The Summer Triangle is an astronomical asterism involving an imaginary triangle drawn on the northern hemisphere's celestial sphere, with its defining vertices at Altair, Deneb, and Vega, being the brightest stars in the three constellations of Aquila, Cygnus, and Lyra.
The English term was popularized by British astronomer Sir Patrick Moore in the 1950s, although he did not invent it. 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 J. J. Littrow, who described it as the "conspicuous triangle" in the text of his atlas (1866), and Bode connected the stars in a map in a book in 1816, although without label.
Near midnight the Summer Triangle lies virtually overhead at mid-northern latitudes during the summer months, but can also be seen during spring in the early morning. In the autumn the summer triangle is visible in the evening well until November. From the southern hemisphere it appears upside down and low in the sky during the winter months. "Northern Triangle" is a more neutral alternative.
| Name | Constellation | Apparent magnitude | Luminosity (× solar) |
Spectral type | Distance (light years) |
|---|---|---|---|---|---|
| Vega | Lyra | 0.03 | 52 | A0 | 25 |
| Deneb | Cygnus | 1.25 | 70000 | A2 | 1550 |
| Altair | Aquila | 0.77 | 10 | A7 | 16.6 |