early 14c., from Old French constellacion "constellation, conjuncture (of planets)," from Late Latin constellationem (nominative constellatio) "set with stars," from constellatus, from Latin com- "with" (see com-) + past participle of stellare "to shine," from stella "star" (see star). Originally in astrology, of position of planets ("stars") in regard to one another on a given day, usually one's birth day, as a determination of one's character. "I folwed ay myn inclinacioun/By vertu of my constillacioun" (Chaucer, "Wife's Prologue," c.1386). Modern astronomical sense is from 1550s.
Our Living Language : Various cultures throughout history have chosen different groups of stars in the night sky to form different constellations. While it was once thought that the Greeks were responsible for determining many of the constellations known today, it is now believed that the mythological origins of the 48 ancient constellations predate the Greeks and originate instead from ancient Middle Eastern civilizations. In the seventeenth and eighteenth centuries another 40 constellations were invented by Europeans for navigational purposes. The boundaries of the 88 constellations currently recognized were defined in the 1920s by the International Astronomical Union. There is no scientific reason why there are exactly 88; the modern constellations are only a convenient way to break up the sky to locate the position of celestial objects or track satellites. Although the stars in any given constellation may look like they're neighbors, they can actually be many light-years apart, and if seen from another part of the galaxy they would form different groups and shapes altogether. Constellation names are usually given in Latin, such as Ursa Major (Great Bear) or Centaurus (Centaur), and individual stars in constellations are named in order of brightness, using the Greek alphabet, with the genitive case of the constellation following. Therefore, Alpha Centauri is the brightest star in the constellation Centaurus, Beta Centauri is the second brightest star, and so on. The stars within our galaxy are rushing through space in various directions, and as the millennia pass, the arrangements of the star groups as seen from Earth will change, inevitably altering the constellations as we know them.
An easily recognized group of stars that appear to be located close together in the sky and that form a picture if lines connecting them are imagined. Constellations are usually named after an animal, a character from mythology, or a common object. (See Big Dipper, Ursa Major, and Ursa Minor.)