The Great Square of Pegasus is one of the highlights of the eastern evening sky in late summer and early autumn. Its four stars are fairly easy to pick out even from the suburbs. The square's brightest star, Alpheratz, actually does double duty. Although it is the brightest star in the classical outline of the flying horse, in modern times it's been pushed into adjoining Andromeda, where it shines as the the constellation's "Alpha" star. This diagram shows the official constellation boundaries, with Alpheratz just across the line in Andromeda.
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In the days before computerized precision, when someone wanted to create a constellation they simply linked up a few stars to make a picture. Some of those pictures are big and bold, such as the Great Square of Pegasus. It’s low in the east as night falls, and is tilted so that it looks more like a diamond than a square.
But as the science of astronomy became more precise, its practitioners needed something a little more organized. So they split the sky into 88 constellations, each with precisely defined borders. That gives each star and galaxy its own constellation to call home.
For the most part, these constellations incorporated the well-known star pictures. But in one case, a prominent star was moved from one constellation to another.
Alpheratz — a name that means “the horse’s shoulder” — is the brightest star of the Great Square. It’s at the left-hand point of the square during the evening hours. For millennia, it was also the brightest star in all of Pegasus.
But when astronomers approved the formal constellation borders in the 1930s, Alpheratz was given a new home. It’s just across the line in the adjoining constellation Andromeda. In fact, it’s tied for brightest-star honors in that constellation.
Yet when you look into the starry night sky, it’s hard to deny the star’s heritage. Alpheratz perfectly completes the Great Square of Pegasus — giving body to the celestial flying horse.
Tomorrow: numbers game.
Script by Damond Benningfield, Copyright 2013