HISTORY:
The oldest known drawings of constellations are motifs on seals, vases, and gaming boards from the Sumerians, indicating that constellations may have been developed as early as 4000 BC. The constellation Aquarius was named by the Sumerians after their god of heaven An, who pours the waters of immortality upon the earth. The division ofthe zodiac into 12 equal signs was known around 450 BC by the Babylonians. The northern constellations known today are little different from those known by the Chaldeans and the ancient Egyptians, Greeks, and Romans. Homer and Hesiod mentioned constellations, and the Greek poet Aratus of Soli (circa 315-c. 245 BC) gave a verse description of 44 constellations in his Phaenomena. The Alexandrian astronomer andmathematician Ptolemy, in his Almagest, described 48 constellations, of which 47 are known today by the same name.

In the past many other peoples have grouped stars in constellations, although their arrangements usually did not correspond to those of the ancients. Some Chineseconstellations, however, resemble those of the ancients, indicating the possibility of a common origin.

EXPLORERS:
At the end of the 16th century the first explorers of the South Seas mapped the southern sky, which was largely unknown to the ancients. New constellations were added by a Dutch navigator, Pieter Dirckz Keyser, who participated in the exploration of the East Indies in 1595. Subsequently, other southern constellations were added by the German astronomer Johann Bayer, who published the first extensive star atlas in the Western world, the Uranometria; by Johannes Hevelius; and by the French astronomerNicolas Louis Lacaille. Many others proposed new constellations, but astronomers finally settled on a list of 88. The boundaries of constellations, however, remained a matter of discussion until 1930, when definitive boundaries were fixed by the International Astronomical Union.

The genitive forms of the names of constellations, preceded by a Greek letter, are used to designate about 1300 bright stars; this system was introduced by Johann Bayer. The famous star Algol in the constellation Perseus, for example, is called Beta Persei. The accompanying table lists the constellations on which separate articles appear in this web site.

CONSTELLATION NAMES AND MEANINGS:
Ancient people and astronomers often saw designs or groupings in the stars and named them after various religious figures, animals, and objects. This table lists some of the more prominent constellations and translates their names.

ZODIAC:
The imaginary belt in the celestial sphere, extending about 8° on either sideof the ecliptic (see ECLIPTIC). The width of the zodiac was determined originally so as to include the orbits of the Sun and Moon and of the five planets (Mercury, Venus, Mars, Jupiter, and Saturn) that were known to the people of ancient times. The zodiac is divided into 12 sections of 30° each, which are called the signs of the zodiac. Starting with the vernal equinox and then proceeding eastward along the ecliptic, each of the divisions is named for the constellation situated within its limits in the 2nd century BC. The names of the zodiacal signs are 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; Capricornus, the Goat; Aquarius, the Water Bearer; and Pisces, the Fishes. Because of the precession of the equinoxes about the ecliptic, a 26,000-year cycle, the first point of Aries retrogrades about 1° in 70 years, so that the sign Aries today lies in the constellation Pisces. In about 24,000 years, when the retrogression will have completed the entire circuit of 360°, the zodiacal signs and constellations will again coincide.

OTHER ZODIACAL SIGNS:
It is believed that the zodiacal signs originated in Mesopotamia as early as 2000 BC. The Greeks adopted the symbols from the Babylonians and passed them on to the other ancient civilizations. The Egyptians assigned other names and symbols to the zodiacal divisions. The Chinese also adopted the 12-fold division, but called the signs rat, ox, tiger, hare, dragon, serpent, horse, sheep, monkey, hen, dog, and pig. Independently, the Aztec people devised a similar system.

ECLIPTIC:
In astronomy, it is the apparent great-circle annual path of the sun in the celestial sphere, as seen from the earth. It is so named because eclipses occur only when the moon is on or near this path. The plane of this path, called the plane of the ecliptic, intersectsthe celestial equator (the projection of the earth's equator on the celestial sphere) at an angle of about 23°27'. This angle is known as the obliquity of the ecliptic and is approximately constant over a period of millions of years, although at present it is decreasing at the rate of 48 seconds of arc in each century and will decrease for several millenniums until it reaches 22°54', after which it will again increase.