At Solstice, the Summer Triangle Is Full of Celestial Drama

It is getting hotter. The days are still lengthening. The sun beats down from its highest point, heralding the long hot afternoons of summer in the Northern Hemisphere. Our star will crest at its northernmost point in the sky on the 21st, and will appear to “stand still” before moving back to the south over the coming days and weeks. This standstill is the northern summer solstice, the official start of summer, and a day for cooking on a grill, sitting outside, and drinking something cold. But at night, when the sun disappears, the solstice sky will yield two other special geometrical arrangements.

This solstice night, the brand-new crescent moon, Venus, and Mars will form a triangle at the onset of darkness, which anyone will be able to see from anywhere. All month, Venus and Mars have been near each other, which is both lovely to see and a beautiful demonstration of celestial mechanics. From the Northern Hemisphere, the planets all appear in the southern sky and seem to sail across the night in a bent arc. They trace this path because of Earth’s tilted rotation axis. The planets all orbit the sun on roughly the same plane, as if they were chasing the grooves of a vinyl record with the sun at the center. We are in one of those grooves, too, between Venus and Mars. This is why those planets occasionally seem to move backward in the sky, known as retrograde motion. Sometimes, the planets appear to pass one another, like a runner on a track lapping someone on an outside lane.

On the 21st, Mars, Venus, and the moon form a triangle, all visible with the naked eye or through binoculars. But there’s another trio that appears around the summer solstice: The Summer Triangle of the three bright stars Altair, Vega, and Deneb. It’s easy and fun to locate, especially late at night now, when the triangle will be high overhead.

First look for Vega. It is one of the brightest stars in the sky, so it’s fairly easy to find; after about 10 p.m. local time, look to the east. The bright bluish star is the highest of the three stars that comprise the triangle. Then look below Vega and slightly to its left, where you’ll find Deneb, smack in the middle of the Milky Way if you’re lucky enough to be somewhere dark enough to see that. The imaginary line that connects them forms one side of the triangle. To complete the triangle, look to the right of the two stars to find Altair, the other bright star in the vicinity.

I have always thought of the Summer Triangle as a stargazing gateway, along with my favorite fall constellation, Orion. The Summer Triangle is an asterism, not a constellation. The latter is a pattern of stars that forms a picture humans have imagined since before antiquity, and which is now recognized by the International Astronomical Union. Our zodiac signs are among the big constellations, as are Orion, Lyra the Harp, Cassiopeia the “W,” and others.

Asterisms don’t form a picture, per se, or a representation of something mythical. They are just arrangements, and nothing more. The Pleiades, also known as Subaru or the seven sisters, form an asterism. The Southern and Northern Crosses are asterisms. The Big Dipper is actually an asterism; the scoop that is so easy to find is actually just a part of the constellation Ursa Major, the Great Bear.

But I think these stellar arrangements are in some ways more interesting. Asterisms do not need stories of gods or queens or monsters to be meaningful. They are just collections of stars, and that is all they need to be. The Summer Triangle tells quite a story of death and power on its own.

Vega is one of the closest stars to the sun, and about two and a half times its size. Vega will only burn for about 650 million more years before puffing into a red giant, eventually fading into a white dwarf. Deneb is a supergiant star, about 200 times the size of the sun, and somewhere between 1,500 and 2,600 light-years away; it’s too hard to measure its precise distance because it is so bright. Deneb is cooling, having burned through its hydrogen, and will probably die in a spectacular supernova in a few million years. Altair is about 16.7 light-years from the sun and spins on its axis every 10 hours—120 miles per second—compared to our sun’s rotation period of 27 days. This spin has flattened Altair so that its poles are smushed inward: Think of how a pizza maker flattens the dough by tossing it in the air and spinning it quickly.

As you look for the Summer Triangle, think about the violence and possibility all taking place within and around these tiny bright beacons in the sky. No mythology is more interesting, in my view.

Is there something you’d like to know about our brilliant night sky? Share your stargazing questions with us and you may see them answered in a future Wondersky column!