What is ayanamsa, and why does it change your sign?
One number: the gap between where the two zodiacs start, currently about 24°. It grows by 50 arcseconds a year, it is the whole computational difference between the systems — and there is more than one of it.
Ayanamsa is one number: the gap between where the Western zodiac starts and where the Vedic one starts. Today it is about 24 degrees. Everything that differs between the two systems’ charts — your sign, your ascendant, your house placements — comes out of that single subtraction.
It is worth understanding properly, because it is the only part of the disagreement that is arithmetic rather than opinion.
Why there is a gap at all
Both systems divide the same circle into twelve equal signs. They disagree about where the circle begins.
- Tropical — 0° Aries is the vernal equinox, the moment the Sun crosses the celestial equator going north. Anchored to the seasons.
- Sidereal — 0° Aries is a fixed point against the background stars. Anchored to the sky.
Those two anchors do not stay together. The Earth’s axis wobbles, so the equinox slides backwards against the stars by about50.3 arcseconds a year — one degree every 72 years, a full circle in roughly 26,000. That slide is precession, and the accumulated distance is the ayanamsa.
The two zodiacs were the same thing once. Under the Lahiri definition they coincided in the third century AD — 285 is the year usually given — and have been drifting apart ever since at a rate you can watch above. Nothing was decided in 285; that is simply where the definition puts zero.
What it does to your chart
Subtract 24° from every tropical position and most things move back one sign. Not all: if your Sun sits past 24° of its tropical sign, it stays put. That is roughly one person in five, which is why “Vedic always moves you back one sign” is wrong often enough to be worth correcting.
Nothing about the planet moves. The Sun is where the Sun is, and both systems agree on that to the arcsecond. Only the label attached to that position changes.
Which ayanamsa — and why that question is awkward
Here is the part rarely volunteered: there is more than one.Lahiri, Raman, Krishnamurti, Fagan–Bradley and others each place zero somewhere slightly different, and they disagree by more than a degree.
A degree is enough to move a planet across a sign boundary for anyone born near a cusp, and enough to shift a dasha boundary by months. So “sidereal” on its own does not specify a chart. The ayanamsa has to be named.
The most widely used, Lahiri, was fixed by a government committee in the 1950s rather than discovered by observation. That is not an accusation — a standard has to come from somewhere, and a shared convention is worth having. But it does mean the zero point is a decision, and anyone presenting it as a measured constant of nature has skipped a step.
The question to ask any chart service is which ayanamsa it used.Most cannot tell you. Ours is stated onthe calculation rules page, along with every other setting, because a chart computed with unnamed settings cannot be checked by anybody — including by us, later.
So which one is correct?
That depends on what you think the signs mean, and it is a real question rather than a rhetorical one. If the signs carry seasonal meaning, tropical is the consistent choice. If they refer to the constellations, sidereal is. They cannot both be describing the same thing.
What can be said without hedging: the two systems are not different opinions about your chart. They are the same chart with a different starting line, and the distance between the lines is a number you can watch move.
Our chart files state the ayanamsa, the house system and the node convention in the file itself, so every position in them can be reproduced or disputed.
Related: why two free birth charts disagree, and Vedic and Western astrology compared.