The Great Square of Pegasus is one of the easiest star patterns, or asterisms, to spot in the autumn night sky — four bright stars forming a huge square that marks the body of the winged horse, Pegasus. Once you’ve found it, the Square becomes a useful launching point for exploring the surrounding sky: Andromeda, Pisces, and even the Andromeda Galaxy are all just a short hop away.
What Does the Great Square Look Like?
The first thing to know about the Great Square is its size. Each side spans roughly 15° of sky — about one and a half fists held at arm’s length. If you’re looking for a neat little square of stars, you’ll probably miss it completely. This is a genuinely huge pattern — it’s called the Great Square for good reason.
Its four corners are marked by Alpheratz, Scheat, Markab and Algenib. Don’t expect a perfect geometric square. Look instead for four bright stars forming a large, slightly uneven box. There are plenty of stars inside it, but most are too faint to see with the naked eye, giving the Square a surprisingly empty appearance. The darker your sky, however, the more stars you’ll begin to pick out inside it.
Despite its name, however, the Great Square doesn’t belong entirely to Pegasus. Three of its corner stars do, but Alpheratz belongs to neighboring Andromeda.
Follow Cassiopeia to the Great Square
During autumn evenings in the Northern Hemisphere, the Great Square is prominent in the eastern and southeastern sky, climbing higher as the evening progresses. If you already recognize its huge four-star shape, it can be surprisingly easy to pick out directly.
If not, start with Cassiopeia. Its distinctive W-shaped pattern is one of the easiest constellations to recognize and lies on the opposite side of Polaris from the Big Dipper. From Cassiopeia, look toward the chain of stars forming Andromeda. Follow this chain away from Cassiopeia and it eventually reaches Alpheratz — the bright star marking the northeast corner of the Great Square.
Once you’ve found Alpheratz, step back and look at a much larger area of sky. The other three corner stars — Scheat, Markab and Algenib — complete the Square.
Andromeda
The Great Square leads naturally into Andromeda. Start at Alpheratz and follow the bright trail of stars stretching away from the Square toward Mirach. Once you see that long, gently curving chain, you’ve found the main outline of the Andromeda constellation.
Finding the constellation Andromeda also puts you within reach of one of the night sky’s most famous sights. Look just above Mirach and, under a dark sky, you may notice a faint, misty smudge that doesn’t look quite like a star. That’s the Andromeda Galaxy (M31), situated about 2.5 million light-years away and the most distant object most people will ever see with the naked eye.
Pisces
The Great Square can also guide you to Pisces, a much fainter constellation lying below it in the autumn sky. Look south of the Square for a small loop of stars known as the Circlet. Pisces is represented by two fish joined by long lines of faint stars, and the Circlet forms the western of the two. It won’t jump out at you like the Great Square, but under a reasonably dark sky its rough ring shape becomes easier to pick out. Once you’ve found the Circlet, you’ve found the most recognizable part of an otherwise faint and sprawling constellation. From there, a long line of dim stars stretches eastward beneath the Great Square toward the second fish.
Farther south, the bright star Fomalhaut is much easier to spot. It belongs to a completely separate constellation, Piscis Austrinus (the Southern Fish), rather than Pisces, but provides another useful landmark below the Great Square.
Pegasus
Pegasus stretches well beyond the Great Square’s four corners. Starting at Markab, follow the curving chain of stars extending westward from the Square. The chain eventually leads to Enif, one of the constellation’s brightest stars at the far end of the winged horse.
Following this chain makes it much easier to trace Pegasus beyond its familiar four-star Square. Enif is particularly easy to remember: its name comes from the Arabic word for “nose,” matching its position in the traditional outline of the winged horse. If you have binoculars or a small telescope, Enif also provides an easy starting point for finding the globular cluster M15, about 4° to its northwest.