Orion is a remarkable constellation, visible from most places on the globe (but not always the whole year long).

A constellation is a group of stars visibly related to each other in a particular configuration. In three-dimensional space, most of the stars we see have little relation to one another, but can appear to be grouped on the celestial sphere of the night sky. Humans excel at finding patterns and throughout history have grouped stars that appear close to one another into constellations. An "unofficial" constellation, that is, one that may be widely known but is not recognized by astronomers or the International Astronomical Union, is also called an asterism (such as The Plough (also known in the US as Big Dipper) and the Little Dipper). The stars in a constellation or asterism rarely have any astrophysical relationship to each other; they just happen to appear close together in the sky as viewed from Earth and typically lie many light years apart in space.

The grouping of stars into constellations is essentially arbitrary, and different cultures have had different constellations, although a few of the more obvious ones tend to recur frequently, e.g., Orion and Scorpio.

The International Astronomical Union (IAU) divides the sky into 88 official constellations with precise boundaries, so that every direction belongs to exactly one constellation. In the northern celestial hemisphere, these are mostly based upon the constellations of the ancient Greek tradition, passed down through the Middle Ages, and contains the signs of the zodiac.

History of the Constellations

Main article:List of Constellations

Our current list is based on those listed by the Roman, Ptolemy. In more recent times this list has been added to, to fill gaps between Ptolemy's patterns (the Greeks considered the sky as including both constellations and dim spaces between). Twelve of the constellations in the southern celestial hemisphere were not observable by the Greeks, and were created by Dutch navigators Pieter Dirkszoon Keyser and Frederick de Houtman in the sixteenth century and first cataloged by Johann Bayer.

Other proposed constellations didn't make the cut, most notably Quadrans Muralis (now part of Boötes) for which the Quadrantid meteors are named. Also the constellation Argo Navis was so big that it was broken up into several different constellations.

Star names

Many stars are named using the genitive of the constellation in which they are found. These names include Bayer designations such as Alpha Centauri, Flamsteed designations such as 61 Cygni, and variable star designations such as RR Lyrae. For more information about star names, see Star designations and the list of stars by constellation.

See also

• List of constellations by area
• Chinese constellation

External links

• The Constellations
• Celestia free 3D realtime space-simulation (OpenGL)
• Data file of official IAU boundaries (Note that the boundaries are defined for equinox 1875.)
• Interactive Sky Charts (Allows navigation through the entire sky with variable star detail, optional constellation lines)
• http://www.astronomical.org/constellations/obs.html
• http://www.seds.org/Maps/Stars_en/Fig/const.html
• Constellations Articles
• Full constellation diagrams resembling their names
• Images of constellations