Ayanamsa

The ayanamsa is the angular difference between the sidereal zodiac (fixed to the actual stars) and the tropical zodiac (fixed to the spring equinox). Because the equinox slowly drifts against the stars — the precession of the equinoxes — the two zodiacs have separated by roughly 24 degrees, and the ayanamsa is the correction used to convert between them.

Vedic (sidereal) astrology applies an ayanamsa; the most common is the Lahiri ayanamsa. It is the technical heart of the tropical-versus-sidereal difference.