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88 modern constellations
In modern astronomy, the sky is divided into 88 regions called constellations, generally based on the asterisms of Greek and Roman mythology.
Those along the ecliptic are the constellations of the zodiac. The ancient Sumerians, and later the Greeks,
established most of the northern constellations in international use today. When explorers mapped the stars of the southern skies, European
and American astronomers proposed new constellations for that region, as well as ones to fill gaps between the traditional constellations.
Not all of these proposals caught on, but in 1922, the International Astronomical Union adopted the modern list of 88 constellations. After this,
Eugène Joseph Delporte drew up precise boundaries for each constellation, so that every point in the sky belonged to exactly one constellation.
History
Some constellations are no longer recognized by the International Astronomical Union, but may appear in older star charts and other references.
Most notable is Argo Navis, which was one of Ptolemy's original 48 constellations.
Abbreviations
Each of the IAU constellations has an official 3 letter abbreviation.
They are actually abbreviations of the genitive form of the constellation names, so some letters appearing in the abbreviation may come
from the genitive form without appearing in the base name.
The majority of the abbreviations are just the first three letters of the constellation, with the first character capitalised: Ori for Orion, Ara
for Ara/Arae, Com for Coma Berenices. In cases where this would not unambiguously identify the constellation, or where the name
and its genitive differ in the first three letters, other letters beyond the initial three are used: Aps for Apus/Apodis, CrA
for Corona Australis, CrB for Corona Borealis, Crv for Corvus. When letters are taken from the second word of a two-word name, the first letter
from the second word is capitalised: CMa for Canis Major, CMi for Canis Minor. The abbreviations are unambiguous, with two exceptions. Leo
for the constellation Leo could be mistaken for Leo Minor, and Tri for Triangulum could be mistaken for Triangulum Australe.
Asterisms
Various other unofficial patterns exist alongside the constellations. These are known as "asterisms". Examples include the Big Dipper/Plough
and the Northern Cross. Some ancient asterisms, for example Coma Berenices, Serpens, and portions of Argo Navis, are now officially constellations.
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