On these March mornings before sunup, look for the Summer Triangle. It’s not summer for our northern temperate latitudes, but the three brilliant stars of the Summer Triangle – Vega, Deneb and Altair – are visible now in the east before sunrise. These three stars are all bright 1st-magnitude stars. They’re the brightest stars in three different constellations: Deneb in Cygnus the Swan, Vega in Lyra the Harp, and Altair in Aquila the Eagle.
The Summer Triangle isn’t one of the officially recognized 88 constellations. Like the Big Dipper, it’s what’s called an asterism, a pattern of stars that’s easy to pick out.
For much of the Northern Hemisphere, the Summer Triangle stars are up for at least part of the night every night of the year. Are you in the Southern Hemisphere? You probably won’t see the entire Summer Triangle yet before sunup from your part of the world. The star Deneb will probably be lost in the glare of sunrise at southern temperate latitudes.
To gauge the size of this signpost star formation, hold a one-foot ruler an arm’s length from your eye. The ruler (about 1/3 of a meter) pretty much fills the gap between Vega and Altair, the Summer Triangle’s first and second brightest stars, respectively.
On a moonless night, an edgewise view of the galactic disk – and the Dark Rift – pass right through the Summer Triangle. Photo via Flickr user cipdatajeffb.
Like all the stars, the stars of the Summer Triangle rise four minutes earlier with each passing day, or two hours earlier with each passing month. Why is this happening? It’s happening because Earth is orbiting the sun, and our night sky is pointing out on an ever-changing panorama of stars.
Around May Day – May 1 – the Summer Triangle will climb over the eastern horizon around local midnight (1 a.m. daylight time). When middle to late June comes rolling along, you’ll see the Summer Triangle sparkling in the east at evening dusk – a sure sign of summer’s return to the Northern Hemisphere.
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Nils Ribi caught this photo of the Summer Triangle on a northern autumn evening – November 8, 2014. View larger and read Nils’ story of this photo.
Bottom line: The Summer Triangle’s three brilliant stars – Vega, Deneb and Altair – are up before dawn in March, before midnight in May and at dusk on the summer solstice. Right now, they’re in the east before sunup, near Venus!
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Source:
https://earthsky.org/sky-archive/the-summer-triangle-a-signpost-for-all-seasons