Guides
What is Ayanamsa?
In short
Why is my Vedic rashi different from my Western star sign?
Because the two systems measure from different starting points. The tropical zodiac begins at the spring equinox; the sidereal zodiac is fixed against the stars. The equinox drifts slowly westward against the stars — precession — and the accumulated gap, currently about 24 degrees, is called the ayanamsa.
Precession, and a gap that grows
The Earth's axis wobbles like a spinning top, completing one turn in about 25,800 years. This makes the equinox point move against the background stars at roughly 50 arcseconds a year — about one degree every 72 years.
The two zodiacs coincided somewhere around the fifth century. Since then the gap has grown to about 24 degrees, which is why someone born in the first three weeks of a Western sign usually falls in the previous rashi in the Vedic system.
Which ayanamsa
There is more than one. Lahiri, also called Chitrapaksha, is the Indian national standard and is what this site uses; Raman and Krishnamurti are also in use and differ by a fraction of a degree. The differences rarely change a rashi, but they can change a nakshatra pada near a boundary.
This is worth knowing when two sites give you slightly different results: they may simply be using different ayanamsas, and neither is wrong.