Principal Features of the Sky

Abstract

About 15,000 stars are detectable by the human eye, most of them near the limit of visibility. At any one time, we may be able to see a few thousand stars in a dark sky, but we tend to remember only striking patterns of them—asterisms such as the Big Dipper or whole constellations such as Ursa Major (the Big Bear) or Orion (the name of a mythological hunter)—and so it has been for millennia. Today, the entire sky has been divided into constellations; they are not defined according to appearance alone but according to location, and there are no boundary disputes. The modern names and locations are more or less those of Argelander (1799–1875) for the Northern Hemisphere and John Herschel (1824–1896) for the Southern, but the present divisions of the constellations were adopted by the International Astronomical Union (IAU), the chief authority on such matters as astronomical nomenclature, in 1930. The IAU has established 88 constellations in the sky; many reflecting an ancient heritage.