp;d 7&thinsp;h 43.1&thinsp;min<ref name="W06"/>)}}
| synodic_period = {{val|29.530589|u=d}} {{nowrap|(29&thinsp;d 12&thinsp;h 44&thinsp;min 2.9&thinsp;s)}}
| avg_speed = {{val|1.022|ul=km/s}}
| inclination = 5.145° to the [[ecliptic]]<ref name="Lang2011"/> (between 18.29° and 28.58° to Earth's [[equator]])<ref name="W06"/><!--Wieczorek et al. 2006; 18.29° when the longitude of the Moon's ascending node is 180°, 28.58° when it is 0°-->
| asc_node = regressing by one [[Orbital revolution|revolution]] in 18.6&nbsp;years
| arg_peri = progressing by one revolution in 8.85&nbsp;years
| satellite_of = [[Earth]]
| flattening = {{val|0.00125}}<!--Calculated from data below-->
| equatorial_radius = {{val|1738.14|u=km}} (0.273&nbsp;Earths)<ref name="NSSDC"/>
| polar_radius = {{val|1735.97|u=km}} {{nowrap|(0.273 Earths)}}<ref name="NSSDC"/>
| mean_radius = {{val|1737.10|u=km}} {{nowrap|(0.273 Earths)}}<ref name="W06"/><ref name="NSSDC"/>
| circumference = {{val|10921|u=km}} ([[equator]]ial)
| surface_area = {{val|3.793|e=7|u=km<sup>2</sup>}} {{nowrap|(0.074 Earths)}}
| volume = {{val|2.1958|e=10|u=km<sup>3</sup>}} {{nowrap|(0.020 Earths)}}
| mass = {{val|7.3477|e=22|u=kg}} {{nowrap|({{val|0.012300}} Earths<ref name="W06"/>)}}
| density = {{val|3.3464|ul=g/cm3}}<ref name="W06"/>
| surface_grav = {{val|1.622|ul=m/s2}} {{nowrap|({{val|0.1654|u=[[G-force|g]]}})}}
| escape_velocity = {{val|2.38|ul=km/s}}
| sidereal_day = {{val|27.321582|u=d}} ([[Synchronous rotation|synchronous]])
<!-- Wieczorek et al. 2006 -->
| rot_velocity = {{nowrap|4.627 m/s}}
| axial_tilt = 1.5424° (to [[ecliptic]])<!-- 1.533° according to Conn 2007 --> <br> 6.687° (to [[Orbital plane (astronomy)|orbit plane]])<ref name="Lang2011"/><!-- 6.4° according to<ref name="Grego2005">Grego, Peter, [http://books.google.ca/books?id=z6Pgt9xEW9gC&pg=PA48 ''The Moon and How to Observe It''], Springer, 2005</ref> 6.6783° according to<ref name="Conn2007">Conn, David (2007); ''Lednorf's Dilemma'', AuthorHouse, Bloomington (IN)</ref>-->
| albedo = 0.136<ref name="Saari"/>
| magnitude = −2.5 to −12.9{{efn|name=maxval}} <br> −12.74 (mean [[full moon]])<ref name="NSSDC"/>
| angular_size = 29.3 to 34.1 [[Minute of arc|arcminutes]]<ref name="NSSDC"/>{{efn|name=angular size}}
| temp_name1 = equator
| min_temp_1 = 100 [[kelvin|K]]
| mean_temp_1 = 220 K
| max_temp_1 = 390 K
| temp_name2 = 85°N{{lower|0.3em|<ref name="Vasavada1999"/>}}
| min_temp_2 = 70 K
| mean_temp_2 = 130 K
| max_temp_2 = 230 K
| atmosphere_ref =<ref name="L06"/>
| surface_pressure = 10<sup>−7</sup> [[Pascal (unit)|Pa]] (day) <br> 10<sup>−10</sup> Pa (night){{efn|name=pressure explanation}}
| atmosphere_composition = [[Argon|Ar]], [[Helium|He]], [[Sodium|Na]], [[Potassium|K]], [[Hydrogen|H]], [[Radon|Rn]]
| atmosphere = yes
| adjectives = [[wikt:lunar|lunar]], [[wikt:selenic|selenic]]
}}
[[File:Space Telescope Science Institute - Crater Copernicus (pd).jpg|thumb|right|250px|Hubble views of [[Copernicus (lunar crater)|Copernicus crater]] (''inset''), which is about 58 miles (93 km) wide]]
[[File:US Navy 041027-N-9500T-001 The moon turns red and orange during a total lunar eclipse.jpg|thumb|right|250px|Red and orange tinted Moon, as seen from Earth during a lunar eclipse, when Earth comes between the Moon and Sun]]
[[File:346627main moonimg 09 full (1).jpg|thumb|The Moon as seen in a digitally processed image from data collected during a spacecraft flyby]]
The '''Moon''' is the only [[natural satellite]] of the [[Earth]]{{efn|name=near-Earth asteroids}}<ref name="Morais2002" /> and the [[List of natural satellites|fifth largest]] moon in the [[Solar System]]. It is the largest natural satellite of a planet in the Solar System relative to the size of its [[primary (astronomy)|primary]],{{efn|name=Charon and Pluto}} having 27% the diameter and 60% the density of Earth, resulting in {{frac|1|81}} its [[mass]]. Among satellites with known densities, the Moon is the second densest, after [[Io (moon)|Io]], a satellite of Jupiter.


The Moon is in [[synchronous rotation]] with Earth, always showing the same face with its [[near side of the Moon|near side]] marked by dark volcanic [[lunar mare|maria]] that fill between the bright ancient crustal highlands and the prominent [[impact crater]]s. It is the most [[luminosity|luminous]] object in the sky after the [[Sun]]. Although it appears a very bright white, its surface is actually dark, with a [[reflectance]] just slightly higher than that of worn asphalt. Its prominence in the sky and its regular cycle of [[lunar phases|phases]] have, since ancient times, made the Moon an important cultural influence on [[Moon#Name and etymology|language]], [[Lunar calendar|calendar]]s, [[Moon in fiction|art]] and [[Lunar deity|mythology]]. The Moon's gravitational influence produces the [[ocean tides]] and the [[tidal acceleration|minute lengthening]] of the day. The Moon's current orbital distance, about thirty times the diameter of Earth, causes it to appear almost the same size in the sky as the Sun, allowing it to cover the Sun nearly precisely in total [[solar eclipse]]s. This matching of apparent visual size is a coincidence. The Moon's linear distance from Earth is currently increasing at a rate of 3.82±0.07&nbsp;cm per year, but this rate is not constant.<ref>http://lasp.colorado.edu/life/GEOL5835/Moon_presentation_19Sept.pdf</ref>
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The Moon is thought to have formed nearly 4.5 billion years ago, not long after Earth. Although there have been several hypotheses for its origin in the past, the current most widely accepted explanation is that the Moon formed from the debris left over after a [[giant impact hypothesis|giant impact]] between Earth and a [[Mars]]-sized body.
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The Moon is the only [[Astronomical object|celestial body]] other than Earth on which [[Moon landing|humans have set foot]]. The [[Soviet Union]]'s [[Luna programme]] was the first to reach the Moon with unmanned [[spacecraft]] in 1959; the United States' [[NASA]] [[Apollo program]] achieved the only manned missions to date, beginning with the first manned lunar orbiting mission by [[Apollo 8]] in 1968, and six manned lunar landings between 1969 and 1972, with the first being [[Apollo 11]]. These missions returned over 380&nbsp;kg of [[Moon rock|lunar rocks]], which have been used to develop a geological understanding of the Moon's origins, the formation of [[internal structure of the Moon|its internal structure]], and [[geology of the Moon|its subsequent history]].
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After the [[Apollo 17]] mission in 1972, the Moon has been visited only by unmanned spacecraft. Of these, orbital missions have dominated: Since 2004, Japan, China, India, the United States, and the [[European Space Agency]] have each sent lunar orbiters, which have contributed to confirming the discovery of [[lunar water|lunar water ice]] in permanently shadowed craters at the poles and bound into the lunar [[regolith]]. The post-Apollo era has also seen two [[Rover (space exploration)|rover]] missions: the final Soviet [[Lunokhod]] mission in 1973, and China's ongoing [[Chang'e 3]] mission, which deployed its [[Yutu (rover)|Yutu rover]] on 14 December 2013.
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Future manned missions to the Moon have been planned, including government as well as privately funded efforts. The Moon remains, under the [[Outer Space Treaty]], free to all nations to explore for peaceful purposes.
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== Name and etymology ==
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The English [[proper name]] for Earth's natural satellite is "the Moon".<ref>
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  | url = http://www.iau.org/public_press/themes/naming/#spelling
|title=Naming Astronomical Objects: Spelling of Names
|publisher=[[International Astronomical Union]]
|accessdate=29 March 2010}}


</ref><ref name="PN-FAQ" /> The noun ''moon'' derives from ''moone'' (around 1380), which developed from ''mone'' (1135), which derives from [[Old English]] ''mōna'' (dating from before 725), which, like all [[Germanic languages|Germanic language]] cognates, ultimately stems from [[Proto-Germanic]] ''*mǣnōn''.<ref name="barnhart1995" />
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The principal modern English adjective pertaining to the Moon is ''lunar'', derived from the Latin ''Luna''. Another less common adjective is ''selenic'', derived from the Ancient Greek ''Selene'' ({{lang|el|''Σελήνη''}}), from which the prefix "seleno-" (as in ''[[selenography]]'') is derived.<ref name="oed" />
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{{main|Origin of the Moon|Giant impact hypothesis}}
 
[[File:Evolution of the Moon.ogv|thumb|250px|left|The evolution of the Moon and a tour of the Moon.]]
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Several mechanisms have been proposed for the Moon's formation {{nowrap|4.527 ± 0.010 billion}} years ago,{{efn|name=age}} some 30–50&nbsp;million years after the origin of the Solar System.<ref>{{cite journal|doi= 10.1126/science.1118842 |journal=[[Science (journal)|Science]] |year=2005 |volume=310 |issue=5754 |pages=1671–1674 |title=Hf–W Chronometry of Lunar Metals and the Age and Early Differentiation of the Moon |last=Kleine |first=T. |coauthors=Palme, H.; Mezger, K.; Halliday, A.N. |pmid=16308422|bibcode = 2005Sci...310.1671K }}</ref> Recent research presented by Rick Carlson indicates a slightly younger age of between 4.4 and 4.45 billion years.<ref>{{cite web |url=http://www.space.com/22894-moon-age-100-million-years-younger.html |title=Carnegie Institution for Science research |accessdate=2013-10-12}}</ref>
 
<ref>{{cite web |url=http://phys.org/news/2013-09-moon-younger-thought.html |title=Phys.org's account of Carlson's presentation to the Royal Society |accessdate=2013-10-13}}</ref> These mechanisms included the fission of the Moon from Earth's crust through [[centrifugal force]]<ref name="Binder" /> (which would require too great an initial spin of Earth),<ref name="BotM" /> the gravitational capture of a pre-formed Moon<ref name="Mitler" /> (which would require an unfeasibly extended [[Earth's atmosphere|atmosphere of Earth]] to [[Dissipation|dissipate]] the energy of the passing Moon),<ref name="BotM"/> and the co-formation of Earth and the Moon together in the primordial [[accretion disk]] (which does not explain the depletion of metallic iron in the Moon).<ref name="BotM"/> These hypotheses also cannot account for the high [[angular momentum]] of the Earth–Moon system.<ref>{{cite journal|last=Stevenson |first=D.J. |title=Origin of the moon–The collision hypothesis |journal=Annual Review of Earth and Planetary Sciences |year=1987 |volume=15|issue=1 |pages=271–315 |bibcode=1987AREPS..15..271S |doi=10.1146/annurev.ea.15.050187.001415}}</ref>
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The prevailing hypothesis today is that the Earth–Moon system formed as a result of a [[Giant impact hypothesis|giant impact]], where a [[Mars]]-sized body (named ''[[Theia (planet)|Theia]]'') collided with the newly formed [[History of the Earth|proto-Earth]], blasting material into orbit around it, which accreted to form the Moon.<ref name="taylor1998" />  This hypothesis perhaps best explains the evidence, although not perfectly. Eighteen months prior to an October 1984 conference on lunar origins, Bill Hartmann, Roger Phillips, and Jeff Taylor challenged fellow lunar scientists: "You have eighteen months. Go back to your Apollo data, go back to your computer, do whatever you have to, but make up your mind. Don’t come to our conference unless you have something to say about the Moon’s birth."  At the 1984 conference at Kona, Hawaii, the giant impact hypothesis emerged as the most popular.  "Before the conference, there were partisans of the three 'traditional' theories, plus a few people who were starting to take the giant impact seriously, and there was a huge apathetic middle who didn’t think the debate would ever be resolved.  Afterward there were essentially only two groups: the giant impact camp and the agnostics."<ref name=Dana-Mackenzie's-book>[http://books.google.com/books?id=omVaG7u4qG8C&pg=PA166&dq=1983+Bill+Hartmann+Roger+Phillips+Jeff+Taylor&hl=en&sa=X&ei=Whv5UebpLoXC9gTmwoHwAQ&ved=0CC8Q6AEwAA#v=onepage&q=1983%20Bill%20Hartmann%20Roger%20Phillips%20Jeff%20Taylor&f=false ''The Big Splat, or How Our Moon Came to Be''], Dana Mackenzie, John Wiley & Sons, Inc., Hoboken, New Jersey, 2003, pages 166-68.</ref>
  <li>[http://pedagogie-differenciee.eu/spip.php?page=auteur&id_auteur=1&lang=fr/ http://pedagogie-differenciee.eu/spip.php?page=auteur&id_auteur=1&lang=fr/]</li>
 
 
Giant impacts are thought to have been common in the early Solar System. Computer simulations modelling a giant impact are consistent with measurements of the angular momentum of the Earth–Moon system and the small size of the lunar core. These simulations also show that most of the Moon came from the impactor, not from the proto-Earth.<ref>{{cite journal|last=Canup |first=R. |coauthors=Asphaug, E. |title=Origin of the Moon in a giant impact near the end of Earth's formation |journal=Nature |volume=412 |pages=708–712 |year=2001 |doi=10.1038/35089010 |pmid=11507633 |issue=6848 |bibcode=2001Natur.412..708C}}</ref> However, more-recent tests suggest more of the Moon coalesced from Earth and not the impactor.<ref>{{cite web|url=http://news.nationalgeographic.com/news/2007/12/071219-moon-collision.html |title=Earth-Asteroid Collision Formed Moon Later Than Thought |publisher=News.nationalgeographic.com |date=28 October 2010 |accessdate=7 May 2012}}</ref><ref>http://digitalcommons.arizona.edu/objectviewer?o=uadc://azu_maps/Volume43/NumberSupplement/Touboul.pdf</ref><ref>{{cite journal | doi = 10.1038/nature06428 | title = Late formation and prolonged differentiation of the Moon inferred from W isotopes in lunar metals | year = 2007 | last1 = Touboul | first1 = M. | last2 = Kleine | first2 = T. | last3 = Bourdon | first3 = B. | last4 = Palme | first4 = H. | last5 = Wieler | first5 = R. | journal = Nature | volume = 450 | issue = 7173 | pages = 1206–9 | pmid = 18097403 |bibcode = 2007Natur.450.1206T }}</ref> [[Meteorite]]s show that other inner Solar System bodies such as [[Mars]] and [[Vesta (asteroid)|Vesta]] have very different oxygen and tungsten [[isotope|isotopic]] compositions to Earth, whereas Earth and the Moon have nearly identical isotopic compositions. Post-impact mixing of the vaporized material between the forming Earth and Moon could have equalized their isotopic compositions,<ref name="Pahlevan2007" /> although this is debated.<ref>{{cite journal|last=Nield |first=Ted |title=Moonwalk (summary of meeting at Meteoritical Society's 72nd Annual Meeting, Nancy, France) |journal=Geoscientist |volume=19 |page=8 |year=2009|url =http://www.geolsoc.org.uk/gsl/geoscientist/geonews/page6072.html}}</ref>
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The large amount of energy released in the giant impact event and the subsequent reaccretion of material in Earth orbit would have melted the outer shell of Earth, forming a magma ocean.<ref name="Warren1985" /><ref>{{cite journal|last=Tonks|first=W. Brian|coauthors=Melosh, H. Jay|year=1993|title=Magma ocean formation due to giant impacts|journal=Journal of Geophysical Research|volume=98|issue=E3|pages=5319–5333|bibcode=1993JGR....98.5319T|doi=10.1029/92JE02726}}</ref> The newly formed Moon would also have had its own [[lunar magma ocean]]; estimates for its depth range from about 500&nbsp;km to the entire radius of the Moon.<ref name="Warren1985" />
 
Despite its accuracy in explaining many lines of evidence, there are still some difficulties that are not fully explained by the giant impact hypothesis, most of them involving the Moon's composition.<ref>{{cite journal | journal = Science | author = Daniel Clery | title = Impact Theory Gets Whacked | volume = 342 | page = 183 | date = 11 October 2013}}</ref>
 
In 2001, a team at the Carnegie Institute of Washington reported the most precise measurement of the isotopic signatures of lunar rocks.<ref name=wiechert>{{Cite journal | title=Oxygen Isotopes and the Moon-Forming Giant Impact | display-authors=1 | last1=Wiechert | first1=U. | last2=Halliday | first2=A. N. | last3=Lee | first3=D.-C. | last4=Snyder | first4=G. A. | last5=Taylor | first5=L. A. | last6=Rumble | first6=D. | volume=294 | issue=12 | pages=345–348 |date=October 2001 | doi=10.1126/science.1063037 | url=http://www.sciencemag.org/cgi/content/abstract/294/5541/345 | accessdate=2009-07-05 | publisher=[[Science (journal)]] | pmid=11598294 | journal=Science |bibcode = 2001Sci...294..345W }}</ref> To their surprise, the team found that the rocks from the [[Apollo program]] carried an isotopic signature that was identical with rocks from Earth, and were different from almost all other bodies in the Solar System. Because most of the material that went into orbit to form the Moon was thought to come from [[Theia (planet)|Theia]], this observation was unexpected. In 2007, researchers from the California Institute of Technology announced that there was less than a 1% chance that Theia and Earth had identical isotopic signatures.<ref name=ps2007>{{Cite journal | last1=Pahlevan | first1=Kaveh | last2=Stevenson | first2=David | title=Equilibration in the Aftermath of the Lunar-forming Giant Impact | journal=EPSL | volume=262 | issue=3–4 |date=October 2007 | pages=438–449 | doi=10.1016/j.epsl.2007.07.055 | bibcode=2007E&PSL.262..438P |arxiv = 1012.5323 }}</ref> Published in 2012, an analysis of titanium isotopes in Apollo lunar samples showed that the Moon has the same composition as Earth,<ref name="test" /> which [[Giant impact hypothesis#Difficulties|conflicts]] with what is expected if the Moon formed far from Earth's orbit or from Theia. Variations on GIH may explain this data.
 
== Physical characteristics ==
 
=== Internal structure ===
 
{{main|Internal structure of the Moon}}
[[File:Moon diagram.svg|thumb|left|300px|Structure of the Moon]]
{| class="wikitable" style="float: right; clear: right; margin-left: 1em; text-align: center;"
|+ Chemical composition of the lunar surface regolith (derived from crustal rocks)<ref>{{cite book
|author=Taylor, Stuart Ross
|title= Lunar science: A post-Apollo view| year=1975
|page=64|publisher=New York, Pergamon Press, Inc.
|url=http://articles.adsabs.harvard.edu//full/1975lspa.book.....T/0000064.000.html}}</ref><!-- After Turkevich, A.L. (1973) PLC 4:1159; Moon. 8:365. -->
! rowspan="2"|Compound
! rowspan="2"|Formula
! colspan="2"|Composition (wt %)
|-
! style="font-size: smaller;" | Maria
! style="font-size: smaller;" | Highlands
|-
| style="text-align: left;" | [[Silicon dioxide|silica]]
| SiO<sub>2</sub>
| 45.4%
| 45.5%
|-
| style="text-align: left;" | [[Aluminium oxide|alumina]]
| Al<sub>2</sub>O<sub>3</sub>
| 14.9%
| 24.0%
|-
| style="text-align: left;" | [[Calcium oxide|lime]]
| CaO
| 11.8%
| 15.9%
|-
| style="text-align: left;" | [[iron(II) oxide]]
| FeO
| 14.1%
| 5.9%
|-
| style="text-align: left;" | [[Magnesium oxide|magnesia]]
| MgO
| 9.2%
| 7.5%
|-
| style="text-align: left;" | [[titanium dioxide]]
| TiO<sub>2</sub>
| 3.9%
| 0.6%
|-
| style="text-align: left;" | [[sodium oxide]]
| Na<sub>2</sub>O
| 0.6%
| 0.6%
|-
! colspan="2" | Total
! 99.9%
! 100.0%
|}
 
The Moon is a [[planetary differentiation|differentiated]] body: it has a [[Geochemistry|geochemically]] distinct [[Crust (geology)|crust]], [[Mantle (geology)|mantle]], and [[Planetary core|core]]. The Moon has a solid iron-rich inner core with a radius of 240 kilometers and a fluid outer core primarily made of liquid iron with a radius of roughly 300 kilometers. Around the core is a partially molten boundary layer with a radius of about 500 kilometers.<ref>{{cite web|url=http://www.nasa.gov/topics/moonmars/features/lunar_core.html|title=NASA Research Team Reveals Moon Has Earth-Like Core|publisher=NASA|date=January 6, 2011}}</ref> This structure is thought to have developed through the [[Fractional crystallization (geology)|fractional crystallization]] of a global [[lunar magma ocean|magma ocean]] shortly after the Moon's formation 4.5&nbsp;billion years ago.<ref>{{cite journal|doi = 10.1038/ngeo417|title =Timing of crystallization of the lunar magma ocean constrained by the oldest zircon|year = 2009|last1 = Nemchin|first1 = A.|last2 = Timms|first2 = N.|last3 = Pidgeon|first3 = R.|last4 = Geisler|first4 = T.|last5 = Reddy|first5 = S.|last6 = Meyer|first6 = C.|journal = Nature Geoscience|volume = 2|issue = 2|pages = 133–136|bibcode = 2009NatGe...2..133N }}</ref>
Crystallization of this magma ocean would have created a [[mafic]] mantle from the [[Precipitation (chemistry)|precipitation]] and sinking of the minerals [[olivine]], [[clinopyroxene]], and [[orthopyroxene]]; after about three-quarters of the magma ocean had crystallised, lower-density [[plagioclase]] minerals could form and float into a crust on top.<ref name="S06" /> The final liquids to crystallise would have been initially sandwiched between the crust and mantle, with a high abundance of [[Compatibility (geochemistry)|incompatible]] and heat-producing elements.<ref name="W06" />
Consistent with this, geochemical mapping from orbit shows the crust is mostly [[anorthosite]],<ref name="L06" /> and [[moon rock]] samples of the flood lavas erupted on the surface from partial melting in the mantle confirm the mafic mantle composition, which is more iron rich than that of Earth.<ref name="W06" />
Geophysical techniques suggest that the crust is on average ~50&nbsp;km thick.<ref name="W06" />
 
The Moon is the second densest satellite in the Solar System after [[Io (moon)|Io]].<ref name="Schubert2004" /> However, the inner core of the Moon is small, with a radius of about 350&nbsp;km or less;<ref name="W06" /> this is only ~20% the size of the Moon, in contrast to the ~50% of most other [[Terrestrial planet|terrestrial bodies]] {{clarify|date=August 2012}}. Its composition is not well constrained, but it is probably metallic iron alloyed with a small amount of [[sulphur]] and nickel; analyses of the Moon's time-variable rotation indicate that it is at least partly molten.<ref>{{cite journal|last = Williams|first = J.G.|coauthors = Turyshev, S.G.; Boggs, D.H.; Ratcliff, J.T.|title = Lunar laser ranging science: Gravitational physics and lunar interior and geodesy|journal = Advances in Space Research|year = 2006|volume = 37|issue = 1|page = 6771|bibcode=2006AdSpR..37...67W|doi = 10.1016/j.asr.2005.05.013|arxiv = gr-qc/0412049 }}</ref>
 
=== Surface geology ===
 
{{main|Geology of the Moon|Moon rocks}}
{{See also|Topography of the Moon|List of features on the Moon}}
{{multiple image
|align = right
|width = 170
|image1 = Moon nearside LRO.jpg
|alt1 = The dark irregular mare lava plains are prominent in the fully illuminated disk. A single bright star of ejecta, with rays stretching a third of the way across the disk, emblazons the lower centre: this is the crater Tycho.
|caption1 = [[Near side of the Moon]]
|image2 = Moon Farside LRO.jpg
|alt2 = This full disk is nearly featureless, a uniform grey surface with almost no dark mare. There are many bright overlapping dots of impact craters.
|caption2 = [[Far side of the Moon]]. Note the almost complete lack of dark [[lunar mare|maria]].<ref>{{cite web|title=Landscapes from the ancient and eroded lunar far side |publisher=esa |url=http://www.esa.int/esaSC/SEMDWNWALPE_index_0.html |accessdate=15 February 2010}}</ref>
}}
 
[[File:MoonTopoLOLA.png|thumb|345px|alt = Topography of the Moon measured from the Lunar Orbiter Laser Altimeter on the mission [[Lunar Reconnaissance Orbiter]], referenced to a sphere of radius 1737.4&nbsp;km|Topography of the Moon]]
 
The [[topography]] of the Moon has been measured with [[laser altimetry]] and [[stereoscopy|stereo image analysis]].<ref>{{cite journal|title=Topography of the South Polar Region from Clementine Stereo Imaging|author=Spudis, Paul D.; Cook, A.; Robinson, M.; Bussey, B.; Fessler, B.| bibcode=1998nvmi.conf...69S|journal=Workshop on New Views of the Moon: Integrated Remotely Sensed, Geophysical, and Sample Datasets|page=69|date=January 1998|last2=Cook|last3=Robinson|last4=Bussey|last5=Fessler}}</ref> The most visible topographic feature is the giant far-side [[South Pole–Aitken basin]], some 2,240&nbsp;km in diameter, the largest crater on the Moon and the largest known crater in the Solar System.<ref name="Spudis1994" /><ref>{{cite journal|doi =10.1029/97GL01718 |first1 = C.M.|last1 = Pieters|first2 =S.|last2 =Tompkins|first3 =J.W.|last3 =Head|first4 =P.C.|last4 =Hess|title = Mineralogy of the Mafic Anomaly in the South Pole‐Aitken Basin: Implications for excavation of the lunar mantle|journal = Geophysical Research Letters|volume = 24|issue = 15|pages = 1903–1906|year =1997|bibcode=1997GeoRL..24.1903P}}</ref> At 13&nbsp;km deep, its floor is the lowest elevation on the Moon.<ref name="Spudis1994" /><ref>{{cite web|url = http://www.psrd.hawaii.edu/July98/spa.html|title = The Biggest Hole in the Solar System|last = Taylor|first = G.J.|date = 17 July 1998|publisher = Planetary Science Research Discoveries, Hawai'i Institute of Geophysics and Planetology|accessdate =12 April 2007}}</ref> The highest elevations are found just to its north-east, and it has been suggested that this area might have been thickened by the oblique formation impact of the South Pole–Aitken basin.<ref>{{cite journal|last=Schultz|first=P. H.|date=March 1997|page=1259|volume=28|title=Forming the south-pole Aitken basin – The extreme games|journal=Conference Paper, 28th Annual Lunar and Planetary Science Conference|bibcode=1997LPI....28.1259S}}</ref> Other large impact basins, such as [[Mare Imbrium|Imbrium]], [[Mare Serenitatis|Serenitatis]], [[Mare Crisium|Crisium]], [[Mare Smythii|Smythii]], and [[Mare Orientale|Orientale]], also possess regionally low elevations and elevated rims.<ref name="Spudis1994" /> The lunar far side is on average about 1.9&nbsp;km higher than the near side.<ref name="W06" />
 
==== Volcanic features ====
 
{{main|Lunar mare}}
 
The dark and relatively featureless lunar plains which can clearly be seen with the naked eye are called ''[[lunar mare|maria]]'' ([[Latin language|Latin]] for "seas"; singular ''mare''), because they were believed by ancient astronomers to be filled with water.<ref>{{cite book| author = Wlasuk, Peter| title = Observing the Moon| url = http://books.google.com/?id=TWtLIOlPwS4C| year = 2000| publisher = Springer| isbn = 978-1-85233-193-1| page = 19 }}</ref> They are now known to be vast solidified pools of ancient [[basalt]]ic lava. Although similar to terrestrial basalts, the mare basalts have much higher abundances of iron and are completely lacking in minerals altered by water.<ref>{{cite web|url = http://www.psrd.hawaii.edu/April04/lunarAnorthosites.html|title = The Oldest Moon Rocks|last = Norman|first = M.|publisher = Planetary Science Research Discoveries|date = 21 April 2004|accessdate =12 April 2007}}</ref><ref>{{cite book| last = Varricchio| first = L.| title = Inconstant Moon| year = 2006| publisher = Xlibris Books| isbn = 978-1-59926-393-9 }}</ref> The majority of these lavas erupted or flowed into the depressions associated with [[impact crater|impact basins]]. Several [[geologic province]]s containing [[shield volcano]]es and volcanic [[lunar dome|domes]] are found within the near side maria.<ref>{{cite journal|last = Head|first = L.W.J.W.|title = Lunar Gruithuisen and Mairan domes: Rheology and mode of emplacement|journal = Journal of Geophysical Research|year = 2003|volume = 108|url = http://www.agu.org/pubs/crossref/2003/2002JE001909.shtml|accessdate =12 April 2007|issue = E2|doi = 10.1029/2002JE001909|page = 5012|bibcode=2003JGRE..108.5012W}}</ref>
 
Maria are found almost exclusively on the near side of the Moon, covering 31% of the surface on the near side,<ref name="worldbook" /> compared with a few scattered patches on the far side covering only 2%.<ref>{{cite journal|last = Gillis|first = J.J.|coauthors = Spudis, P.D.|title = The Composition and Geologic Setting of Lunar Far Side Maria|journal = Lunar and Planetary Science|year = 1996|volume = 27|pages = 413–404|bibcode = 1996LPI....27..413G|last2 = Spudis}}</ref> This is thought to be due to a [[KREEP|concentration of heat-producing elements]] under the crust on the near side, seen on geochemical maps obtained by ''[[Lunar Prospector]]'''s gamma-ray spectrometer, which would have caused the underlying mantle to heat up, partially melt, rise to the surface and erupt.<ref name="S06" /><ref>{{cite journal|url=http://www.sciencemag.org/cgi/content/full/281/5382/1484|title=Global Elemental Maps of the Moon: The Lunar Prospector Gamma-Ray Spectrometer|author=Lawrence|journal=[[Science (journal)|Science]]|volume=281|issue=5382|pages=1484–1489|doi=10.1126/science.281.5382.1484|issn=1095-9203|publisher=HighWire Press|date=11 August 1998|accessdate=29 August 2009|pmid=9727970|bibcode = 1998Sci...281.1484L|author-separator=,|author2=D. J.|display-authors=2|last3=Barraclough|first3=BL|last4=Binder|first4=AB|last5=Elphic|first5=RC|last6=Maurice|first6=S|last7=Thomsen|first7=DR }}</ref><ref>{{cite web|url = http://www.psrd.hawaii.edu/Aug00/newMoon.html|title = A New Moon for the Twenty-First Century|last = Taylor|first = G.J.|publisher = Planetary Science Research Discoveries, Hawai'i Institute of Geophysics and Planetology|date = 31 August 2000|accessdate =12 April 2007}}</ref> Most of the Moon's [[lunar mare|mare basalts]] erupted during the Imbrian period, 3.0–3.5&nbsp;billion years ago, although some radiometrically dated samples are as old as 4.2&nbsp;billion years,<ref name="Papike" /> and the youngest eruptions, dated by [[crater counting]], appear to have been only 1.2&nbsp;billion years ago.<ref name="Hiesinger" />
 
The lighter-coloured regions of the Moon are called ''terrae'', or more commonly ''highlands'', because they are higher than most maria. They have been radiometrically dated as forming 4.4&nbsp;billion years ago, and may represent [[plagioclase]] [[Cumulate rock|cumulates]] of the [[lunar magma ocean]].<ref name="Papike" /><ref name="Hiesinger" /> In contrast to Earth, no major lunar mountains are believed to have formed as a result of tectonic events.<ref>{{cite web|last = Munsell|first = K.|publisher = NASA|work = Solar System Exploration|title = Majestic Mountains|url = http://sse.jpl.nasa.gov/educ/themes/display.cfm?Item=mountains|date = 4 December 2006|accessdate =12 April 2007}}</ref>
 
The concentration of mare on the Near Side likely reflects the substantially thicker crust of the highlands of the Far Side, which may have formed in a slow-velocity impact of a second terran moon a few tens of millions of years after the formations of the moons themselves.<ref>{{cite web|author=Richard Lovett |url=http://www.nature.com/news/2011/110803/full/news.2011.456.html#B1 |title=Early Earth may have had two moons : Nature News |publisher=Nature.com |date= |accessdate=2012-11-01}}</ref><ref>{{cite web|url=http://theconversation.edu.au/was-our-two-faced-moon-in-a-small-collision-2659 |title=Was our two-faced moon in a small collision? |publisher=Theconversation.edu.au |date= |accessdate=2012-11-01}}</ref>
 
==== Impact craters ====
[[File:Moon-craters.jpg|right|thumb|alt=A grey, many-ridged surface from high above. The largest feature is a circular ringed structure with high walled sides and a lower central peak: the entire surface out to the horizon is filled with similar structures that are smaller and overlapping.|Lunar crater [[Daedalus (crater)|Daedalus]] on the Moon's far side]]
{{See also|List of craters on the Moon}}
 
The other major geologic process that has affected the Moon's surface is [[impact crater]]ing,<ref>{{cite book| last = Melosh| first = H. J.| title = Impact cratering: A geologic process| year = 1989| publisher = Oxford Univ. Press| isbn = 978-0-19-504284-9 }}</ref> with craters formed when asteroids and comets collide with the lunar surface. There are estimated to be roughly 300,000 craters wider than 1&nbsp;km on the Moon's near side alone.<ref>{{cite web|title=Moon Facts|url=http://planck.esa.int/science-e/www/object/index.cfm?fobjectid=31412|work=SMART-1|publisher=European Space Agency|year=2010|accessdate=12 May 2010}}</ref> Some of these are [[Selenography#Mapping and naming the Moon|named]] for scholars, scientists, artists and explorers.<ref name="gazetteer" /> The [[lunar geologic timescale]] is based on the most prominent impact events, including [[Nectarian|Nectaris]], [[Lower Imbrian|Imbrium]], and [[Mare Orientale|Orientale]], structures characterized by multiple rings of uplifted material, typically hundreds to thousands of kilometres in diameter and associated with a broad apron of ejecta deposits that form a regional [[stratigraphy|stratigraphic horizon]].<ref name="geologic" /> The lack of an atmosphere, weather and recent geological processes mean that many of these craters are well-preserved. Although only a few multi-ring basins have been definitively dated, they are useful for assigning relative ages. Because impact craters accumulate at a nearly constant rate, counting the number of craters per unit area can be used to estimate the age of the surface.<ref name="geologic" /> The radiometric ages of impact-melted rocks collected during the [[Apollo missions]] cluster between 3.8 and 4.1&nbsp;billion years old: this has been used to propose a [[Late Heavy Bombardment]] of impacts.<ref>{{cite journal|last=Hartmann |first=William K. |last2=Quantin |first2=Cathy |last3=Mangold |first3=Nicolas |year=2007 |volume=186|issue=1 |pages=11–23 |journal=Icarus |title=Possible long-term decline in impact rates: 2. Lunar impact-melt data regarding impact history |doi=10.1016/j.icarus.2006.09.009 |postscript=<!--None--> |bibcode=2007Icar..186...11H}}</ref>
 
Blanketed on top of the Moon's crust is a highly [[Comminution|comminuted]] (broken into ever smaller particles) and [[impact gardening|impact gardened]] surface layer called [[regolith]], formed by impact processes. The finer regolith, the [[lunar soil]] of [[silicon dioxide]] glass, has a texture like snow and smell like spent [[gunpowder]].<ref>{{cite web|date = 30 January 2006|accessdate =15 March 2010|url = http://science.nasa.gov/headlines/y2006/30jan_smellofmoondust.htm|title = The Smell of Moondust|publisher = NASA}}</ref> The regolith of older surfaces is generally thicker than for younger surfaces: it varies in thickness from 10–20&nbsp;m in the highlands and 3–5&nbsp;m in the maria.<ref>{{cite book| last = Heiken| first = G.| coauthors = Vaniman, D.; French, B. (eds.)| title = Lunar Sourcebook, a user's guide to the Moon| year = 1991| publisher = Cambridge University Press| location = New York| isbn = 978-0-521-33444-0| page = 736 }}</ref>
Beneath the finely comminuted regolith layer is the ''megaregolith'', a layer of highly fractured bedrock many kilometres thick.<ref>{{cite journal|last = Rasmussen|first = K.L.|coauthors = Warren, P.H.|title = Megaregolith thickness, heat flow, and the bulk composition of the Moon|journal = Nature|year = 1985|volume = 313|issue = 5998|pages = 121–124|bibcode = 1985Natur.313..121R|doi = 10.1038/313121a0}}</ref>
 
==== Presence of water ====
 
{{main|Lunar water}}
[[File:Moon South Pole.jpg|thumbnail|alt=Twenty degrees of latitude of the Moon's disk, completely covered in the overlapping circles of craters. The illumination angles are from all directions, keeping almost all the crater floors in sunlight, but a set of merged crater floors right at the south pole are completely shadowed.|Mosaic image of the lunar south pole as taken by ''[[Clementine (spacecraft)|Clementine]]'': note permanent polar shadow.]]
 
Liquid water cannot persist on the lunar surface. When exposed to solar radiation, water quickly decomposes through a process known as [[photodissociation]] and is lost to space. However, since the 1960s, scientists have hypothesized that water ice may be deposited by impacting [[comets]] or possibly produced by the reaction of oxygen-rich lunar rocks, and hydrogen from [[solar wind]], leaving traces of water which could possibly survive in cold, permanently shadowed craters at either pole on the Moon.<ref name="Margot1999" /><ref>
 
{{cite journal
| first = William R. | last = Ward
| title = Past Orientation of the Lunar Spin Axis
| journal = Science
| date = 1 August 1975
| volume = 189
| issue = 4200 | pages = 377–379
| doi = 10.1126/science.189.4200.377
| pmid = 17840827
| bibcode = 1975Sci...189..377W }}
 
</ref> Computer simulations suggest that up to 14,000&nbsp;km<sup>2</sup> of the surface may be in permanent shadow.<ref name="M03" /> The presence of usable quantities of water on the Moon is an important factor in rendering [[Colonization of the Moon|lunar habitation]] as a cost-effective plan; the alternative of transporting water from Earth would be prohibitively expensive.<ref name="seedhouse2009" />
 
In years since, signatures of water have been found to exist on the lunar surface.<ref name="moonwater_18032010" /> In 1994, the [[Clementine mission#Bistatic Radar Experiment|bistatic radar experiment]] located on the ''[[Clementine (spacecraft)|Clementine]]'' spacecraft, indicated the existence of small, frozen pockets of water close to the surface. However, later radar observations by [[Arecibo Observatory|Arecibo]], suggest these findings may rather be rocks ejected from young impact craters.<ref>{{cite web| last= Spudis|first = P.|title = Ice on the Moon|url = http://www.thespacereview.com/article/740/1|publisher = The Space Review|date = 6 November 2006|accessdate =12 April 2007}}</ref> In 1998, the [[Lunar Prospector#Neutron Spectrometer (NS)|neutron spectrometer]] located on the ''Lunar Prospector'' spacecraft, indicated that high concentrations of hydrogen are present in the first meter of depth in the regolith near the polar regions.<ref name="Feldman1998" /> In 2008, an analysis of volcanic lava beads, brought back to Earth aboard Apollo 15, showed small amounts of water to exist in the interior of the beads.<ref name="Saal2008" />
 
The 2008 ''[[Chandrayaan-1]]'' spacecraft has since confirmed the existence of surface water ice, using the on-board [[Moon Mineralogy Mapper]]. The spectrometer observed absorption lines common to [[hydroxyl]], in reflected sunlight, providing evidence of large quantities of water ice, on the lunar surface. The spacecraft showed that concentrations may possibly be as high as 1,000&nbsp;[[parts per million|ppm]].<ref name="Pieters2009" /> In 2009, ''[[LCROSS]]'' sent a 2300&nbsp;kg impactor into a permanently shadowed polar crater, and detected at least 100&nbsp;kg of water in a plume of ejected material.<ref name="Planetary" /><ref name="Colaprete" /> Another examination of the LCROSS data showed the amount of detected water, to be closer to 155 kilograms (± 12&nbsp;kg).<!--, or 5.6% (±2.9%) by mass.--This seems too technical for this overview--><ref name="Colaprete2010" />
 
In May 2011, Erik Hauri et al. reported<ref name="hauri" /> 615–1410 ppm water in [[melt inclusions]] in lunar sample 74220, the famous high-titanium "orange glass soil" of volcanic origin collected during the [[Apollo 17]] mission in 1972. The inclusions were formed during explosive eruptions on the Moon approximately 3.7 billion years ago. This concentration is comparable with that of magma in Earth's [[upper mantle]]. Although of considerable selenological interest, Hauri's announcement affords little comfort to would-be lunar colonists—the sample originated many kilometers below the surface, and the inclusions are so difficult to access that it took 39 years to find them with a state-of-the-art ion microprobe instrument.
 
=== Gravitational field ===
{{main|Gravity of the Moon}}
[[File:PIA16587 - GRAIL's Gravity Field of the Moon.jpg|thumb|Gravity field of the Moon]]
The gravitational field of the Moon has been measured through tracking the [[Doppler effect|Doppler shift]] of radio signals emitted by orbiting spacecraft.
The main lunar gravity features are [[mass concentration (astronomy)|mascons]], large positive gravitational anomalies associated with some of the giant [[impact crater|impact basins]], partly caused by the dense mare basaltic lava flows that fill these basins.<ref>{{cite journal|last = Muller|first = P.|coauthors = Sjogren, W.|title = Mascons: lunar mass concentrations|journal = Science|volume = 161 |pages = 680–684|year = 1968|doi = 10.1126/science.161.3842.680|pmid = 17801458|issue = 3842|bibcode = 1968Sci...161..680M }}</ref><ref>{{cite journal | journal = Science | author = Richard A. Kerr | title = The Mystery of Our Moon's Gravitational Bumps Solved? | volume = 340 | page = 128 | date = 12 April 2013}}</ref> These anomalies greatly influence the orbit of spacecraft about the Moon. There are some puzzles: lava flows by themselves cannot explain all of the gravitational signature, and some mascons exist that are not linked to mare volcanism.<ref>{{cite journal|last = Konopliv|first = A.|coauthors = Asmar, S.; Carranza, E.; Sjogren, W.; Yuan, D.|title = Recent gravity models as a result of the Lunar Prospector mission|journal = Icarus|volume = 50|issue = 1|pages = 1–18|year = 2001|doi = 10.1006/icar.2000.6573|bibcode=2001Icar..150....1K}}</ref>
 
===Magnetic field===
{{Main|Magnetic field of the Moon}}
The Moon has an external [[magnetic field]] of about 1–100 [[Tesla (unit)|nanoteslas]], less than one-hundredth [[Earth's magnetic field|that of Earth]]. It does not currently have a global [[dipole|dipolar]] magnetic field, as would be generated by a liquid metal core [[geodynamo]], and only has crustal magnetization, probably acquired early in lunar history when a geodynamo was still operating.<ref name="GB2009" /><ref>{{cite web|url = http://lunar.arc.nasa.gov/results/magelres.htm|publisher = Lunar Prospector (NASA)|title = Magnetometer / Electron Reflectometer Results|year = 2001|accessdate =17 March 2010}}</ref> Alternatively, some of the remnant magnetization may be from transient magnetic fields generated during large impact events, through the expansion of an impact-generated plasma cloud in the presence of an ambient magnetic field—this is supported by the apparent location of the largest crustal magnetizations near the [[antipodes]] of the giant impact basins.<ref>{{cite journal|last = Hood|first = L.L.|coauthors = Huang, Z.|title = Formation of magnetic anomalies antipodal to lunar impact basins: Two-dimensional model calculations|journal = J. Geophys. Res.|volume = 96|issue = B6|pages = 9837–9846|year = 1991|doi = 10.1029/91JB00308|bibcode=1991JGR....96.9837H}}</ref>
 
=== Atmosphere ===
[[File:Apollo 17 twilight ray sketch.jpg|thumb|At sunrise and sunset many Apollo crews saw glows and light rays.<ref>{{cite web|url=http://science.nasa.gov/science-news/science-at-nasa/2005/07dec_moonstorms/ |title=Moon Storms |publisher=Science.nasa.gov |date=2013-09-27 |accessdate=2013-10-03}}</ref>]]
{{main|Atmosphere of the Moon}}
 
The Moon has an atmosphere so tenuous as to be nearly [[vacuum]], with a total mass of less than 10&nbsp;metric tons.<ref>{{cite book| editor=Richard D. Johnson & Charles Holbrow |last = Globus|first = Ruth|title=Space Settlements: A Design Study|chapter = Chapter 5, Appendix J: Impact Upon Lunar Atmosphere|publisher=NASA|url = http://settlement.arc.nasa.gov/75SummerStudy/5appendJ.html| year = 1977|accessdate=17 March 2010}}</ref> The surface pressure of this small mass is around 3{{esp|−15}}&nbsp;[[Atmosphere (unit)|atm]] (0.3&nbsp;[[nanopascal|nPa]]); it varies with the lunar day. Its sources include [[outgassing]] and [[sputtering]], the release of atoms from the bombardment of lunar soil by [[solar wind]] ions.<ref name="L06" /><ref>{{cite journal|last=Crotts |first=Arlin P.S. |title=Lunar Outgassing, Transient Phenomena and The Return to The Moon, I: Existing Data |year=2008 |publisher=Department of Astronomy, Columbia University |url=http://www.astro.columbia.edu/~arlin/TLP/paper1.pdf |format=PDF|accessdate=29 September 2009|bibcode=2008ApJ...687..692C|volume=687|page=692|journal=The Astrophysical Journal|doi=10.1086/591634|arxiv = 0706.3949 }}</ref> Elements that have been detected include [[sodium]] and [[potassium]], produced by sputtering, which are also found in the atmospheres of [[Mercury (planet)|Mercury]] and [[Io (moon)|Io]]; [[helium-4]] from the solar wind; and [[Argon|argon-40]], [[Radon|radon-222]], and [[polonium-210]], outgassed after their creation by [[radioactive decay]] within the crust and mantle.<ref name="Stern1999" /><ref>{{cite journal|last = Lawson|first = S.|coauthors = Feldman, W.; Lawrence, D.; Moore, K.; Elphic, R.; Belian, R.|title = Recent outgassing from the lunar surface: the Lunar Prospector alpha particle spectrometer|journal = J. Geophys. Res.|volume = 110|issue = E9|page=1029|year = 2005|doi = 10.1029/2005JE002433|bibcode=2005JGRE..11009009L}}</ref> 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.<ref name="Stern1999" /> Water vapour has been detected by ''[[Chandrayaan-1]]'' and found to vary with latitude, with a maximum at ~60–70&nbsp;degrees; it is possibly generated from the [[sublimation (chemistry)|sublimation]] of water ice in the regolith.<ref name="Sridharan2010" /> These gases can either return into the regolith due to the Moon's gravity or be lost to space, either through solar radiation pressure or, if they are ionized, by being swept away by the solar wind's magnetic field.<ref name="Stern1999" />
 
=== Seasons ===
[[File:The Moon's North Pole.jpg|thumb|left|The Moon's north pole during summer.]]
 
The Moon's [[axial tilt]] with respect to the [[ecliptic]] is only 1.5424°,<ref name="SolarViews" /> much less than the 23.44° of Earth. Because of this, the Moon's solar illumination varies much less with season, and topographical details play a crucial role in seasonal effects.<ref name="bbc" /> From images taken by ''[[Clementine (spacecraft)|Clementine]]'' in 1994, it appears that four mountainous regions on the rim of [[Peary (crater)|Peary crater]] at the Moon's north pole may remain illuminated for the entire lunar day, creating [[Peak of Eternal Light|peaks of eternal light]]. No such regions exist at the south pole. Similarly, there are places that remain in permanent shadow at the bottoms of many polar craters,<ref name="M03" /> and these dark craters are extremely cold: ''[[Lunar Reconnaissance Orbiter]]'' measured the lowest summer temperatures in craters at the southern pole at 35&nbsp;K (−238&nbsp;°C)<ref>{{cite web |date=17 September 2009 |url=http://www.diviner.ucla.edu/blog/?p=123 |title=Diviner News |publisher=UCLA |accessdate=17 March 2010 }}</ref> and just 26&nbsp;K close to the winter solstice in north polar [[Hermite (crater)|Hermite Crater]]. This is the coldest temperature in the Solar System ever measured by a spacecraft, colder even than the surface of [[Pluto]].<ref name="bbc" />
 
== {{anchor|Orbit and relationship to Earth}} Relationship to Earth ==
 
[[File:Earth-Moon.PNG|thumb|350px|alt=Earth has a pronounced axial tilt; the Moon's orbit is not perpendicular to Earth's axis, but lies close to Earth's orbital plane.|Schematic of the Earth–Moon system (without a consistent scale)]]
 
=== Orbit ===
 
{{main|Orbit of the Moon|Lunar theory}}
 
The Moon makes a complete orbit around Earth with respect to the fixed stars about once every 27.3&nbsp;days{{efn|name=orbpd}} (its [[sidereal period]]). However, because Earth is moving in its orbit around the Sun at the same time, it takes slightly longer for the Moon to show the same [[lunar phase|phase]] to Earth, which is about 29.5&nbsp;days{{efn|name=synpd}} (its [[synodic period]]).<ref name="worldbook"/> Unlike most satellites of other planets, the Moon orbits closer to the [[ecliptic plane]] than to the planet's [[equatorial plane]]. The Moon's orbit is subtly [[perturbation (astronomy)|perturbed]] by the Sun and Earth in many small, complex and interacting ways. For example, the plane of the Moon's orbital motion [[precession|gradually rotates]], which affects other aspects of lunar motion. These follow-on effects are mathematically described by [[Cassini's Laws|Cassini's laws]].<ref name="Beletskii2" />
 
<!-- {{unreferenced section|date=July 2013}}
The Moon is unique among natural satellites in that it experiences a stronger gravitational attraction to the Sun than to its primary, Earth. As a consequence, its path is always concave to the Sun. It can be argued that this makes the Moon a planet, orbiting the Sun, rather than a satellite of Earth. Usually, it is considered to be in orbit around Earth, but its orbit is substantially distorted from a simple elliptical shape by the gravity of the Sun, which includes a tidal gradient that causes the Moon to be attracted less strongly in the direction of Earth at full and new moon than at the quarter phases (in a frame of reference in which Earth is stationary). This perturbs the orbit so as to make its curvature more acute in the directions of the quarter phases than elsewhere. If the orbit were otherwise circular, this perturbation would make it approximately elliptical, with its major axis lying along the direction of Earth's motion around the Sun. Earth would be at the centre of this ellipse, rather than at one of its foci. In reality, this perturbation is superimposed on the elliptical orbit of the Moon, rotating with the seasons. The Moon's motion is therefore quite complex, and can be calculated only very approximately by assuming the orbit to be an ellipse. Likewise, the parameters of the orbit, eccentricity, semimajor axis, etc., can be stated only as approximate averages.-->
 
[[File:Speed of light from Earth to Moon.gif|thumb|600px|center|Earth and Moon, showing their sizes and distance to scale. The yellow bar represents a pulse of light traveling from Earth to Moon (approx. 400,000 km or 250,000 mi) in 1.26 seconds.]]
{{clear}}
 
=== Relative size ===
[[File:Earth and Moon Visualization.ogg|thumb|300px|A view of the Moon orbiting Earth from a point above the north ecliptic pole. Objects are to scale.]]
 
The Moon is exceptionally large relative to Earth: a quarter the diameter of the planet and 1/81 its mass.<ref name="worldbook" /> It is the largest moon in the Solar System relative to the size of its planet, though [[Charon (moon)|Charon]] is larger relative to the [[dwarf planet]] [[Pluto]], at 1/9 Pluto's mass.<ref>{{cite web|url=http://www.planetary.org/explore/topics/pluto/|title=Space Topics: Pluto and Charon|publisher=The Planetary Society|accessdate=6 April 2010}}</ref>
 
However, Earth and the Moon are still considered a planet–satellite system, rather than a [[double planet|double-planet]] system, because their [[Earth–Moon barycenter|barycentre]], the common centre of mass, is located 1,700&nbsp;km (about a quarter of Earth's radius) beneath Earth's surface.<ref>{{cite web|title=Planet Definition Questions & Answers Sheet|publisher= International Astronomical Union|year=2006|url=http://www.iau.org/public_press/news/release/iau0601/q_answers/|accessdate=24 March 2010}}</ref>
 
=== Appearance from Earth{{Anchor|Observation}}{{Anchor|Appearance from Earth}} ===
[[File:Mountain Moonset.jpg|thumb|right|280px|Moon setting in western sky over [[High Desert (California)]]]]
 
{{See also|Lunar phase|Earthshine|Observing the Moon}}
 
The Moon is in [[synchronous rotation]]: it rotates about its axis in about the same time it takes to [[orbit]] Earth. This results in it nearly always keeping the same face turned towards Earth. The Moon used to rotate at a faster rate, but early in its history, its rotation slowed and became [[Tidal locking|tidally locked]] in this orientation as a result of [[friction]]al effects associated with [[tidal force|tidal]] deformations caused by Earth.<ref>{{cite journal|last = Alexander|first = M. E.|title = The Weak Friction Approximation and Tidal Evolution in Close Binary Systems|journal = Astrophysics and Space Science|year = 1973|volume = 23|issue = 2|pages = 459–508|bibcode = 1973Ap&SS..23..459A|doi = 10.1007/BF00645172}}</ref> The side of the Moon that faces Earth is called the [[Near side of the Moon|near side]], and the opposite side the [[Far side of the Moon|far side]]. The far side is often inaccurately called the "dark side", but in fact, it is illuminated as often as the near side: once per lunar day, during the new moon phase we observe on Earth when the near side is dark.<ref>{{cite web|title=Dark Side of the Moon|author=[[Phil Plait]]|publisher=Bad Astronomy:Misconceptions |url=http://www.badastronomy.com/bad/misc/dark_side.html |accessdate=15 February 2010}}</ref>
 
The Moon has an exceptionally low [[albedo]], giving it a reflectance that is slightly brighter than that of worn asphalt. Despite this, it is the brightest object in the sky after the [[Sun]].<ref name="worldbook" />{{efn|name=brightness}} This is partly due to the brightness enhancement of the [[opposition effect]]; at quarter phase, the Moon is only one-tenth as bright, rather than half as bright, as at full moon.<ref name="Moon" />
 
Additionally, [[colour constancy]] in the [[visual system]] recalibrates the relations between the colours of an object and its surroundings, and because the surrounding sky is comparatively dark, the sunlit Moon is perceived as a bright object. The edges of the full moon seem as bright as the centre, with no [[limb darkening]], due to the [[Lambert's cosine law#Lambertian scatterers|reflective properties]] of [[lunar soil]], which reflects more light back towards the Sun than in other directions. The Moon does appear larger when close to the horizon, but this is a purely psychological effect, known as the [[Moon illusion]], first described in the 7th century BC.<ref>{{cite book| last = Hershenson| first = Maurice| title = The Moon illusion| year = 1989| publisher = Routledge| isbn = 978-0-8058-0121-7| page = 5 }}</ref> The full moon subtends an arc of about 0.52° (on average) in the sky, roughly the same apparent size as the Sun (see [[Solar and lunar eclipses|eclipses]]).
 
{{wide image|Moon_phases_en.jpg|800px|The monthly changes of angle between the direction of illumination by the Sun and viewing from Earth, and the phases of the Moon that result}}
The highest [[altitude (astronomy)|altitude]] of the Moon in the sky varies: although it has nearly the same limit as the Sun, it alters with the lunar phase and with the season of the year, with the full moon highest during winter. The 18.6-year [[lunar node|nodes cycle]] also has an influence: when the [[orbital node|ascending node]] of the lunar orbit is in the [[vernal equinox]], the lunar [[declination]] can go as far as 28° each month. This means the Moon can go overhead at latitudes up to 28° from the equator, instead of only 18°. The orientation of the Moon's crescent also depends on the latitude of the observation site: close to the equator, an observer can see a smile-shaped crescent Moon.<ref>{{cite web|url = http://curious.astro.cornell.edu/question.php?number=393|publisher = Curious About Astronomy|title = Is the Moon seen as a crescent (and not a "boat") all over the world?|date = 18 October 2002|first = K.| last = Spekkens|accessdate =16 March 2010}}</ref>
 
The distance between the Moon and Earth varies from around 356,400&nbsp;km to 406,700&nbsp;km at the extreme [[Apsis|perigees]] (closest) and apogees (farthest). On 19 March 2011, it was closer to Earth when at full phase than it has been since 1993, 14% closer than its farthest position in apogee.<ref>{{cite web|url=http://www.pe.com/localnews/stories/PE_News_Local_D_moon19.23a6364.html|title=Full moon tonight is as close as it gets|date=18 March 2011|accessdate=19 March 2011|publisher=The Press Enterprise}}</ref> Reported as a "[[Supermoon|super moon]]", this closest point coincides within an hour of a [[full moon]], and it was 30% more luminous than when at its greatest distance due to its angular diameter being 14% greater, because <math>\scriptstyle1.14^2\approx1.30</math>.<ref>
 
{{cite web
  | title = Super Full Moon
  | date = 16 March 2011
  | author = Dr. Tony Phillips
  | publisher = NASA
  | accessdate =19 March 2011
  | url = http://science.nasa.gov/science-news/science-at-nasa/2011/16mar_supermoon/}}
 
</ref><ref>
 
{{cite web
  | title = Full moon tonight is as close as it gets
  | date = 18 March 2011
  |author = Richard K. De Atley
  |publisher = The Press-Enterprise
  | accessdate =19 March 2011
  | url = http://www.pe.com/localnews/stories/PE_News_Local_D_moon19.23a6364.html
}}</ref><ref>
 
{{cite web|url=http://www.guardian.co.uk/science/2011/mar/19/super-moon-closest-point-years|title='Super moon' to reach closest point for almost 20 years|work=The Guardian |date=19 March 2011|accessdate=19 March 2011}}</ref> At lower levels, the human perception of reduced brightness as a percentage is provided by the following formula:<ref>
{{cite web
  |last=Georgia State University, Dept. of Physics (Astronomy)
  |title=Perceived Brightness
  |url=http://hyperphysics.phy-astr.gsu.edu/hbase/vision/bright.html
  |work=Brightnes and Night/Day Sensitivity
  |publisher=Georgia State University, GA, USA
  |accessdate=25 January 2014
}}</ref><ref>
{{cite web
  |last=Lutron
  |title=Measured light vs. perceived light
  |url=http://www.lutron.com/TechnicalDocumentLibrary/Measured_vs_Perceived.pdf
  |work=From IES Lighting Handbook 2000, 27-4
  |publisher=Lutron.com
  |accessdate=25 January 2014
}}</ref>
 
<math>perceived-reduction % = 100 \times \sqrt{actual-reduction % \over 100}</math>
 
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% between apogee and perigee moons of the same phase.<ref>
{{cite web
  |last=Walker
  |first=John
  |title=Inconstant Moon
  |url=http://www.fourmilab.ch/earthview/moon_ap_per.html
  |work=Earth and Moon Viewer
  |publisher=Fourmilab, Switzerland
  |accessdate=23 January 2014
  |location=Fourth paragraph of "How Bright the Moonlight"
  |date=May 1997
  |quote=14% [...] due to the logarithmic response of the human eye.
}}</ref>
 
There has been historical controversy over whether features on the Moon's surface change over time. Today, many of these claims are thought to be illusory, resulting from observation under different lighting conditions, poor [[astronomical seeing]], or inadequate drawings. However, [[outgassing]] does occasionally occur, and could be responsible for a minor percentage of the reported [[transient lunar phenomenon|lunar transient phenomena]]. Recently, it has been suggested that a roughly 3&nbsp;km diameter region of the lunar surface was modified by a gas release event about a million years ago.<ref>{{cite web|url = http://www.psrd.hawaii.edu/Nov06/MoonGas.html|last = Taylor|first = G.J.|title = Recent Gas Escape from the Moon|publisher = Planetary Science Research Discoveries, Hawai'i Institute of Geophysics and Planetology|date = 8 November 2006|accessdate =4 April 2007}}</ref><ref>{{cite journal|last = Schultz|first = P.H.|coauthors = Staid, M.I.; Pieters, C.M.|year = 2006|title = Lunar activity from recent gas release|journal= Nature |volume = 444 |pages = 184–186|doi = 10.1038/nature05303|pmid = 17093445|issue = 7116|bibcode = 2006Natur.444..184S }}</ref> The Moon's appearance, like that of the Sun, can be affected by Earth's atmosphere: common effects are a 22° [[Halo (optical phenomenon)|halo ring]] formed when the Moon's light is refracted through the ice crystals of high [[cirrostratus]] cloud, and smaller [[Corona (meteorology)|coronal rings]] when the Moon is seen through thin clouds.<ref>{{cite web|url=http://ww2010.atmos.uiuc.edu/%28Gh%29/guides/mtr/opt/ice/halo/22.rxml|title=22 Degree Halo: a ring of light 22 degrees from the sun or moon |publisher=Department of Atmospheric Sciences at the University of Illinois at Urbana-Champaign.|accessdate=13 April 2010}}</ref>
 
=== Tidal effects ===
 
{{main|Tidal force|Tidal acceleration|Tide|Theory of tides}}
 
The tides on Earth are mostly generated by the gradient in intensity of the Moon's gravitational pull from one side of Earth to the other, the [[tidal forces]]. This forms two tidal bulges on Earth, which are most clearly seen in elevated sea level as [[tide|ocean tides]].<ref name="Lambeck1977" /> Because Earth spins about 27&nbsp;times faster than the Moon moves around it, the bulges are dragged along with Earth's surface faster than the Moon moves, rotating around Earth once a day as it spins on its axis.<ref name="Lambeck1977" /> The ocean tides are magnified by other effects: frictional coupling of water to Earth's rotation through the ocean floors, the [[inertia]] of water's movement, ocean basins that get shallower near land, and oscillations between different ocean basins.<ref>{{cite journal|last=Le Provost|first=C.|coauthors=Bennett, A. F.; Cartwright, D. E.|year=1995|title=Ocean Tides for and from TOPEX/POSEIDON|pages=639–42|journal=Science|pmid=17745840|volume=267|issue=5198|bibcode=1995Sci...267..639L|doi=10.1126/science.267.5198.639}}</ref> The gravitational attraction of the Sun on Earth's oceans is almost half that of the Moon, and their gravitational interplay is responsible for [[Spring tide|spring and neap tides]].<ref name="Lambeck1977" />
[[File:Lunar libration with phase Oct 2007 450px.gif|thumb|thumbtime=0:02|alt=Over one lunar month more than half of the Moon's surface can be seen from the surface of Earth.|The [[libration]] of the Moon over a single lunar month.]]
 
Gravitational coupling between the Moon and the bulge nearest the Moon acts as a [[torque]] on Earth's rotation, draining [[angular momentum]] and rotational [[kinetic energy]] from Earth's spin.<ref name="Lambeck1977" /><ref name="touma1994" /> In turn, angular momentum is added to the Moon's orbit, accelerating it, which lifts the Moon into a higher orbit with a longer period.
As a result, the distance between Earth and Moon is [[tidal acceleration|increasing]], and Earth's spin slowing down.<ref name="touma1994" /> Measurements from [[lunar laser ranging experiment|lunar ranging experiments]] with laser reflectors left during the Apollo missions have found that the Moon's distance to Earth increases by 38&nbsp;mm per year<ref>{{cite journal|last=Chapront|first=J.|coauthors=Chapront-Touzé, M.; Francou, G.|year=2002|title=A new determination of lunar orbital parameters, precession constant and tidal acceleration from LLR measurements|journal=Astronomy and Astrophysics|volume=387|issue=2|pages=700–709|doi=10.1051/0004-6361:20020420|bibcode = 2002A&A...387..700C}}</ref> (though this is only 0.10&nbsp;[[Parts per billion|ppb]]/year of the radius of the Moon's orbit).
[[Atomic clock]]s also show that Earth's day lengthens by about 15&nbsp;[[microsecond]]s every year,<ref>{{cite web|last = Ray|first = R.|date = 15 May 2001|url = http://bowie.gsfc.nasa.gov/ggfc/tides/intro.html|title = Ocean Tides and the Earth's Rotation|publisher = IERS Special Bureau for Tides|accessdate =17 March 2010}}</ref> slowly increasing the rate at which [[Coordinated Universal Time|UTC]] is adjusted by [[leap second]]s.
Left to run its course, this tidal drag would continue until the spin of Earth and the orbital period of the Moon matched. However, the Sun will become a [[red giant#The Sun as a red giant|red giant]] long before that, engulfing Earth.<ref>{{cite book| author = Murray, C.D. and Dermott, S.F.| coauthors = Stanley F. Dermott| title = Solar System Dynamics| year = 1999| publisher = Cambridge University Press| isbn = 978-0-521-57295-8| page = 184 }}</ref><ref>{{cite book| last = Dickinson| first = Terence| authorlink = Terence Dickinson| title = From the Big Bang to Planet X| year = 1993| publisher = [[Camden House]]| location = Camden East, Ontario| isbn = 978-0-921820-71-0| pages = 79–81 }}
</ref>
 
The lunar surface also experiences tides of amplitude ~10&nbsp;cm over 27&nbsp;days, with two components: a fixed one due to Earth, because they are in [[synchronous rotation]], and a varying component from the Sun.<ref name="touma1994"/> The Earth-induced component arises from [[libration]], a result of the Moon's orbital eccentricity; if the Moon's orbit were perfectly circular, there would only be solar tides.<ref name="touma1994" /> Libration also changes the angle from which the Moon is seen, allowing about 59% of its surface to be seen from Earth (but only half at any instant).<ref name="worldbook" /> The cumulative effects of stress built up by these tidal forces produces [[moonquakes]]. Moonquakes are much less common and weaker than earthquakes, although they can last for up to an hour—a significantly longer time than terrestrial earthquakes—because of the absence of water to damp out the seismic vibrations. The existence of moonquakes was an unexpected discovery from [[seismometer]]s placed on the Moon by [[Apollo program|Apollo]] [[astronaut]]s from 1969 through 1972.<ref>{{cite journal|last1 = Latham|first1 = Gary|year = 1972|last2 = Ewing|first2 = Maurice|last3 = Dorman|first3 = James|last4 = Lammlein|first4 = David|last5 = Press|first5 = Frank|last6 = Toksőz|first6 = Naft|last7 = Sutton|first7 = George|last8 = Duennebier|first8 = Fred|last9 = Nakamura|first9 = Yosio|displayauthors=9|title = Moonquakes and lunar tectonism |journal = Earth, Moon, and Planets|volume = 4|issue = 3–4| pages = 373–382|doi = 10.1007/BF00562004|bibcode = 1972Moon....4..373L}}</ref>
 
=== Eclipses ===
 
{{main|Solar eclipse|Lunar eclipse|Eclipse cycle}}
 
{{multiple image
|width = 150
|footer = From Earth, the Moon and Sun appear the same size. From a satellite in an Earth-trailing orbit, the Moon may appear smaller than the Sun.
|image1 =Solar_eclipse_1999_4_NR.jpg
|alt1 = The fiercely bright disk of the Sun is completely obscured by the exact fit of the disk of the dark, non-illuminated Moon, leaving only the radial, fuzzy, glowing coronal filaments of the Sun around the edge.
|caption1 = The [[Solar eclipse of August 11, 1999|1999 solar eclipse]]|image2 = STEREO-B solar eclipse.jpg
|alt2 = The bright disk of the Sun, showing many coronal filaments, flares and grainy patches in the wavelength of this image, is partly obscured by a small dark disk: here, the Moon covers less than a fifteenth of the Sun.
|caption2 = The Moon passing in front of the Sun, from the STEREO-B spacecraft.<ref>{{cite web|author=Phillips, Tony|work=Science@NASA|title= Stereo Eclipse|date=12 March 2007|accessdate=17 March 2010|url=http://science.nasa.gov/headlines/y2007/12mar_stereoeclipse.htm}}</ref>
}}
Eclipses can only occur when the Sun, Earth, and Moon are all in a straight line (termed "[[Syzygy (astronomy)|syzygy]]"). [[Solar eclipse]]s occur at [[new moon]], when the Moon is between the Sun and Earth. In contrast, [[lunar eclipse]]s occur at [[full moon]], when Earth is between the Sun and Moon. The apparent size of the Moon is roughly the same as that of the Sun, with both being viewed at close to one-half a degree wide. The Sun is much larger than the Moon but it is the precise vastly greater distance that coincidentally gives it the same apparent size as the much closer and much smaller Moon from the perspective of Earth. The variations in apparent size, due to the non-circular orbits, are nearly the same as well, though occurring in different cycles. This makes possible both [[total eclipse|total]] (with the Moon appearing larger than the Sun) and [[annular eclipse|annular]] (with the Moon appearing smaller than the Sun) solar eclipses.<ref>{{cite web|first = F.|last = Espenak|year = 2000|url = http://www.mreclipse.com/Special/SEprimer.html|title = Solar Eclipses for Beginners|publisher = MrEclipse|accessdate =17 March 2010}}</ref> In a total eclipse, the Moon completely covers the disc of the Sun and the solar [[corona]] becomes visible to the [[naked eye]]. Because the distance between the Moon and Earth is very slowly increasing over time,<ref name="Lambeck1977" /> the angular diameter of the Moon is decreasing. Also, as it evolves toward becoming a [[red giant]], the size of the Sun, and its apparent diameter in the sky, are slowly increasing.{{efn|name=size changes}} The combination of these two changes means that hundreds of millions of years ago, the Moon would always completely cover the Sun on solar eclipses, and no annular eclipses were possible. Likewise, hundreds of millions of years in the future, the Moon will no longer cover the Sun completely, and total solar eclipses will not occur.<ref name = fourmilab>{{cite web|last=Walker|first=John|url=http://www.fourmilab.ch/images/peri_apo/|title=Moon near Perigee, Earth near Aphelion|publisher=Fourmilab|date=July 10, 2004|accessdate=December 25, 2013}}</ref>
 
Because the Moon's orbit around Earth is inclined by about 5° to the [[ecliptic|orbit of Earth around the Sun]], eclipses do not occur at every full and new moon. For an eclipse to occur, the Moon must be near the intersection of the two orbital planes.<ref name="eclipse" /> The periodicity and recurrence of eclipses of the Sun by the Moon, and of the Moon by Earth, is described by the [[Saros (astronomy)|saros]], which has a period of approximately 18&nbsp;years.<ref>{{cite web|url = http://sunearth.gsfc.nasa.gov/eclipse/SEsaros/SEsaros.html|last = Espenak|first = F. |title = Saros Cycle|publisher = NASA|accessdate =17 March 2010}}</ref>
 
As the Moon is continuously blocking our view of a half-degree-wide circular area of the sky,{{efn|name=area}}<ref>{{cite journal|title=The Square Degree as a Unit of Celestial Area|author=Guthrie, D.V. |year=1947|journal=Popular Astronomy|volume=55|pages=200–203|url=http://adsabs.harvard.edu/abs/1947PA.....55..200|bibcode = 1947PA.....55..200G }}</ref> the related phenomenon of [[occultation]] occurs when a bright star or planet passes behind the Moon and is occulted: hidden from view. In this way, a solar eclipse is an occultation of the Sun. Because the Moon is comparatively close to Earth, occultations of individual stars are not visible everywhere on the planet, nor at the same time. Because of the [[precession]] of the lunar orbit, each year different stars are occulted.<ref>{{cite web|url = http://occsec.wellington.net.nz/total/totoccs.htm|title = Total Lunar Occultations|publisher = Royal Astronomical Society of New Zealand|accessdate =17 March 2010}}</ref>
 
== {{Anchor|Exploration}} Study and exploration ==
 
{{See also|Robotic exploration of the Moon|List of current and future lunar missions|Colonization of the Moon|List of man-made objects on the Moon}}
[[File:Moon by Johannes hevelius 1645.PNG|thumb|alt=On an open folio page is a carefully drawn disk of the full moon. In the upper corners of the page are waving banners held aloft by pairs of winged cherubs. In the lower left page corner a cherub assists another to measure distances with a pair of compasses; in the lower right corner a cherub views the main map through a handheld telescope, whereas another, kneeling, peers at the map from over a low cloth-draped table.|Map of the Moon by [[Johannes Hevelius]] from his ''[[Selenographia]]'' (1647), the first map to include the [[libration]] zones.]]
 
=== Early studies ===
 
{{main|Exploration of the Moon#Early history|l1=Exploration of the Moon: Early history|Selenography|Lunar theory}}
 
Understanding of the Moon's cycles was an early development of astronomy: by the {{nowrap|5th century BC}}, [[Babylonian astronomy|Babylonian astronomers]] had recorded the 18-year [[Saros cycle]] of [[lunar eclipse]]s,<ref>{{cite journal|doi=10.2307/1006543|title=Saros Cycle Dates and Related Babylonian Astronomical Texts|first1=A. |last1 = Aaboe|first2= J. P. |last2 =Britton|first3= J. A. |last3 =Henderson, |first4= Otto|last4 = Neugebauer| authorlink4 = Otto Neugebauer |first5= A. J. |last5 =Sachs|journal=Transactions of the American Philosophical Society |volume=81|issue=6 |year=1991|pages=1–75 |publisher=[[American Philosophical Society]]|quote=One comprises what we have called "Saros Cycle Texts", which give the months of eclipse possibilities arranged in consistent cycles of 223&nbsp;months (or 18&nbsp;years).|postscript=<!--None-->|jstor=1006543}}</ref> <!--The texts discussed in that article are more recent than 490 BC and, as mentioned in the paper, the observations can have occurred no earlier than that. The earliest reference for the Metonic cycle in Neugubauer's (1957) ''The Exact Sciences in Antiquity'' is 380 BC (p. 140).--> and [[Indian astronomy|Indian astronomers]] had described the Moon's monthly elongation.<ref name="Sarma-Ast-Ind" /> The [[Chinese astronomy|Chinese astronomer]] [[Shi Shen]] {{nowrap|(fl. 4th century BC)}} gave instructions for predicting solar and lunar eclipses.{{sfn|Needham|1986|p=411}}
Later, the physical form of the Moon and the cause of [[moonlight]] became understood. The [[ancient Greece|ancient Greek]] philosopher [[Anaxagoras]] {{nowrap|(d. 428 BC)}} reasoned that the Sun and Moon were both giant spherical rocks, and that the latter reflected the light of the former.<ref>{{cite web|last = O'Connor|first = J.J.|coauthors = Robertson, E.F.|date=February 1999|url = http://www-history.mcs.st-andrews.ac.uk/Biographies/Anaxagoras.html|title = Anaxagoras of Clazomenae|publisher = University of St Andrews|accessdate =12 April 2007}}</ref>{{sfn|Needham|1986|p=227}} Although the Chinese of the [[Han Dynasty]] believed the Moon to be energy equated to ''[[qi]]'', their 'radiating influence' theory also recognized that the light of the Moon was merely a reflection of the Sun, and [[Jing Fang]] (78–37&nbsp;BC) noted the sphericity of the Moon.{{sfn|Needham|1986|p=413–414}} In 2nd century AD [[Lucian]] wrote a novel where the heroes travel to the Moon, which is inhabited. In 499&nbsp;AD, the Indian astronomer [[Aryabhata]] mentioned in his ''[[Aryabhatiya]]'' that reflected sunlight is the cause of the shining of the Moon.<ref>{{cite web|url=http://www-history.mcs.st-andrews.ac.uk/Biographies/Aryabhata_I.html|title=Aryabhata the Elder |last1 =Robertson|first1 = E. F.|date = November 2000|publisher=School of Mathematics and Statistics, University of St Andrews|accessdate=15 April 2010|location=Scotland}}</ref> The astronomer and physicist [[Alhazen]] (965–1039) found that [[sunlight]] was not reflected from the Moon like a mirror, but that light was emitted from every part of the Moon's sunlit surface in all directions.<ref>{{cite book| location=Detroit|year=2008|publisher = Charles Scribner's Sons|title = Dictionary of Scientific Biography|chapter= Ibn Al-Haytham, Abū ʿAlī Al-Ḥasan Ibn Al-Ḥasan|author=A. I. Sabra|pages=189–210, at 195}}</ref> [[Shen Kuo]] (1031–1095) of the [[Song Dynasty]] created an allegory equating the waxing and waning of the Moon to a round ball of reflective silver that, when doused with white powder and viewed from the side, would appear to be a crescent.{{sfn|Needham|1986|p=415–416}}
 
In [[Aristotle|Aristotle's]] (384–322&nbsp;BC) [[On the Heavens|description of the universe]], the Moon marked the boundary between the spheres of the mutable elements (earth, water, air and fire), and the imperishable stars of [[aether (classical element)|aether]], an [[Aristotelian physics|influential philosophy]] that would dominate for centuries.<ref>{{cite book| last = Lewis| first = C. S.| authorlink = C. S. Lewis| title = The Discarded Image| year = 1964| publisher = Cambridge University Press| location = Cambridge| isbn = 978-0-521-47735-2| page = 108 }}</ref> However, in the {{nowrap|2nd century BC}}, [[Seleucus of Seleucia]] correctly theorized that [[tide]]s were due to the attraction of the Moon, and that their height depends on the Moon's position relative to the [[Sun]].<ref>{{cite journal|first = Bartel Leendert|last = van der Waerden| authorlink = Bartel Leendert van der Waerden|year = 1987|title = The Heliocentric System in Greek, Persian and Hindu Astronomy|journal = Annals of the New York Academy of Sciences|volume = 500|pages = 1–569|pmid = 3296915|bibcode = 1987NYASA.500....1A |doi = 10.1111/j.1749-6632.1987.tb37193.x }}</ref> In the same century, [[Aristarchus of Samos|Aristarchus]] [[Aristarchus On the Sizes and Distances|computed the size and distance]] of the Moon from Earth, obtaining a value of about twenty times the [[Earth radius|radius of Earth]] for the distance. These figures were greatly improved by [[Ptolemy]] (90–168&nbsp;AD): his values of a mean distance of 59&nbsp;times Earth's radius and a diameter of 0.292&nbsp;Earth diameters were close to the correct values of about 60 and 0.273 respectively.<ref>{{cite book| last = Evans| first = James| title = The History and Practice of Ancient Astronomy| year = 1998| publisher = Oxford University Press| location = Oxford & New York| isbn = 978-0-19-509539-5| pages = 71, 386 }}</ref> [[Archimedes]] (287–212 BC) invented a planetarium calculating motions of the Moon and the known planets.<ref>{{cite news| url=http://www.nytimes.com/2008/07/31/science/31computer.html?hp|work=The New York Times|title=Discovering How Greeks Computed in 100 B.C.|date=31 July 2008|accessdate=27 March 2010}}</ref>
 
During the [[Middle Ages]], before the invention of the telescope, the Moon was increasingly recognised as a sphere, though many believed that it was "perfectly smooth".<ref>{{cite web|last = Van Helden|first = A.|year = 1995|url = http://galileo.rice.edu/sci/observations/moon.html|title = The Moon|publisher = Galileo Project|accessdate =12 April 2007}}</ref> In 1609, [[Galileo Galilei]] drew one of the first telescopic drawings of the Moon in his book {{lang|la|''[[Sidereus Nuncius]]''}} and noted that it was not smooth but had mountains and craters. Telescopic mapping of the Moon followed: later in the 17th century, the efforts of [[Giovanni Battista Riccioli]] and [[Francesco Maria Grimaldi]] led to the system of naming of lunar features in use today. The more exact 1834–36 {{lang|la|''Mappa Selenographica''}} of [[Wilhelm Beer]] and [[Johann Heinrich Mädler]], and their associated 1837 book {{lang|de|''Der Mond''}}, the first [[trigonometry|trigonometrically]] accurate study of lunar features, included the heights of more than a thousand mountains, and introduced the study of the Moon at accuracies possible in earthly geography.<ref>{{cite journal|last=Consolmagno|first=Guy J.|year=1996|title=Astronomy, Science Fiction and Popular Culture: 1277 to 2001 (And beyond) |journal=Leonardo|publisher=The MIT Press|volume=29|issue=2|page=128|jstor=1576348|doi=10.2307/1576348}}</ref> Lunar craters, first noted by Galileo, were thought to be [[volcanic]] until the 1870s proposal of [[Richard Proctor]] that they were formed by collisions.<ref name="worldbook" /> This view gained support in 1892 from the experimentation of geologist [[Grove Karl Gilbert]], and from comparative studies from 1920 to the 1940s,<ref name="Hall1977" /> leading to the development of [[lunar geologic timescale|lunar stratigraphy]], which by the 1950s was becoming a new and growing branch of [[astrogeology]].<ref name="worldbook" />
 
=== First direct exploration: 1959–1976 ===
 
==== Soviet missions ====
 
{{main|Luna program|Lunokhod programme}}
[[File:Lunokhod 1 (high resolution).jpg|thumb|220px|[[Lunokhod 1]] (lit. ''moonwalker''), the first successful [[rover (space exploration)|space rover]].]]
 
The [[Cold War]]-inspired [[Space Race]] between the Soviet Union and the U.S. led to an acceleration of interest in [[exploration of the Moon]]. Once launchers had the necessary capabilities, these nations sent unmanned probes on both flyby and impact/lander missions. Spacecraft from the Soviet Union's [[Luna programme|''Luna'' program]] were the first to accomplish a number of goals: following three unnamed, failed missions in 1958,<ref>{{cite web|url= http://www.russianspaceweb.com/spacecraft_planetary_lunar.html| first = Anatoly|last = Zak |year = 2009|title = Russia's unmanned missions toward the Moon|accessdate=20 April 2010}}</ref> the first man-made object to escape Earth's gravity and pass near the Moon was ''[[Luna 1]]''; the first man-made object to impact the lunar surface was ''[[Luna 2]]'', and the first photographs of the normally occluded far side of the Moon were made by ''[[Luna 3]]'', all in 1959.
 
The first spacecraft to perform a successful lunar [[lander (spacecraft)|soft landing]] was ''[[Luna 9]]'' and the first unmanned vehicle to orbit the Moon was ''[[Luna 10]]'', both in 1966.<ref name="worldbook" /> [[Moon rock|Rock and soil samples]] were brought back to Earth by three ''Luna'' [[sample return mission]]s (''[[Luna 16]]'' in 1970, ''[[Luna 20]]'' in 1972, and ''[[Luna 24]]'' in 1976), which returned 0.3&nbsp;kg total.<ref>{{cite web|url=http://curator.jsc.nasa.gov/lunar/index.cfm|title=Rocks and Soils from the Moon|publisher=NASA|accessdate=6 April 2010}}</ref> Two pioneering robotic [[rover (space exploration)|rovers]] landed on the Moon in 1970 and 1973 as a part of Soviet [[Lunokhod programme]].
 
==== United States missions ====
 
{{main|Apollo program|Moon landing}}
[[File:NASA-Apollo8-Dec24-Earthrise.jpg|thumb|alt=The small blue-white semicircle of Earth, almost glowing with colour in the blackness of space, rising over the limb of the desolate, cratered surface of the Moon.|[[Earth]] as viewed from Lunar orbit during the [[Apollo 8]] mission, Christmas Eve, 1968. Africa is at the sunset terminator, both Americas are under cloud, and Antarctica is at the left end of the terminator.]]
[[File:As11-40-5886, uncropped.jpg|thumb|[[Neil Armstrong]] and a U.S. flag]]
The United States launched unmanned probes to develop an understanding of the lunar surface for an eventual manned landing: the [[Jet Propulsion Laboratory]]'s [[Surveyor Program|''Surveyor'' program]] landed [[Surveyor 1|its first spacecraft]] four months after ''Luna 9''. [[NASA]]'s manned [[Apollo program]] was developed in parallel; after a series of unmanned and manned tests of the Apollo spacecraft in Earth orbit, and spurred on by a potential [[Soviet moonshot|Soviet lunar flight]], in 1968 [[Apollo 8]] made the first crewed mission to lunar orbit. The subsequent landing of the first humans on the Moon in 1969 is seen by many as the culmination of the Space Race.<ref name="CNN" /> [[Neil Armstrong]] became the first person to walk on the Moon as the commander of the American mission [[Apollo 11]] by first setting foot on the Moon at 02:56&nbsp;UTC on 21 July 1969.<ref>{{cite web|url=http://history.nasa.gov/ap11ann/ap11events.html|title=Record of Lunar Events, 24 July 1969|work=Apollo 11 30th anniversary|publisher=NASA.|accessdate=13 April 2010}}</ref> The Apollo missions 11 to 17 (except [[Apollo 13]], which aborted its planned lunar landing) returned 382&nbsp;kg of lunar rock and soil in 2,196 separate samples.<ref>{{cite web|url=http://www.psrd.hawaii.edu/Dec09/PSRD-Apollo-lunar-samples.pdf|title=Celebrated Moon Rocks --- Overview and status of the Apollo lunar collection: A unique, but limited, resource of extraterrestrial material.|last=Martel|first=Linda M. V.|date=21 December 2009|publisher=Planetary Science and Research Discoveries|accessdate=6 April 2010}}</ref> The American [[Moon landing]] and return was enabled by considerable technological advances in the early 1960s, in domains such as [[ablation]] chemistry, [[software engineering]] and [[atmospheric re-entry]] technology, and by highly competent management of the enormous technical undertaking.<ref>{{cite web|url=http://history.nasa.gov/ap11ann/legacy.htm|title=The Legacy of Project Apollo|last=Launius|first=Roger D.|date=July 1999|publisher=NASA History Office|accessdate=13 April 2010}}</ref><ref>{{cite book|title=SP-287 What Made Apollo a Success? A series of eight articles reprinted by permission from the March 1970 issue of Astronautics & Aeronautics, a publicaion of the American Institute of Aeronautics and Astronautics.|publisher=Scientific and Technical Information Office, National Aeronautics and Space Administration|location=Washington, D.C.|year=1971}}</ref>
 
Scientific instrument packages were installed on the lunar surface during all the Apollo landings. Long-lived [[Apollo Lunar Surface Experiments Package|instrument stations]], including heat flow probes, [[seismometer]]s, and [[magnetometer]]s, were installed at the [[Apollo 12]], [[Apollo 14|14]], [[Apollo 15|15]], [[Apollo 16|16]], and [[Apollo 17|17]] landing sites. Direct transmission of data to Earth concluded in late 1977 due to budgetary considerations,<ref>{{cite press release|title = NASA news release 77-47 page 242| date = 1 September 1977| url = http://www.nasa.gov/centers/johnson/pdf/83129main_1977.pdf|accessdate =16 March 2010|format=PDF}}</ref><ref>{{cite news|url = http://www.ast.cam.ac.uk/~ipswich/Miscellaneous/Archived_spaceflight_news.htm|accessdate =29 August 2007| location = NASA Turns A Deaf Ear To The Moon|year = 1977|title = OASI Newsletters Archive|last = Appleton|first = James|coauthors = Radley, Charles; Deans, John; Harvey, Simon; Burt, Paul; Haxell, Michael; Adams, Roy; Spooner N.; Brieske, Wayne|archiveurl = http://web.archive.org/web/20071210143103/http://www.ast.cam.ac.uk/~ipswich/Miscellaneous/Archived_spaceflight_news.htm |archivedate = 10 December 2007|deadurl=yes}}</ref> but as the stations' [[Lunar laser ranging experiment|lunar laser ranging]] corner-cube retroreflector arrays are passive instruments, they are still being used. Ranging to the stations is routinely performed from Earth-based stations with an accuracy of a few centimetres, and data from this experiment are being used to place constraints on the size of the lunar core.<ref>{{cite journal|last = Dickey|first = J.|year = 1994|title = Lunar laser ranging: a continuing legacy of the Apollo program|journal = Science|volume = 265 |pages = 482–490|doi = 10.1126/science.265.5171.482|pmid = 17781305|issue = 5171|bibcode=1994Sci...265..482D|display-authors = 1|last2 = Bender|first2 = P. L.|last3 = Faller|first3 = J. E.|last4 = Newhall|first4 = X X|last5 = Ricklefs|first5 = R. L.|last6 = Ries|first6 = J. G.|last7 = Shelus|first7 = P. J.|last8 = Veillet|first8 = C.|last9 = Whipple|first9 = A. L.}}</ref>
 
=== Current era: 1990–present ===
[[File:EarthAndMoon.jpg|thumb|left|280px|alt=The dark shadowed disk of the Moon moves across the face of the quarter-phase Earth, covering only a small part of the cloud-swirled semicircle.|Earth and the Moon, as imaged by [[Deep Impact (spacecraft)|Deep Impact]] in September 2008 at a separation of {{nowrap|50 million km}}<ref>[http://antwrp.gsfc.nasa.gov/apod/ap080903.html '' Astronomy Picture of the Day''], 2008 September 3</ref>]]
Post-Apollo and ''Luna'', many more countries have become involved in direct exploration of the Moon. In 1990, Japan became the third country to place a spacecraft into lunar orbit with its ''[[Hiten]]'' spacecraft. The spacecraft released a smaller probe, ''Hagoromo'', in lunar orbit, but the transmitter failed, preventing further scientific use of the mission.<ref>{{cite web|title=Hiten-Hagomoro|publisher=NASA|url=http://solarsystem.nasa.gov/missions/profile.cfm?MCode=Hiten&Display=ReadMore|accessdate=29 March 2010}}</ref> In 1994, the U.S. sent the joint Defense Department/NASA spacecraft ''[[Clementine (spacecraft)|Clementine]]'' to lunar orbit. This mission obtained the first near-global topographic map of the Moon, and the first global [[Multi-spectral image|multispectral]] images of the lunar surface.<ref>{{cite web|title=Clementine information|publisher=NASA|year=1994|url=http://nssdc.gsfc.nasa.gov/planetary/cleminfo.html|accessdate=29 March 2010}}</ref> This was followed in 1998 by the ''[[Lunar Prospector]]'' mission, whose instruments indicated the presence of excess hydrogen at the lunar poles, which is likely to have been caused by the presence of water ice in the upper few meters of the regolith within permanently shadowed craters.<ref>{{cite web|title=Lunar Prospector: Neutron Spectrometer|publisher=NASA|url=http://lunar.arc.nasa.gov/results/neutron.htm|year=2001|accessdate=29 March 2010}}</ref>
 
The European spacecraft ''[[SMART-1]]'', the second [[ion propulsion|ion-propelled]] spacecraft, was in lunar orbit from 15 November 2004 until its lunar impact on 3 September 2006, and made the first detailed survey of chemical elements on the lunar surface.<ref>{{cite web|url=http://www.esa.int/SPECIALS/SMART-1/SEMSDE1A6BD_0.html|title=SMART-1 factsheet|date=26 February 2007|publisher=European Space Agency|accessdate=29 March 2010}}</ref>
 
China has pursued an ambitious [[Chinese Lunar Exploration Program|program of lunar exploration]], beginning with ''[[Chang'e 1]]'', which successfully orbited the Moon from 5 November 2007 until its controlled lunar impact on 1 March 2008.<ref name="xinhua_20090301" /> In its sixteen-month mission, it obtained a full image map of the Moon. China followed up this success with ''[[Chang'e 2]]'' beginning in October 2010, which reached the Moon over twice as fast as ''Chang'e 1'', mapped the Moon at a higher resolution over an eight month period, then left lunar orbit in favor of an extended stay at the Earth–Sun L2 [[Lagrangian point]], before finally performing a flyby of asteroid [[4179 Toutatis]] on 13 December 2012, and then heading off into deep space. On 14 December 2013, ''[[Chang'e 3]]'' improved upon its orbital mission predecessors by landing a lunar [[lander (spacecraft)|lander]] onto the Moon's surface, which in turn deployed a [[lunar rover]], named ''Yutu'' (Chinese: 玉兔; literally "Jade Rabbit"). In so doing, ''Chang'e 3'' made the first lunar [[Soft landing (rocketry)|soft landing]] since ''[[Luna 24]]'' in 1976, and the first lunar rover mission since ''[[Lunokhod 2]]'' in 1973. China intends to launch another rover mission (''[[Chang'e 4]]'') in 2015, followed by a [[sample return mission]] (''[[Chang'e 5]]'') in 2017.
 
Between 4 October 2007 and 10 June 2009, the [[Japan Aerospace Exploration Agency]]'s ''[[SELENE|Kaguya]] (Selene)'' mission, a lunar orbiter fitted with a [[high-definition video]] camera, and two small radio-transmitter satellites, obtained lunar geophysics data and took the first high-definition movies from beyond Earth orbit.<ref>{{cite web|url=http://www.selene.jaxa.jp/en/profile/index.htm|title=KAGUYA Mission Profile|publisher=JAXA|accessdate=13 April 2010}}</ref><ref>{{cite web|url=http://www.jaxa.jp/press/2007/11/20071107_kaguya_e.html|title=KAGUYA (SELENE) World's First Image Taking of the Moon by HDTV|date=7 November 2007|publisher=Japan Aerospace Exploration Agency (JAXA) and NHK (Japan Broadcasting Corporation)|accessdate=13 April 2010}}</ref>
India's first lunar mission, ''[[Chandrayaan-1|Chandrayaan I]]'', orbited from 8 November 2008 until loss of contact on 27 August 2009, creating a high resolution chemical, mineralogical and photo-geological map of the lunar surface, and confirming the presence of water molecules in lunar soil.<ref>{{cite web|url=http://www.isro.org/Chandrayaan/htmls/mission_sequence.htm|title=Mission Sequence|date=17 November 2008|publisher=Indian Space Research Organisation|accessdate=13 April 2010}}</ref> The [[Indian Space Research Organisation]] planned to launch ''[[Chandrayaan II]]'' in 2013, which would have included a Russian robotic lunar rover.<ref>{{cite web|url=http://www.isro.org/scripts/futureprogramme.aspx#Space|title=Indian Space Research Organisation: Future Program|publisher=Indian Space Research Organisation|accessdate=13 April 2010}}</ref><ref>{{cite web|url=http://isro.org/pressrelease/Nov14_2007.htm |title=India and Russia Sign an Agreement on Chandrayaan-2|date=14 November 2007|publisher=Indian Space Research Organisation|accessdate=13 April 2010 |archiveurl = http://web.archive.org/web/20071217203828/http://isro.org/pressrelease/Nov14_2007.htm |archivedate = 17 December 2007}}</ref> However, the failure of Russia's ''[[Fobos-Grunt]]'' mission has delayed this project.
[[File:Shrinking, Growing Moon.ogv|thumb|250px|right|Explore what the discovery of lunar valleys tells us about the moon's evolution.]]
The U.S. co-launched the ''[[Lunar Reconnaissance Orbiter]]'' (LRO) and the ''[[LCROSS]]'' impactor and follow-up observation orbiter on 18 June 2009; ''LCROSS'' completed its mission by making a planned and widely observed impact in the crater [[Cabeus (crater)|Cabeus]] on 9 October 2009,<ref>{{cite web|url=http://lcross.arc.nasa.gov/observation.htm|title=Lunar CRater Observation and Sensing Satellite (LCROSS): Strategy & Astronomer Observation Campaign|date=October 2009|publisher=NASA|accessdate=13 April 2010}}</ref> whereas ''LRO'' is currently in operation, obtaining precise lunar [[altimetry]] and high-resolution imagery. In November 2011, the LRO passed over the [[Aristarchus (crater)|Aristarchus crater]], which spans 40 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 geologically diverse places on the Moon: a mysterious raised flat plateau, a giant rille carved by enormous outpourings of lava, fields of explosive volcanic ash, and all surrounded by massive flood basalts", said Mark Robinson, principal investigator of the Lunar Reconnaissance Orbiter Camera at [[Arizona State University]]. NASA released photos of the crater on 25 December 2011.<ref>{{cite news|url = http://www.msnbc.msn.com/id/45901428#.Twe_flbWxvY|title = Giant moon crater revealed in spectacular up-close photos|publisher=MSNBC|agency = Space.com|date = 6 January 2012}}</ref>
 
Two [[NASA]] [[Gravity Recovery and Interior Laboratory|GRAIL]] spacecraft begin orbiting the Moon around 1 January 2012,<ref>{{cite news|url = http://www.thesunnews.com/2011/12/26/2569001/twin-probes-to-circle-moon-to.html|title = Twin probes to circle moon to study gravity field|last = Chang|first = Alicia |work = [[The Sun News]]|agency = [[Associated Press]]|date = 26 December 2011|accessdate =27 December 2011}}</ref> on a mission to learn more about the Moon's internal structure. NASA's ''[[Lunar Atmosphere and Dust Environment Explorer|LADEE]]'' probe, designed to study the lunar [[exosphere]], achieved orbit on 6 October 2013. 
[[File:Earth-Moon system as seen from Saturn (PIA14949).jpg|left|thumb|250px|NASA's [[Cassini spacecraft]] photographs [[Earth]] and the Moon (both visible in centre of image, if it is enlarged) from [[Saturn]] (July 19, 2013). The surrounding sky is not dark, because the image was taken through one of Saturn's rings]] 
Other upcoming lunar missions include Russia's ''[[Luna-Glob]]'': an unmanned lander, set of seismometers, and an orbiter based on its failed Martian ''[[Fobos-Grunt]]'' mission.<ref>{{cite web|url = http://www.aviationnow.com/avnow/news/channel_awst_story.jsp?id=news/aw060506p2.xml|title = Russia Plans Ambitious Robotic Lunar Mission|last = Covault|first = C.|publisher = Aviation Week|date = 4 June 2006|accessdate =12 April 2007}}</ref><ref>{{cite web|url=http://rt.com/Top_News/2009-02-25/Russia_to_send_mission_to_Mars_this_year__Moon_in_three_years_.html|title=Russia to send mission to Mars this year, Moon in three years |date=25 February 2009|publisher="TV-Novosti"|accessdate=13 April 2010}}</ref>
Privately funded lunar exploration has been promoted by the [[Google Lunar X Prize]], announced 13 September 2007, which offers US$20&nbsp;million to anyone who can land a robotic rover on the Moon and meet other specified criteria.<ref>{{cite web|title=About the Google Lunar X Prize|publisher=X-Prize Foundation|url=http://www.googlelunarxprize.org/lunar/about-the-prize|year=2010|accessdate=24 March 2010|archiveurl=http://web.archive.org/web/20100228024532/http://www.googlelunarxprize.org/lunar/about-the-prize <!--Added by H3llBot-->|archivedate=28 February 2010}}</ref> [[Shackleton Energy Company]] is building a program to establish operations on the south pole of the Moon to harvest water and supply their [[Propellant Depot]]s.<ref>{{cite web
  | title = Mining the Moon's Water: Q&A with Shackleton Energy's Bill Stone
  | publisher = Space News
  | last = Wall
  | first = Mike
  | date = 14 January 2011
  | url = http://www.spacenews.com/10619-mining-moon-water-bill-stone-110114.html}}</ref>
    
    
NASA began to [[Vision for Space Exploration|plan to resume manned missions]] following the call by U.S. President [[George W. Bush]] on 14 January 2004 for a manned mission to the Moon by 2019 and the construction of a lunar base by 2024.<ref>{{cite press release|url = http://www.nasa.gov/missions/solarsystem/bush_vision.html| title = President Bush Offers New Vision For NASA|date = 14 December 2004| publisher = NASA|accessdate =12 April 2007}}</ref> The [[Constellation program]] was funded and construction and testing begun on a [[Orion (Constellation program)|manned spacecraft]] and [[Ares (rocket)|launch vehicle]],<ref>{{cite web|url=http://www.nasa.gov/mission_pages/constellation/main/index.html|title=Constellation|publisher=NASA|accessdate=13 April 2010}}</ref> and design studies for a lunar base.<ref>{{cite press release|title = NASA Unveils Global Exploration Strategy and Lunar Architecture|publisher = NASA|date = 4 December 2006| url = http://www.nasa.gov/home/hqnews/2006/dec/HQ_06361_ESMD_Lunar_Architecture.html|accessdate =12 April 2007}}</ref> However, that program has been cancelled in favor of a manned asteroid landing by 2025 and a manned [[Mars]] orbit by 2035.<ref>{{cite web|author=NASAtelevision |url=https://www.youtube.com/watch?v=3rNn_cUrlmE |title=President Obama Pledges Total Commitment to NASA |publisher=YouTube |date=15 April 2010 |accessdate=7 May 2012}}</ref> [[India]] has also expressed its hope to send a manned mission to the Moon by 2020.<ref>{{cite web|url = http://www.space.com/news/061110_india_mannedspace.html| publisher = SPACE.com|title = India's Space Agency Proposes Manned Spaceflight Program| date = 10 November 2006| accessdate =23 October 2008}}</ref>
  </ul>
 
== Astronomy from the Moon ==
[[File:Earthinuvfrommoon.jpg|thumb|right|This is a picture of Earth in ultraviolet light, taken from the surface of the Moon. The day-side reflects a lot of UV light from the Sun, but the night-side shows bands of UV emission from the aurora caused by charged particles.<ref>{{cite web|url=http://science.hq.nasa.gov/kids/imagers/ems/uv.html |title=NASA - Ultraviolet Waves |publisher=Science.hq.nasa.gov |date=2013-09-27 |accessdate=2013-10-03}}</ref>]]
For many years, the Moon has been recognized as an excellent site for telescopes.<ref>{{cite web
  | last = Takahashi
  | first = Yuki
  | title = Mission Design for Setting up an Optical Telescope on the Moon
  | publisher = California Institute of Technology
  | date = September 1999
  | url = http://www.ugcs.caltech.edu/~yukimoon/MoonTelescope/
  | accessdate =27 March 2011}}</ref> It is relatively nearby; [[astronomical seeing]] is not a concern; certain craters near the poles are permanently dark and cold, and thus especially useful for [[infrared telescope]]s; and [[radio telescope]]s on the far side would be shielded from the radio chatter of Earth.<ref>{{cite web
  | last = Chandler
  | first = David
  | title = MIT to lead development of new telescopes on moon
  | work = MIT News
  | date = 15 February 2008
  | url = http://web.mit.edu/newsoffice/2008/moonscope-0215.html
  | accessdate =27 March 2011}}</ref> The [[lunar soil]], although it poses a problem for any moving parts of [[telescope]]s, can be mixed with [[carbon nanotube]]s and [[Epoxy|epoxies]] in the construction of mirrors up to 50 meters in diameter.<ref>{{cite web
  | last = Naeye
  | first = Robert
  | title = NASA Scientists Pioneer Method for Making Giant Lunar Telescopes
  | publisher = Goddard Space Flight Center
  | date = 6 April 2008
  | url = http://www.nasa.gov/centers/goddard/news/topstory/2008/lunar_telescopes.html
  | accessdate =27 March 2011}}</ref> A lunar [[zenith telescope]] can be made cheaply with [[ionic liquid]].<ref>{{cite web
  | last = Bell
  | first = Trudy
  | title = Liquid Mirror Telescopes on the Moon
  | work = Science News
  | publisher = NASA
  | date = 9 October 2008
  | url = http://science.nasa.gov/science-news/science-at-nasa/2008/09oct_liquidmirror/
  | accessdate =27 March 2011}}</ref>
 
In April 1972, the [[Apollo 16]] mission recorded various astronomical photos and spectra in ultraviolet with the [[Far Ultraviolet Camera/Spectrograph]].<ref>{{cite web|url=http://www.lpi.usra.edu/lunar/missions/apollo/apollo_16/experiments/f_ultra/ |title=Far Ultraviolet Camera/Spectrograph |publisher=Lpi.usra.edu |date= |accessdate=2013-10-03}}</ref>
 
== Legal status ==
 
{{main|Space law}}
Although ''[[Luna program|Luna]]'' landers scattered pennants of the [[Soviet Union]] on the Moon, and [[Lunar Flag Assembly|U.S. flag]]s were symbolically planted at their landing sites by the [[List of Apollo astronauts|Apollo astronauts]], no nation currently claims ownership of any part of the Moon's surface.<ref name="unoosa_q6" /> Russia and the U.S. are party to the 1967 [[Outer Space Treaty]],<ref name="unoosa_q4" /> which defines the Moon and all outer space as the "[[common heritage of mankind|province of all mankind]]".<ref name="unoosa_q6" /> This treaty also restricts the use of the Moon to peaceful purposes, explicitly banning military installations and [[weapons of mass destruction]].<ref name="unoosa_q5" />
The 1979 [[Moon Treaty|Moon Agreement]] was created to restrict the exploitation of the Moon's resources by any single nation, but it has not been signed by any of the [[space-faring nations]].<ref name="unoosa_moon" /> Although several individuals have made [[Extraterrestrial real estate|claims to the Moon]] in whole or in part, none of these are considered credible.<ref name="unoosa_q7" /><ref name="iisl_2004" /><ref name="iisl_2009" />
 
== In culture ==
 
{{Further|Moon in fiction|Lunar calendar|Metonic cycle|Lunar deity|Lunar effect|Blue moon}}
<!--[[File:Máni and Sól by Lorenz Frølich.jpg|thumb|The gods [[Máni]] (left) and [[Sól (Sun)|Sól]] (right), the personified Moon and Sun in [[Norse mythology]], as depicted in an illustration by [[Lorenz Frølich]] (1895)]]-->
[[File:Moon-bonatti.png|thumb|Luna, the Moon, from a 1550 edition of [[Guido Bonatti]]'s ''Liber astronomiae''.]]
The Moon's regular phases make it a very convenient timepiece, and the periods of its waxing and waning form the basis of many of the oldest calendars. [[Tally stick]]s, notched bones dating as far back as 20–30,000 years ago, are believed by some to mark the phases of the Moon.<ref name="Marshack" /><ref>Brooks, A. S. and Smith, C. C. (1987): "Ishango revisited: new age determinations and cultural interpretations", ''The African Archaeological Review'', 5 : 65–78.</ref><ref>{{cite book| last = Duncan| first = David Ewing| title = The Calendar| year = 1998| publisher = Fourth Estate Ltd.| isbn = 978-1-85702-721-1| pages = 10–11 }}</ref>
The ~30-day month is an approximation of the [[lunar cycle]]. The English noun ''month'' and its cognates in other Germanic languages stem from Proto-Germanic ''*mǣnṓth-'', which is connected to the above mentioned Proto-Germanic ''*mǣnōn'', indicating the usage of a [[lunar calendar]] among the [[Germanic peoples]] ([[Germanic calendar]]) prior to the adoption of a [[solar calendar]].<ref name="barnhart-and-germania" /> The same [[Proto-Indo-European language|Indo-European root]] as ''moon'' led, via [[Latin]], to ''measure'' and ''menstrual'', words which echo the Moon's importance to many ancient cultures in measuring time (see [[Latin]] {{lang|la|''mensis''}} and [[Ancient Greek]] {{lang|el|''μήνας''}} (''mēnas''), meaning "month").<ref>{{cite book| author = Smith, William George| title = Dictionary of Greek and Roman Biography and Mythology: Oarses-Zygia| url = http://books.google.com/?id=PJ0YAAAAIAAJ| accessdate = 29 March 2010| volume = 3| year = 1849| publisher = J. Walton| page = 768 }}</ref><ref>{{cite book| author = Estienne, Henri| title = Thesaurus graecae linguae| url = http://books.google.com/?id=0qQ_AAAAcAAJ| accessdate = 29 March 2010| volume = 5| year = 1846| publisher = Didot| page = 1001 }}</ref>
 
[[File:Moonstar.jpg|thumb|260px|left|A [[Star and crescent|crescent Moon]] and "star" (here the planet [[Venus]]) are a common [[Symbols of Islam|symbol of Islam]], appearing in flags like: {{flag|Turkey|name=(Turkey)}}, {{flag|Algeria|name=(Algeria)}} and {{flag|Pakistan|name=(Pakistan)}}.]]
 
The Moon has been the subject of many works of art and literature and the inspiration for countless others. It is a motif in the visual arts, the performing arts, poetry, prose and music. A 5,000-year-old rock carving at [[Knowth]], Ireland, may represent the Moon, which would be the earliest depiction discovered.<ref name="spacetoday" /> The contrast between the brighter highlands and the darker maria creates the patterns seen by different cultures as the [[Man in the Moon]], the [[Moon rabbit|rabbit]] and the buffalo, among others. In many prehistoric and ancient cultures, the Moon was personified as [[Lunar deity|a deity]] or other [[supernatural]] phenomenon, and [[Moon (astrology)|astrological views of the Moon]] continue to be propagated today.
 
[[File:Mohammed Splits the Moon.jpg|thumb|right|240px|Muhammad points out the splitting moon. Anonymous 16th-century watercolor from a ''[[Falnama]]'', a Persian book of prophecy. Muhammad is the veiled figure on the right.]]
 
The Moon plays an important role in [[Islam]]; the [[Islamic calendar]] is strictly lunar, and in many Muslim countries the months are determined by the visual sighting of the [[Hilal (crescent moon)|hilal]], or earliest crescent moon, over the horizon.<ref>{{cite web|title=Islamic Calendars based on the Calculated First Visibility of the Lunar Crescent|publisher=University of Utrecht|url=http://www.staff.science.uu.nl/~gent0113/islam/islam_tabcal_hilal.htm|accessdate=2014-01-11}}</ref> The [[star and crescent]], initially a symbol of the [[Ottoman Empire]], has recently been adopted as a wider symbol for the Muslim community. The [[splitting of the moon]] ({{lang-ar|انشقاق القمر}}) was a miracle attributed to the prophet [[Muhammad]].<ref>"Muhammad." [[Encyclopædia Britannica]]. 2007. Encyclopædia Britannica Online, p.13</ref>
 
The Moon has a long association with insanity and irrationality; the words ''lunacy'' and ''lunatic'' (popular shortening ''loony'') are derived from the Latin name for the Moon, ''Luna''. Philosophers [[Aristotle]] and [[Pliny the Elder]] argued that the full moon induced insanity in susceptible individuals, believing that the brain, which is mostly water, must be affected by the Moon and its power over the tides, but the Moon's gravity is too slight to affect any single person.<ref name="sciam" /> Even today, people insist that admissions to psychiatric hospitals, traffic accidents, homicides or suicides increase during a full moon, although there is no scientific evidence to support such claims.<ref name="sciam" />
{{clr}}
 
== See also ==
 
{{Portal|Solar System|Moon}}
{{Wikipedia books
|1=The Moon
  |3=Solar System
}}
* [[Other moons of Earth]]
* [[Moon illusion]]
* [[2006 RH120]]
 
== References ==
 
'''Notes'''
 
{{notes
| notes =
 
{{efn
| name = maxval
| The ''maximum value'' is given based on scaling of the brightness from the value of −12.74 given for an equator to Moon-centre distance of 378&nbsp;000&nbsp;km in the NASA factsheet reference to the minimum Earth–Moon distance given there, after the latter is corrected for Earth's equatorial radius of 6&nbsp;378&nbsp;km, giving 350&nbsp;600&nbsp;km. The ''minimum value'' (for a distant [[new moon]]) is based on a similar scaling using the maximum Earth–Moon distance of 407&nbsp;000&nbsp;km (given in the factsheet) and by calculating the brightness of the [[earthshine]] onto such a new moon. The brightness of the earthshine is {{nowrap|[ Earth [[albedo]] ×}} {{nowrap|([[Earth radius]] /}} Radius of [[Orbit of the Moon|Moon's orbit]])<sup>2</sup>&nbsp;] relative to the direct solar illumination that occurs for a full moon. ({{nowrap|Earth albedo {{=}} 0.367}}; {{nowrap|Earth radius {{=}} (polar}} radius&nbsp;× equatorial {{nowrap|radius)<sup>½</sup> {{=}} 6 367 km}}.)
}}
 
{{efn
| name = angular size
| The range of angular size values given are based on simple scaling of the following values given in the fact sheet reference: at an Earth-equator to Moon-centre distance of 378&nbsp;000&nbsp;km, the [[Angular diameter|angular size]] is 1896&nbsp;[[arcsecond]]s. The same fact sheet gives extreme Earth–Moon distances of 407&nbsp;000&nbsp;km and 357&nbsp;000&nbsp;km. For the maximum angular size, the minimum distance has to be corrected for Earth's equatorial radius of 6&nbsp;378&nbsp;km, giving 350&nbsp;600&nbsp;km.
}}
 
{{efn
| name = pressure explanation
| Lucey ''et al.'' (2006) give {{nowrap|10<sup>7</sup> particles cm<sup>−3</sup>}} by day and {{nowrap|10<sup>5</sup> particles cm<sup>−3</sup>}} by night. Along with equatorial surface temperatures of 390&nbsp;[[Kelvin|K]] by day and 100&nbsp;K by night, the [[ideal gas law]] yields the pressures given in the infobox (rounded to the nearest [[order of magnitude]]): 10<sup>−7</sup>&nbsp;[[Pascal (unit)|Pa]] by day and 10<sup>−10</sup>&nbsp;Pa by night.
}}
 
{{efn
| name = near-Earth asteroids
| There are a number of [[near-Earth asteroid]]s, including [[3753 Cruithne]], that are [[co-orbital]] with Earth: their orbits bring them close to Earth for periods of time but then alter in the long term (Morais et al, 2002). These are [[quasi-satellite]]s&nbsp;– they are not moons as they do not orbit Earth. For more information, see [[Other moons of Earth]].
}}
 
{{efn
| name = Charon and Pluto
| [[Charon (moon)|Charon]] is proportionally larger in comparison to [[Pluto]], but Pluto has been reclassified as a [[dwarf planet]].
}}
 
{{efn
| name = age
| This age is calculated from isotope dating of lunar rocks.
}}
 
{{efn
| name = orbpd
| More accurately, the Moon's mean sidereal period (fixed star to fixed star) is 27.321661&nbsp;days {{nowrap|(27d 07h 43m 11.5s)}}, and its mean tropical orbital period (from equinox to equinox) is 27.321582&nbsp;days {{nowrap|(27d 07h 43m 04.7s)}} (''Explanatory Supplement to the Astronomical Ephemeris'', 1961, at p.107).
}}
 
{{efn
| name = synpd
| More accurately, the Moon's mean synodic period (between mean solar conjunctions) is 29.530589&nbsp;days {{nowrap|(29d 12h 44m 02.9s)}} (''Explanatory Supplement to the Astronomical Ephemeris'', 1961, at p.107).
}}
 
{{efn
| name = brightness
| The Sun's [[apparent magnitude]] is −26.7, and the full moon's apparent magnitude is −12.7.
}}
 
{{efn
| name = area
| On average, the Moon covers an area of {{nowrap|0.21078 square degrees}} on the night sky.
}}
 
{{efn
| name = size changes
| See graph in [[Sun#Life phases]]. At present, the diameter of the Sun is increasing at a rate of about five percent per billion years. This is very similar to the rate at which the apparent angular diameter of the Moon is decreasing as it recedes from Earth.
}}
 
}}
 
'''Citations'''
 
{{reflist
| colwidth = 30em
| refs =
 
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{{cite journal
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{{cite journal
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{{cite book| last = Stroud| first = Rick| title = The Book of the Moon| year = 2009| publisher = Walken and Company| isbn = 978-0-8027-1734-4| pages = 24–27 }}
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{{cite journal
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{{cite web
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{{cite journal
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{{cite web
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{{cite journal
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{{cite journal
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{{cite journal
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{{cite journal
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<ref name="SolarViews">
Hamilton, Calvin J.; Hamilton, Rosanna L., ''The Moon'', [http://www.solarviews.com/eng/moon.htm Views of the Solar System], 1995–2011
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<ref name="bbc">
{{cite news
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{{cite book| author = V V Belet︠s︡kiĭ| title = Essays on the Motion of Celestial Bodies| url = http://books.google.com/?id=byWZusmVSecC| year = 2001| publisher = Birkhäuser| isbn = 978-3-7643-5866-2| page = 183 }}
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<ref name="Moon">
{{cite web
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{{cite journal
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{{cite journal
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{{cite web
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{{cite book| last = Sarma| first = K. V.| authorlink = K. V. Sarma| editor = Helaine Selin| editor-link = Helaine Selin| title = Encyclopaedia of the History of Science, Technology, and Medicine in Non-Western Cultures| edition = 2| year = 2008| publisher = [[Springer Science+Business Media|Springer]]| isbn = 978-1-4020-4559-2| pages = 317–321| contribution = Astronomy in India }}
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{{cite web
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<ref name="CNN">
{{cite news
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<ref name="xinhua_20090301">
{{cite news
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<ref name="unoosa_q6">
{{cite web
| url = http://www.unoosa.org/oosa/en/FAQ/splawfaq.html#Q6
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}}
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<ref name="unoosa_q4">
{{cite web
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| title = How many States have signed and ratified the five international treaties governing outer space?
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<ref name="unoosa_q5">
{{cite web
| url = http://www.unoosa.org/oosa/en/FAQ/splawfaq.html#Q5
| title = Do the five international treaties regulate military activities in outer space?
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<ref name="unoosa_moon">
{{cite web
| url = http://www.unoosa.org/oosa/en/SpaceLaw/moon.html
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| accessdate =28 March 2010
}}
</ref>
 
<ref name="unoosa_q7">
{{cite web
| url = http://www.unoosa.org/oosa/en/FAQ/splawfaq.html#Q7
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| publisher = United Nations Office for Outer Space Affairs
| accessdate =28 March 2010
}}
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<ref name="iisl_2004">
{{cite web
| url = http://www.iislweb.org/docs/IISL_Outer_Space_Treaty_Statement.pdf
| title = Statement by the Board of Directors of the IISL On Claims to Property Rights Regarding The Moon and Other Celestial Bodies (2004)
| year = 2004
| publisher = International Institute of Space Law
| accessdate =28 March 2010
}}
</ref>
 
<ref name="iisl_2009">
{{cite web
| url = http://www.iislweb.org/docs/Statement%20BoD.pdf
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| date = 22 March 2009
| publisher = International Institute of Space Law
| accessdate =28 March 2010
}}
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<ref name="Marshack">
Marshack, Alexander (1991): ''The Roots of Civilization'', Colonial Hill, Mount Kisco, NY.
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<ref name="barnhart-and-germania">
For etymology, see {{cite book| last = Barnhart| first = Robert K.| title = The Barnhart Concise Dictionary of Etymology| year = 1995| publisher = Harper Collins| isbn = 978-0-06-270084-1| page = 487 }} For the lunar calendar of the Germanic peoples, see {{cite book| last = Birley| first = A. R. (Trans.)| title = ''Agricola and Germany''| series = Oxford World's Classics| year = 1999| publisher = Oxford| location = USA| isbn = 978-0-19-283300-6| page = 108 }}
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<ref name="spacetoday">
{{cite web
| url = http://www.spacetoday.org/SolSys/Earth/OldStarCharts.html
| title = Carved and Drawn Prehistoric Maps of the Cosmos
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}}
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<ref name="sciam">
{{cite web
| title = Lunacy and the Full Moon
| publisher = Scientific American
| year = 2009
| first1 =Scott O.
| last1 = Lilienfeld
| first2 = Hal
| last2 = Arkowitz
| url = http://www.scientificamerican.com/article.cfm?id=lunacy-and-the-full-moon
| accessdate =13 April 2010
}}
</ref>
 
}}
 
'''Bibliography'''
 
{{Refbegin}}
* {{Anchor|CITEREFNeedham1986}}{{cite book| last = Needham| first = Joseph| title = Science and Civilization in China, Volume III: Mathematics and the Sciences of the Heavens and Earth| url = http://books.google.com/?id=jfQ9E0u4pLAC| year = 1986| publisher = Caves Books| isbn = 978-0-521-05801-8| place = Taipei }}
{{Refend}}
 
== Further reading ==
 
{{Refbegin}}
* [http://www.bbc.co.uk/worldservice/specials/948_discovery_2008/page4.shtml The Moon]. ''Discovery 2008''. BBC World Service.
* {{cite book| last = Bussey| first = B.| coauthors = [[Paul Spudis|Spudis, P.D.]]| title = The Clementine Atlas of the Moon| year = 2004| publisher = Cambridge University Press| isbn = 0-521-81528-2 }}
* {{cite web |last = Cain|first = Fraser|title= Where does the Moon Come From? |publisher = Universe Today |url=http://www.astronomycast.com/astronomy/episode-17-where-does-the-moon-come-from/ |accessdate =1 April 2008}} (podcast and transcript)
* {{cite journal| last = Jolliff| first = B.| coauthors = Wieczorek, M.; Shearer, C.; Neal, C. (eds.)| title = New views of the Moon| url = http://www.minsocam.org/msa/RIM/Rim60.html| accessdate = 12 April 2007| volume = 60| year = 2006| publisher = Min. Soc. Amer.| location = Chantilly, Virginia| isbn = 0-939950-72-3| doi = 10.2138/rmg.2006.60.0| page = 721| issue = 1| journal = Rev. Mineral. Geochem. }}
* {{cite web|last = Jones|first = E.M.|title = Apollo Lunar Surface Journal|publisher = NASA|year = 2006|url = http://www.hq.nasa.gov/office/pao/History/alsj/|accessdate =12 April 2007}}
* {{cite web|title = Exploring the Moon|publisher = Lunar and Planetary Institute|url = http://www.lpi.usra.edu/expmoon/|accessdate =12 April 2007}}
* {{cite book| last = Mackenzie| first = Dana| title = The Big Splat, or How Our Moon Came to Be| year = 2003| publisher = John Wiley & Sons, Inc| location = Hoboken, New Jersey| isbn = 0-471-15057-6 }}
* {{cite book| last = [[Patrick Moore|Moore, P.]]| title = On the Moon| year = 2001| publisher = Sterling Publishing Co.| location = Tucson, Arizona| isbn = 0-304-35469-4 }}
* {{cite web|title = Moon Articles|publisher = Planetary Science Research Discoveries|url = http://www.psrd.hawaii.edu/Archive/Archive-Moon.html}}
* {{cite book| last = Spudis| first = P. D.| title = The Once and Future Moon| year = 1996| publisher = Smithsonian Institution Press| isbn = 1-56098-634-4 }}
* {{cite book|last = Taylor|first = S.R.|title = Solar system evolution|publisher = Cambridge Univ. Press|page = 307|year = 1992|isbn = 0-521-37212-7}}
* {{cite web|last = Teague|first = K.|title = The Project Apollo Archive|year = 2006|url = http://www.apolloarchive.com/apollo_archive.html|accessdate =12 April 2007}}
* {{cite journal|last = Wilhelms|first = D.E.|title = Geologic History of the Moon|journal = U.S. Geological Survey Professional paper|year = 1987|volume = 1348|url = http://ser.sese.asu.edu/GHM/|accessdate =12 April 2007}}
* {{cite book| last = Wilhelms| first = D.E.| title = To a Rocky Moon: A Geologist's History of Lunar Exploration| url = http://www.lpi.usra.edu/publications/books/rockyMoon/| accessdate = 10 March 2009| year = 1993| publisher = University of Arizona Press| location = Tucson, Arizona| isbn = 0-8165-1065-2 }}
{{Refend}}
 
== External links ==
 
{{Sister project links|Moon|voy=Moon}}
*[http://apod.nasa.gov/apod/ap130129.html APOD - Video of lunar drive]
 
=== Cartographic resources ===
 
* {{cite web|title = Consolidated Lunar Atlas|publisher = Lunar and Planetary Institute|url = http://www.lpi.usra.edu/resources/cla/ | accessdate =26 February 2012}}
* [http://planetarynames.wr.usgs.gov/jsp/FeatureTypes2.jsp?system=Earth&body=Moon&systemID=3&bodyID=11 Gazetteer of Planetary Nomenclature (USGS)] List of feature names.
* {{cite web|title = Clementine Lunar Image Browser|publisher = U.S. Navy|date = 15 October 2003|url = http://www.cmf.nrl.navy.mil/clementine/clib/|accessdate =12 April 2007}}
* 3D zoomable globes:
** {{cite web|title = Google Moon|publisher = Google|year = 2007|url = http://moon.google.com|accessdate =12 April 2007}}
** {{cite web|title = Moon|work = World Wind Central|publisher = NASA|year = 2007|url = http://www.worldwindcentral.com/wiki/Moon|accessdate =12 April 2007}}
* {{cite web|last = Aeschliman|first = R|title = Lunar Maps|work = Planetary Cartography and Graphics|url = http://ralphaeschliman.com/id26.htm|accessdate =12 April 2007}} Maps and panoramas at Apollo landing sites
* [http://wms.selene.jaxa.jp/index_e.html Japan Aerospace Exploration Agency (JAXA)] [[Selene|Kaguya (Selene)]] images
 
=== Observation tools ===
 
* {{cite web|title = NASA's SKYCAL—Sky Events Calendar|publisher = NASA Eclipse Home Page|url = http://sunearth.gsfc.nasa.gov/eclipse/SKYCAL/SKYCAL.html|accessdate =27 August 2007}}
* {{cite web|title = Find moonrise, moonset and moonphase for a location|year = 2008|url =http://www.timeanddate.com/worldclock/moonrise.html|accessdate =18 February 2008}}
* {{cite web|title = HMNAO's Moon Watch|year = 2005|url = http://www.crescentmoonwatch.org/nextnewmoon.htm|accessdate =24 May 2009}} See when the next new crescent moon is visible for any location.
 
{{The Moon}}
{{Earth}}
{{Solar System moons (compact)}}
{{Solar System}}
 
{{Featured article}}
 
[[Category:Moon| ]]
 
{{Link FA|af}}
{{Link FA|ar}}
{{Link FA|bg}}
{{Link FA|da}}
{{Link FA|de}}
{{Link FA|hu}}
{{Link FA|ml}}
{{Link FA|mr}}
{{Link FA|sk}}
{{Link FA|tr}}
{{Link FA|vi}}
{{Link GA|oc}}
{{Link GA|pl}}
{{Link FA|hy}}

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