Why Vedic and Western signs differ: the ayanamsa

Two reference points

The tropical zodiac (Western) starts at the March equinox. The sidereal zodiac (Vedic) is fixed against the stars. The angle between them is the ayanamsa.

Cause: precession

Earth's rotation axis precesses with a period of about 25,800 years, so the equinox moves backwards against the stars by about 50.3 arcseconds per year, roughly one degree every 72 years. The Lahiri ayanamsa was 23.857° at 1 January 2000 and is about 24.2° in 2026.

Formula

sidereal longitude = tropical longitude − ayanamsa. A body in the first ~24° of a tropical sign is therefore in the previous sign sidereally. Because that covers about 80% of each sign, roughly four in five Sun, Moon and ascendant placements change sign between the systems.

Worked example

A tropical Moon at 12° Scorpio (222°) minus a 24° ayanamsa gives 198°, which is 18° Libra in the sidereal zodiac.

In Jyotishya

Positions are computed tropically with Astronomy Engine, then the Lahiri ayanamsa (23.85709° at J2000 plus general precession) is subtracted for Vedic positions. The Vedic, Western and Chinese charts are shown side by side, with any sign differences listed.