What is ayanamsa? Lahiri and Chitra Paksha explained

Planets are measured in degrees from the start of the zodiac, from 0 to 360. Many Western astrologers use the sayana zodiac, the tropical or 'moving' zodiac, while Vedic astrology uses the nirayana zodiac, the sidereal or 'fixed' zodiac.

The difference exists because of precession, a slow wobble of the earth's axis. Because of it, the starting point of the tropical zodiac drifts slowly against the fixed stars.

P.V.R. Narasimha Rao (PVR), author of Vedic Astrology: An Integrated Approach, offers an analogy.

The tropical zodiac is like measuring the positions of trees and buildings while sitting in a slowly moving bus. The sidereal zodiac takes the bus's motion into account and adjusts for it.

The difference between the two zodiacs is called the ayanamsa, which means 'sidereal difference'.

It varies with time. Precession changes it every year, so the gap between the two zodiacs grows slowly.

PVR and B.V. Raman, author of Hindu Predictive Astrology, give the same rule. A modern ephemeris, a table of planet positions, gives tropical positions; to convert them, the ayanamsa is subtracted.

Raman's wording: subtract the ayanamsa for the year of birth from the Western positions to get the sidereal chart.

The ayanamsa is not a fixed number. Raman notes that it grows by roughly 50 and one-third seconds of arc a year.

In practice, the Vedic sign of a planet or of the ascendant (lagna), the sign rising on the eastern horizon at birth, is often one sign earlier than the Western one. But not always: a position more than about 24 degrees into a tropical sign stays in the same sign.

The two worked examples show both cases. A planet at 10 degrees tropical Leo sits at about 16 degrees sidereal Cancer, one sign earlier. A planet at 28 degrees tropical Leo stays in Leo, at about 4 degrees sidereal Leo.

PVR writes: 'There are many opinions on the correct value of ayanamsa, because nobody knows exactly which star is supposed to be the starting point of the real zodiac.'

Raman gives a similar picture. When exactly the two zodiacs coincided 'is doubted by a number of astronomers', so the ayanamsa used by different authorities varied 'from 19 to 23 degrees' in his day.

Chitra Paksha and Lahiri are two names for the same family of ayanamsa. It is anchored to the star Chitra (Spica). PVR uses it in his book, 'which is accepted by most Vedic astrologers of India'.

Other texts differ. Raman's own book gives a separate simple method for calculating the ayanamsa, so his figures do not match Lahiri exactly. This is one example of the 'many opinions'.

PVR, in his chapter on rational thinking, is candid about the field: astrologers 'don't agree even on the basics, such as the zodiac to be used and the ayanamsa.'

The choice of ayanamsa shifts every position by the same amount, so PVR says it 'matters only to a small extent' in one sense.

He adds a warning, though. A crude straight-line formula for the ayanamsa can cause serious errors in sensitive calculations, so the correct non-linear formula of whichever ayanamsa you choose should be used.

Different ayanamsas differ by small amounts. When a planet or the ascendant sits within about a degree of a sign boundary, the ayanamsa chosen can decide which sign it lands in.

Skybond uses the True Chitra Paksha ayanamsa, which keeps the star Chitra (Spica) as its reference point.

Because positions near a sign boundary are sensitive to the ayanamsa chosen, the Skybond calculator warns you when the ascendant is within a degree of the next sign.