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Definitions

In colloquial usage, a constellation is what astronomers call an asterism: a group of celestial bodies (usually stars) that appear to form a pattern in the sky or appear visibly related to each other. Examples are Orion (which appears like a human figure with a belt, often referred to as "The Hunter"), Leo (which contains bright stars that outline the form of a lion), Scorpius (which can seem reminiscent of a scorpion), and Crux (a cross).

In astronomy, however, a constellation is an area of the sky, and contains all the stars and other celestial objects within that area. The International Astronomical Union (IAU) divides the sky into 88 official constellations with exact boundaries, so that every direction or place in the sky is defined by one constellation. Most of these constellations are centered on the traditional constellations of Western culture.
Constellations were devised by ancient people to be able to recognize stars in the sky. The shapes of constellations resemble objects familiar to those people.

Human perception versus reality

Constellations are normally the product of human perception rather than astronomical realities. 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. However, there are some exceptions. The famous star pattern known as the Big Dipper is almost entirely created by stars that are genuinely close together in astronomical terms; they are known as the Ursa Major moving group.

The grouping of stars into constellations is essentially arbitrary, as different cultures have seen different patterns in the sky, although a few of the more obvious ones tend to recur frequently, e.g., Orion and Scorpius.

Official constellations

The 88 official constellations defined by the IAU (International Astronomical Union) are mostly based upon those of the ancient Greek tradition, passed down through the Middle Ages, which includes the 'signs of the zodiac,' twelve constellations through which the sun passes and which thus have had special cultural significance. The rest consist of constellations which were defined in the early modern era by astronomers who studied the southern hemisphere's skies, which were invisible to the Greeks.

Boundaries

The constellation boundaries now used by the International Astronomical Union were drawn up in 1930 by Eugène Delporte. He drew them along vertical and horizontal lines of right ascension and declination. However, he did so for the epoch B1875.0, the era when Benjamin A. Gould made the proposal on which Delporte based his work. The consequence of this early date is that due to precession of the equinoxes, the borders on a modern star map (e.g., for epoch J2000) are already somewhat skewed and no longer perfectly vertical or horizontal. This skew will increase over the years and centuries to come.

A star pattern may be widely known but may not be used by the International Astronomical Union. One famous example is the asterism known as the Big Dipper; this term is not used by the IAU as the stars are considered part of the larger constellation of Ursa Major.

Constellation systems across the world

Western

In the Western world, the sky of the northern hemisphere is traditionally divided into constellations based on those described by the Ancient Greeks. The first ancient Greek works which dealt with the constellations were books of star myths. The oldest of these was a poem composed by Hesiod in or around the eighth century BC, of which only fragments survive. The most complete existing works dealing with the mythical origins of the constellations are by the Hellenistic writer termed pseudo-Eratosthenes and an early Roman writer styled pseudo-Hyginus.

In the 2nd century AD, the Greek astronomer Ptolemy described the constellations in great detail in his influential work the Almagest.

Chinese

Chinese constellations are different from the Western constellations due to the independent development of ancient Chinese astronomy One difference is that the Chinese counterpart of the 12 western zodiac constellations is the 28 "Xiu" (宿) or "mansions" (a literal translation). The Western and Chinese too have some similarities.

Indian

In Vedic astrology, the 12 zodiac constellations are called raasis. The twelve raasis along the ecliptic correspond directly to the twelve western star signs. These are however divided into 27 Nakshatras, or lunar houses.

Dark cloud constellations

The "Emu in the sky," a constellation defined by dark clouds rather than the stars. A western interpretation would recognise Crux (the Southern Cross) above the emu's head and Scorpius on the left. The head of the emu is the Coalsack.

In the southern hemisphere, it is possible to discern dark patches in the Milky Way. Some cultures have discerned shapes in these patches and have given names to these "dark cloud constellations." Members of the Inca civilization identified various dark areas or dark nebulae in the Milky Way as animals, and associated their appearance with the seasonal rains. Australian Aboriginal astronomy also describes dark cloud constellations, the most famous being the "emu in the sky" whose head is formed by the Coalsack.