We’ve just seen Orion’s return to the east before dawn, which means our northern summer is beginning to draw to the a close. But the Summer Triangle asterism still rules the skies. You can see it overhead this evening. As seen from mid-northern latitudes, Vega – the Summer Triangle’s brightest star – shines high overhead around 10 p.m. local daylight saving time (9 p.m. local standard time). Altair resides to the lower left (southeast) of Vega, and Deneb lies to Vega’s left (east).
The Summer Triangle is not a constellation. It’s three bright stars in three different constellations, as the wonderful photo below – by Susan Jensen in Odessa, Washington – shows.
As the stars drift westward during the night Deneb will swing upward, to replace Vega as the overhead star some two hours later. Of course, the stars aren’t really moving. It’s the Earth’s rotation that causes the stars to move westward during the night, and the sun to go westward during the day.
If you’re lucky enough to be under a dark starry sky, you’ll see the great swath of stars known as the Milky Way passing in between the Summer Triangle stars Vega and Altair. The star Deneb bobs in the middle of this river of stars that passes through the Summer Triangle, and arcs across the sky. Although every star that you see with the unaided eye is actually a member of our Milky Way galaxy, oftentimes the term Milky Way refers to the cross-sectional view of the galactic disk, whereby innumerable far-off suns congregate into a cloudy trail of stars.
By the way, you can see the Summer Triangle in the Southern Hemisphere, too – although there do you call it the Winter Triangle? I wonder. South of the equator, people see an upside-down version of tonight’s sky scene, in contrast to our northern perspective. Late tonight, Southern Hemisphere residents will see Altair at the top of the Summer Triangle, and Vega and Altair sparkling at bottom.
Bottom line: The Summer Triangle asterism can be seen overhead at late evening now. The Summer Triangle is not a constellation. It’s three bright stars in three different constellations. These stars are Vega in the constellation Lyra the Harp, Deneb in the constellation Cygnus the Swan, and Altair in the constellation Aquila the Eagle.