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Constellations in the universe help us describe areas of the universe.
Constellations in the universe, refers to an area of the celestial sphere, defined by exact boundaries.
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 belongs within one constellation. Most of these constellations are centred on the traditional constellations of Western culture
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 in North America or the Plough in the UK 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 in the universe.
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 of constellations.
The constellation boundaries now used by the International Astronomical Union were drawn up in 1930 by Eug? 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 in North America or the Plough in the UK; this term is not used by the IAU as the stars are considered part of the larger constellation of Ursa Major.
Names and star designations of constellations.
All modern constellation names are Latin proper names or words, and some stars are named using the genitive, or sometimes the ablative [specify] of the constellation in which they are found. These are formed by using the usual rules of Latin grammar. Some examples include: Aries - Arietis; Taurus - Tauri; Gemini - Geminorum; Virgo - Virginis; Libra - Librae; Pisces - Piscium; Lepus - Leporis. In addition, all constellation names have a standard three-letter abbreviation assigned by the International Astronomical Union; for example, Aries becomes Ari, Pisces becomes Psc, Sagittarius becomes Sgr and Ursa Major becomes UMa.
Identification of stars within a given constellation includes use of Bayer designations such as Alpha Centauri, Flamsteed designations such as 61 Cygni, and variable star designations such as RR Lyrae. However, many fainter stars will just be given a catalog number designation (in each of various star catalogs) that does not incorporate the constellation name. Frequently, the abbreviated form of the constellation name is used in the star designation, e.g., Alpha Cen, 61 Cyg, RR Lyr.
For more information about star names, see star designations and the list of stars by constellation.
Constellation systems across the world.
In the Western world, the constellation 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 CE, the Greek astronomer Ptolemy described the constellations in great detail in his influential work the Almagest. The 48 constellations he described are still used by modern astronomers today.
Chinese constellations are different from the Western constellations due to the independent development of ancient Chinese astronomy, although there are also similarities. One difference is that the Chinese counterpart of the 12 western zodiac constellations is the 28 "Xiu" or "mansions" (a literal translation).
In Hindu/Vedic astronomy, the 12 zodiac constellations are called raasi's. The twelve raasi's 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; and 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.
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