Precession of the equinoxes — Astrology glossary

Precession of the equinoxes is the slow conical wobble of Earth's rotational axis, completing one full circuit roughly every 25,800 years. Because the equinox points shift gradually westward against the background stars, the position of the spring equinox moves about one degree every seventy-two years. In astrology this is the reason the tropical zodiac, fixed to the seasons, has drifted away from the sidereal zodiac, fixed to the constellations, by around twenty-four degrees over the past two millennia. It matters because it accounts for the gap between a person's tropical Sun sign and the constellation the Sun genuinely occupied at their birth, and it is the technical basis of the ayanamsha used to convert between the two zodiacs. For example, the equinox sat in Aries in ancient times but lies in Pisces today.