Favorite Star Patterns

Great Square of Pegasus gallops into autumn sky

Great Square of Pegasus: Sky chart showing the constellation Pegasus the Winged Horse with stars labeled.
The Great Square of Pegasus consists of 4 stars of nearly equal brightness: Scheat, Alpheratz, Markab and Algenib. The Great Square is an asterism within the constellation Pegasus.

Great Square of Pegasus

Look east just after dark for the Great Square of Pegasus around the time of the September equinox (in 2022, it is on September 22). Throughout late September and October evenings, the Great Square will be ascending in the eastern evening sky. The Great Square consists of four stars of nearly equal brightness. Specifically, they are Scheat, Alpheratz, Markab and Algenib. By the way, the constellation Pegasus represents a Flying Horse. But the Great Square – a prominent asterism within Pegasus – is a landmark of the Northern Hemisphere’s autumn sky.

First of all, to find the Great Square, use the Big Dipper to star-hop to Polaris the North Star. Then draw an imaginary line from any Big Dipper handle star through Polaris, and going twice the distance, you’ll always land on the W or M-shaped constellation Cassiopeia the Queen.

Finally, a line from Polaris through the star Caph of Cassiopeia faithfully escorts you to the Great Square of Pegasus.

Chart: Line drawn from Polaris through Cassiopeia star Caph to Great Square.
Look for Polaris, then Cassiopeia, to find the Great Square of Pegasus.

Great Square is an asterism, not a constellation

As well as the Big Dipper, the Great Square of Pegasus isn’t a constellation. Instead, it’s an asterism, or noticeable pattern on our sky’s dome. And, like the Big Dipper, the Great Square can be used to help you find other sky treasures, the most notable being the Andromeda galaxy, another famous sight on autumn evenings in the Northern Hemisphere.

Read more: 2 ways to find the Andromeda galaxy

Constellation Andromeda and asterism Great Square with the Andromeda galaxy labeled.
As a matter of fact, you can use the Great Square of Pegasus to find the Andromeda galaxy. Here’s how to do it.

Great Square of Pegasus: A big square of nothing

Often, at events where many are stargazing for the first time, one may hear:

… the Great Square has nothing in it.

But, of course, the Great Square isn’t empty. Undoubtedly, no part of the night sky is really empty. Because the stars inside the Square are so faint, the unaided eye can’t easily detect them. But if you use binoculars or small telescopes you can see many stars within the Great Square. In fact, counting stars inside the Great Square is a good way to determine how dark your sky is when observing.

Dense star field with four labeled brighter stars at corners of Square.
View larger. | You often hear people say the Great Square is “empty” of stars. Of course, it’s not. Charles White created this composite on November 20, 2017. It consists of 10 images, each a 30-second exposure. Rokinon 35mm lens, f2.0 ISA1600. Camera: Sony QX1 ILCE. Iptron Sky Tracker.

First discovery of an exoplanet orbiting a sun-like star

One of the most famous faint stars near the Great Square is 51 Pegasi. In 1995, astronomers announced in the journal Nature they’d discovered a planet around this star, which is now called 51 Pegasi b. After a few months of skepticism from the astronomical community, it was confirmed that the first planet beyond our solar system had been discovered.

Now we know that at least two planets orbit this star, and astronomers have confirmed more than 5,000 exoplanets. The astronomers who found 51 Pegasi b – Michel Mayor of the University of Geneva and Didier Queloz at the Cavendish Laboratory and Geneva University – received the 2019 Nobel Prize in Physics for their discovery.

Some books say that 51 Pegasi is visible with the unaided eye alone. But it’s a challenge. Using binoculars, look roughly halfway between Scheat and Markab in the Great Square of Pegasus. The chart below is courtesy of Professor Jim Kaler. Note that you won’t be able to see the planets. Pegasus 51 is approximately 50 light-years away from Earth.

Chart of constellation Pegasus with 10 stars labeled.
The star 51 Pegasi in the Great Square, via Jim Kaler.

Mythology of the Great Square of Pegasus

You might recall that Pegasus was a winged horse in Greek mythology. The constellation Pegasus is one of seven constellations in the autumn sky that explain why it’s never a good idea to claim a mortal’s beauty is greater than that of the gods. This story is plastered all over the autumn night sky, and Pegasus figures into it prominently.

The story goes that Cassiopeia the Queen bragged that she (or her daughter Andromeda the Princess) was more beautiful than the immortal Nereids, or sea nymphs. This angered the gods, who asked the sea-god Poseidon to take revenge. The punishment was that Cepheus the King and Cassiopeia had to sacrifice Andromeda to Cetus the Whale (sometimes called a sea monster). Andromeda was chained to a rock at sea. She was about to be gobbled up by Cetus, when she saw Perseus the Hero – riding Pegasus the Flying Horse – swooping toward her.

Perseus had a trick up his sleeve. He faced Cetus, holding up the head of the Gorgon Medusa. It’s said the sight of the Medusa turned Cetus to stone. Then Perseus whacked the chains that bound Andromeda and freed her. They rode off on Pegasus the Flying Horse and lived happily ever after. Later, Zeus placed all of them in the sky as stars, along with Delphinus the Dolphin, who’d provided comfort to Andromeda.

Star chart with black stars on white of constellation Pegasus with galaxy as small red oval.
The Great Square of Pegasus makes up the eastern (left) half of the constellation Pegasus. Image via International Astronomical Union/ Wikimedia Commons.

Bottom line: How to see the Great Square of Pegasus star pattern.

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Posted 
September 21, 2022
 in 
Favorite Star Patterns

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Deborah Byrd

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