Precession

Precession is the slow wobble of the Earth's axis, one full turn in about 26,000 years. As it turns, the point where the Sun sits at the March equinox slides backwards through the constellations, about one degree every seventy-two years. It is the reason the tropical zodiac, tied to the equinox, and the constellations, tied to the stars, no longer line up. The ayanamsa is the running measure of that drift.