The zodiac, or "circle of animals" is a zone or belt in space projected onto the celestial sphere through which, from our viewpoint, the planets move. A symbolic geometric construction around 15 to 18 degrees wide, it is divided into 12 signs, each of 30 degrees longitude (making 360 degrees in all), with the ecliptic, the apparent path of the Sun, as its middle line. The tropical zodiac used by most Western astrologers has its beginning at the exact moment that the Sun crosses the celestial equator and enters the zodiacal sign of Aries. Some Western astrologers use the sidereal zodiac favoured by Indian ("jyotish") astrologers, which is based more closely on actual positions of constellations in the heavens, as opposed to the tropical zodiac, which is a moveable format based on the seasons.
The tropical zodiac defines the vernal point (the first day of spring in the northern hemisphere) as the first degree of Aries, but the sidereal zodiac allows it to precess. Many people are confused regarding the difference between the sidereal zodiac and the tropical zodiac signs. Because of a "wobble" in the Earth's axis of rotation over a period of about 26,000 years (often called a "great year"), the rate at which the vernal equinox precesses in the heavens is approximately 0 deg, 0 min, 50.23 seconds a year, drifting by one degree every 72 years. Precession of the equinoxes thus occurs at a rate of roughly 5 arc minutes of a degree every 6 years. The tropical signs relate to the seasons and not the stars.
