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Some of the stars in the sky are brighter and more noticeable than others are, and some of these bright stars appear to the eye to be grouped together. Ancient civilizations imagined that groups of stars represented figures in the sky. The oldest known representations of these groups of stars, called constellations, are from ancient Sumer (now Iraq) from about 4000 bc. The constellations recorded by ancient Greeks and Chinese resemble the Sumerian constellations. The northern hemisphere constellations that astronomers recognize today are based on the Greek constellations. Explorers and astronomers developed and recorded the official constellations of the southern hemisphere in the 16th and 17th centuries. The International Astronomical Union (IAU) officially recognizes 88 constellations. The IAU defined the boundaries of each constellation, so the 88 constellations divide the sky without overlapping.

Ancient astronomers noted that the Sun makes a yearly journey across the celestial sphere, part of which is represented in this picture by the blue band. The ancient astronomers associated dates with the constellations in this narrow belt (which is known as the zodiac), assigning to each constellation of stars the dates when the Sun was in the same region of the celestial sphere as the constellation. The twelve zodiacal signs for these constellations were named by the 2nd-century astronomer Ptolemy, as follows: Aries (ram), Taurus (bull), Gemini (twins), Cancer (crab), Leo (lion), Virgo (virgin), Libra (balance), Scorpio (scorpion), Sagittarius (archer), Capricorn (goat), Aquarius (water-bearer), and Pisces (fishes).

A familiar group of stars in the northern hemisphere is called the Big Dipper. The Big Dipper is actually part of an official constellation—Ursa Major, or the Great Bear. Groups of stars that are not official constellations, such as the Big Dipper, are called asterisms. While the stars in the Big Dipper appear in approximately the same part of the sky, they vary greatly in their distance from Earth.

This is true for the stars in all constellations or asterisms—the stars making up the group do not really occur close to each other in space; they merely appear together as seen from Earth. The patterns of the constellations are figments of humans’ imagination, and different artists may connect the stars of a constellation in different ways, even when illustrating the same myth.


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