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Science EncyclopediaScience & Philosophy: Condensation to Cosh

A constellation is a group of stars that form a long-recognized pattern in the sky. The names of many constellations are Greek in origin and are related to ancient mythology. The stars that make up a constellation may be at very different distances from the earth and from one another. The pattern is one that we as humans choose to see and has no physical significance.

Novice stargazers are often taught that the pattern of stars in a constellation resembles an animal or a person engaged in some activity. For example, Sagittarius is supposed to be an archer, Ursa Major a large bear, and Ursa Minor a small bear. However, most people locate Sagittarius by looking for a group of stars that resemble an old-fashioned coffee pot. Ursa Major is more commonly seen as a Big Dipper and Ursa Minor as a Little Dipper. In fact, it is more likely that ancient stargazers named constellations to honor people, objects, or animals that were a part of their mythology and not because they thought the pattern resembled the honoree.

Today's modern stars divide the sky into 88 constellations that are used by astronomers to identify regions where stars and other objects are located in the celestial sphere (sky). Just as you might tell someone that Pike's Peak is near Colorado Springs, Colorado, so an astronomer refers to nebula (M 42) as the Orion Nebula, or speaks of galaxy M 31 in Andromeda and the globular cluster M 13 in Hercules.

The constellations that you see in the Northern Hemisphere's winter sky—Orion, Taurus, Canis Major, and others—gradually move westward with time, rising above the eastern horizon approximately four minutes earlier each evening. By late spring and early summer, the winter constellations are on the western horizon in the early evening and Leo, Bootes, Cygnus, and Sagittarius dominates the night sky. In the fall, Pegasus, Aquila, and Lyra brighten the heavens. A number of polar constellations (Cephus, Cassiopeia, and Ursa Minor in the north and Crux, Centaurus, and Pavo in the south) are visible all year as they rotate about points directly above the North and South Poles.

The westward movement of the constellations is the result of Earth's motion along its orbit. With each passing day and month, we see a different part of the celestial sphere at night. From our frame of reference on a planet with a tilted axis, the sun, moon and planets follow a path along the celestial sphere called ecliptic, which makes an angle of 23.5° with the celestial equator. As the sun moves along the ecliptic, it passes through 12 constellations, which ancient astronomers referred to as the Signs of the Zodiac—Aries, Taurus, Gemini, Cancer, The constellation Orion, the Great Hunter. The three closely placed stars just left of center in this photo are Alnilam, Alnitak, and Mintaka, and they mark Orion's belt. The short line of less brilliant stars beneath the belt are his scabbard. In the upper left, at Orion's right shoulder, is the star Betelgeuse. His left shoulder is the star Bellatrix. His left foot is the star Rigel, and the bright stars to the right of top center are an animal skin that he carries as a shield. U.S. National Aeronautics and Space Administration (NASA). Leo, Libra, Scorpius, Sagittarius, Capricorn, Aquarius, and Pisces. The planets also move along the ecliptic, but because they are much closer to us than the stars constellations along the zodiac their paths change with respect to the constellations. These wanderers, which is what the ancients called the planets, led to astrology—the belief that the motion of the sun, moon, and planets along the zodiac has some influence on human destiny. While there is no evidence to support such belief, the careful observations of early astronomers owes much to the pseudoscience of astrology.

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