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 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's 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, so the two systems agree entirely on the astronomy and disagree entirely on the sign names.
Lahiri, and stop arguing. A degree of disagreement between schools does not deserve a lifetime of debate.
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's a reasonable choice and it's 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 don't settle it.
So the disagreement isn't carelessness. It's 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 isn't 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.
- Don't average them. Combining a tropical Sun with a sidereal Moon is using two coordinate systems in one sentence.
Where AsterNym sits
AsterNym's Western natal chart uses the tropical zodiac and therefore applies no ayanamsa, because the interpretive traditions it draws on were written for the tropical zodiac. AsterNym's Jyotish calculator uses the Lahiri (Chitrapaksha) ayanamsa to convert those same astronomical longitudes into its sidereal zodiac. Both pages state which zodiac they use.
The underlying astronomy is identical: one ecliptic longitude for every body, and subtracting the ayanamsa from it is what produces the Vedic chart. The Vedic page prints the ayanamsa it used for your birth instant, so you can do the subtraction yourself.
Mira Hale