How to find it
The Great Square of Pegasus gallops into the fall sky just after dark around the September equinox, which falls annually on or near September 23. Look to the east to northeast just above the horizon. If you are in dark skies, the river of stars known as the Milky Way streams overhead through the Summer Triangle.
Four stars of nearly equal brightness make up the Great Square of Pegasus: Scheat, Alpheratz, Markab and Algenib. A landmark of the Northern Hemisphere’s autumn sky, it looks like a celestial baseball diamond or a great big square. To find it, first of all use the Big Dipper to star-hop to Polaris the North Star. By drawing 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. A line from Polaris through the star Caph of Cassiopeia faithfully escorts you to the Great Square of Pegasus.
Sky chart of Pegasus and Great Square
The Great Square is used much like the Big Dipper to help you find other sky treasures, the most notable being the Andromeda Galaxy. Also, like the Big Dipper, the Great Square of Pegasus is an asterism or part of a constellation. The four second-magnitude stars that make up the square are Scheat, Alpheratz, Markab and Algenib.
A great big square of nothing
Often at events where many are stargazing for the first time, one may hear, “this great big square has nothing in it.” The square isn’t exactly empty. The stars in the square are faint enough that the unaided eye can’t easily detect them. If you have binoculars or small telescopes many stars pop up within the square.
One of the most famous faint stars near the Great Square is 51 Pegasi. In 1995 astronomers announced they discovered a planet around this star. After a few months of skepticism from the astronomical community, it was confirmed the first planet outside of our solar system was discovered. Now we know that two planets orbit the star. Some books say that 51 Pegasi is naked-eye but it is a bit of a challenge. Using binoculars, look roughly halfway between Scheat and Markab. Here is a chart, 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.
As one may remember, Pegasus was a winged horse in Greek mythology. The constellation Pegasus is one of seven constellations in the sky that tells why it is not good to say that a mortal is more beautiful than the gods. This story is plastered all over the autumn night sky.
Queen Cassiopeia bragged that she (or her daughter Andromeda) was more beautiful than immortal Nereids, or sea nymphs. This angered the gods, who asked the sea-god Poseidon to take revenge. The punishment was that King Cepheus and the Queen had to sacrifice their only daughter Andromeda to Cetus the sea monster. Andromeda while chained down to a rock at sea, and about to be gobbled up by the sea monster, saw Perseus riding Pegasus the flying horse. Perseus swooped down and showed the head of the Medusa to the Cetus, the sea monster, then Cetus immediately turned to stone. Then he whacked the chains holding Andromeda and freed her.
They flew off into the sunset to live happily ever after. The mortal horse on the last day of his life was given the honor of becoming a constellation for his loyal service. The dolphin that provided comfort to Andromeda was also granted immortality in the heavens by Zeus with the Delphinus constellation.