Why your Vedic sign is different from your western one
Almost everyone who generates a Vedic chart for the first time asks the same question: why is my sign different from the one I have known all my life? The answer is not that one system is wrong. They are measuring from two different starting points, and the gap between those points is currently around 24 degrees.
Two zodiacs
Western astrology uses the tropical zodiac, which begins at the spring equinox. Indian astrology uses the sidereal zodiac, which is fixed against the stars.
These once coincided. The Earth's axis wobbles slowly — precession — and the equinox has drifted backwards against the constellations at roughly one degree every 72 years. After many centuries the two zodiacs have separated by about 24°, which is most of a whole sign.
What an ayanamsa is
The ayanamsa is that gap: the correction applied to a tropical position to get a sidereal one. It is not a constant, because it grows every year.
Because a sign is 30° and the gap is about 24°, most people's sidereal sign is the one before their tropical sign. Someone born in the last three-quarters of tropical Leo is usually sidereal Cancer.
Which ayanamsa this site uses
We use Lahiri, also called Chitrapaksha — the standard adopted by the Indian government calendar and the most widely used in Indian practice. Every chart on this site states it in the footer.
Other ayanamsas exist and are seriously argued for: Raman, Krishnamurti, Fagan–Bradley among them. They differ by a fraction of a degree to a couple of degrees, which almost never changes a Moon sign but can change a nakshatra near a boundary. If a site gives you a different nakshatra from ours, this is usually why.
Which one is right
They answer different questions and have separate traditions of interpretation behind them. What matters practically is that a chart is internally consistent and states its method. A reading that mixes tropical positions with sidereal techniques is the only version of this that is straightforwardly wrong.
Common questions
Why is my Vedic sign different from my western sign?
Because Vedic astrology measures against the fixed stars and western astrology measures from the spring equinox. Precession has separated those by about 24°, which is enough to move most people back one sign.
What is the Lahiri ayanamsa?
The Chitrapaksha ayanamsa, the correction between tropical and sidereal positions adopted as standard by the Indian government calendar. It is what this site uses for every calculation.
Which ayanamsa is the most accurate?
There is no settled answer, and serious practitioners differ. Lahiri is the most widely used in India. Differences between the main ayanamsas are small enough to rarely change a sign, but can change a nakshatra near a boundary.