This is the one disagreement in astrology that is not a matter of interpretation. Two traditions measure the same circle from two different zeroes, and one of the zeroes moves.
Almost everybody meets this read the same way: they open a Vedic chart for the first time, see a Sun sign that is not theirs, and assume the software is broken.
It is not broken. You have run into the one place where the two astrologies disagree about something you can go outside and check.
Two zeroes
Both traditions cut a circle into twelve parts of thirty degrees. They disagree about where the circle starts.
Western astrology uses the tropical zodiac. Zero degrees of Aries is defined as the March equinox — the moment the Sun crosses the celestial equator — wherever that happens to fall against the background stars.
Vedic astrology uses the sidereal zodiac. Zero is anchored to the stars themselves. A sign here is a stretch of actual sky.
Those two zeroes agreed once, roughly seventeen hundred years ago, and have been separating ever since, because the Earth's axis slowly wobbles. The wobble is called precession, it runs at about fifty arcseconds a year, and the accumulated gap is now around twenty-four degrees — most of a whole sign.
The practical consequence: if your birthday falls in roughly the first three weeks of a Western sign, your sidereal Sun sits in the sign before it. A late March Aries is usually a Pisces here.
Neither system is doing the arithmetic wrong. They are measuring from different zeroes and each is consistent with its own body of technique. What does not work is mixing them — reading a sidereal chart with tropical meanings gives you the worst of both.
The number that measures the gap
The gap has a name — ayanamsa — and it is not a matter of taste. It is a measured quantity, published, and there is more than one published value.
This app uses Lahiri, the official Indian standard. You will also see it labelled "N.C. Lahiri" and "Lahiri (Chitrapaksha)"; those are the same ayanamsa under different vendors' names, not different systems.
Why the precision matters more than it looks: your entire life-timetable is set by a fraction of a star-segment, so an error in the sky-measurement scales straight into an error in your dates. One degree of ayanamsa error moves every period of your life by about a year and a half. Even a tenth of a degree is nearly two months. This is why the value is a fixed engine constant here and why nobody gets to quietly tune it.
What is actually in the sky here
Nine bodies, called grahas. Seven you can point at: the Sun, the Moon, Mars, Mercury, Jupiter, Venus and Saturn. Two you cannot.
The last two are Rahu and Ketu, the north and south lunar nodes — the two opposite points where the Moon's path crosses the Sun's, which is why eclipses happen there and why the tradition calls them shadow bodies rather than planets. They own no sign of their own and they always run backwards. They are real geometry, not mythology with coordinates attached; this app computes the true node rather than the smoothed average, which are not the same point.
You will notice who is missing: Uranus, Neptune and Pluto. They are not part of this system. That is not an oversight to be corrected — the technique was built on nine and the rules are written for nine.
Why we are telling you where to check us
Everything above is falsifiable, which is unusual for astrology copy, so here is the invitation: open any almanac you trust — Drik Panchang is the reference this project matches its conventions against — and compare a planet's sign against ours on any date you like.
We say that with some confidence and no swagger, because we have already lost that bet in public.
For its entire life until August 2026 this app placed the five visible planets a fraction of a degree behind where they really were: 0.37° in 2026, growing by about fifty arcseconds a year. The Sun and the Moon came from one frame of reference and the five planets from another, and the same ayanamsa was subtracted from both. Nothing inside the app ever contradicted anything — everything was wrong by the same amount in the same direction, which is exactly why it survived eighteen releases.
It was not caught by a test. It was caught by a reader who opened Drik Panchang and saw Venus in a different sign.
That story is in this Journal in full, with the four-line fix and the test that now runs on every build. We keep it up for the same reason we are telling you which almanac to check us against: an instrument that cannot be caught being wrong is not an instrument.
And a limit, since we are here
Our positions for the Sun, the Moon and the two nodes sit within about forty arcseconds of reference — and those are the four that set your Moon sign, your star-segment and your whole timetable. The five visible planets use a lower-precision model and can be several arcminutes out, which almost never changes a sign and can, within about half a degree of a boundary, change a star-segment. Where that matters, we say so rather than rounding it away.