Editing Moon (section)

==Position and appearance==
{{anchor|Observation|Appearance from Earth}}
{{See also|Lunar observation}}
[[File:Lunar libration with phase Oct 2007 HD.gif|alt=Over one lunar month more than half of the Moon's surface can be seen from Earth's surface.|thumb|[[Libration]], the slight variation in the Moon's [[apparent size]] and viewing angle over a single lunar month as viewed from somewhere on the Earth's northern hemisphere.]]
The Moon's highest [[altitude (astronomy)|altitude]] at [[culmination]] varies by its [[lunar phase]], or more correctly its orbital position, and time of the year, or more correctly the position of the Earth's axis. The full moon is highest in the sky during winter and lowest during summer (for each hemisphere respectively), with its altitude changing towards dark moon to the opposite.

At the [[North Pole|North]] and [[South Pole]]s the Moon is 24 hours above the horizon for two weeks every [[tropical month]] (about 27.3 days), comparable to the [[polar day]] of the [[tropical year]]. [[Zooplankton]] in the [[Arctic]] use [[moonlight]] when the Sun is [[polar night|below the horizon]] for months on end.<ref>{{cite web |date=January 16, 2016 |title=Moonlight helps plankton escape predators during Arctic winters |url=https://www.newscientist.com/article/dn28738-moonlight-helps-plankton-escape-predators-during-arctic-winters/ |url-status=live |archive-url=https://web.archive.org/web/20160130112225/https://www.newscientist.com/article/mg22930562-500-moonlight-helps-plankton-escape-predators-during-arctic-winters/ |archive-date=January 30, 2016 |work=[[New Scientist]]}}</ref>

The apparent orientation of the Moon depends on its position in the sky and the hemisphere of the Earth from which it is being viewed. In the [[northern hemisphere]] it appears upside down compared to the view from the [[southern hemisphere]].<ref>{{cite web |last=Howells |first=Kate |date=September 25, 2020 |title=Can the Moon be upside down? |url=https://www.planetary.org/articles/can-the-moon-be-upside-down |url-status=live |archive-url=https://web.archive.org/web/20220102132012/https://www.planetary.org/articles/can-the-moon-be-upside-down |archive-date=January 2, 2022 |access-date=January 2, 2022 |publisher=The Planetary Society}}</ref> Sometimes the "horns" of a crescent moon appear to be pointing more upwards than sideways. This phenomenon is called a [[wet moon]] and occurs more frequently in the [[tropics]].<ref>{{cite web |last=Spekkens |first=K. |author1-link=Kristine Spekkens |date=October 18, 2002 |title=Is the Moon seen as a crescent (and not a "boat") all over the world? |url=http://curious.astro.cornell.edu/our-solar-system/46-our-solar-system/the-moon/observing-the-moon/124-is-the-moon-seen-as-a-crescent-and-not-a-boat-all-over-the-world-is-the-same-phase-of-the-moon-visible-from-the-northern-and-southern-hemispheres-advanced |url-status=live |archive-url=https://web.archive.org/web/20151016011356/http://curious.astro.cornell.edu/our-solar-system/46-our-solar-system/the-moon/observing-the-moon/124-is-the-moon-seen-as-a-crescent-and-not-a-boat-all-over-the-world-is-the-same-phase-of-the-moon-visible-from-the-northern-and-southern-hemispheres-advanced |archive-date=October 16, 2015 |access-date=September 28, 2015 |publisher=Curious About Astronomy}}</ref>

The [[Lunar distance|distance between the Moon and Earth]] varies from around {{convert|356,400|km|mi|abbr=on}} ([[perigee]]) to {{convert|406,700|km|mi|abbr=on}} (apogee), making the Moon's distance and apparent size fluctuate up to 14%.<ref name="size1"/><ref name="size2"/> On average the Moon's [[angular diameter]] is about 0.52°, roughly the same apparent size as the Sun (see {{section link||Eclipses}}). In addition, a purely psychological effect, known as the [[Moon illusion]], makes the Moon appear larger when close to the horizon.<ref>{{cite book |last=Hershenson |first=Maurice |title=The Moon illusion |date=1989 |publisher=[[Routledge]] |isbn=978-0-8058-0121-7 |page=5}}</ref>

===Rotation===
[[File:Tidal locking of the Moon with the Earth.gif|thumb|Comparison between the Moon on the left, rotating tidally locked (correct), and with the Moon on the right, without rotation (incorrect)]]
The [[synchronous rotation|tidally locked synchronous rotation]] of the Moon as it orbits the Earth results in it always keeping nearly the same face turned towards the planet. The side of the Moon that faces Earth is called the [[near side]], and the opposite the [[far side of the Moon|far side]]. The far side is often inaccurately called the "dark side", but it is in fact illuminated as often as the near side: once every 29.5 Earth days. During [[dark moon]] to [[new moon]], 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 |access-date=February 15, 2010 |url-status=live |archive-url=https://web.archive.org/web/20100412192834/http://www.badastronomy.com/bad/misc/dark_side.html |archive-date=April 12, 2010 |author-link=Phil Plait}}</ref>

The Moon originally rotated at a faster rate, but early in its history its rotation slowed and became [[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]] |date=1973 |volume=23 |issue=2 |pages=459–508 |bibcode=1973Ap&SS..23..459A |doi=10.1007/BF00645172 |s2cid=122918899}}</ref> With time, the energy of rotation of the Moon on its axis was dissipated as heat, until there was no rotation of the Moon relative to Earth. In 2016, planetary scientists using data collected on the 1998–99 NASA ''[[Lunar Prospector]]'' mission found two hydrogen-rich areas (most likely former water ice) on opposite sides of the Moon. It is speculated that these patches were the poles of the Moon billions of years ago before it was tidally locked to Earth.<ref>{{cite news |title=Moon used to spin 'on different axis' |url=https://www.bbc.com/news/science-environment-35883576 |access-date=March 23, 2016 |url-status=live |archive-url=https://web.archive.org/web/20160323203442/http://www.bbc.com/news/science-environment-35883576 |archive-date=March 23, 2016 |work=BBC News |date=March 23, 2016}}</ref>

===Illumination and phases===
{{See also|Lunar phase|Moonlight|Halo (optical phenomenon)}}
[[File:Moon phases en.jpg|center|thumb|550x550px|The monthly changes in the angle between the direction of sunlight and view from Earth, and the [[Lunar phase|phases of the Moon]] that result, as viewed from the [[Northern Hemisphere]]. The [[Earth–Moon distance]] is not to scale.]]

The Moon rotates, as it orbits Earth, changing orientation toward the Sun, experiencing a [[lunar day]]. A lunar day is equal to one [[lunar month]] (one synodic orbit around Earth) due to it being [[tidal locking|tidally locked]] to Earth. Since the Moon is not tidally locked to the Sun, lunar daylight and night times both occur around the Moon. The changing position of the illumination of the Moon by the Sun during a lunar day is observable from Earth as the changing [[lunar phase]]s, waxing crescent being the sunrise and the waning crescent the sunset phase of a day observed from afar.<ref name="r040">{{cite web | title=Phases of the Moon explained | website=BBC Sky at Night Magazine | date=January 21, 2025 | url=https://www.skyatnightmagazine.com/space-science/why-does-the-moons-appearance-change | access-date=April 29, 2025}}</ref>

Lunar night is the darkest on the far side and during lunar eclipses on the near side (and darker than a moonless night on Earth). The near side is during its night illuminated by [[Earthlight (astronomy)|Earthlight]], making the near side illuminated enough by the Earthlight to see lunar surface features from Earth where it is dark during its night phase due to Earthlight being reflected back to Earth. Earthshine makes the night on the near side about 43 times brighter, and sometimes even 55 times brighter than a night on Earth illuminated by the light of the full moon.<ref name="u988">{{cite web | last=Siegel | first=Ethan | title=Ask Ethan: How Bright Is The Earth As Seen From The Moon? | website=Forbes | date=March 18, 2017 | url=https://www.forbes.com/sites/startswithabang/2017/03/18/ask-ethan-how-bright-is-the-earth-as-seen-from-the-moon/ | access-date=April 29, 2025}}</ref>

In Earth's sky brightness and apparent size of the Moon changes also due to its elliptic [[Orbit of the Moon|orbit around Earth]]. At [[perigee]] (closest), since the Moon is up to 14% closer to Earth than at [[apogee]] (most distant), it subtends a [[solid angle]] which is up to 30% larger. Consequently, given the same phase, the Moon's brightness also varies by up to 30% between apogee and perigee.<ref>{{cite web |url=http://www.space.com/34515-supermoon-guide.html |title=Supermoon November 2016 |date=November 13, 2016 |access-date=November 14, 2016 |publisher=Space.com |url-status=live |archive-url=https://web.archive.org/web/20161114220725/http://www.space.com/34515-supermoon-guide.html |archive-date=November 14, 2016}}</ref> A full (or new) moon at such a position is called a [[supermoon]].<ref name="size1">{{cite web |title=Super Full Moon |date=March 16, 2011 |author=Tony Phillips |publisher=NASA |access-date=March 19, 2011 |url=https://science.nasa.gov/science-news/science-at-nasa/2011/16mar_supermoon/ |url-status=dead |archive-url=https://web.archive.org/web/20120507035348/https://science.nasa.gov/science-news/science-at-nasa/2011/16mar_supermoon/ |archive-date=May 7, 2012}}</ref><ref name="size2">{{cite news |title=Full moon tonight is as close as it gets |date=March 18, 2011 |author=Richard K. De Atley |newspaper=[[The Press-Enterprise]] |access-date=March 19, 2011 |url=http://www.pe.com/localnews/stories/PE_News_Local_D_moon19.23a6364.html |url-status=dead |archive-url=https://web.archive.org/web/20110322161600/http://www.pe.com/localnews/stories/PE_News_Local_D_moon19.23a6364.html |archive-date=March 22, 2011}}</ref><ref>{{cite news |url=https://www.theguardian.com/science/2011/mar/19/super-moon-closest-point-years |title='Super moon' to reach closest point for almost 20 years |newspaper=The Guardian |date=March 19, 2011 |access-date=March 19, 2011 |url-status=live |archive-url=https://web.archive.org/web/20131225175506/http://www.theguardian.com/science/2011/mar/19/super-moon-closest-point-years |archive-date=December 25, 2013}}</ref>

====Observational phenomena====
There has been historical controversy over whether observed 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 [[lunar transient phenomena]]. Recently, it has been suggested that a roughly {{convert|3|km|abbr=on}} diameter region of the lunar surface was modified by a gas release event about a million years ago.<ref>{{cite journal |last=Taylor |first=G. J. |date=November 8, 2006 |title=Recent Gas Escape from the Moon |url=http://www.psrd.hawaii.edu/Nov06/MoonGas.html |url-status=dead |journal=Planetary Science Research Discoveries |page=110 |bibcode=2006psrd.reptE.110T |archive-url=https://web.archive.org/web/20070304055515/http://www.psrd.hawaii.edu/Nov06/MoonGas.html |archive-date=March 4, 2007 |access-date=April 4, 2007}}</ref><ref>{{cite journal |last1=Schultz |first1=P. H. |last2=Staid |first2=M. I. |last3=Pieters |first3=C. M. |date=2006 |title=Lunar activity from recent gas release |journal=[[Nature (journal)|Nature]] |volume=444 |issue=7116 |pages=184–186 |bibcode=2006Natur.444..184S |doi=10.1038/nature05303 |pmid=17093445 |s2cid=7679109}}</ref>

===Albedo and color===
[[File:Two Lunar Phases.jpg|thumb|right|upright=1.35|The changing apparent color of the Moon, filtered by Earth's atmosphere]]
The Moon has an exceptionally low [[albedo]], giving it a [[diffuse reflection|reflectance]] that is slightly brighter than that of worn [[Asphalt concrete|asphalt]]. Despite this, it is the [[List of brightest natural objects in the sky|brightest object in the sky]] after the [[Sun]].<ref name="worldbook" />{{efn|name=brightness}} This is due partly to the brightness enhancement of the [[opposition surge]]; the Moon at quarter phase is only one-tenth as bright, rather than half as bright, as at [[full moon]].<ref name="Moon" /> Additionally, [[color constancy]] in the [[visual system]] recalibrates the relations between the colors 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 center, without [[limb darkening]], because of the [[Lambert's cosine law#Lambertian scatterers and radiators|reflective properties]] of [[lunar soil]], which [[retroreflector|retroreflects]] light more towards the Sun than in other directions. The Moon's color depends on the light the Moon reflects, which in turn depends on the Moon's surface and its features, having for example large darker regions. In general, the lunar surface reflects a brown-tinged gray light.<ref name="Science Mission Directorate 2020">{{cite web |date=November 11, 2020 |title=Colors of the Moon |url=https://science.nasa.gov/colors-moon |url-status=live |archive-url=https://web.archive.org/web/20220409212600/https://science.nasa.gov/colors-moon |archive-date=April 9, 2022 |access-date=April 9, 2022 |website=Science Mission Directorate}}</ref>

At times, the Moon can appear red or blue.
It may appear red during a [[lunar eclipse]], because of the red spectrum of the Sun's light being [[refracted]] onto the Moon by Earth's atmosphere. Because of this red color, lunar eclipses are also sometimes called [[Blood Moon (eclipse)|blood moons]]. The Moon can also seem red when it appears at low angles and through a thick atmosphere.

The Moon may appear blue depending on the presence of certain particles in the air,<ref name="Science Mission Directorate 2020" /> such as volcanic particles,<ref>{{cite web |last=Gibbs |first=Philip |date=May 1997 |title=Why is the sky blue? |url=http://math.ucr.edu/home/baez/physics/General/BlueSky/blue_sky.html |url-status=live |archive-url=https://web.archive.org/web/20151102085211/http://math.ucr.edu/home/baez/physics/General/BlueSky/blue_sky.html |archive-date=November 2, 2015 |access-date=November 4, 2015 |website=math.ucr.edu |quote=... may cause the moon to have a blue tinge since the red light has been scattered out.}}</ref> in which case it can be called a [[Blue moon#Whole blue moon|blue moon]].

Because the words "red moon" and "blue moon" can also be used to refer to specific [[full moon]]s of the year, they do not always refer to the presence of red or blue [[moonlight]].

=== Eclipses ===
{{Main |Solar eclipse |Lunar eclipse |Solar eclipses on the Moon|Eclipse cycle}}
{{multiple image
| total_width       = 330
| image1            = Solar_eclipse_1999_4_NR.jpg
| image2            = Full Eclipse of the Moon as seen in from Irvine, CA, USA (52075715442) (cropped).jpg
| caption1          = A [[solar eclipse]] causes the Sun to be covered, revealing the white [[Stellar corona|corona]].
| caption2          = The Moon, tinted reddish, during a [[lunar eclipse]]
}}

Eclipses 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 vastly greater distance that 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 |date=2000 |url=http://www.mreclipse.com/Special/SEprimer.html |title=Solar Eclipses for Beginners |publisher=MrEclip |access-date=March 17, 2010 |archive-date=May 24, 2015 |archive-url=https://web.archive.org/web/20150524172606/http://www.mreclipse.com/Special/SEprimer.html |url-status=dead}}</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. 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 |access-date=December 25, 2013 |url-status=live |archive-url=https://web.archive.org/web/20131208153430/http://www.fourmilab.ch/images/peri_apo/ |archive-date=December 8, 2013}}</ref>

As the Moon's orbit around Earth is inclined by about 5.145° (5° 9') 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 |access-date=March 17, 2010 |url-status=dead |archive-url=https://web.archive.org/web/20071030225501/http://sunearth.gsfc.nasa.gov/eclipse/SEsaros/SEsaros.html |archive-date=October 30, 2007}}</ref>

Because the Moon continuously blocks the view of a half-degree-wide circular area of the sky,{{Efn |name=area}}<ref>{{cite magazine |title=The Square Degree as a Unit of Celestial Area |author=Guthrie, D.V. |date=1947 |magazine=[[Popular Astronomy (US magazine)|Popular Astronomy]] |volume=55 |pages=200–203 |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]] |access-date=March 17, 2010 |url-status=dead |archive-url=https://web.archive.org/web/20100223022627/http://occsec.wellington.net.nz/total/totoccs.htm |archive-date=February 23, 2010}}</ref>