Precession
Why the equinox drifts against the stars
Earth’s axis turns slowly, like a top, once in about 25,800 years. See how that moves the March equinox and why the ayanamsa keeps growing.
01 · The drifting equinox
The equinox slides slowly backwards along the stars.
The tropical zodiac starts at the March equinox, and the sidereal zodiac at a point fixed against the stars. The equinox drifts westward along the ecliptic by about a degree every 72 years, so the gap between the two starting points, the ayanamsa, keeps growing.
02 · The cause
Earth’s axis turns like a slow top.
Earth bulges at the equator, and the Sun and Moon pull on the bulge. Earth’s spin turns that pull into a slow sideways swing of the axis, the way a spinning top’s axis circles instead of falling over.
A slowly turning axis
Earth spins on an axis tilted about 23.4° from the perpendicular to its orbit. The tilt stays nearly the same, but its direction does not. The Sun and Moon pull on the bulge around Earth’s equator, and because Earth is spinning, that pull makes the axis swing slowly round in a cone, as a spinning top’s axis circles instead of falling over. One full turn takes about 25,800 years. This motion is precession.
The equinox moves with it
The celestial equator is set by Earth’s axis, so it turns with the axis. The March equinox, where the Sun crosses the equator heading north, is one of the two points where the equator meets the ecliptic. As the equator turns, those crossing points slide westward along the ecliptic, against the stars, by about 50″ a year: one degree in about 72 years.
The seasons stay with the equinox. The tropical zodiac starts at the equinox, so it drifts with it, and the Sun always enters tropical Aries in March. The sidereal zodiac stays fixed against the stars. The gap between them, the ayanamsa, therefore grows steadily.
How the ayanamsa has grown
On the site’s reference, True Chitrapaksha, which places Spica at 180°, the two zodiacs coincided around the year 285. Since then the gap has grown:
| Year | Ayanamsa |
|---|---|
| 1000 | 9°55′ |
| 1900 | 22°27′ |
| 2000 | 23°50′ |
| 2026 | 24°12′ |
| 2100 | 25°14′ |
Values from goastro’s True Chitrapaksha function. Other sidereal traditions pick a different zero point: the Lahiri formula differs from it by about a minute of arc today, KP by about four and a half, and the Raman reference by about 1°26′.
The drift also moves sidereal events through the calendar. The Sun reaches sidereal Capricorn, the moment of Makar Sankranti, about a day later every 72 years or so.
The pole star changes too
The same motion moves the point in the sky that Earth’s axis points to. Today it lies close to Polaris, which is why Polaris is the pole star. In about 12,000 years the axis will point near the bright star Vega instead, and it returns towards Polaris after a full turn.
Read how the two zodiacs label the same sky in why two zodiacs give different signs, or explore another topic in Learn.