Ayanamsa

Ayanamsa (Sanskrit ayanāṃśa: ayana "movement" + aṃśa "component"), also ayanabhāga (Sk. bhāga "portion"), is the Sanskrit term in Indian astronomy for the amount of precession. In astrology, this is the longitudinal difference between the Tropical (Sāyana) and Sidereal (Nirayana) zodiacs.

The above is a modern definition of ayanamsha, based on arguments of Colebrooke, Burgess, etc. But ancient definition of ayanamsha had no relation with precession of equinoxes. Suryasiddhanta (iii, 9-10) defines ayanamsha as the to and fro motion of the circle of asterisms (Nakshatra-chakra or Bhachakra) within a maximum range of + and - 27 degrees at an annual rate of 54". Burgess could not digest the idea of trepidating Nakshatra-chakra, and assumed that some error had creeped in the text. On the basis of this assumption, he advocated the use of precession of equinoxes to define ayanamsha, following the arguments of his predecessors like Colebrooke. They assumed that ancient Indians did not know how to measure precession accurately and therefore invented a wrong concept of trepidating precession. But Bhaskar-ii in Siddhanta Shiromani gives equationjs for measurement of precession of equinoxes, and says his equations are based on some lost equations of Suryasiddhanta plus the equation of Munjaala.

Ancient concepts

In the chapter "Direction, Place and Time" (Suryasiddhānta, Ch.iii), E. Burgess writes:

“ The (Surya Siddhāntic) theory which the passage (verses 9-12), in its present form, is actually intended to put forth is as follows : the vernal equinox librates westward and eastward from the fixed point, war Piscium, assumed as the commencement of the sidereal sphere— the limits of the libratory movement being 27 degrees in either direction from that point, and the time of a complete revolution of libration being the six-hundredth part of the period called the Great Age (ie, Mahāyuga as defined by Burgess in chapter i,15-17, where he gave it a span of 4320000 years), or 7200 years; so that the annual rate of motion of the equinox is 54″. ”

This is the interpretation of existing version of Surya Siddhānta (त्रिंशत्कृत्यो युगे भानां चक्रे प्राक् परिलम्बते ..., SS,iii.9) in the words of E. Burgess, "as it is actually intended to put forth" by all traditional commentators.

The moot point is this: Burgess knew the traditional interpretation (भानां चक्रे.., i.e. pendulum-like motion of nakshatra orbit itself) , but gave his own meaning based upon modern concept of precession of equinoxes, and tried to create doubts about the authenticity of these verses (iii, 9-12) by putting forth deliberately false arguments. Let us examine Burgess.

Overview

Ayanamsa is now defined as the angle by which the sidereal ecliptic longitude of a celestial body is less than its tropical ecliptic longitude. Ayanamsa is mostly assumed to be close to be 24° today, according to N. C. Lahiri[who?] 23.85° as of 2000. This value would correspond to a coincidence of the sidereal with the tropical zodiac in or near the year 293 AD, roughly compatible with the assumption that the tradition of the tropical zodiac as current in Western astrology was fixed by Ptolemy in the 3rd century.

The sidereal ecliptic longitude of a celestial body is its longitude on the ecliptic defined with respect to the "fixed" stars.

The tropical ecliptic longitude of a celestial body is its longitude on the ecliptic defined with respect to the vernal equinox point.

Since the vernal equinox point precesses westwards at a rate of about 50".29 per year (the rate has been accelerating) with respect to the fixed stars, the longitude of a fixed body defined with respect to it will increase slowly. On the other hand, since the stars "do not move" (this ignores the effect of proper motion) the longitude of a fixed body defined with respect to them will never change.

Traditional Vedic astrology (Jyotisha) uses a system of sidereal longitude. When the practitioners of these schools of astrology use modern astronomical calculations to determine the position of celestial bodies, they need to take into account the difference caused by the different reference point used in specifying the longitude, and this they call the ayanamsa.