In modern astronomy, a constellation is an internationally defined area of the celestial sphere. These areas are grouped around asterisks (which themselves are generally referred to in non-technical language as "constellations"), which are patterns formed by prominent stars within apparent proximity to one another on Earth's night sky.
There are 88 standard constellations recognized by the International Astronomical Union (IAU ) since 1922. The majority of these go back to the 48 constellations defined by Ptolemy in his Almagst (2nd century). The remaining ones were defined in the 17th and 18th century; the most recent ones are found on the southern sky, defined in Coelum australe stelliferum by Nicolas Louis de Lacaille (1763).
There are also numerous historical constellations not recognized by the IAU, or constellations recognized in regional traditions of astronomy or astrology, such as Chinese, Hindu or Australian Aboriginal
The Late Latin term "constellātiō" can be translated as "set with stars". The term was first used in astrology, of asterisms that supposedly exerted influence, attested in Ammianus (4th century). In English the term was used from the 14th century, also in astrology, of conjunctions of planets. The modern astronomical sense of "area of the celestial sphere around a specific asterism" dates to the mid 16th century.
Colloquial usage does not distinguish the senses of "asterism" and "area surrounding an asterism". The modern system of constellations used in astronomy focuses primarily on constellations as grid-like segments of the celestial sphere rather than as patterns, while the term for a star-pattern is asterism. For example, the asterism known as the Big Dipper corresponds to the seven brightest stars of the larger IAU constellation of Ursa Major.
The term circumpolar constellation is used for any constellation that, from a particular latitude on Earth, never sets below the horizon. From the north pole, all constellations north of the celestial equator are circumpolar constellations. In the northern latitudes, the informal term equatorial constellation has sometimes used for constellations that lie to the south of the circumpolar constellations.Depending on the definition, equatorial constellations can include those that lie entirely between declinations 45° north and 45° south,or those that pass overhead between the tropics of Cancer and Capricorn. They generally include all constellations that intersect the celestial equator.
The current list of 88 constellations recognised by the International Astronomical Union since 1922 is based on the 48 listed by Ptolemy in his Almagst
in the 2nd century. Ptolemy's catalogue is informed by Eudoxus
of Cnidus
, a Greek astronomer of the 4th century BC who introduced earlier Babylonian astronomy to the Hellenistic culture. Of the 48 constellations listed by Ptolemy, thirty can be shown to have a much longer history, reaching back into at least the Late Bronze Age. This concerns the zodiacal constellations
