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Away from city lights on a clear, moonless night, the naked eye can see 2000-3000 stars. As you look at the stars, your mind may group them into different shapes of patterns. People of nearly every culture throughout history have looked at the stars and given names to shapes they saw and invented stories to go with them. The pattern that the Greeks named Orion, the hunter, was seen by the ancient Chinese who called it a supreme warrior named Shen. The Chemehuevi Native Americans of the California desert saw the same group of stars as a line of three sure-footed mountain sheep.
The patterns of stars seen in the sky are usually called constellations, although more acurately, a group of stars that forms a pattern in the sky is called an asterism. Astronomers use the term constellation to refer to an area of the sky, which 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 place in the sky belongs within a constellation. Most of the constellations in the northern hemisphere are based on the constellations invented by the ancient Greeks, while most in the southern hemisphere are based on names given to them by seventeenth century European explorers.