The Precession of the Equinoxes
Traditionally, the vernal equinox date (March 21st) has served as a reference point for calendars, for most ancient cultures. The ancient astronomers were aware that position of the vernal equinox sun, relative to the fixed stars of the zodiac, drifts slowly backwards along the zodiac, completing an entire 360° circuit around the zodiac in about 25,800 years. This slow movement of the equinoctial point backwards along the zodiac is called the “precession of the equinoxes” or simply, precession, while the cycle duration of 25,800 years is called the “precession cycle” of the earth.