Eri · Eridani
Eridanus Constellation
IAU constellation mapEriEridanus (Eri) is one of the 88 constellations recognised by the International Astronomical Union. It covers 1138 square degrees of the equatorial sky in quadrant SQ1. Its brightest star is Achernar. The Latin genitive of the name is Eridani, used to form star names within the constellation. The IAU standardised all constellation boundaries in 1930.
Quick facts
- IAU name
- Eridanus
- Abbreviation
- Eri
- Latin genitive
- Eridani
- Hemisphere
- equatorial
- Area
- 1,138 sq deg
- Brightest star
- Achernar
- Quadrant
- SQ1
- Family
- Heavenly Waters
- Discovery era
- Ptolemy's 48 (Almagest, c. 150 CE)
Overview
Eridanus is one of the 88 constellations formally recognised by the International Astronomical Union (IAU). It occupies 1138 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 Heavenly Waters family of constellations. Its Latin genitive is Eridani, the form used when naming stars within the constellation — for example, the brightest star Achernar may appear in catalogues as Alpha Eridani 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 Eridanus is Achernar. Individual stars within Eridanus are conventionally named using Bayer designation — Greek letters followed by the Latin genitive of the constellation name (Eridani) — introduced by Johann Bayer in his 1603 star atlas Uranometria. Flamsteed numbers (numeric designators followed by Eridanus) were added by John Flamsteed in his 1712 catalogue. Variable stars, double stars, and deep-sky objects within the Eridanus boundary are catalogued by the IAU with the abbreviation Eri.
History and catalogue
Eridanus was introduced or documented by Ptolemy's 48 (Almagest, c. 150 CE). The modern IAU constellation boundaries were formally established in 1930. The designation Eri is the official three-letter IAU abbreviation used in star catalogues, variable star designations, and positional references. The family classification 'Heavenly Waters' 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 Eridanus mean?
Eridanus 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 Eridanus are designated using the genitive form of the name (e.g. Alpha Eridanus 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 Eridanus compared to other constellations?
Eridanus covers 1138 square degrees of sky. The full sphere of the sky contains 41,253 square degrees, so Eridanus occupies about 2.8% 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 Eridanus visible?
Eridanus 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, Achernar, 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.
