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Why Earth axis traces a cone over 25772 years changing the pole star and ayanamsha
Imagine a spinning top on a table. As it spins, its axis slowly traces a circle in the air. Earth does exactly this — its rotation axis traces a cone in space, completing one full circle every 25,772 years. This is called precession, and it is the single most important astronomical phenomenon for Jyotish.
The cause is gravitational: the Sun and Moon pull on Earth equatorial bulge (Earth is ~43km wider at the equator than pole-to-pole). This torque slowly tilts the rotation axis, causing it to precess like a gyroscope. The rate is about 50.3 arcseconds per year — roughly 1 degree every 71.6 years.
The practical consequence: the point where the Sun crosses the celestial equator (the equinox) slowly shifts westward along the ecliptic. This means the tropical zodiac (tied to the equinox) drifts relative to the stars. Today the gap is about 24.2 degrees — the ayanamsha.
The Surya Siddhanta (Ch.3) describes precession but used a trepidation model (oscillating back and forth) rather than the correct steady increase. This was its one major error. Varahamihira in Pancha Siddhantika also discussed precession. The Indian discovery was independent of the Greek discovery by Hipparchus (~150 BCE).