Why Do We Call It the 'Summer' Triangle?

Step outside this week as soon as darkness falls and look directly overhead; you will see the famous and very distinctive trio of bright stars — Vega, Altair and Deneb — collectively known as The Summer Triangle. 

The Triangle is not recognized officially as a constellation but instead as an asterism, a prominent pattern or group of stars that typically have a popular name and are composed of stars either within a recognized constellation (such as the Big Dipper asterism belonging to the constellation Ursa Major, the Big Bear) or, in the case of the Triangle, three stars from three different constellations. 

And so far as most asterisms go, the Summer Triangle is a big one. If you recall that your clenched fist held out at arm's length measures roughly 10 degrees, then the apparent distance from Altair to Deneb measures 38 degrees, 34 degrees from Altair to Vega and 24 degrees from Vega to Deneb.

Related: Famous Star Patterns Explained (Images) 

Who christened it?

You might believe that this big triangle has always been noticed as a stand-out star pattern, but that's not so. Most astronomy guidebooks from the early 20th century made no reference to it! The legendary British astronomer Sir Patrick Moore often claimed that he was the first to make reference to the Summer Triangle on his weekly television program in the late 1950s. 

However, several years earlier, H.A. Rey also made reference to it in his popular sky guides , first as a Right Triangle and later using the popular moniker we're all accustomed to today. So, it appears that we cannot say with surety who was first to coin the term, but today virtually every astronomy or constellation guide makes reference to it.

Meet the trio

The brightest of the stellar trio is brilliant, bluish-white Vega, in the constellation of Lyra (the Lyre). Next comes yellow-white Altair in the constellation of Aquila (the Eagle), and finally there is the whitish Deneb in the constellation of Cygnus (the Swan). 

While it is plainly obvious that Vega appears nearly twice as bright as Altair and more than three times brighter than Deneb, the truth is that of the trio, Deneb is the true powerhouse. Situated roughly 2,600 light-years away from Earth, it is about 200 times larger and shines with a luminosity nearly 200,000 times that of our sun. But Vega is more than a hundred times closer to Earth than Deneb, at just 25 light-years away. Vega is only about twice as large as the sun, and about 40 times more luminous; as you can see, that is no comparison with Deneb. 

We could use the analogy that comparing Deneb to Vega would be like comparing the Phare du Creach lighthouse in France (one of the world's most powerful) to a penlight. Incidentally, I think about this analogy every time I see the movie "Cast Away," when Tom Hanks desperately tries to signal a distant ship using a pocket flashlight. 

Or, put another way, if we could somehow move Deneb as close to us as Vega is, it would appear to shine about 40 times brighter than Venus in Earth's sky, casting distinct shadows while being readily visible, even in broad daylight!

So, the only reason that Vega shines so much more brightly than Deneb is because of its close proximity to us. 

As for the third member of the Triangle, Altair, it turns out to be the smallest (less than twice as large as the sun), the least luminous (just 10 times that of the sun), and the closest at only 17 light-years away. We could not see Altair at all if it were as far away as Deneb is; it would appear no brighter than a very faint star of 12th magnitude.

Why not "Autumn" Triangle?

Some might wonder why we call this the "Summer Triangle." After all, the summer season will soon be drawing to a close and yet, the Triangle remains so very prominent in our evening sky. 

It turns out that the Triangle was at opposition — appearing directly opposite to the sun in the sky — on July 17, and for more than a month before and after this date, the Triangle is visible all night long, from dusk until dawn, right on through much of the summertime. Although this time of year we still have these three stars readily visible for much of the night, Altair is dropping out of sight below the western horizon by around 4 a.m. local daylight time.

Now, here is something that's rather strange. Fast-forward to the first week of November, which marks the midpoint of autumn. Now step outside as soon as gets dark. Where would we look to see the Summer Triangle? 

Would you believe that it would still appear almost directly overhead? How is that possible?

The earlier arrival of dusk at this time of year almost offsets the seasonal progression of the constellations (caused by the sun's apparent eastward motion against the background of stars). The result is that the constellations we see at early November nightfalls are only slightly advanced from what we are now seeing in September. 

So, even while temperatures begin to drop, summer continues to linger in our early evening sky. 

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Joe Rao serves as an instructor and guest lecturer at New York's Hayden Planetarium. He writes about astronomy for Natural History magazine, the Farmers' Almanac and other publications, and he is also an on-camera meteorologist for Verizon FiOS1 News in New York's lower Hudson Valley. Follow us on Twitter @Spacedotcom and on Facebook.