Cnc · Cancri
Cancer Constellation
IAU constellation mapCncCancer (Cnc) is one of the 88 constellations recognised by the International Astronomical Union. It covers 506 square degrees of the northern sky in quadrant NQ2. Its brightest star is Al Tarf. The Latin genitive of the name is Cancri, used to form star names within the constellation. In Greek mythology, Cancer is associated with greek tradition. The IAU standardised all constellation boundaries in 1930.
Quick facts
- IAU name
- Cancer
- Abbreviation
- Cnc
- Latin genitive
- Cancri
- Hemisphere
- northern
- Area
- 506 sq deg
- Brightest star
- Al Tarf
- Quadrant
- NQ2
- Family
- Zodiac
- Mythology origin
- Greek
- Discovery era
- Ptolemy's 48 (Almagest, c. 150 CE)
Mythology
Cancer is the faintest of the 12 zodiacal constellations, yet its mythological role is memorable: a minor but charming cameo in the story of Heracles. When Heracles was battling the many-headed Lernaean Hydra as his Second Labour, the goddess Hera — implacably hostile to Heracles — sent a giant crab to distract him by pinching at his feet. The crab was quickly crushed underfoot by the hero but earned immortality in the sky as a reward for its service to the goddess. In Babylonian astronomy the area corresponded to a tortoise, and later to a crayfish or crab. At the time of Hipparchus and Ptolemy the Sun was positioned in Cancer at the summer solstice — the most northerly point of the ecliptic — which is why the latitude of maximum solar elevation is still called the Tropic of Cancer, even though precession has shifted the solstice into Taurus. The most remarkable deep-sky object within Cancer is the Beehive Cluster (Messier 44, also called Praesepe), a bright open cluster of about 1000 stars visible to the naked eye under dark skies and an important calendrical marker for ancient observers.
Sources: Apollodorus Bibliotheca 2.5.2; Aratus Phaenomena 497-503; Hyginus Astronomica 2.23.
Overview
Cancer is one of the 88 constellations formally recognised by the International Astronomical Union (IAU). It occupies 506 square degrees of sky in the northern hemisphere sky, best visible at latitudes between +90 and -60 degrees in March. The constellation belongs to the Zodiac family of constellations. Its Latin genitive is Cancri, the form used when naming stars within the constellation — for example, the brightest star Al Tarf may appear in catalogues as Alpha Cancri 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 Cancer is Al Tarf. Individual stars within Cancer are conventionally named using Bayer designation — Greek letters followed by the Latin genitive of the constellation name (Cancri) — introduced by Johann Bayer in his 1603 star atlas Uranometria. Flamsteed numbers (numeric designators followed by Cancer) were added by John Flamsteed in his 1712 catalogue. Variable stars, double stars, and deep-sky objects within the Cancer boundary are catalogued by the IAU with the abbreviation Cnc.
History and mythology
The figure of Cancer 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 Cancer mean?
Cancer 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 Cancer are designated using the genitive form of the name (e.g. Alpha Cancer 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 Cancer compared to other constellations?
Cancer covers 506 square degrees of sky. The full sphere of the sky contains 41,253 square degrees, so Cancer occupies about 1.2% 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 Cancer visible?
Cancer 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, Al Tarf, 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.
