Precession of the equinoxes

The precession of the equinoxes is the slow motion of the Earth's rotational axis that makes the vernal point drift backward along the zodiac by about one degree every seventy-two years, a full circuit in some twenty-six thousand years. Its discovery is credited to Hipparchus in the second century BCE. It accounts for the gap between the tropical and sidereal zodiacs, and underlies the idea of astrological ages.