The ayanamsa: the maths behind the sidereal zodiac
One number that converts a Western chart into a Vedic one — and several competing versions of it.
4 min read · Published 22 August 2026
The short answer
The ayanamsa is the angular difference between the tropical zodiac, which starts at the March equinox, and the sidereal zodiac, which starts from a fixed point among the stars. It is currently about 24 degrees and grows by roughly one degree every 72 years because of precession. Several competing values exist — Lahiri is the most common and is India's official standard — and they differ by up to about a degree, which is enough to change a nakshatra near a boundary.
What it is doing
Both zodiacs divide the ecliptic into twelve equal 30-degree signs. They differ only in where the first one starts.
Tropical starts at the March equinox — the point where the Sun crosses the celestial equator going north. Sidereal starts from a fixed point among the stars, defined differently by different schools.
Because Earth's axis precesses over roughly 25,800 years, the equinox drifts backwards against the stars at about 50 arcseconds a year — one degree every 72 years. The accumulated gap is the ayanamsa.
To convert: sidereal longitude = tropical longitude − ayanamsa. That single subtraction turns a Western chart into a Vedic one, which is why the two systems agree entirely on the astronomy and disagree entirely on the sign names.
The competing values
| Ayanamsa | Anchored to | Notes |
|---|---|---|
| Lahiri (Chitrapaksha) | The star Spica at 180° sidereal | The most widely used, and India's official government standard |
| Raman | A slightly different zero point | Associated with B. V. Raman; differs from Lahiri by under a degree |
| Krishnamurti (KP) | Its own zero point | Used in the KP system, which has its own methods throughout |
| Fagan-Bradley | A different stellar anchor | Predominant in Western sidereal astrology rather than Indian practice |
The spread between these is under about a degree in most cases — small, but not negligible. A degree is enough to move a planet across a nakshatra boundary, and a nakshatra boundary near your Moon changes your entire dasha timeline.
So two Vedic astrologers using different ayanamsas can give you different life periods. This is the sidereal counterpart to the house system and rulership disputes in Western practice: a convention that changes the answer and is rarely disclosed.
Why there is no agreed value
Because defining the sidereal zodiac requires choosing a zero point among the stars, and nothing in the sky marks one.
The Lahiri approach anchors to Spica — placing that star at exactly 180 degrees sidereal, which sets zero at the opposite point. It is a reasonable choice and it is a choice. Other schools anchor to different stars or to reconstructions of where the zodiac's zero sat at a particular historical date.
There is also genuine uncertainty about what the zero point historically was. Determining when the tropical and sidereal zodiacs coincided requires knowing what ancient astronomers meant by the start of Aries, and the sources do not settle it.
So the disagreement is not carelessness. It is a real problem with no observational solution.
What this means practically
- Ask which ayanamsa a chart used. Any competent tool will tell you. Most default to Lahiri.
- Placements near a boundary are uncertain. If your Moon is within a degree of a nakshatra edge, your dasha sequence depends on a convention rather than on your birth.
- The tropical chart is not wrong. The two zodiacs measure from different zeros, and both are computed from the same astronomical positions. Neither is a corrupted version of the other.
- Do not average them. Combining a tropical Sun with a sidereal Moon is not synthesis; it is using two coordinate systems in one sentence.
Where AsterNym sits
We compute tropical positions and apply no ayanamsa. That is a choice made because the interpretive traditions we draw on were written for the tropical zodiac, and it is stated on every page rather than assumed.
The underlying astronomy is identical to what a sidereal tool starts from — your natal chart gives the exact ecliptic longitude of every body, and subtracting an ayanamsa from those figures is what produces a Vedic chart. If you want the sidereal reading, a Jyotisha tool will do that subtraction; see Far Far Away astrology.
Common questions
- What is the ayanamsa?
- The angular difference between the tropical zodiac, which begins at the March equinox, and the sidereal zodiac, which begins from a fixed point among the stars. It is currently a little over 24 degrees and increases by about one degree every 72 years.
- Which ayanamsa should be used?
- Lahiri, also called Chitrapaksha, is the most widely used and is India's official government standard. Raman, Krishnamurti and Fagan-Bradley are also in use and differ from Lahiri by under a degree in most cases.
- Why do ayanamsa values disagree?
- Because defining a sidereal zodiac requires choosing a zero point among the stars, and nothing marks one. Lahiri anchors to Spica at 180 degrees; other schools anchor differently. There is also genuine historical uncertainty about where the zero point originally sat.
- Does a different ayanamsa change my chart much?
- Usually by under a degree, which rarely changes a sign but can move a planet across a nakshatra boundary. Since your Moon's nakshatra sets your entire dasha timeline, a boundary case can produce materially different life periods.