Editing Sky

{{Short description|Unobstructed view upward from Earth}}
{{About|Earth's sky|other skies|extraterrestrial skies|other uses}}
{{Pp|small=yes}}
{{Use dmy dates|date=July 2014}}
[[File:July night sky (35972569256).jpg|upright=1.25|thumb|The [[night sky]] over Slovenia in July]]
[[File:Sky over Munich 02.jpg|upright=1.25|thumb|[[cirrus cloud|Cirriform clouds]] and a [[lunar phase|quarter]] [[Moon]] in the blue [[daytime]] sky over Germany]]

The '''sky''' is an unobstructed view upward from the [[planetary surface|surface]] of the [[Earth]]. It includes the [[atmosphere of Earth|atmosphere]] and [[outer space]]. It may also be considered a place between the ground and outer space, thus distinct from outer space.

In the field of [[astronomy]], the sky is also called the [[celestial sphere]]. This is an abstract sphere, [[concentric objects|concentric]] to the Earth, on which the [[Sun]], [[Moon]], [[planet]]s, and [[fixed stars|stars]] appear to be [[Diurnal motion|drifting]]. The celestial sphere is conventionally divided into [[IAU designated constellations|designated areas]] called [[constellation]]s.

Usually, the term ''sky'' informally refers to a perspective from the Earth's surface; however, the meaning and usage can vary. An observer on the surface of the Earth can see a small part of the sky, which resembles a dome (sometimes called the ''sky bowl'') appearing flatter during the day than at [[night]].<ref>{{cite journal |author1=Baird, J. C. |author2=Wagner, M. |year=1982 |title=The moon illusion: I. How high is the sky? |journal=[[Journal of Experimental Psychology: General]] |volume=111 |issue=3 |pages=296–303|doi=10.1037/0096-3445.111.3.296 |pmid=6215460 }}</ref> In some cases, such as in discussing the weather, the sky refers to only the lower, denser layers of the atmosphere.

The [[daytime]] sky appears blue because air molecules [[Rayleigh scattering|scatter]] shorter wavelengths of [[sunlight]] more than longer ones (redder light).<ref>{{cite journal |first=John |last=Tyndall |date=December 1868 |title=On the Blue Colour of the Sky, the Polarization of Skylight, and on the Polarization of Light by Cloudy Matter Generally |journal=[[Proceedings of the Royal Society]] |volume=17 |pages=223–33 |doi=10.1098/rspl.1868.0033 |jstor=112380 |bibcode=1868RSPS...17..223T |doi-access=|s2cid=121593427 }}</ref><ref>{{cite journal |author=Lord Rayleigh |date=June 1871 |title=On the scattering of light by small particles |journal=[[Philosophical Magazine]] |volume=41 |issue=275 |pages=447–51}}</ref><ref>{{cite journal |first=J. G. |last=Watson |date=June 2002 |title=Visibility: Science and Regulation |journal=J. Air & Waste Manage. Assoc. |volume=52 |issue=6 |pages=628–713 |url= |doi=10.1080/10473289.2002.10470813 |pmid=12074426 |doi-access= |bibcode=2002JAWMA..52..628W |s2cid=1078961 }}</ref><ref name="bluesky">{{cite web |url=http://math.ucr.edu/home/baez/physics/General/BlueSky/blue_sky.html |title=Why is the sky Blue? |work=[[Usenet]] Physics FAQ |date=May 1997 |first=Philip |last=Gibbs |access-date=11 December 2012 |archive-date=2 November 2015 |archive-url=https://web.archive.org/web/20151102085211/http://math.ucr.edu/home/baez/physics/General/BlueSky/blue_sky.html |url-status=live }}</ref> The [[night sky]] appears to be a mostly dark surface or region spangled with stars. The Sun and sometimes the Moon are visible in the daytime sky unless obscured by [[cloud]]s. At night, the Moon, planets, and stars are similarly visible in the sky.

Some of the natural phenomena seen in the sky are clouds, [[rainbow]]s, and [[aurora]]e. [[Lightning]] and [[precipitation]] are also visible in the sky. Certain birds and insects, as well as human inventions like [[aircraft]] and [[kite]]s, can fly in the sky. Due to [[human impact on the environment|human activities]], [[smog]] during the day and [[light pollution]] during the night are often seen above large cities.

==Etymology==
The word ''sky'' comes from the [[Old Norse]] {{lang|non|sky}}, meaning 'cloud, abode of [[God]]'. The Norse term is also the source of the [[Old English]] {{lang|ang|scēo}}, which shares the same [[Indo-European]] base as the classical [[Latin]] {{lang|la|obscūrus}}, meaning 'obscure'.

In Old English, the term ''[[heaven]]'' was used to describe the observable expanse above the earth. During the period of  [[Middle English]], "heaven" began shifting toward its current, religious meaning.<ref>{{Cite OED|sky, n.1}}</ref>

== During daytime ==
{{See also|Atmospheric optics|Diffuse sky radiation}}
[[File:Blue Sky, Dhaka, Bangladesh.jpg|thumb|left|Sky during day time]]
[[File:Rayleigh sunlight scattering.png|thumb|[[Earth's atmosphere]] scatters a greater proportion of blue light than of red light.]]

Except for direct [[sunlight]], most of the [[Daylight|light]] in the [[daytime]] sky is caused by [[Light scattering by particles|scattering]], which is dominated by a small-particle limit called [[Rayleigh scattering]]. The scattering due to molecule-sized particles (as in air) is greater in the directions both [[Forward scatter|toward]] and [[Backscatter|away]] from the source of light than it is in directions [[Phase angle (astronomy)|perpendicular]] to the incident path.<ref>{{cite book |url=https://books.google.com/books?id=2x4CAQMEKigC&pg=PA205 |title=Theoretical Fundamentals of Atmospheric Optics |author1=Yu Timofeev |author2=A. V. Vasilʹev |name-list-style=amp |page=174 |date=1 May 2008 |publisher=Cambridge International Science Publishing |isbn=978-1-904602-25-5 }}</ref> Scattering is significant for light at all visible wavelengths, but is stronger at the shorter (bluer) end of the [[visible spectrum]], meaning that the scattered light is bluer than its source: the [[Sun]]. The remaining direct sunlight, having lost some of its shorter-wavelength components, appears slightly less blue.<ref name="bluesky" />

[[Mie scattering|Scattering]] also occurs even more strongly in clouds. Individual water droplets refract white light into a set of [[Corona (optical phenomenon)|colored rings]]. If a cloud is thick enough, scattering from multiple water droplets will wash out the set of colored rings and create a washed-out white color.{{clarify|date=October 2014|reason=What in the world is "a washed-out white color"?}}<ref>{{cite book |url=https://books.google.com/books?id=4Yi8r4qgMV4C&pg=PA427 |title=Fundamentals of Atmospheric Radiation: An Introduction with 400 Problems |author1=Craig F. Bohren |author2=Eugene Edmund Clothiaux |name-list-style=amp |page=427 |publisher=[[Wiley-VCH]] |date=2006 |isbn=978-3-527-40503-9 |bibcode=2006fari.book.....B}}</ref>

The sky can turn a multitude of colors such as red, orange, purple, and yellow (especially near sunset or sunrise) when the light must travel a much longer [[Optical path|path]] (or [[optical depth]]) through the atmosphere. Scattering effects also partially [[Polarization (waves)|polarize]] light from the sky and are most pronounced at an angle 90° from the Sun. Scattered light from the horizon travels through as much as 38 times the [[Air mass (astronomy)|air mass]] as does light from the [[zenith]], causing a blue [[Image gradient|gradient]] looking [[Colorfulness|vivid]] at the zenith and pale near the horizon.<ref>{{cite news|url=http://usatoday30.usatoday.com/news/science/wonderquest/2001-04-11-bluer-sky.htm|title=Bluer on top|work=[[USA Today]]|date=11 April 2001|access-date=3 June 2015|archive-date=21 March 2016|archive-url=https://web.archive.org/web/20160321200530/http://usatoday30.usatoday.com/news/science/wonderquest/2001-04-11-bluer-sky.htm|url-status=live}}</ref> Red light is also scattered if there is enough air between the source and the observer, causing [[Belt of Venus|parts of the sky]] to change color as the Sun rises or sets. As the air mass nears infinity, scattered daylight appears whiter and whiter.<ref name="Lynch" />

Apart from the Sun, distant clouds or snowy mountaintops may appear yellow. The effect is not very obvious on clear days, but is very pronounced when clouds cover the line of sight, reducing the blue hue from scattered sunlight.<ref name="Lynch">{{cite book |url=https://books.google.com/books?id=4Abp5FdhskAC&pg=PA31 |title=Color and Light in Nature |author=David K. Lynch |author2=William Charles Livingston |page=31 |publisher=[[Cambridge University Press]] |isbn=978-0-521-77504-5 |date=2001 }}</ref> At higher altitudes, the sky tends toward darker colors since scattering is reduced due to lower [[density of air|air density]]. An extreme example is the Moon, where no atmospheric scattering occurs, making the [[Extraterrestrial sky#The Moon|lunar sky]] black even when the Sun is visible.<ref>{{cite web |author=National Weather Service |author-link=National Weather Service |location=[[Anchorage, Alaska]] |date=15 July 2005 |url=http://pafc.arh.noaa.gov/classroom/mountain_weather/CH3.pdf |title=Chapter 3: Radiation and Temperature |archive-url=https://web.archive.org/web/20111016195402/http://pafc.arh.noaa.gov/classroom/mountain_weather/CH3.pdf |archive-date=16 October 2011 |publisher=[[National Oceanic and Atmospheric Administration|NOAA]] |access-date=15 February 2012 }}</ref>

Sky [[luminance]] distribution models have been recommended by the [[International Commission on Illumination]] (CIE) for the design of [[Daylighting (architecture)|daylighting]] schemes. Recent developments relate to "all sky models" for modelling sky [[luminance]] under weather conditions ranging from clear to overcast.<ref>{{cite web |author1=Darula, S. |author2=Kittler, R. |url=http://www.ustarch.sav.sk/ustarch/download/Darula_Kittler_Proc_Conf_eSim_2002.pdf |title=General Sky Standard Defining Luminance Distributions |work=Proc. Conf. eSim 2002 |pages=11–13 |date=September 2002 |location=[[Montreal]] |access-date=26 August 2013 |archive-date=23 October 2014 |archive-url=https://web.archive.org/web/20141023222348/http://www.ustarch.sav.sk/ustarch/download/Darula_Kittler_Proc_Conf_eSim_2002.pdf |url-status=live }}</ref>

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== During twilight ==
{{main|Twilight|Dawn|Dusk}}
{{see also|Sky brightness|Earth's shadow|Chappuis absorption}}
[[File:Moon in Sunrise Sky 2.jpg|thumb|upright|left|The [[crescent]] Moon remains visible just moments before sunrise.]]
[[File:Twilight subcategories.svg|thumb|Civil, nautical, and astronomical [[twilight]]. [[Dusk]] is the end of evening twilight.<ref name="Low precision formulae">{{cite journal |bibcode=1979ApJS...41..391V |last=Van Flandern |first=T. |author2=K. Pulkkinen |title=Low precision formulae for planetary positions |journal=[[The Astrophysical Journal|Astrophysical Journal Supplement Series]] |volume=31 |issue=3 |pages=391 |date=1980 |doi=10.1086/190623}}</ref>]]
[[File:Twilight description full day.svg|thumb|[[Dawn]] is the beginning of morning twilight.]]

The brightness and color of the sky vary greatly over the course of a day, and the primary cause of these properties differs as well. When the [[Sun]] is well above the [[horizon]], direct [[Light scattering by particles|scattering]] of [[sunlight]] ([[Rayleigh scattering]]) is the overwhelmingly dominant source of light. However, during [[twilight]], the period between [[sunset]] and night or between night and [[sunrise]], the situation is more complex.

[[Green flash]]es and green rays are optical phenomena that occur shortly after sunset or before sunrise, when a green spot is visible above the Sun, usually for no more than a second or two, or it may resemble a green ray shooting up from the sunset point. Green flashes are a group of phenomena that stem from different causes,<ref name="young">{{cite news |author=Young, A. |url=http://mintaka.sdsu.edu/GF/papers/Zenit/glance.html |title=Green flashes at a glance |date=2006 |publisher=[[San Diego State University]] |access-date=5 March 2009 |archive-date=5 February 2009 |archive-url=https://web.archive.org/web/20090205040740/http://mintaka.sdsu.edu/GF/papers/Zenit/glance.html |url-status=live }}</ref> most of which occur when there is a temperature [[Inversion (meteorology)|inversion]] (when the temperature increases with altitude rather than the normal decrease in temperature with altitude). Green flashes may be observed from any altitude (even from an aircraft). They are usually seen above an unobstructed horizon, such as over the ocean, but are also seen above clouds and mountains. Green flashes may also be observed at the horizon in association with the [[Moon]] and bright planets, including [[Venus]] and [[Jupiter]].<ref>{{cite web |last=Nave |first=C. R. |title=Red Sunset, Green Flash |url=http://hyperphysics.phy-astr.gsu.edu/hbase/atmos/redsun.html |work=[[Georgia State University]] |publisher=HyperPhysics |access-date=11 August 2010 |archive-date=15 August 2010 |archive-url=https://web.archive.org/web/20100815024525/http://hyperphysics.phy-astr.gsu.edu/hbase/atmos/redsun.html |url-status=live }}</ref><ref>{{cite journal |last=O'Connell |first=D. J. K. |title=The Green Flash and Other Low-Sun Phenomena |journal=Castel Gandolfo: Vatican Observatory, Ricerche Astronomiche |date=1958 |volume=4 |page=7 |bibcode=1958RA......4.....O}}</ref>

[[Earth's shadow]] is the shadow that the planet casts through its atmosphere and into outer space. This atmospheric phenomenon is visible during civil twilight (after sunset and before sunrise). When the weather conditions and the [[Scenic viewpoint|observing site]] permit a clear view of the horizon, the shadow's fringe appears as a dark or dull bluish band just above the horizon, in the low part of the sky opposite of the (setting or rising) Sun's direction. A related phenomenon is the [[Belt of Venus]] (or antitwilight arch), a pinkish band that is visible above the bluish band of Earth's shadow in the same part of the sky. No defined line divides Earth's shadow and the Belt of Venus; one colored band fades into the other in the sky.<ref name=AO>{{cite web |title=Earth's shadow |first=Les |last=Cowley |date=2 August 2009 |access-date=15 February 2012 |url=http://www.atoptics.co.uk/atoptics/earshad.htm |archive-date=19 December 2020 |archive-url=https://web.archive.org/web/20201219085648/https://www.atoptics.co.uk/atoptics/earshad.htm |url-status=live }}</ref><ref name=earth-shadow>{{cite book |title=Color and light in nature |first1=David K. |last1=Lynch |first2= William Charles |last2=Livingston |url=https://books.google.com/books?id=4Abp5FdhskAC&pg=PA38 |pages=38, 39 |publisher=Cambridge University Press |edition=2nd |date=July 2001 |isbn=978-0-521-77504-5}}</ref>

Twilight is divided into three stages according to the Sun's depth below the horizon, measured in segments of 6°. After sunset, the [[civil twilight]] sets in; it ends when the Sun drops more than 6° below the horizon. This is followed by the [[nautical twilight]], when the Sun is between 6° and 12° below the horizon (depth between −6° and −12°), after which comes the [[astronomical twilight]], defined as the period between −12° and −18°. When the Sun drops more than 18° below the horizon, the sky generally attains its minimum brightness.<ref>{{cite web |url=http://individual.utoronto.ca/kalendis/twilight/index.htm |publisher=[[University of Toronto]] |first=Irv |last=Bromberg |date=4 April 2011 |access-date=15 February 2012 |title=The Duration of Twilight |archive-date=20 February 2012 |archive-url=https://web.archive.org/web/20120220082706/http://individual.utoronto.ca/kalendis/twilight/index.htm |url-status=live }}</ref>

Several sources can be identified as the source of the intrinsic brightness of the sky, namely [[airglow]], indirect scattering of sunlight, scattering of [[starlight]], and artificial [[light pollution]].

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== During the night ==
{{Main|Night sky}}
[[File:Deathvalleysky nps big.jpg|thumb|upright=1.25|The [[Milky Way]] can be seen as a large band across the night sky, and is distorted into an arch in this 360° panorama.]]

The term night sky refers to the sky as seen at night. The term is usually associated with [[skygazing]] and [[astronomy]], with reference to views of [[celestial bodies]] such as stars, the Moon, and planets that become visible on a clear night after the Sun has set. Natural light sources in a night sky include moonlight, starlight, and airglow, depending on location and timing.  The fact that the sky is not completely dark at night can be easily observed. Were the sky (in the absence of moon and city lights) absolutely dark, one would not be able to see the silhouette of an object against the sky.

The night sky and studies of it have a historical place in both ancient and modern cultures.  In the past, for instance, farmers have used the state of the night sky as a calendar to determine when to plant crops.  The ancient belief in [[astrology]] is generally based on the belief that relationships between heavenly bodies influence or convey information about events on Earth.  The ''scientific'' study of the night sky and bodies observed within it, meanwhile, takes place in the science of astronomy.

Within [[visible-light astronomy]], the visibility of celestial objects in the night sky is affected by light pollution. The presence of the [[Moon]] in the night sky has historically hindered astronomical observation by increasing the amount of ambient lighting. With the advent of artificial light sources, however, light pollution has been a growing problem for viewing the night sky. Special filters and modifications to light fixtures can help to alleviate this problem, but for the best views, both professional and amateur optical astronomers seek viewing sites located far from major urban areas.

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== Use in weather forecasting ==
{{See also|Weather forecasting}}
[[File:Appearance of sky for weather forecast, Dhaka, Bangladesh.JPG|thumb|left|White cumulus clouds appeared over Dhaka, Bangladesh, when significant flooding was underway in many parts of the country.]]

Along with pressure tendency, the condition of the sky is one of the more important parameters used to forecast weather in mountainous areas. Thickening of cloud cover or the invasion of a higher cloud deck is indicative of rain in the near future. At night, high thin [[cirrostratus cloud]]s can lead to [[halo (optical phenomenon)|halos]] around the Moon, which indicate the approach of a [[warm front]] and its associated rain.<ref>{{cite journal|url=https://books.google.com/books?id=KtkDAAAAMBAJ&pg=PA148|journal=[[Popular Mechanics]]|page=148|title=Make Your Own Weather Forecasts|author=Dennis Eskow|date=March 1983|volume=159|issue=3}}</ref> Morning fog portends fair conditions and can be associated with a [[marine layer]], an indication of a stable atmosphere.<ref>{{cite web|url=http://www.wrh.noaa.gov/lox/climate/climate_intro.php|title=Climate of Los Angeles|author=National Weather Service Office, Oxnard, California|publisher=National Weather Service Western Region Headquarters|date=2012|access-date=16 February 2012|archive-date=21 January 2012|archive-url=https://web.archive.org/web/20120121124215/http://www.wrh.noaa.gov/lox/climate/climate_intro.php|url-status=live}}</ref> Rainy conditions are preceded by wind or clouds which prevent fog formation. The approach of a line of thunderstorms could indicate the approach of a [[cold front]]. Cloud-free skies are indicative of fair weather for the near future.<ref>{{cite web |first=Mark |last=Moore |url=http://www.nwac.us/education_resources/Field_forecasting.pdf |archive-url=https://web.archive.org/web/20090325034756/http://www.nwac.us/education_resources/Field_forecasting.pdf |title=Field Forecasting – A Short Summary |archive-date=25 March 2009 |access-date=15 February 2012 }}</ref> The use of sky cover in weather prediction has led to various [[weather lore]] over the centuries.<ref>{{cite web |first=Jerry |last=Wilson |date=2012 |url=http://wilstar.com/skywatch.htm#indicators |title=Skywatch: Signs of the Weather |archive-url=https://web.archive.org/web/20040407171716/http://wilstar.com/skywatch.htm |archive-date=7 April 2004 |access-date=25 May 2008 }}</ref>

=== Tropical cyclones ===
[[File:Tropical cyclone eyewall P-3.jpg|thumb|Picture of the sky in the eye of a tropical cyclone]]

Within 36&nbsp;hours of the passage of a [[tropical cyclone]]'s center, the pressure begins to fall and a veil of white cirrus clouds approaches from the cyclone's direction.  Within 24&nbsp;hours of the closest approach to the center, low clouds begin to move in, also known as the [[bar (tropical cyclone)|bar]] of a tropical cyclone, as the barometric pressure begins to fall more rapidly and the winds begin to increase. Within 18&nbsp;hours of the center's approach, squally weather is common, with sudden increases in wind accompanied by rain showers or thunderstorms. Within six hours of the center's arrival, rain becomes continuous. Within an hour of the center, the rain becomes very heavy and the highest winds within the tropical cyclone are experienced. When the center arrives with a strong tropical cyclone, weather conditions improve and the sun becomes visible as the [[Eye (cyclone)|eye]] moves overhead. Once the system departs, winds reverse and, along with the rain, suddenly increase. One day after the center's passage, the low overcast is replaced with a higher overcast, and the rain becomes intermittent. By 36&nbsp;hours after the center's passage, the high overcast breaks and the pressure begins to level off.<ref>[[Central Pacific Hurricane Center]] (23 July 2006). [http://www.prh.noaa.gov/cphc/pages/FAQ/Observations.php Tropical Cyclone Observations.] {{Webarchive|url=https://web.archive.org/web/20170322035214/http://www.prh.noaa.gov/cphc/pages/FAQ/Observations.php |date=22 March 2017 }} [[National Oceanic and Atmospheric Administration]]. Retrieved on 5 May 2008.</ref>

== Use in transportation ==
{{Main|Flight}}

Flight is the process by which an object  moves through or beyond the sky (as in the case of spaceflight), whether by generating [[Lift (force)|aerodynamic lift]], [[Air propulsion|propulsive thrust]], [[Lighter than air|aerostatically]] using buoyancy, or by [[Ballistics|ballistic]] movement, without any direct mechanical support from the ground.  The engineering aspects of flight are studied in [[aerospace engineering]] which is subdivided into [[aeronautics]], which is the study of vehicles that travel through the air, and [[astronautics]], the study of vehicles that travel through space, and in [[ballistics]], the study of the flight of projectiles.  While human beings have been capable of flight via [[hot air balloon]]s since 1783,<ref>{{cite web
 |url=http://www.centennialofflight.gov/essay/Lighter_than_air/Early_Balloon_Flight_in_Europe/LTA1.htm
 |title=U.S. Centennial of Flight Commisstion: Early Balloon Flight in Europe
 |access-date=4 June 2008
 |url-status=dead
 |archive-url=https://web.archive.org/web/20080602012700/http://www.centennialofflight.gov/essay/Lighter_than_air/Early_Balloon_Flight_in_Europe/LTA1.htm
 |archive-date=2 June 2008
}}</ref> other species have used flight for significantly longer.  Animals, such as birds, bats, and insects are capable of flight.  [[Spore]]s and seeds from plants use flight, via use of the wind, as a method of propagating their species.<ref>{{cite book|author=J. Gurevitch|author2=S. M. Scheiner|author3=G. A. Fox|name-list-style=amp|date=2006|title=Plant Ecology|edition=2nd|publisher=Sinauer Associates |location=Massachusetts}}</ref>

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== Significance in mythology ==
{{See also|Astrology|Sky father}}
[[File:Júpiter y Tetis, por Dominique Ingres.jpg|thumb|upright=0.55|[[Jupiter (mythology)|Jupiter]], Ancient Roman [[sky deity]]]]

Many mythologies have deities especially associated with the sky.  In [[Ancient Egyptian religion|Egyptian religion]], the sky was deified as the goddess [[Nut (goddess)|Nut]] and as the god [[Horus]]. [[Dyeus]] is reconstructed as the god of the sky, or the sky personified, in [[Proto-Indo-European religion]], whence [[Zeus]], the god of the sky and thunder in [[Greek mythology]] and the [[Roman mythology|Roman]] god of sky and thunder [[Jupiter (mythology)|Jupiter]].

In [[Australian Aboriginal mythology]], [[Altjira]] (or Arrernte) is the main sky god and also the creator god. In [[Iroquois mythology]], [[Atahensic]] was a sky goddess who fell down to the ground during the creation of the Earth. Many cultures have drawn constellations between stars in the sky, using them in association with legends and mythology about their deities.

== See also ==
* [[Cyanometer]]
* [[Archaeoastronomy]]

== References ==
{{Reflist}}

== External links ==
{{Wikiquote}}
* {{Commons category-inline}}

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