Science & history · Asterism guide
The Summer Triangle
The Summer Triangle is three bright stars in three different constellations: Vega in Lyra, Deneb in Cygnus and Altair in Aquila. It is an asterism, and its name is a Northern Hemisphere name.
How to spot it: Three bright stars in a big triangle: Vega, Deneb and Altair. Source (opens in a new tab)
Three constellations, one shape
The NASA Night Sky Network (opens in a new tab) describes the triangle as Vega in Lyra, Deneb in Cygnus and Altair in Aquila, prominent on warm Northern Hemisphere nights. It belongs to no single constellation, which is exactly what makes it an asterism rather than a constellation.
Neighbours only in the sky
The three corners are nowhere near each other in space. Altair is 16.8 light-years (opens in a new tab) away and Vega 25.3, while Deneb is about 1,400 (opens in a new tab), the most distant first-magnitude star. Deneb looks as bright as it does only because it is an enormously luminous supergiant.
Summer where?
"Summer" is a northern name. From the Southern Hemisphere the same three stars are low in the north on winter evenings. The computed ranges below give the latitudes; the finder gives the times for your own place.
The weaver and the cowherd
Two of the corners, Vega and Altair, are the lovers of a Chinese story, parted by the Milky Way that runs through the triangle: the Weaver and the Cowherd.
Where on Earth, and when
The whole key pattern rises from anywhere north of about 45°S.
South of about 81°S, none of it rises.
North of about 81°N, the whole key pattern never sets.
Its middle climbs at least 20° high, the finder's threshold for a suggestion, from anywhere north of about 38°S.
It is highest in the evening sky, at about 9 p.m. local time, in mid-September, from either hemisphere, wherever it rises. With daylight saving time, that is nearer 10 p.m. on the clock.
Worked out by this site from the Bright Star Catalogue, for a flat, clear horizon. Hills, buildings, haze and city light all cut into this; the air also lifts objects near the horizon by about half a degree. For your own place and night, use the sky finder.
Sources
- Astronomy Kat Troche (Astronomical Society of the Pacific), Connecting the Dots with Asterisms, NASA Night Sky Network (2024) (opens in a new tab) for the three stars and their constellations
- Astronomy ESA, Hipparcos: The 50 Brightest Stars (opens in a new tab) for the distances of Vega and Altair
- History Ian Ridpath, Star Tales: Cygnus (opens in a new tab) for Deneb's distance