Constellation
In modern astronomy, a constellation is an internationally defined area of the celestial sphere. Historically, the term was also used to refer to a pattern formed by prominent stars within apparent proximity to one another, and this practice is still common today.
Definition
In colloquial usage, a constellation is a group of celestial bodies, usually stars, which appear to form a pattern in the sky. Astronomers today still utilize the term, though the current system focuses primarily on constellations as grid-like segments of the celestial sphere rather than as patterns. A star-pattern that is not officially classed as a constellation is referred to as an asterism. One famous example is the asterism known as the Big Dipper, a term unused by the International Astronomical Union (IAU) as the stars are considered part of the larger constellation of Ursa Major.
In 1922, Eugène Delporte aided the IAU in dividing the celestial sphere into 88 official constellations. Typically, these modern constellations share the names of their Graeco-Roman predecessors, such as Orion, Leo and Scorpius. While such celestial formations were originally linked to a mythical event, creature or person, the categorization of the night sky into recognizable patterns was important in early land and naval navigation prior to the invention of the compass during the Age of Discovery. With the technical advancement of astronomy, it became important to move from a pattern-based system of constellations to one based on area-mapping, which led to several historic formations becoming obsolete.
Boundaries
In 1930, the boundaries between the 88 official constellations were devised by Delporte along vertical and horizontal lines of right ascension and declination. However, the data he used originated back to epoch B1875.0, which was when Benjamin A. Gould first made the proposal to designate boundaries for the celestial sphere, a suggestion upon which Delporte would base his work. The consequence of this early date is that due to the precession of the equinoxes, the borders on a modern star map, such as epoch J2000, are already somewhat skewed and no longer perfectly vertical or horizontal. This effect will increase over the years and centuries to come.
Proximity
The stars within a constellation rarely have any substantial astrophysical relationship to each other, and their apparent proximity when viewed from Earth disguises the fact that they typically lie light years apart. However, there are some exceptions: the constellation of Ursa Major is almost entirely constituted by stars approximate to one another, a phenomenon known as the Ursa Major moving group.
Constellation systems across the world
Graeco-Roman
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).
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
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
Links and References
http://en.wikipedia.org/wiki/Constellation
Star Tales origins and mythology of the constellations (Ian Ridpath)
Astrology | Constellations | Astronomical objects
