Oph · Ophiuchi
Ophiuchus Constellation
IAU constellation mapOphOphiuchus (Oph) is one of the 88 constellations recognised by the International Astronomical Union. It covers 948 square degrees of the equatorial sky in quadrant SQ3. Its brightest star is Rasalhague. The Latin genitive of the name is Ophiuchi, used to form star names within the constellation. The IAU standardised all constellation boundaries in 1930.
Quick facts
- IAU name
- Ophiuchus
- Abbreviation
- Oph
- Latin genitive
- Ophiuchi
- Hemisphere
- equatorial
- Area
- 948 sq deg
- Brightest star
- Rasalhague
- Quadrant
- SQ3
- Family
- Hercules
- Discovery era
- Ptolemy's 48 (Almagest, c. 150 CE)
Overview
Ophiuchus is one of the 88 constellations formally recognised by the International Astronomical Union (IAU). It occupies 948 square degrees of sky in the equatorial sky region, crossing both hemispheres, best visible at latitudes between +45 and -90 degrees in June. The constellation belongs to the Hercules family of constellations. Its Latin genitive is Ophiuchi, the form used when naming stars within the constellation — for example, the brightest star Rasalhague may appear in catalogues as Alpha Ophiuchi 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 Ophiuchus is Rasalhague. Individual stars within Ophiuchus are conventionally named using Bayer designation — Greek letters followed by the Latin genitive of the constellation name (Ophiuchi) — introduced by Johann Bayer in his 1603 star atlas Uranometria. Flamsteed numbers (numeric designators followed by Ophiuchus) were added by John Flamsteed in his 1712 catalogue. Variable stars, double stars, and deep-sky objects within the Ophiuchus boundary are catalogued by the IAU with the abbreviation Oph.
History and catalogue
Ophiuchus was introduced or documented by Ptolemy's 48 (Almagest, c. 150 CE). The modern IAU constellation boundaries were formally established in 1930. The designation Oph is the official three-letter IAU abbreviation used in star catalogues, variable star designations, and positional references. The family classification 'Hercules' 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 Ophiuchus mean?
Ophiuchus 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 Ophiuchus are designated using the genitive form of the name (e.g. Alpha Ophiuchus 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 Ophiuchus compared to other constellations?
Ophiuchus covers 948 square degrees of sky. The full sphere of the sky contains 41,253 square degrees, so Ophiuchus occupies about 2.3% 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 Ophiuchus visible?
Ophiuchus 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, Rasalhague, 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.
