Cet · Ceti
Cetus Constellation
IAU constellation mapCetCetus (Cet) is one of the 88 constellations recognised by the International Astronomical Union. It covers 1231 square degrees of the equatorial sky in quadrant SQ1. Its brightest star is Diphda. The Latin genitive of the name is Ceti, used to form star names within the constellation. In Greek mythology, Cetus is associated with greek tradition. The IAU standardised all constellation boundaries in 1930.
Quick facts
- IAU name
- Cetus
- Abbreviation
- Cet
- Latin genitive
- Ceti
- Hemisphere
- equatorial
- Area
- 1,231 sq deg
- Brightest star
- Diphda
- Quadrant
- SQ1
- Family
- Perseus
- Mythology origin
- Greek
- Discovery era
- Ptolemy's 48 (Almagest, c. 150 CE)
Mythology
Cetus, the Sea Monster (or Whale), is the creature sent by Poseidon to devastate the coast of Ethiopia as punishment for Queen Cassiopeia's boastful claim that her daughter Andromeda surpassed the beauty of the sea-nymphs. In Greek the creature was called Ketos, a generic term for any large sea creature; the Latin Cetus is similarly broad, and it was identified with the biblical 'great fish' that swallowed Jonah in early Christian tradition, eventually becoming conflated with a whale. The hero Perseus killed Cetus by showing it the petrifying head of Medusa, turning it to stone. Cetus covers 1231 square degrees, making it one of the largest constellations. Within it lies Mira (Omicron Ceti), the prototype of the long-period pulsating variable stars: Mira varies from magnitude 2 (naked-eye visibility) to magnitude 10 (requiring binoculars) over a period of approximately 332 days, a variability first formally documented by David Fabricius in 1596 — the first variable star to be documented as such. Tau Ceti is a Sun-like star only 11.9 light-years from Earth and one of the nearest solar analogues.
Sources: Apollodorus Bibliotheca 2.4.3; Aratus Phaenomena 353-360; Hyginus Astronomica 2.31.
Overview
Cetus is one of the 88 constellations formally recognised by the International Astronomical Union (IAU). It occupies 1231 square degrees of sky in the equatorial sky region, crossing both hemispheres, best visible at latitudes between +45 and -90 degrees in December. The constellation belongs to the Perseus family of constellations. Its Latin genitive is Ceti, the form used when naming stars within the constellation — for example, the brightest star Diphda may appear in catalogues as Alpha Ceti 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 Cetus is Diphda. Individual stars within Cetus are conventionally named using Bayer designation — Greek letters followed by the Latin genitive of the constellation name (Ceti) — introduced by Johann Bayer in his 1603 star atlas Uranometria. Flamsteed numbers (numeric designators followed by Cetus) were added by John Flamsteed in his 1712 catalogue. Variable stars, double stars, and deep-sky objects within the Cetus boundary are catalogued by the IAU with the abbreviation Cet.
History and mythology
The figure of Cetus 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 Cetus mean?
Cetus 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 Cetus are designated using the genitive form of the name (e.g. Alpha Cetus 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 Cetus compared to other constellations?
Cetus covers 1231 square degrees of sky. The full sphere of the sky contains 41,253 square degrees, so Cetus occupies about 3.0% 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 Cetus visible?
Cetus straddles the celestial equator and is visible from both hemispheres, though part of it may be low on the horizon depending on the observer's latitude. The brightest star, Diphda, 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.
