Ayanamsa calculation is of high importance for planetary positions, divisional charts, Dasas, transits, etc. Switching the Ayanamsa can result in major changes in Dasa balance (e.g. for quite sensitive Kalachakra Dasa) or high level Varga charts like Shastiamsa (D-60).
If we calculate the same chart e.g. with Lahiri and Raman Ayanamsa, we can get a difference in Vimsottari Dasa balance of 2 years, Kalachakra Dasa will be completely different, and Shastiamsa charts will have no common planetary positions.
There are different opinions about the correct Ayanamsa. The main stream versions of Ayanamsa in Vedic astrology differ about 1:30 degrees. This section exposes the most important versions of Ayanamsa, compares them and gives examples for the effects on Dasa balance.
Ayanamsa is the difference between tropical and sidereal zodiac. The actual value is about 23:56, i.e. this value must be deducted from the tropical length of each planet to get the sidereal length.
This kind of zodiac is called tropical or in sanskrit sayana. Tropical zodiac plays only a small role in Vedic astrology; 99% of calculations depend on the sidereal zodiac. This kind of zodiac has the reference point in the stars. The point 0 Aries is interpreted as the beginning of the sign Aries (or the Nakshatra Aswini) in the sky.
Western astrology, as well as astronomical ephemeris, use the tropical zodiac. This kind of calculation sets the point 0 Aries (vernal point) to the position of the Sun at the time of spring equinox.
Because of the movement in earth’s precession, the starting point of the tropical zodiac changes slowly (with respect to fixed stars). Tropical (sayana) zodiac is analogous to measuring the positions of trees and buildings sitting in a slowly moving bus. Sidereal (nirayana) zodiac, on the other hand, considers a fixed zodiac. It considers the motion of the tropical zodiac (bus in our analogy) and makes an adjustment accordingly. We use the sidereal zodiac in Vedic astrology. The difference between the tropical zodiac and the sidereal zodiac is of great importance. What modern ephemeris gives us is the tropical positions of planets. To convert these positions, we have to subtract the difference between the two zodiacs. This difference varies with time.
As earth’s precession changes every year, the difference between the tropical zodiac and the sidereal zodiac changes. This difference is called “ayanamsa” (sidereal difference). 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.