It’s getting hotter. The days keep getting longer. The sun is blazing from its highest point, heralding the long hot afternoons of summer in the Northern Hemisphere. Our star will rise at its northernmost point in the sky on the 21st and appear to ‘stand still’ before moving back south over the coming days and weeks. This lull is the northern summer solstice, the official start of summer and a day for grilling, sitting outside and drinking something cold. But at night, when the sun disappears, the solstice sky will give two other special geometric arrangements.
On this night of the solstice, the new crescent moon, Venus and Mars will form a triangle at the beginning of darkness that everyone will be able to see from anywhere. Throughout the month, Venus and Mars have been close together, which is both beautiful to behold and a beautiful display of celestial mechanics. From the northern hemisphere, all the planets appear in the southern sky and seem to float through the night in a curved arc. They track this path because of the tilted axis of the Earth’s rotation. All the planets orbit the sun in roughly the same plane, as if following the grooves of a vinyl record with the sun in the center. We are also in one of these grooves between Venus and Mars. This is why these planets sometimes appear to move backwards in the sky, known as retrograde motion. Sometimes the planets seem to pass each other, like a track runner tripping someone on an outside lane.
On the 21st, Mars, Venus and the Moon form a triangle, all visible to the naked eye or through binoculars. But there is 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 find, especially late at night now that the triangle will be high overhead.
First look for Vega. It’s one of the brightest stars in the sky, so it’s pretty easy to find; after about 22 hours local time, look east. The bright bluish star is the highest of the three stars that make up the triangle. Then look below Vega and slightly to the left of it, where you’ll find Deneb, in the middle of the Milky Way, if you’re lucky enough to be somewhere dark enough to see this. The imaginary line connecting 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 nearby.
I’ve always thought of the Summer Triangle as a portal to stargazing, 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 that people have imagined since ancient times and that is now recognized by the International Astronomical Union. Our zodiac signs are among the major constellations, as well as Orion, Lyra the Harp, Cassiopeia “W” and others.
Asterisms in themselves do not form a picture or representation of anything mythical. They are just arrangements and nothing more. The Pleiades, also known as Subaru or the Seven Sisters, form an asterism. The Southern Cross and the Northern Cross are asterisms. The Big Dipper is actually an asterism; the spoon that’s so easy to find is actually just part of the constellation Ursa Major, the Big Dipper.
But I think these star arrangements are in some ways more interesting. Asterisms don’t need stories about gods, queens, or monsters to make sense. They’re just a collection of stars, and that’s all they should be. The Summer Triangle tells quite a story about 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 another 650 million years or so before becoming a red giant that will eventually fade to 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 from us; it is too difficult to measure its exact distance because it is so bright. Deneb is cooling after burning through its hydrogen and will likely die in a spectacular supernova in a few million years. Altair is about 16.7 light-years from the sun and rotates on its axis every 10 hours — 120 miles per second — compared to our sun’s rotation period of 27 days. This rotation has flattened Altair so that its poles are squashed inward: think of a pizza maker flattening dough by tossing it in the air and spinning it rapidly.
As you search for the summer triangle, consider the violence and opportunity that takes place in and around these tiny bright beacons in the sky. No mythology is more interesting in my opinion.
Is there anything you’d like to know about our brilliant night sky? Share your star questions with us and you might see their answers in a future Wondersky column!