Constellation

Constellation

A constellation is a group of stars that form a longrecognized pattern in the sky. The names of many constellations are Greek in origin and are related to ancient mythology. (The ancient Chinese astronomers also divided stars into constellations that, today, are called Xiu, or mansions.) 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 humans choose to see and has no physical significance. In fact, in three-dimensional space, most of the stars within a constellation are far distant from one another—having little or nothing in common with respect to their distance from Earth or from one another.

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, not because they thought the pattern resembled the honoree.

Today, the International Astronomical Union divides the stars in 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 a person 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 seen 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 dominate 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 about the sun. With each passing day and month, humans see a different part of the celestial sphere at night. From this frame of reference, on a planet with a tilted axis, the sun, moon, and planets follow a path along the celestial sphere called the 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, Leo, Libra, Scorpius, Sagittarius, Capricorn, Aquarius, and Pisces. The planets also move along the ecliptic, but because they are much closer to the Earth than the stars, 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 pseudoscience of astrology led to the careful observations of early astronomers.

See also Celestial coordinates; Milky Way; Star.