Cep · Cephei
Cepheus Constellation
IAU constellation mapCepCepheus (Cep) is one of the 88 constellations recognised by the International Astronomical Union. It covers 588 square degrees of the northern sky in quadrant NQ4. Its brightest star is Alderamin. The Latin genitive of the name is Cephei, used to form star names within the constellation. In Greek mythology, Cepheus is associated with greek tradition. The IAU standardised all constellation boundaries in 1930.
Quick facts
- IAU name
- Cepheus
- Abbreviation
- Cep
- Latin genitive
- Cephei
- Hemisphere
- northern
- Area
- 588 sq deg
- Brightest star
- Alderamin
- Quadrant
- NQ4
- Family
- Perseus
- Mythology origin
- Greek
- Discovery era
- Ptolemy's 48 (Almagest, c. 150 CE)
Mythology
Cepheus was the king of Ethiopia and husband of Cassiopeia in the Perseus mythology cycle. He reluctantly agreed to chain his daughter Andromeda to a rock as a sacrifice to Cetus when his wife's boastfulness endangered the kingdom. After Perseus freed Andromeda and killed the sea monster, Cepheus gave his daughter in marriage to Perseus; the entire family was later placed in the sky together. Cepheus is depicted as a king wearing a crown, though the constellation is not particularly prominent. Delta Cephei, however, is one of the most historically important stars in astronomy: it is the prototype of the Cepheid variable class of pulsating stars, which pulse in brightness with periods directly related to their intrinsic luminosity. This period-luminosity relationship, discovered by Henrietta Leavitt in 1908 studying Cepheid variables in the Magellanic Clouds, became the first reliable 'standard candle' for measuring extragalactic distances and is fundamental to determining the scale and expansion rate of the universe. Delta Cephei was first identified as variable by John Goodricke in 1784.
Sources: Apollodorus Bibliotheca 2.4.3; Aratus Phaenomena 179-187; Hyginus Astronomica 2.9.
Overview
Cepheus is one of the 88 constellations formally recognised by the International Astronomical Union (IAU). It occupies 588 square degrees of sky in the northern hemisphere sky, best visible at latitudes between +90 and -60 degrees in September. The constellation belongs to the Perseus family of constellations. Its Latin genitive is Cephei, the form used when naming stars within the constellation — for example, the brightest star Alderamin may appear in catalogues as Alpha Cephei 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 Cepheus is Alderamin. Individual stars within Cepheus are conventionally named using Bayer designation — Greek letters followed by the Latin genitive of the constellation name (Cephei) — introduced by Johann Bayer in his 1603 star atlas Uranometria. Flamsteed numbers (numeric designators followed by Cepheus) were added by John Flamsteed in his 1712 catalogue. Variable stars, double stars, and deep-sky objects within the Cepheus boundary are catalogued by the IAU with the abbreviation Cep.
History and mythology
The figure of Cepheus 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 Cepheus mean?
Cepheus 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 Cepheus are designated using the genitive form of the name (e.g. Alpha Cepheus 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 Cepheus compared to other constellations?
Cepheus covers 588 square degrees of sky. The full sphere of the sky contains 41,253 square degrees, so Cepheus 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 Cepheus visible?
Cepheus 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, Alderamin, 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.
