Western astrology commonly uses a tropical zodiac tied to the seasons; Vedic astrology commonly uses a sidereal zodiac referenced to the stars. The difference is accounted for by an ayanamsa, so a person's sign may differ between systems. AstroSutra uses Lahiri ayanamsa for its sidereal chart calculations. Both approaches have interpretive traditions; a different sign reflects a different coordinate system rather than a changed birth moment.
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