Constellations

A constellation is an easily identifiable configuration of the brightest stars in a moderately small region of the night sky. Originally there was not a single set of constellations recognized by all astronomers. Rather, early astronomers in many regions of the world often defined and named the particular collections of bright stars they observed in relatively small regions of the sky after specific heroes, events, and creatures from their ancient cultures and mythologies. The ancient people used the stars to tell stories, to honor heroes and ferocious creatures such as Orion the hunter and Ursa Major (the Great Bear) and to remind each new human generation that the heavens were the abode of the gods.

Early Greek astronomers adopted the constellations they found in Mesopotamia, embellished them with their own myths and religious beliefs, and eventually created a set of 48 ancient constellations. Eudoxus of Cnidus was one of the first Greeks to formally codify these ancient constellations. Hipparchus reinforced his work about a century later. Finally, Ptolemy cast the 48 ancient constellations in their present form about 150 C.E. in his great compilation of astronomical knowledge, Syntaxis. The early Greek astronomers were also keenly aware that these 48 constellations did not account for all the stars in the night sky. The Ancient Constellations table contains a list of the 48 ancient Greek constellations. Today astronomers officially recognize all but one of these ancient star patterns. The somewhat cumbersome constellation Argo Navis has now been broken up into four new Constellations.

As part of the explosive interest in astronomy that occurred at the start of the scientific revolution, the German astronomer Johann Bayer published the important work Uranometria in 1603. This book was the first major star catalog for the entire celestial sphere. Bayer charted more than 2,000 stars visible to the naked eye and introduced the practice of assigning Greek letters to the main stars in each constellation, usually in an approximate order of their brightness. Expanding the legacy of 48 constellations from ancient Greece, Bayer named 12 new southern hemisphere constellations: Apus, Chamaeleon, Dorado, Grus, Hydrus, Indus, Musca, Pavo, Phoenix, Triangulum Australe, Tucana, and Volans. The Modern Constellations Table describes these new constellations as well as the other modern constellations officially recognized by the International Astronomical Union since 1929.

Using the newly invented astronomical telescope the Polish-German astronomer Johannes Hevelius filled in some of the empty spaces in the Northern Hemisphere of the celestial sphere by identifying the following new constellations: Canes Venatici, Lacerta, Lynx, and Leo Minor. Then, in the 18th century the French astronomer Abbé Nicolas Louis De Lacaille described 14 new constellations he found in the Southern Hemisphere and named some of them after scientific artifacts and instruments emerging during the period. His newly identified constellations included: Antlia, Caelum, Circinus, Fornax, Horologium, Mensa, Microscopium, Norma, Octans, Pictor, Pyxis, Reticulum, Sculptor, and Telescopium.

Lacaille and other astronomers of his era also dismantled the cumbersome ancient constellation Argo Navis and carved up the stars in this large Southern Hemisphere constellation into four smaller, more manageable ones whose names retain the original nautical theme: Carina (the Keel), Puppis (the Stern), Pyxis (the Nautical Compass), and Vela (the Sail).

Today astronomers have 88 officially recognized constellations. All of these constellations may be found by combining the two Constellations Tables. The tables include the position of each constellation on the celestial sphere as expressed in the equatorial coordinates (right ascension [RA] and declination [δ]) that correspond to the approximate center of the constellation.

Questions to Ponder

• Why do constellations look the same after several years even though all the stars are moving?
• What was the purpose behind grouping stars into Constellations?