Ayanamsa

Ayanamsa is the angle of difference between the tropical zodiac (anchored to the vernal equinox) and the sidereal zodiac (anchored to fixed stars), caused by the precession of the equinoxes. Currently approximately 23–24°. The most widely used ayanamsa in India is the Lahiri ayanamsa (officially adopted by the Indian government). Arcanavana uses Lahiri ayanamsa for all Vedic calculations. This is why your Vedic Sun sign is often one sign earlier than your Western Sun sign.