CONSTELLATIONS

For untold thousands of years men have traced the outlines

of familiar things among the stars. These patterns in the night

sky are called constellations, from Latin words meaning together

and stars.

Many of the constellations have names that are very old. The

Sumerian shepherds and farmers of Mesopotamia 7,000 years ago may

have called the Bull, the Ram, the Lion, and many other

constellations by the same names we use. Students of history are

sure these names started in Mesopotamia because the choice of

animals suggests this. If the names had first been used in Egypt,

there should be a hippopotamus or elephant among the stars. If

they had started in ancient India, there should be a tiger or

crocodile. (See also Astronomy.)

The later people of Mesopotamia took over the old Sumerian

names for the constellations and still later the Greeks adopted

them. The Greeks added many names of heroes and demigods to the

list of constellations. The Romans used the Greek list but

translated the names into Latin.

About AD 150 the Egyptian astronomer Ptolemy listed the 48

constellations known to him in his book the 'Almagest'. His list

did not cover the entire sky. There were blank spaces between

constellations; and there were no constellations at all for the

southernmost stars because these could not be seen from the

Mediterranean region. In later centuries astronomers added

constellations to Ptolemy's list. Some of these later

constellations are named for scientific instruments, such as the

Sextant, the Compasses, and the Microscope. Others bear the names

of birds and beasts in tropical regions (the Giraffe, the

Chameleon, the Toucan). Today 88 constellations are recognized by

astronomers.

To most people a constellation is a group of bright stars,

but to an astronomer it is a definite area in the sky. Thus every

star, no matter how dim, lies in one constellation or another,

just as any point in the world is in one country. Although all

the stars in the same constellation look close together in the

sky, they are not necessarily close in space because some may be

much farther out in space than others. The boundaries of the

constellations used to be very irregular and had many curved

lines. In 1928 astronomers straightened them out so that the

outline of any constellation includes only straight lines running

north and south or east and west. Astronomers use the

constellation names to identify most bright stars and all

variable stars, so it was important to make the boundaries clear

and precise (see Star).

The constellations are useful to astronomers today not for

their connections with ancient myths, but for telling where in

the sky different stars can be found. Many of the brightest stars

have individual names that come from Greek, Latin, or Arabic, and

the navigators of ships and aircraft call them by these names.

Astronomers, however, find it more convenient to name them by

their constellations, with a Greek letter to distinguish the

different stars in each constellation. For example, Polaris, the

Pole Star, in the Northern Hemisphere, is the brightest star in

the constellation Ursa Minor, the Little Bear, and the

astronomers' name for it is alpha Ursae Minoris, or a UMi for

short.

The path of the sun among the stars is called the ecliptic.

The twelve constellations that lie along the ecliptic form the

Zodiac, or birth-sign constellations. The other constellations

are divided into those north of the Zodiac and those south of it.

From one place on the Earth different constellations are

seen at different times of the year. This happens because, as

well as turning on its axis, the Earth is always moving around

the sun, making one orbit each year. A star that is visible at

night during one part of the year may appear close to the sun six

months later; it would not then be seen at night.

Some constellations can be seen only from the Northern

Hemisphere and some only from the Southern Hemisphere. The

constellations of the Zodiac can be seen from both hemispheres.

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