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Dalriada n:
Kingdom of the Scots,
home of the residents

How many stars in the Great Square of Pegasus?

You can just about make out the constellation of Pegasus as an upside down horse in the southern sky at the moment, see the Stellarium screenshot below:

pegasusscreenshot3

Its most prominent feature is an asterism known as the Great Square of Pegasus. An asterism is a recognisable pattern of bright starts in the firmament, like the Plough, or the Summer Triangle. The stars may, or may not, belong to the same constellation (e.g. the Summer Triangle comprises Vega, Deneb and Altair in the constellations Lyra, Cygnus and Aquila). The Great Square of Pegasus is not quite as prominent as the Summer Triangle and the stars within it are even less so. So Great Square of Pegasus is actually used as a handy way for checking your sky clarity. Sky at Night magazine says if you can count 7 or more your sky is pretty good. I knew our light polluted sky in Glasgow was not much good for counting dim stars in the Great Square of Pegasus so I set up a camera to see if a long exposure on an equatorial mount would do the trick.

Continuing my quest for astrophotography the easy way I set up the tripod close to the conservatory window, led some control cables inside, and switched on the heater. Then I can steer the mount and control the camera in comfort from my laptop.

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Here is my result, with a clip from the Stellarium screenshot for comparison. Having processed my image by subtracting a dark frame we can just about count 7 stars within the square, but I don’t think that really qualifies as a pretty good sky by the normal naked eye standards.

pegasus2gpegasuscroppedscreenshot4

If you want to try it yourself here’s a handy clip from freestarcharts.com.

Inside the Square

The square of Pegasus is large and the four corner stars easily identifiable. Despite this, there are few easily visible naked eye stars inside the square, but there are many stars on the fringe of naked eye visibility. Consequently counting the total number of stars you can see inside the square is an excellent guide to how dark / good your night skies are.

Seeing conditions

Limiting Magnitude Number of naked eye stars visible in the square Seeing
6.5 35 Exceptional
6.25 21 Excellent
6 13 Superb
5.75 9 Very good
5.5 7 Good
5.25 5 Above Average
5 4 Average
4.75 3 Below Average
4.5 1 Poor
<=4.00 0 Very Poor