constellation

constellation in common usage, group of stars that appear to form a configuration in the sky; properly speaking, a constellation is a definite region of the sky in which the configuration of stars is contained. Identifiable groupings of bright stars have been recognized and named since ancient times, the names corresponding to mythological figures (e.g., Perseus, Andromeda, Hercules, Orion), animals (e.g., Leo the Lion, Cygnus the Swan, Draco the Dragon), or objects (e.g., Libra the Balance, Corona the Crown). Ptolemy listed 48 constellations in his Almagest (2d cent. AD).

As systematic observations were extended to the entire southern sky from the 17th cent. on, more constellations were added to the list by J. Bayer, N. L. de Lacaille, and others. For example, Ptolemy's 48th constellation, Argo Navis, representing a ship, was divided into four smaller constellations corresponding to different parts of the ship. The final list consists of 88 constellations, each associated with a definite region of the sky. Thus, the entire celestial sphere is divided according to a plan prepared by Eugene Delporte, with the boundaries fixed by international agreement in 1930, along lines of right ascension and declination (see equatorial coordinate system ). The 12 constellations located along or near the ecliptic , the apparent path of the sun through the heavens, are known as the constellations of the zodiac ; the remaining constellations are officially classified as northern (28 constellations) or southern (48 constellations).

The table entitled Constellations lists the constellations according to their official Latin names, with the English equivalents and the approximate positions given. In some cases, the English name for a constellation is not an exact translation of the Latin; e.g., the English name for Pictor reflects the fact that the figure in the constellation is not the painter himself but his easel. Certain familiar star groups, or asterisms, are not listed as constellations because they form only part of a larger constellation; the Big Dipper and Little Dipper are parts of the constellations Ursa Major and Ursa Minor, and the Northern Cross is part of Cygnus.

Bright stars within a constellation are designated according to a system originated by Bayer in 1603: the brightest star is designated by the Greek letter alpha followed by the genitive form of the Latin name for the constellation, the second brightest star by beta, and so on, with Roman letters and pairs of Roman letters being used after the Greek letters have all been assigned. For example, the brightest star in Taurus, Aldebaran, is designated Alpha Tauri, the second brightest, Elnath, is designated Beta Tauri, and so on. The alphabetical order does not always indicate the stars' relative brightness: in a few cases, e.g., Ursa Major, the assignment of a Bayer name is according to position rather than brightness.

constellation Any of the 88 areas into which the celestial sphere is divided for the purposes of identifying objects, as adopted by the International Astronomical Union in 1922 (see Table 3, Appendix). In 1930 the IAU went on to adopt official constellation boundaries, defined by lines of right ascension and declination for the epoch 1875. This epoch was chosen because it had already been used by B. A.Gould to draw boundaries for the southern constellations. The IAU boundaries, defined by the Belgian astronomer Eugène Joseph Delporte (1882–1955), were published in Délimitation Scientifique des Constellations (1930) and the related Atlas Céleste (1930).

 The brightest stars in a constellation are identified by a Greek letter (the system of Bayer letters) or by a number (its Flamsteed number). When referring to stars identified in this way, the genitive case of the constellation's name is always used, as in Alpha Orionis, 61 Cygni, or Zeta Ursae Majoris. Three-letter abbreviations of constellation names, as laid down by the IAU, are also frequently encountered (e.g. Ori, Cyg, or UMa).

 The constellations officially recognized today are based on a group of 48 Greek figures listed by Ptolemy in the 2nd century ad, with subsequent additions by various others. At the end of the 16th century two Dutch navigators, Pieter Dirkszoon Keyser (c.1540–96) and Frederick de Houtman (1571–1627), created twelve new constellations in the far southern part of the sky, below the horizon to Greek astronomers. The Dutch celestial cartographer Petrus Plancius (1552–1622), the Latinized form of Pieter Platevoet, added three more constellations in the spaces between those known to the Greeks, and he separated the stars of Crux from Centaurus.

 The northern constellations that we know today were completed by J.Hevelius in 1687, who introduced several new figures, seven of which are still recognized today. In the 1750s the southern sky was filled out with fourteen new constellations by N. L. de Lacaille, who also split the large Greek figure Argo Navis into three parts.

constellation †(astrol.) relative position of the stars; (astron.) number of fixed stars artificially grouped together. XIV. — (O)F. — late L. constellātiō, -ōn-, f. CON- + stella STAR; see -ATION.