3 February – today’s theme is Pegasus

Pegasus is a legendary flying horse, the steed of Perseus, and the constellation with that name dates back at least to Ancient Greek times. The most distinctive feature of the constellation is the large square, known as the Square of Pegasus, in the autumn and winter skies, but there are two oddities. One is that only the head and front half of the body appear in the sky, and the other is that they are upside down.

The figure of Pegasus in the sky, from Bode’s atlas of 1801. Original from ETH-Zurich.

Where to find Pegasus

During National Astronomy Week, the Square of Pegasus is above and to the right of Venus, over to the west. Look at our video of the stars to see exactly where. Compare it with the illustration above and you’ll see that there’s more to it than just the Square, but the head is dipping low in the sky during NAW.

Fun with the Square of Pegasus

You can use the Square of Pegasus to work out the faintest stars you can see. This depends on your eyesight and the darkness of your sky, and provides a fun way to compare your eyesight with others. All you have to do is to count the number of stars that you can see within the square, not counting the four stars that make it up. Really you should do this without looking at the maps below, so that you aren’t biased as to where you know the stars are.

The brightness of a star is called its magnitude. It works the opposite way round from most other scales of measurement, with the brightest stars having the smallest numbers, but this isn’t so odd – think of it as a contest, with the winner being number 1, the next being number 2 and so on. It’s all thanks to the Ancient Greeks, who called the brightest stars magnitude 1 and the faintest magnitude 6.

Pegasus to mag 4.5Pegasus mag 5.0
Pegasus mag 5.5Pegasus mag 6.0
Pegasus mag 6.5

Count the stars that you can see inside the Square of Pegasus and then use these maps to estimate what must be the faintest star you can see. Typical urban skies are worse than magnitude 4.5, while a good country sky is about magnitude 5.5 to 5.8.

Despite what the Greeks said about the faintest star visible being magnitude 6, on the modern magnitude scale some people with really good eyes can see fainter stars, so we have included a map for magnitude 6.5 as well.

Fun facts about Pegasus

The first-ever planet of an ordinary star beyond the Solar System lies in Pegasus, around the star 51 Pegasi. It was found in 1995 by Swiss astronomers, who found that the planet was causing the star to wobble very slightly in it movement across the sky.

See if you can find 51 Pegasi (circled) in the sky. Pic: Robin Scagell

Pegasus shares a star with Andromeda. The top left-hand star of the Square of Pegasus has two astronomical designations, Delta Pegasi and Alpha Andromedae. Strictly speaking it is in Andromeda, hence the name Alpha Andromedae, but until 1922 it was also Delta Pegasi because it is part of the Square of Pegasus.

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