Editing Summer Triangle

{{Short description|Asterism}}
[[File:Map-summer-triangle.png|thumb|right|alt=Map of the Summer Triangle|Map of the Summer Triangle]]

The '''Summer Triangle''' is an astronomical [[asterism (astronomy)|asterism]] in the [[northern celestial hemisphere]]. The defining [[vertex (geometry)|vertices]] of this imaginary [[triangle]] are at [[Altair]], [[Deneb]], and [[Vega]], each of which is the [[list of brightest stars|brightest star]] of its [[constellation]] ([[Aquila (constellation)|Aquila]], [[Cygnus (constellation)|Cygnus]], and [[Lyra]], respectively). The greatest declination is  +45° and lowest is +9° meaning the three can be seen from all places in the Northern Hemisphere and from the home of most people resident in the Southern Hemisphere. The two stars in Aquila and Cygnus represent the head of an eagle and tail of a swan that looks east inscribed into the triangle and forming the [[Altitude (triangle)|altitude of the triangle]]. Two small constellations, [[Sagitta]] and [[Vulpecula]], lie between Aquila in the south of the triangle and Cygnus and Lyra to the north.

==History==
The term was popularized by the American author [[H. A. Rey]] and the British astronomer [[Patrick Moore]] in the 1950s.<ref name=moore>{{cite book|author=Patrick Moore|title=Patrick Moore's History of astronomy|url=https://books.google.com/books?id=HXTvAAAAMAAJ|date=20 October 1983|publisher=Macdonald|isbn=978-0-356-08607-1}}</ref> The name can be found in constellation guidebooks as far back as 1913.<ref>Alice Mary Matlock Griffiths (1913), ''The Stars and Their Stories: A Book for Young People''.</ref> The Austrian astronomer [[Oswald Thomas]] described these stars as ''Grosses Dreieck'' (Great Triangle) in the late 1920s and ''Sommerliches Dreieck'' (Summerly Triangle) in 1934.  The asterism was remarked upon by [[Joseph Johann von Littrow]], who described it as the "conspicuous triangle" in the text of his atlas (1866), and [[Johann Elert Bode]] connected the stars in a map in a book in 1816, although without a label. These are the same stars recognized in the Chinese legend of ''[[The Cowherd and the Weaver Girl]]'', a story dating back some 2,600 years, celebrated in the [[Qixi Festival]]. The stars also bear ceremonial significance in the related celebrations of [[Tanabata]], [[Chilseok]], and Thất Tịch, derived from Qixi. In the mid- to late-20th century, before [[inertial navigation system]]s and other electronic and mechanical equipment took their places in military aircraft, the [[United States Air Force]] navigators referred to this asterism as the "Navigator's Triangle".<ref>{{cite book |author=Lt. Col. William E. Hubert, USAF (Ret.) |url=https://books.google.com/books?isbn=1425956890 |title=Pilot Here Or Pile It There: A Memoir |publisher=[[AuthorHouse]] |chapter=Chapter Eleven: "Triple Rated" Copilot, (Ugh)! |page=115 |date=December 1, 2006 |isbn=978-142595689-9}}</ref>

==Visibility==
[[File:Altaïr Vega & Deneb.gif|thumb|right|The Summer Triangle in the context of the [[night sky]], with dimmer stars fading out first and then fading in last]]

From [[middle latitudes|mid]]-to-tropical [[Northern Hemisphere|northern latitudes]]:
*the centre of the triangle appears about [[zenith|overhead]] around [[solar midnight]] during [[summer]], and exactly so at about the [[27th parallel north]]. This means it rises at sunset in the east and sets at sunrise in the west. 
*it is visible in the eastern sky in early mornings during [[spring (season)|spring]]. 
*In autumn and winter evenings, it is visible in the western sky until January. 

From mid-[[Southern Hemisphere|southern latitudes]], the asterism is in the north during the culmination season described above.

==The stars of the Summer Triangle==
Both Altair and Vega are bluish-white, rapidly-rotating A-type main sequence stars in the local neighbourhood of the sun. However, Deneb is a white supergiant star over 100 times as distant, and one of the most luminous stars in the entire galaxy.
{| class="wikitable"
!Name
!Constellation
![[Apparent magnitude]]
!Luminosity<br/>({{solar luminosity|link=y}})
![[Stellar classification|Spectral type]]
!Distance<br/>([[light year]]s)
!Radius<br/>({{solar radius|link=y}})
|-
![[Vega]]
|[[Lyra]]||0.03||52||A0 V||25||2.36 to 2.82
|-
![[Deneb]]
|[[Cygnus (constellation)|Cygnus]]||1.25||200,000||A2 Ia||3550||203 ± 17
|-
![[Altair]]
|[[Aquila (constellation)|Aquila]]||0.77||10||A7 V||16.6||1.63 to 2.03
|}

==See also==
* [[Northern Cross (asterism)|Northern Cross]]
* Spring [[Great Diamond]]
* [[Spring Triangle]]
* [[Winter Triangle]]
* [[Winter Hexagon]]
* [[Heavenly Market enclosure]]

==References==
{{reflist}}

==External links==
{{commons category|Summer triangle}}
* [https://web.archive.org/web/20030823181320/http://homepage.mac.com/kvmagruder/bcp/aster/constellations/sumt.htm Summer Triangle] at [https://web.archive.org/web/20031013145549/http://homepage.mac.com/kvmagruder/bcp/ Basic Celestial Phenomena] by Kerry Magruder
* [http://www.astronomy.net/articles/24/ Summer Triangle] at [http://www.astronomy.net/ The Astronomy Net]
* [http://www.daviddarling.info/encyclopedia/S/Summer_Triangle.html Summer Triangle] at [http://www.daviddarling.info/ DavidDarling.info]
* {{APOD |date=25 June 2010 |title=Summer Triangle}}
* {{APOD |date=3 July 2017 |title= The Summer Triangle over the Great Wall }}
{{Portal bar|Astronomy|Stars|Outer space}}

[[Category:Asterisms (astronomy)]]
[[Category:Vega]]
[[Category:Altair]]