The Great Square of Pegasus: Autumn's Signpost

Every season has a signature pattern that anchors the sky, and for autumn it is the Great Square of Pegasus. This large, unmistakable quadrilateral rides high in the southern sky during October and November evenings, serving as a starting point for exploring the surrounding constellations and finding the autumn sky's finest deep-sky objects.

Finding the Great Square

The Great Square is easy to spot once you know what to look for:

1. Face south during an autumn evening around 9-10 PM local time.
2. Look for a large, tilted square of four moderately bright stars high in the sky. The square spans about 15 degrees on a side -- roughly the width of your fist at arm's length.
3. Note the empty interior. The area inside the square is remarkably devoid of bright stars. Under light-polluted skies, it may appear completely empty. From dark sites, you might count five to ten faint stars within.

The emptiness of the interior is actually a useful test of sky quality. Counting how many stars you can see inside the square with the naked eye gives a rough measure of your limiting magnitude and sky darkness.

The Four Corner Stars

Markab (Alpha Pegasi)

The southwestern corner at magnitude +2.49. Markab is a hot blue-white star about 133 light-years from Earth.

Scheat (Beta Pegasi)

The northwestern corner at magnitude +2.42. Scheat is a red giant whose orange tint is visible through binoculars. It is slightly variable, ranging between magnitudes +2.31 and +2.74.

Algenib (Gamma Pegasi)

The southeastern corner at magnitude +2.84. A blue-white star and the faintest of the four corners.

Alpheratz (Alpha Andromedae)

The northeastern corner at magnitude +2.06 and the brightest of the four. Despite marking a corner of Pegasus, Alpheratz officially belongs to the constellation Andromeda. It serves as the link between the two constellations.

Using the Great Square as a Springboard

The Great Square connects to several important constellations:

Andromeda

From Alpheratz, a chain of stars extends to the northeast. Following this chain leads to the Andromeda Galaxy (M31), the nearest large galaxy to our Milky Way. M31 is visible to the naked eye from dark sites and is the premier deep-sky target of autumn.

Cassiopeia

The W-shaped circumpolar constellation sits high above the Great Square. A line from Scheat through Alpheratz and extended northward points toward Cassiopeia.

Perseus

Beyond Andromeda toward the east, Perseus contains the Double Cluster and the variable star Algol.

Pisces

Below and around the Great Square, the faint zodiac constellation Pisces sprawls. Planets frequently pass through this region along the ecliptic.

Aquarius

To the southwest of the Great Square, Aquarius hosts several globular clusters and the Helix Nebula.

Deep-Sky Objects Near the Great Square

• M31 (Andromeda Galaxy): About 15 degrees from Alpheratz along the Andromeda chain. The most distant object visible to the naked eye at 2.5 million light-years. Through binoculars, its elongated glow extends several degrees. See the autumn sky guide for more details.
• M33 (Triangulum Galaxy): A face-on spiral galaxy between Andromeda and Triangulum. Fainter and more diffuse than M31 but rewarding from dark sites.
• M15: A bright, compact globular cluster in Pegasus, easily found with binoculars about 4 degrees northwest of Enif (Pegasus's nose star).
• NGC 7331: A galaxy in Pegasus sometimes called the "Milky Way's twin" due to its similar structure. Visible in medium telescopes.
• Stephan's Quintet: A famous group of five galaxies near NGC 7331, challenging but achievable through larger amateur telescopes.

The Mythological Connection

The Great Square represents the body of Pegasus, the winged horse of Greek mythology. The constellation is usually depicted upside-down in the sky, with the horse's body above and legs extending upward (northward). The chain of stars forming Andromeda extends from the horse's body, linking the two constellations and their mythological stories.

Visibility Through the Year

The Great Square is best seen from September through December. In early autumn, it rises in the east during evening hours. By late autumn, it is high in the south at dusk. During winter, it sinks into the western sky. In spring and summer, it is below the horizon during convenient evening hours.

Photographing the Region

The Great Square and surrounding region are excellent targets for smartphone astrophotography. A wide-angle shot from a dark site can capture the square, the Andromeda chain, and even a hint of M31's glow. Longer exposures through a tracking mount can reveal the Andromeda Galaxy beautifully.

Common Questions

Why is the square so empty inside?

It is not truly empty -- it contains many faint stars. But the four corner stars are bright enough and evenly spaced enough that the interior appears conspicuously vacant by contrast, especially under light-polluted skies.

Is it really a perfect square?

Not quite. The sides are slightly unequal and the angles are not precisely 90 degrees, but it is close enough to look square to the eye.

Find the Great Square Tonight

Open StarGlobe to locate the Great Square of Pegasus and use it as your launchpad into the autumn sky. From there, trace the chain to the Andromeda Galaxy, find Cassiopeia overhead, and explore the quiet beauty of the autumn constellations. For seasonal guides to other parts of the year, see the Winter Hexagon, Spring Arc to Arcturus, and Summer Triangle.