Constellation

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 imaginary plane of the night sky. People are very good at finding patterns and throughout history have grouped stars that appear close to one another into constellations.

The grouping of stars into contellations 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 Scorpius.

The International Astronomical Union (IAU) divides the sky into 88 official constellations with precise boundaries, so that every direction belongs to exactly one constellation. These are mostly based upon the constellations of the ancient Greek tradition, passed down through the Middle Ages. The zodiac includes the following 12:

• Aries, the ram
• Taurus, the bull
• Gemini, the twins
• Cancer, the crab
• Leo, the lion
• Virgo, the virgin
• Libra, the scales
• Scorpius (also called Scorpio), the scorpion
• Sagittarius, the archer
• Capricornus (also called Capricorn), the sea goat
• Aquarius, the water carrier
• Pisces, the fish

In addition to these 12, Ptolemy listed also the following 36 (which now count as 38, due to the break-up of Argo Navis):

• Andromeda
• Aquila, the eagle
• Ara, the altar
• Argo Navis, the ship of the argonauts, since divided into Carina, Puppis and Vela
• Auriga, the charioteer
• Boötes, the herdsman
• Canis Major, the greater dog
• Canis Minor, the lesser dog
• Cassiopeia
• Centaurus, the centaur
• Cepheus
• Cetus, the whale
• Corona Australis, the southern crown
• Corona Borealis, the northern crown
• Corvus, the raven
• Crater, the cup
• Cygnus, the swan
• Delphinus, the dolphin
• Draco, the dragon
• Equuleus, the little horse
• Eridanus, the river
• Hercules
• Hydra, the sea monster
• Lepus, the hare
• Lupus, the wolf
• Lyra, the lyre
• Ophiuchus, the serpent-bearer - actually intersects the ecliptic
• Orion, the hunter
• Pegasus
• Perseus
• Piscis Austrinus, the southern fish
• Sagitta, the arrow
• Serpens, the serpent
• Triangulum, the triangle
• Ursa Major, the greater bear
• Ursa Minor, the lesser bear

In more recent times this list has been added to, first to fill gaps between Ptolemy's patterns (the Greeks considered the sky as including both constellations and dim spaces between) and second to fill up the southern sky as explorers sailed where they could see it. The 38 new constellations are:

• Antlia, the pump
• Apus, the bird of paradise
• Caelum, the chisel
• Camelopardalis, the giraffe
• Canes Venatici, the hunting dogs
• Chamaeleon
• Circinus?, the drawing compasses
• Columba, the dove
• [Coma Berenices]? (traditional asterism), Berenice's hair
• Crux, the cross
• Dorado, the swordfish
• Fornax, the furnace
• Grus, the crane
• Horologium?, the clock
• Hydrus, the water snake
• Indus, the Indian
• Lacerta?, the lizard
• [Leo Minor]?, the lesser lion
• Lynx
• Mensa - originally Mons Mensae, table mountain
• Microscopium, the microscope
• Monoceros?, the unicorn
• Musca?, the fly
• Norma?, the square
• Octans, the octant
• Pavo?, the peacock
• Phoenix
• Pictor? - originally Equuleus Pictoris, the painter's easel
• Pyxis, the compass
• Reticulum?, the reticle
• Sculptor
• Scutum?, the shield
• Sextans, the sextant
• Telescopium, the telescope
• Triangulum Australe, the southern triangle
• Tucana, the toucan
• Volans? - originally Piscis Volans, the flying fish
• Vulpecula? - originally Vulpecula Cum Ansere, the fox with the goose

There were also other constellations that didn't make the cut, most notably Quadrans Muralis (now part of Boötes) for which the [quadrantid meteors]? are named. Various other less official patterns have existed alongside the constellations called asterisms, such as the Big Dipper and Little Dipper. Note that in any of these figures the stars rarely have anything to do with one another - they are just along roughly the same lines of sight, and are typically very far apart.

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External links:

• [The Constellations]
• [Data file of official IAU boundaries] (Note that the boundaries are defined for equinox 1875.)