Ayanamsa is the number that separates two astrologies. Feed the same birth moment to a tropical chart and a sidereal one and the planets land about 24 degrees apart, which is usually a whole sign.
Ayanamsa is the angular distance between the tropical zodiac, measured from the spring equinox, and the sidereal zodiac, measured from the fixed stars. The gap exists because the equinox drifts westward against the stars at roughly 50 arc seconds per year, a full degree about every 72 years, completing a circuit in some 26,000 years. Around the early centuries CE the two frames nearly coincided; by 2026 they differ by approximately 23 degrees 50 minutes. Because the drift is continuous, an ayanamsa is a formula rather than a constant, and several competing definitions exist: Lahiri, Raman, Krishnamurti and Fagan-Bradley among them. Lahiri, which fixes the star Spica near 180 degrees sidereal, is the standard in Indian practice and the only value Astrolium's Jyotish modules use.
Why the gap exists
The Earth's axis wobbles like a slowing top, one full turn in roughly 26,000 years. The effect, called precession of the equinoxes, moves the equinox point backward along the ecliptic at about 50.3 arc seconds per year.
The tropical zodiac is pinned to that moving point: 0 degrees Aries is wherever the Sun stands at the March equinox, by definition, in any year. The sidereal zodiac is pinned to the stars instead. Two frames, one of them drifting, and the gap between them is the ayanamsa.
When Hellenistic and early Indian astrology were being written down, the gap was near zero, which is why the two traditions can use the same sign names and still mean different arcs today. Anyone with a tropical Sun in the first three weeks of a sign will usually find a sidereal Sun in the sign before it.
Competing values
There is no single correct ayanamsa, because there is no agreed marker for where the sidereal zodiac should start. Each definition picks a different anchor.
- Lahiri, also called Chitrapaksha, places the star Spica (Chitra) at or near 180 degrees of sidereal longitude. The Indian Calendar Reform Committee adopted it in 1955 for the national almanac, and it dominates modern Jyotish.
- Raman and Krishnamurti differ from Lahiri by small amounts, the Krishnamurti value by a matter of arc minutes, and each belongs to a specific teaching lineage.
- Fagan-Bradley is the Western sidereal value, anchored on a different star convention, and differs from Lahiri by roughly 0.9 degrees.
The practical consequence is small for most placements and decisive at the edges. A planet within a degree of a sign boundary, or a Moon near a nakshatra boundary, can change sign, mansion and therefore dasha lord depending on which value the software used. Charts should carry their ayanamsa the way they carry their house system.
In Astrolium
Astrolium's Jyotish workspace is Lahiri only. There is no selector, and the chart frame is stamped on the PDF so the reader knows the convention behind the placements. Nodes are mean, houses are whole sign, and those three choices are stated rather than buried. The separate Western sidereal setting is where other ayanamsa values live for practitioners working in the Fagan-Bradley lineage.
The free Vedic birth chart calculator also runs Lahiri, which is why its output matches the workspace for the same birth data. For how the offset reshapes signs and mansions, see rashi and the Vedic astrology guide. For the star-based work that precession affects in a different way, see the fixed stars calculator.