The Zodiac is a zone of the sky that includes about 8 degrees 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 degrees each, each part being named for a constellation and represented by a sign.Because of the precession of the equinoxes, the equinox and solstice points have each moved westward about 30 degrees in the last 2,000 years; thus the Tropical Zodiac months, which were named in ancient times, no longer correspond to the constellations of the Sidereal Zodiac represented by their signs.
However the Tropical Zodiac still accurately divides the year into twelve 30° months; 4 seasons of 3 months uniformly measuring the points between solstice and equinox.
Adapted from The Concise Columbia Encyclopedia by Columbia University Press, 1991.