Constellations

Constellations

In ancient times when people looked up at the starry night sky, they thought they saw shapes in groups of stars. We call these shapes constellations. The ancient Sumerians, Babylonians, Egyptians, Greeks, and Chinese named constellations of stars after heroes and beasts from stories and after everyday objects.

You probably know some constellations. The Big Dipper looks like a giant pot with a long handle. The constellation Orion is named after a hunter in Greek mythology. You can see his belt, marked by three bright stars, and his sword, which hangs from his belt.

The stars that form constellations are not really near each other. Some of the stars in a constellation are much farther from us than others. The stars just happen to form patterns as we view them from the Earth.

WHICH CONSTELLATION IS WHICH?

When you look up at the sky, you see shapes much like those which the ancient stargazers named after characters from magical tales. You can use a star chart, a map that shows where stars appear in the sky, to become familiar with the shapes and names of the constellations. As the night passes, these great shapes seem to move through the sky, just as the Sun appears to cross the sky during the day. But it’s actually Earth that’s moving, not the Sun and stars.

You can only see the brightest stars with the naked eye. Try looking at the sky through binoculars or a telescope. Thousands of fainter stars come into view. You can no longer see the shapes of constellations.

DO CONSTELLATIONS ALWAYS LOOK THE SAME?

Because Earth tilts as it circles the Sun, you see different constellations at different times of year. The Big Dipper, for example, is easiest to find during summer. Orion is most visible during winter.

People in Australia and elsewhere in the Southern Hemisphere see completely different constellations than people in Canada or the United States. One of the most famous southern constellations is Crux, the Southern Cross.

The shapes of constellations slowly change over very long periods of time. The familiar forms will look quite different many thousands of years from now.

HOW DO ASTRONOMERS USE CONSTELLATIONS?

Astronomers divide the sky into 88 constellations. Even though the constellations do not represent real groupings of stars, astronomers still find them useful for naming stars and mapping the sky.

Astronomers use letters of the Greek alphabet to name stars. They also use a form of the name of the constellation the star is in. The brightest star in a constellation has alpha in its name, because alpha is the first letter of the Greek alphabet. For example, the brightest star in the constellation Perseus is called Alpha Persei. And the second brightest is Beta Persei. (Beta is the second letter in the Greek alphabet.) The star closest to the Sun is Alpha Centauri, the brightest star in the southern constellation of Centaurus.

Some objects that are not stars are also named after the constellations in which they appear. Such objects include the Andromeda galaxy and the Orion nebula. At certain times of the year, the Earth passes through showers of meteors (shooting stars). Even these meteor showers, such as the Perseids and Geminids, are named after the constellations from which they seem to fall.

WHAT ARE THE CONSTELLATIONS OF THE ZODIAC?

The ancient Babylonians noticed that the Sun’s position in the sky changes through the year. They divided the stars along the Sun’s path into 12 constellations. We call these 12 the constellations of the zodiac. They consist of Aries, the Ram; Taurus, the Bull; Gemini, the Twins; Cancer, the Crab; Leo, the Lion; Virgo, the Virgin; Libra, the Balance; Scorpio, the Scorpion; Sagittarius, the Archer; Capricorn, the Goat; Aquarius, the Water Bearer; and Pisces, the Fishes.