Pegasus

Pegasus's Great Square marks the winged horse's body, and its corner stars, as its own article notes, double as lunar-mansion markers in several other sky-reckoning traditions. A different star entirely within the same constellation opened an entire field of modern astronomy.

In 1995, the Swiss astronomers Michel Mayor and Didier Queloz announced the discovery of 51 Pegasi b, a planet orbiting the Sun-like star 51 Pegasi, the first exoplanet ever confirmed around a star broadly similar to our own Sun. The planet itself was strange by the standards later exoplanet science expected, a gas giant roughly the mass of Jupiter orbiting closer to its star than Mercury orbits the Sun, a configuration nobody had predicted and that forced astronomers to rethink assumptions about how planetary systems form. Mayor and Queloz shared the 2019 Nobel Prize in Physics for the discovery, three decades after making it, once thousands of further exoplanets had confirmed the method and the field it launched.

None of this involves Bellerophon or Medusa or the tamed horse the constellation depicts; 51 Pegasi is simply one star among the many making up the winged horse's outline, chosen by two astronomers for reasons of instrument sensitivity and target selection rather than myth.

The horse that carried a hero to Olympus in Greek legend turns out, in one small corner of its own outline, to have opened the modern search for other worlds.

Pegasus is a large, prominent northern constellation, one of Ptolemy's forty-eight, representing the winged horse of Greek myth, born of the blood of the slain Medusa and tamed by the hero Bellerophon. Its most recognisable feature is the Great Square of Pegasus, a large asterism marking the body of the horse, though one of the square's corner stars is now assigned to neighbouring Andromeda. The horse is drawn upside down and incomplete, only its forequarters shown, joined to Andromeda whom Perseus rescued. In astronomical tradition the constellation is a pattern of stars rather than a zodiac sign. These are the features of Pegasus as the classical and modern traditions describe it, distinct from any astrological reading; the Great Square is one of the signposts of the autumn sky, its corner stars also forming lunar-mansion markers in other traditions.

The lunar-mansion marker role noted for the Great Square's corner stars belongs to the Arabic manazil al-qamar, the twenty-eight lunar mansions, a wholly separate system from the Chinese twenty-eight xiu despite the identical count of twenty-eight, and from the twenty-seven Indian nakshatras, three independent traditions of dividing the Moon's monthly path that this atlas treats at length under their own tables.