Sky
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Reverted edits by [[Special:Contributions/68.61.193.235|68.61.193.235]] to last version by SoWhy (using [[WP:HG|Huggle]])
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[[Image:Skyshot.jpg|thumb|200px|right|When seen from [[altitude]], as here from an [[airplane]], the sky's [[color]] varies from pale to dark at elevations approaching the [[zenith]]]]
[[Image:Turbulent skies.jpg|thumb|190px|right|Turbulent skies]]
[[Image:Trees-sky.jpg|thumb|190px|right|The sky's zenith appears centered in this daytime photograph taken looking up though [[trees]]]]
[[Image:Sunset02.jpg|thumb|190px|right|Clouds made [[Orange (colour)|orange]] by a sunset]]
The '''sky''' is the part of the [[atmosphere]] or of [[outer space]] visible from the surface of any [[astronomical object]]. It is difficult to define precisely for several reasons. During [[daylight]] the sky of [[Earth]] has the appearance of a deep [[blue]] surface, as the result of the [[air]]'s [[scattering]] of [[sunlight]].<ref>{{cite journal |last=Tyndall |first=John |authorlink=John Tyndall |year=1868 |month=December |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 of London]] |volume=17 |pages=pp. 223–233 |url=http://links.jstor.org/sici?sici=0370-1662%281868%2F1869%2917%3C223%3AOTBCOT%3E2.0.CO%3B2-X&size=LARGE |doi=10.1098/rspl.1868.0033}} </ref><ref> {{cite journal |last=Rayleigh |first=Lord|authorlink=Lord Rayleigh |year=1871 |month=June |title=On the scattering of light by small particles |journal=[[Philosophical Magazine]] |volume=41, 275 |pages=pp. 447–451}} </ref><ref> {{cite journal |last=Watson |first=JG |year=2002 |month=June |title=Visibility: Science and Regulation |journal=J. Air & Waste Manage. Assoc |volume=52 |pages=pp. 628–713 |url=http://scholar.google.com/scholar?hl=en&lr=&q=cache:aulPiqN6uTUJ:www.awma.org/journal/pdfs/2002/6/Crit_Review.pdf+ |accessdate = 2007-04-19}} </ref><ref>[http://math.ucr.edu/home/baez/physics/General/BlueSky/blue_sky.html Why is the sky Blue?<!-- Bot generated title -->]</ref> The sky is sometimes defined as the denser [[gas]]eous zone of a [[planet]]'s [[celestial body's atmosphere|atmosphere]]. At night the sky has the appearance of a black surface or region scattered with [[star]]s.
During the day the [[Sun]] can be seen in the sky, unless covered by [[cloud]]s. In the [[night sky]] (and to some extent during the day) the [[moon]], [[planet]]s and [[star]]s are visible in the sky. Some of the [[natural]] phenomena seen in the sky are [[cloud]]s, [[rainbow]]s, and [[Polar Aurora|aurorae]]. [[Lightning]] and [[Precipitation (meteorology)|precipitation]] can also be seen in the sky during [[storm]]s. On [[Earth]], [[bird]]s, [[insect]]s, [[aircraft]], and [[kite flying|kite]]s are often considered to [[Flight|fly]] in the sky. As a result of [[human]] activities, [[smog]] during the day and light radiance during the night are often seen above large [[city|cities]] (see also [[light pollution]]).
In the field of [[astronomy]], the sky is also called the [[celestial sphere]]. This is an imaginary dome where the [[sun]], [[star]]s, [[planet]]s, and the [[moon]] are seen to be travelling. The celestial sphere is divided into regions called [[constellation]]s.
See [[skies of other planets]] for descriptions of the skies of various planets and moons in the [[solar system]].
<!-- I feel pretty....GREEK -->== Sky luminance and colors ==
The light from the sky is a result of the [[diffuse sky radiation|scattering]] of sunlight, which results in a light blue [[color]] perceived by the human eye. On a sunny day [[Rayleigh Scattering]] gives the sky a blue [[Image gradient|gradient]] — dark in the [[zenith]], light near the [[horizon]]. Light that comes in from overhead encounters an air mass 1/38th of the mass that of a sunbeam coming along a horizon paths. So, fewer particles scatter the zenith sunbeam, and, therefore the light remains a darker blue.<ref>[http://weatherquesting.com/bluer-on-top.htm Why is the sky bluer on top than at the horizon]</ref>
The sky can turn a multitude of colors such as red, orange and yellow (especially near [[sunset]] or [[sunrise]]) and black at [[night]]. Scattering effects also partially [[polarization|polarize]] light from the sky.
Sky [[luminance]] distribution models have been recommended by the [[International Commission on Illumination]] (CIE) for the design of [[daylighting]] schemes. Recent developments relate to “all sky models” for modelling sky [[luminance]] under [[weather]] conditions ranging from clear sky to [[overcast]].<ref>{{PDFlink|[http://www.esim.ca/2002/documents/Proceedings/other2.pdf CIE General Sky Standard Defining Luminance Distributions]|710 [[Kibibyte|KiB]]<!-- application/pdf, 727333 bytes -->}}</ref>
== Dark Skies==
Dark Skies is the name usually given to the campaign to reduce and eventually eliminate [[light pollution]] from as much of the planet as possible. The campaign is led by the [http://www.darksky.org/ International Dark Sky Association] (IDA) and supported by organizations in many countries such as [http://www.rasnz.org.nz/darkskies/index.htm The Royal Astronomical Society of New Zealand - RASNZ] and [http://www.sky.org.nz Dark Sky Taonga]. Light pollution is defined by the IDA as; "Any adverse effect of artificial light including sky glow, glare, light trespass, light clutter, decreased visibility at night, and energy waste.
== See also ==
* [[Air]]
* [[Diffuse sky radiation]] — Why the sky is blue.
* [[Sky brightness]]
* [[Skygazing]]
== References ==
<references />
== External links ==
{{Commons|Sky|Sky}}
* [http://www.rayching.co.nz/sky.htm Day Sky Images]
* [http://www.astronomy.net.nz/ Night Sky Images]
* [http://www.hanifworld.com/Sky.htm Sky Photo Gallery]
* [http://math.ucr.edu/home/baez/physics/General/BlueSky/blue_sky.html Why is the sky blue?]
[[Category:Basic meteorological concepts and phenomena]]
[[Category:Astronomy]]
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