zodiac (zōˈdēăk) [key] [Gr. zoion = animal], in astronomy, zone of the sky that includes about 8° on either side of the ecliptic. The apparent paths of the sun, the moon, and the major planets all fall within this zone. The zodiac is divided into 12 equal parts of 30° each, each part being named for a constellation, each of which is represented by a sign and many of which have animal names. The constellations and their corresponding symbols and dates are listed in the table entitled The Signs of the Zodiac. The zodiac serves as a convenient means of indicating the positions of the heavenly bodies. When the constellations of the zodiac were named about 2,000 years ago, the vernal equinox coincided with the beginning of the constellation Aries. For this reason, the first 30° section of the zodiac is called Aries; it extends eastward 30° from the vernal equinox, which is therefore called the first point of Aries. However, because of the precession of the equinoxes, the vernal equinox has moved westward about 30° and now lies in the constellation Pisces; the zodiacal constellations thus no longer correspond to the segments of the zodiac represented by their signs. The constellations will again coincide with the sections of the zodiac in about 25,800 years. The zodiac probably had its origins among the Assyrians or Chaldaeans, although it may have originated among the Babylonians as early as 2000 B.C. It is of importance in astrology.
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