View source for Astronomy:Sunbeam

{{short description|Rays of sunlight that appear to radiate from the point in the sky where the sun is located}}
[[File:Crepuscular Rays Reno Nevada USA.jpg|thumb|Sunbeams in Nevada during a sunset]]
[[File:Crepuscular Rays, India.JPG|thumb|Daytime sunbeams as seen from the ISS, illustrating their parallel nature]]
A '''sunbeam''', in meteorological optics, is a beam of [[Astronomy:Sunlight|sunlight]] that appears to radiate from the [[Astronomy:Position of the Sun|position of the Sun]]. Shining through openings in clouds or between other objects such as mountains and buildings, these beams of [[Physics:Light scattering by particles|particle-scattered]] sunlight are essentially [[Parallel (geometry)|parallel]] shafts separated by darker [[Physics:Shadow|shadow]]ed volumes. Their apparent convergence in the sky is a visual illusion from linear perspective. The same illusion causes the apparent convergence of parallel lines on a long straight road or hallway at a distant [[Vanishing point|vanishing point]].<ref>{{cite book |title=A Field Guide to the Atmosphere |first1=Vincent J. |last1=Schaefer |first2=John A. |last2=Day |first3=Jay |last3=Pasachoff |publisher=Houghton Mifflin Harcourt |year=1998 |page=169|isbn=9781417657094}}</ref> The scattering particles that make sunlight visible may be air molecules or [[Earth:Particulates|particulates]].<ref name="Lynch&1995">{{cite book |last1=Lynch |first1=D. K. |last2=Livingston |first2=W. |year=1995 |title=Color and Light in Nature |location=Cambridge |publisher={{wipe|Cambridge University Press}}|isbn=9780521468367}}</ref>

==Crepuscular rays==
{{main|Astronomy:Crepuscular rays}} 
[[File:Taipei crepuscular rays 臺北雲隙光 2018.jpg|thumb|Crepuscular rays as seen from [[Place:Taipei|Taipei]], Taiwan (2018)]]

''[[Astronomy:Crepuscular rays|Crepuscular rays]]'' or ''god rays'' are sunbeams that originate when the sun is just below the horizon, during [[Earth:Twilight|twilight]] hours.<ref>{{cite book|url=https://books.google.com/books?id=ZQOJGerV2IcC&pg=PA77|title=Out of the Blue: A 24-Hour Skywatcher's Guide|first=John|last= Naylor|publisher=Cambridge University Press|year=2002|pages=77–79|isbn=9780521809252}}</ref> Crepuscular rays are noticeable when the contrast between light and dark is most obvious. Crepuscular comes from the Latin word "crepusculum", meaning twilight.<ref>{{cite web |url= http://www.weatherscapes.com/album.php?cat=optics&subcat=crepuscular_rays |title= Crepuscular rays |first= Harald |last= Edens |work= Weather Photography lightning, clouds, atmospheric optics & astronomy |access-date= November 1, 2011}}</ref> Crepuscular rays usually appear orange because the path through the atmosphere at sunrise and sunset passes through up to 40 times as much air as rays from a high midday sun. Particles in the air scatter short wavelength light (blue and green) through [[Physics:Rayleigh scattering|Rayleigh scattering]] much more strongly than longer wavelength yellow and red light.

Loosely, the term "crepuscular rays" is sometimes extended to the general phenomenon of rays of sunlight that appear to converge at a point in the sky, irrespective of time of day.<ref>{{Cite web|url=https://www.atoptics.co.uk/atoptics/ray1.htm|title = Crepuscular Rays}}</ref><ref>{{Cite web|url=https://www.weatheronline.co.uk/reports/wxfacts/Crepuscular-rays.htm|title = Weather Facts: Crepuscular rays &#124; weatheronline.co.uk}}</ref>

==Anticrepuscular rays==
{{main|Earth:Anticrepuscular rays}}
[[File:Anticrepuscular rays from above.jpg|thumb|right|These anticrepuscular rays appear to converge at the antisolar point, as viewed from an aircraft above the clouded ocean.]]

In some cases, sunbeams may extend across the sky and appear to converge at the [[Astronomy:Antisolar point|antisolar point]], the point on the [[Celestial sphere|celestial sphere]] opposite of the Sun's direction. In this case, they are called ''antisolar rays'' (anytime not during astronomical night) or ''[[Earth:Anticrepuscular rays|anticrepuscular rays]]'' (during the [[Earth:Twilight|twilight]] period).<ref>{{cite web |url=http://www.atoptics.co.uk/atoptics/anti1.htm |title=Anti-solar (anti-crepuscular) rays |first=Les |last=Cowley |work=Atmospheric Optics |access-date=March 19, 2015}}</ref> This apparent dual convergence (at both the solar and the antisolar points) is a [[Perspective (graphical)|perspective]] effect analogous to the apparent dual convergence of the parallel lines of a long straight road or hallway at directly opposite points (to an observer above the ground).<ref name="cresp">{{cite book |pages=124–27 |url=https://books.google.com/books?id=0cpQGHqxQBUC&pg=PA124 |first=John A. |last=Day |title=The Book of Clouds |year=2005 |publisher=Sterling |isbn=9781402728136 |access-date=2010-10-09}}</ref>

==Alternative names==

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* Backstays of the sun,<ref>{{cite book|last=Heuer|first=K|year=1978|title=Rainbows, Halos, and Other Wonders: Light and Color in the Atmosphere|location=United States|publisher=Dodd, Mead|page=94|isbn=9780396075578}}</ref> a nautical term, from the fact that [[Engineering:Backstay|backstay]]s that brace the mast of a sailing ship converge in a similar way
* Buddha rays<ref name="Lynch&1995" />
* God rays, used by some members of the computer graphics industry<ref>E.g. this term is mentioned in: {{cite conference|last1=Krüger|first1=Jens|last2=Bürger|first2=Kai|last3=Westermann|first3=Rüdiger|year=2006|title=Interactive screen-space accurate photon tracing on GPUs|url=http://wwwcg.informatik.tu-muenchen.de/fileadmin/user_upload/Lehrstuehle/Lehrstuhl_XV/Research/Publications/2006/egsr06.pdf|book-title=Proceedings of the 17th Eurographics conference on Rendering Techniques (EGSR'06)}}</ref>
* [[Religion:Jacob's Ladder|Jacob's Ladder]]<ref name="Lynch&1995" />
* Light shafts, sometimes used in the computer graphics industry, such as the game engine [[Unreal Engine]]<ref name=":0">{{Cite web|url=https://docs.unrealengine.com/en-us/Engine/Rendering/LightingAndShadows/LightShafts|title=Light Shafts|website=Unreal Engine 4 Documentation|language=en-US|archive-url=https://web.archive.org/web/20181117180933/https://docs.unrealengine.com/en-us/Engine/Rendering/LightingAndShadows/LightShafts|archive-date=2018-11-17|access-date=2018-11-17}}</ref>
* Ropes of Maui, originally ''taura a Maui''{{snd}}from the Maori tale of Maui Potiki restraining the sun with ropes to make the days longer<ref name="Lynch&1995" />
* Sun drawing water, from the {{wipe|Ancient Greece}} belief that sunbeams drew water into the sky (an early description of [[Physics:Evaporation|evaporation]])<ref name="Lynch&1995" />
{{div col end}}

==See also==

* {{annotated link|Astronomy:Airglow}}
* {{annotated link|Earth:Anticrepuscular rays}}
* {{annotated link|Physics:Light beam}}
* {{annotated link|Earth:Earth's shadow}}
* {{annotated link|Physics:Rayleigh scattering}}
* {{annotated link|Physics:Tyndall effect}}
* {{annotated link|Astronomy:Sunlight}}
* {{annotated link|Volumetric lighting}}

==References==

{{reflist|32em}}

==External links==
{{commons and category}}

* [http://www.atoptics.co.uk/ Sunrays – Crepuscular rays, Explanation & Images]
* [http://www.demark.org/essays/CrepuscularRays.html Detailed description of how crepuscular rays occur]

[[Category:Atmospheric optical phenomena]]
[[Category:Sun]]

{{Sourceattribution|Sunbeam}}