ZO'DIAC (in Greek, d Zepascisa s abnAos, "the Zodiac circle") is a name given to a zone of the visible heavens, extending in breadth to • certain equal distances on both sides of a great circle of the celestial sphere, In the plane of the earth's orbit produced. This circle, with which the apparent annual path of the sun coincides, is called the ecliptic ; at present It makes, with the plane of the earth's equator, an angle equal to about 23° 27' 35*, and it is divided into twelve equal parts, called slips, which receive their denominations from those of the figures intended to designate the constellations or groups of stars about it Most of the figures being those of animals, the name of zodiac (from Cs'ithoe, Wien, the diminutive of Giles, then, "an animal") has, in consequence, been applied to the zone.
The planes of the orbits of all the planets, when produced to the celestial sphere, are supposed to be comprehended within the breadth of the zodiac, and that breadth is determined by two small circles parallel to the plane of the ecliptic. Before the discovery of Ceres, Pallas, and the other asteroids, the greatest inclination of the orbit of a planet to the ecliptic scarcely exceeded 7 degrees, and there fore the breadth of the zodiacal zone was to be about 10 degrees, or 8 degrees on each side northward and southward of the ecliptic. The orbit of Pallas (that which deviates most from the ecliptic) is inclined about 35 degrees to that plane ; and it might now be \mderstood that the breadth of the zone is about 70 degrees.
The line In which the plane of the ecliptic intersects that of the terrestrial equator, being produced indefinitely, cuts the celestial sphere in two points diametrically opposite to each other ; and one of these meeting the heavens, in the age of the earliest Greek astronomy, near ccrtaiu stars forming a constellation to which the figure of a rain (Aries) was assigned, is generally called the first poiut of Aries. From this point are reckoned, on the ecliptic, the longitudes of celestial bodies; and on the equator, their right ascensions. The twelve equal parts or signs into which the ecliptic is divided are distinguished by the names of the constellations which, in the age above alluded to, fell within their respective extents in longitude; and the names both of the signs and constellations are as follow :—Aries, Taurus, Gemini, Cancer. Leo, Virgo, Libra, Scorpio, Sagittarius, Capricoruus, Aquarius, and Pisces.
The distribution of all the visible stars into groups or constellations was the first step in laying the foundation of astronomical science. It must have taken place in the earliest ages of the world ; and there is the highest degree of probability that the zodiacs of all nations have been derived from a common source, though in passing from one people to another it may be easily conceived that the figures would suffer changes from the vanity or caprice of individuals. Among the
ancients, the place of the sun in the zodiac served to regulate the seasons of the year; the representations of the figures associated with the groups of stars were almost constant ornaments of the religious edifices; and superstition assigned to the regions of space influences on the lives and characters of men depending upon the qualities of tho animals or objects which distinguished the constellations in the corre sponding parts of the celestial sphere.
The loss of the writings of the ancient Babylonians, and the entire destruction of their edifices, have deprived the world of every menu mcut by which light might be thrown on the state of astronomy among a people whose priests, accordiug to Strabo, were mostly engaged in the study of the science. In fact, our information respeoting it consists chiefly of the notices given by Ptolemy concerning a few of their obser vations, and from the evidence of Geminus. Diodorus Siculus states Biblloth. Histor.; ii. 30) "that the Babylonians had twelve chief deities, to each of whom they assigned a month, and one of the so-called twelve animals," by which he means the twelve parts of the zodiac ; and from this it may be inferred that they divided the zodiac into twelve signs. Sextus Empiricus (` Adversue Math.; lib, v.) makes a like statement, and shows how, by means of a clepsydra, the division was or may have been made. With respect to the astronomical tuouu meuts which adorned their edifices, a few fragments only of stone, having on them figures which may or may not have been inteuded as representations of those which distinguish the constellations, have been dug up near Bagdad; and the most remarkable of these is one having on its face a solar disc accompanied by a serpent : the figure may have designated Ophiuchus, and it is possible that it may have been part of a ChalffiTan planisphere. The division of the zodiac into twelve signs may have been originally made for the conveuience of distinguishing the portions which the sun passes through iu the several mouths; and a division into twenty-eight parts is alluded to by Geminus (Eicareyfi Fir 4mie4asea), which was probably made to denote the space daily described by the moon by her proper motion. The former division was used by the Egyptians, the Greeks, and by all the civilised nations of Asia ; and the latter is found among the Persians, Arabs, Hindus, and Chinese : the twenty-eight parts are called the stations or abodes of the moon, because this luminary is in some one of them each day.