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Definition: constellation from Philip's Encyclopedia

Grouping of stars, forming an imaginary figure traced on the sky. The groupings have no physical basis as each star is a different distance from Earth. There are 88 constellations that have been assigned boundaries on the celestial sphere by the International Astronomical Union in 1930.


Summary Article: constellation
from The Hutchinson Unabridged Encyclopedia with Atlas and Weather Guide

One of the 88 areas into which the sky is divided for the purposes of identifying and naming celestial objects. The first constellations were simple, arbitrary patterns of stars in which early civilizations visualized gods, sacred beasts, and mythical heroes.

The constellations used in scientific astronomy today are derived from a list of 48 known to the ancient Greeks, who inherited some from the Babylonians. The current list of 88 constellations was adopted by the International Astronomical Union, astronomy's governing body, in 1930. Traditional Chinese astronomy used different constellations; for example, the zodiac was divided into 28 ‘mansions’ rather than the 12 constellations familiar in the West.

Ancient origins Some of the current constellations can be traced back to the inhabitants of the Euphrates valley, from whom they were handed down through the Greeks and Arabs. Few pictorial records of the ancient constellation figures have survived, but in the AlmagestAD 150, Ptolemy catalogued the positions of 1,022 of the brightest stars both in terms of celestial latitude and longitude, and of their places in 48 constellations. The Ptolemaic constellations left a blank area centred not on the present south pole but on a point which, because of precession, would have been the south pole c. 2800 BC, a fact that is consistent with the belief that the constellation system had its origin about 5,000 years ago.

Gradual rationalization In 1515 the German artist Albrecht Dürer published two maps, one for each hemisphere, with the plotted positions of Ptolemy's stars incorporated into appropriate figures. These figures were copied by most subsequent cartographers, in particular by Johann Bayer of Augsburg in his Uranometria 1603. Bayer devotes a full page to each of the constellations in the Almagest, and identifies the individual stars within each constellation by Greek and Latin letters assigned partly in order of brightness, and partly with regard to their position in the constellation figure. Bayer also gave a chart of the south polar region, based on the observations of the Dutch navigator Pieter Dirchsz Keyser, who formalized the 12 constellations of southern stars used by early ocean navigators. Except for Triangulum Australe, the names given to the Keyser constellations refer to living or mythical creatures that the navigators might have encountered on their travels.

Bayer did not assign Greek letters to the stars within these new constellations. This was done by Nicolas de Lacaille, who also added 14 fainter constellations to the southern sky as a result of the systematic observations he made at the Cape of Good Hope 1751–53. Bayer's letters still provide the usual way of referring to many of the brightest stars. Others less bright are frequently identified by the number assigned to them in Flamsteed's Historia Coelestis Britannica 1729, in which the stars in each constellation were enumerated in the order of their then right ascension.

Originally a constellation was regarded as being restricted to the conventional figure, thus leaving many unattached stars unidentified. The Ptolemaic constellations include only the brighter stars visible from the northern latitudes. Fainter stars between them were gathered into a variety of new constellations by different map makers, but few have survived. Of those that have, two were formed by Bartschius (1624) and seven by Hevelius (1689). The first cartographer to draw boundaries between adjacent constellations and thus fill up the sky was J E Bode in his Uranographia 1801, with figures based on those of Dürer. These boundaries became more important than the figures themselves and were gradually rationalized, particularly by F W A Argelander in Uranometria Nova 1843, and by B A Gould in Uranometria Argentina 1877.

Modern system The final stage of rationalization came 1930 with the formal adoption by the International Astronomical Union of a system in which the definitive boundaries are arcs of constant right ascension or declination for the equinox of 1875. Each constellation includes not only the historic star grouping but also all the variable stars that have become associated with it. The genitive, or possessive, form of the name is used when an object is being identified by its constellation letter or number. For example, the star Polaris is Alpha Ursae Minoris, which is usually contracted to alpha UMi.

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