Every Vedic chart begins with one correction, the ayanamsa. It converts planetary positions from the tropical zodiac, which is tied to the seasons, to the sidereal zodiac, which is tied to the stars. Get it wrong and every sign, nakshatra and dasha date shifts with it.
Why a correction is needed
Astronomers measure positions from the spring equinox. But the Earth's axis wobbles like a slowing top, completing one cycle in roughly 25,800 years. As a result the equinox drifts backwards along the zodiac by about 50 seconds of arc a year, roughly one degree every 72 years. The ayanamsa is the size of that drift since the two zodiacs coincided. Today it is a little over 24 degrees. See Vedic vs Western astrology for what this means for your sign.
Lahiri (Chitrapaksha): the Indian standard
In 1955 the Government of India's Calendar Reform Committee, chaired by the astrophysicist Meghnad Saha, adopted the ayanamsa proposed by N. C. Lahiri. It fixes the star Chitra (Spica) at 0° Libra, hence the name Chitrapaksha. It is the standard for Indian almanacs (panchangs) and for most Vedic astrologers, and it is the one we use.
Other ayanamsas
- KP (Krishnamurti) ayanamsa differs from Lahiri by only a few minutes of arc. It is used in KP astrology, where the precise sub lord of a cusp matters.
- Raman ayanamsa, popularised by B. V. Raman, is about 1.5° smaller than Lahiri.
- Others, such as Yukteshwar and Fagan-Bradley (used in Western sidereal astrology), differ from Lahiri by up to about a degree.
How much difference does it make?
A degree or so changes little for most planets. But it can:
- move a planet near the edge of a sign into the next sign;
- change the nakshatra or pada of the Moon, which shifts every dasha date by months;
- change the Navamsa of planets near a boundary.
That is why it matters to know which ayanamsa your astrologer uses, and to compare charts calculated the same way.
Our charts use the Lahiri (Chitrapaksha) ayanamsa with NASA JPL planetary positions; the KP section uses the KP ayanamsa. Calculate your free birth chart.