Cae · Caeli
Caelum Constellation
IAU constellation mapCaeCaelum (Cae) is one of the 88 constellations recognised by the International Astronomical Union. It covers 125 square degrees of the southern sky in quadrant SQ1. Its brightest star is Alpha Caeli. The Latin genitive of the name is Caeli, used to form star names within the constellation. The IAU standardised all constellation boundaries in 1930.
Quick facts
- IAU name
- Caelum
- Abbreviation
- Cae
- Latin genitive
- Caeli
- Hemisphere
- southern
- Area
- 125 sq deg
- Brightest star
- Alpha Caeli
- Quadrant
- SQ1
- Family
- Lacaille
- Discovery era
- Nicolas Louis de Lacaille, 1751-1752
Overview
Caelum is one of the 88 constellations formally recognised by the International Astronomical Union (IAU). It occupies 125 square degrees of sky in the southern hemisphere sky, best visible at latitudes between +45 and -90 degrees in December. The constellation belongs to the Lacaille family of constellations. Its Latin genitive is Caeli, the form used when naming stars within the constellation — for example, the brightest star Alpha Caeli may appear in catalogues as Alpha Caeli 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 Caelum is Alpha Caeli. Individual stars within Caelum are conventionally named using Bayer designation — Greek letters followed by the Latin genitive of the constellation name (Caeli) — introduced by Johann Bayer in his 1603 star atlas Uranometria. Flamsteed numbers (numeric designators followed by Caelum) were added by John Flamsteed in his 1712 catalogue. Variable stars, double stars, and deep-sky objects within the Caelum boundary are catalogued by the IAU with the abbreviation Cae.
History and catalogue
Caelum was introduced or documented by Nicolas Louis de Lacaille, 1751-1752. The modern IAU constellation boundaries were formally established in 1930. The designation Cae is the official three-letter IAU abbreviation used in star catalogues, variable star designations, and positional references. The family classification 'Lacaille' groups this constellation with others sharing a region of sky or a common historical source in astronomical literature.
Sources & further reading (2)
- official-iau — accessed 2026-05-06
- encyclopedia — accessed 2026-05-06
Frequently asked questions
What does the name Caelum mean?
Caelum 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 Caelum are designated using the genitive form of the name (e.g. Alpha Caelum 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 Caelum compared to other constellations?
Caelum covers 125 square degrees of sky. The full sphere of the sky contains 41,253 square degrees, so Caelum occupies about 0.3% 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 Caelum visible?
Caelum is primarily a southern hemisphere constellation. Observers at high northern latitudes cannot see it; it is best appreciated from the southern tropics or high southern latitudes. The brightest star, Alpha Caeli, 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.
