Cas · Cassiopeiae
Cassiopeia Constellation
IAU constellation mapCasCassiopeia (Cas) is one of the 88 constellations recognised by the International Astronomical Union. It covers 598 square degrees of the northern sky in quadrant NQ1. Its brightest star is Schedar. The Latin genitive of the name is Cassiopeiae, used to form star names within the constellation. In Greek mythology, Cassiopeia is associated with greek tradition. The IAU standardised all constellation boundaries in 1930.
Quick facts
- IAU name
- Cassiopeia
- Abbreviation
- Cas
- Latin genitive
- Cassiopeiae
- Hemisphere
- northern
- Area
- 598 sq deg
- Brightest star
- Schedar
- Quadrant
- NQ1
- Family
- Perseus
- Mythology origin
- Greek
- Discovery era
- Ptolemy's 48 (Almagest, c. 150 CE)
Mythology
Cassiopeia, the vain queen of Ethiopia, is depicted sitting on her throne in the sky — forever rotating around the celestial pole, sometimes sitting upright, sometimes hanging upside down, a punishment for her hubris. Her boast that she (or her daughter Andromeda) was more beautiful than the sea-nymphs set in motion the chain of events that required the sacrifice of Andromeda to the sea monster Cetus. After Perseus killed Cetus and rescued Andromeda, the gods placed the whole royal family — Cassiopeia, Cepheus, Andromeda, and Perseus — in the sky together. Cassiopeia is best known for its distinctive W or M shape (depending on orientation) formed by its five brightest stars. The constellation contains two historical supernova remnants: Cassiopeia A, the remnant of a supernova that exploded around 1680 CE, is the brightest radio source in the sky outside the Solar System. Tycho Brahe observed another supernova in the constellation in 1572 (SN 1572), which was bright enough to be visible in daylight and seriously challenged the Aristotelian doctrine of celestial immutability.
Sources: Apollodorus Bibliotheca 2.4.3; Hyginus Astronomica 2.10; Aratus Phaenomena 188-195.
Overview
Cassiopeia is one of the 88 constellations formally recognised by the International Astronomical Union (IAU). It occupies 598 square degrees of sky in the northern hemisphere sky, best visible at latitudes between +90 and -60 degrees in December. The constellation belongs to the Perseus family of constellations. Its Latin genitive is Cassiopeiae, the form used when naming stars within the constellation — for example, the brightest star Schedar may appear in catalogues as Alpha Cassiopeiae or similar. The IAU standardised the boundaries of all 88 constellations in 1930 under the direction of Eugène Delporte, using straight lines of right ascension and declination to eliminate the ambiguities of earlier variable-boundary systems.
Notable stars
The brightest star in Cassiopeia is Schedar. Individual stars within Cassiopeia are conventionally named using Bayer designation — Greek letters followed by the Latin genitive of the constellation name (Cassiopeiae) — introduced by Johann Bayer in his 1603 star atlas Uranometria. Flamsteed numbers (numeric designators followed by Cassiopeia) were added by John Flamsteed in his 1712 catalogue. Variable stars, double stars, and deep-sky objects within the Cassiopeia boundary are catalogued by the IAU with the abbreviation Cas.
History and mythology
The figure of Cassiopeia appears in the classical astronomical tradition. Ptolemy included this constellation in the Almagest (c. 150 CE), the definitive ancient catalogue of 48 constellations, which formed the foundation for Islamic, medieval European, and Renaissance astronomy. The constellation's figure and mythology were transmitted through works such as Aratus's Phaenomena, Eratosthenes's Catasterismi, and Hyginus's Astronomica. See the Mythology section above for the full narrative.
Sources & further reading (2)
- official-iau — accessed 2026-05-06
- encyclopedia — accessed 2026-05-06
Frequently asked questions
What does the name Cassiopeia mean?
Cassiopeia is the Latin name used by the IAU to designate this constellation. The name comes from classical Latin and Greek astronomical tradition. In star catalogues, stars within Cassiopeia are designated using the genitive form of the name (e.g. Alpha Cassiopeia or similar Latin genitive forms) — a naming convention introduced by Johann Bayer in his 1603 atlas Uranometria and still in use today.
How large is Cassiopeia compared to other constellations?
Cassiopeia covers 598 square degrees of sky. The full sphere of the sky contains 41,253 square degrees, so Cassiopeia occupies about 1.4% of the total sky. For comparison, the largest constellation is Hydra at 1303 square degrees; the smallest is Crux at 68 square degrees.
When and where is Cassiopeia visible?
Cassiopeia is primarily a northern hemisphere constellation, best seen from mid-northern latitudes. Southern hemisphere observers may see it low on the horizon depending on their latitude. The brightest star, Schedar, serves as the main visual anchor for locating the constellation. As with all constellations, the best viewing conditions are a dark sky away from artificial light pollution, with the constellation high enough above the horizon to minimise atmospheric absorption.
