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The Great Square itself is the find: four modestly bright stars — Markab, Scheat, Algenib, and Alpheratz — enclosing a patch of sky that looks, at a glance, almost starless. From Markab, a bent line running through Zeta and Theta leads southwest to Enif, marking the horse's nose. Best seen September through November evenings, when the square stands high in the south after dark.

RA is in decimal hours, Dec in decimal degrees, both J2000 — the same figures the site's own chart is drawn from. Magnitudes are visual.

Four stars close the square: Markab, Scheat, and Algenib belong to Pegasus, but the fourth corner, Alpheratz, was formally reassigned to Andromeda when the International Astronomical Union fixed the modern 88 constellation boundaries in the 1920s. It still visually completes the square, and the site's own chart draws it once and shares it between both figures' outlines — a nice case of the sky not caring where a human committee drew a line.

The square's real usefulness, beyond star-hopping, is as a naked-eye light-pollution gauge. It covers a wide patch of sky that is genuinely low on bright stars, so how many faint ones you can count inside its four corners is a rough, quick read on how dark your sky is: roughly zero to two stars is a badly light-polluted sky, three to seven is average suburban, eight to eleven is good, and a dozen or more — up into the 20s or 30s under truly dark skies — is excellent. It costs nothing and takes under a minute, which makes it one of the few astronomy exercises anyone can do standing in a driveway.

On 6 October 1995, Swiss astronomers Michel Mayor and Didier Queloz, using the ELODIE spectrograph at the Observatoire de Haute-Provence, announced a planet orbiting 51 Pegasi, a Sun-like star roughly 50 light years away — the first exoplanet ever confirmed around a star like our own. (A pair of planets around a pulsar had been found in 1992, but a dead star's remnant is nothing like the Sun.) The planet, a "hot Jupiter" roughly half Jupiter's mass on an orbit of just four days, was a genuine surprise: nothing like it existed in the solar system, and its discovery kicked off three decades of exoplanet hunting. Mayor and Queloz shared the 2019 Nobel Prize in Physics for the find. In 2015, as part of the IAU's first public NameExoWorlds vote, the star and planet were formally named Helvetios (Latin for "Swiss," a nod to the discovery team) and Dimidium ("half," for the planet's mass).

Pegasus was born, according to Hesiod's Theogony (c. 700 BCE), from the blood of Medusa at the instant Perseus beheaded her — sprung fully formed along with the warrior Chrysaor, his brother. Later tradition has Pegasus striking the ground with a hoof on Mount Helicon and opening the spring Hippocrene, sacred to the Muses, and carrying Zeus's thunderbolts. One popular pairing is worth correcting plainly: Perseus never rides Pegasus in any ancient source. That role belongs to Bellerophon, a separate hero of a later generation, who tamed Pegasus with a golden bridle from Athena and rode him against the Chimera; the idea of Perseus astride Pegasus is a modern, post-classical conflation of two different stories. Ptolemy lists Pegasus among the Almagest's 48 constellations, and Eratosthenes and Hyginus both record its catasterism.

The Great Square has a documented Babylonian identity, unlike some of its neighbors: MUL.APIN, the Babylonian star-list compiled by around 1000 BCE, records Iku, "the Field" — a square marked out by these same four stars, used as a fixed marker for the start of the agricultural year. It is one of the oldest continuously recognizable star patterns on record, predating the Greek horse by many centuries. Chinese astronomers split the same territory differently again: Alpheratz and Algenib together formed Bi, "the Wall" — read as the eastern wall of the Emperor's palace grounds, or his private library — while Markab and Scheat formed Shi, "the Encampment," one of the imperial palace buildings; both gave their names to lunar mansions used to track the Moon's monthly path. The neighboring Babylonian fish figures, which also touched on this stretch of sky, are described in more detail on the Pisces page.

• Andromeda — shares its corner star, Alpheratz, with the Great Square.
• Perseus — the hero at whose hand Pegasus was born.
• Cepheus — Andromeda's father, and the fourth corner of the same story.
• Pisces — the Babylonian fish figures that bordered this same stretch of sky.