The Summer Triangle is a Northern Hemisphere asterism (stars of similar brightness recognized in a distinctive shape). Unlike many other asterisms, the Summer Triangle is actually an amalgamation of stars from three separate constellations.
Three stars make up the triangle: Deneb, Vega and Altair. Deneb is the farthest away from Earth among these three, and is the brightest star in the constellation Cygnus; it forms the tail of the Swan. Coincidentally, Deneb is also the head of another asterism known as the Northern Cross, which is contained in Cygnus.
Vega is the brightest star of an otherwise dim and small constellation, Lyra (the Harp). Vega is one of the brightest stars in the night sky. (Sirius is the brightest in the night sky, but appears in the winter of the Northern Hemisphere.) About 12,000 years ago, it used to be the North Star due to an effect called precession, where the Earth's north-pointing direction changes due to a wobbling axis.
Rounding out the asterism is Altair, which is the brightest star in the constellation Aquilia (the Eagle.) Altair is one of the brightest close stars to Earth.
The name of the asterism became widely used in the 1950s, with the British astronomer Sir Patrick Moore and American author H.A. Rey responsible for popularising the term. The Summer Triangle can be found in astronomy guidebooks dating back to 1913. 19th century astronomers Bode and J.J. Littrow noted the asterism, but did not give it a label.
It was referred to as the Navigator’s Triangle by U.S. military navigators before GPS systems and other navigational equipment took over.
In the summer months, the Summer Triangle can be found directly overhead in mid-northern latitudes, but it is visible at other times of year, too. In spring, it lies in the east in the early morning hours and in fall it can be seen in the west in the evening until November. In mid-northern latitudes, the asterism can be seen at any time of year at some point in the night. In southern latitudes, the Summer Triangle appears upside down and is visible low in the sky in winter.
