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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. In astronomy too, this is the difference between the length of a tropical year (365.2422 rotations of the earth) and a sidereal year (365.2563 rotations) required to complete one orbit relative to the sun (tropical) or stars (sidereal).
The above is a modern definition of ayanamsha, based on arguments of Colebrooke, Burgess, etc. The ancient definition of ayanamsha was not called the precession of equinoxes but in measuring the length of the tropical year versus the sidereal year the ancients were essentially tracking the precession observable. 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" (versus the modern value of 50.3"). Burgess could not digest the idea of trepidating Nakshatra-chakra, and assumed that some error had crept 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 equations for measurement of precession of equinoxes, and says his equations are based on some lost equations of Suryasiddhanta plus the equation of Munjaala.
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 late N. C. Lahiri 23.85° as of 2000. This value would correspond to a coincidence of the sidereal with the tropical zodiac in or near the year 285 AD, roughly compatible with the assumption that the tradition of the tropical zodiac as current in Western astrology was fixed by Ptolemy in the 2nd century (Ptolemy actually did not fix it, he only stated indirectly that tropical and sidereal zodiacs coincided during his times ; the credit of discovering precession in Europe should go to Hipparchus and not to Ptolemy).
To be precise, the so-called "Lahiri Ayanamsha" is a misnomer because N. C. Lahiri borrowed this Chitra-pakshiya Ayanamsha from its inventors Ketkar Brothers who propounded this idea three decades before him, and Lahiri never claimed any credit. But he popularized it due to his influence on Pt Jawaharlal Nehru who allowed Lahiri's ideas to dominate in reforming national calendar of India. According to this theory, the sidereal position of Spica (alpha-Virginis, assumed to be the ancient Chitra) should be exactly 180 degrees as stated in Suryasiddhaanta, while both sidereal and tropical zodiacs should coincide at the time of zero ayanamsha. Although Suryasiddhaanta and other ancient texts state that ayanamsha was zero in 499 AD (Mesha Samkranti), N C Lahiri insisted on Spica's identification as Chitra and concluded that Spica was the nearest bright star adjacent to 180 degrees, hence resting on Spica he concluded that tropical position of Spica being zero in 285 AD must be the zero point of Ayanamsha too.
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.
Some orthodox schools of Vedic astrology reject modern astronomy and still base their computations upon traditional texts and treatises, mostly following the Surya Siddhanta or treatises based on it. They use ayanāmsa according to Surya Siddhānta, in which ayanāmsa rises from 0° to +27° during 1800 years, then decreases to 0° and further to -27°, thereafter rising again, thus oscillating within a rage of ±27° instead of cyclically moving in a circle as modern concept of ayanāmsa suggests.
Manjula advocated a cyclical concept of ayanāmsa, but it could not gain currency among almanac makers. In West Theon (ca. 4th century AD) was the earliest known advocate of Surya Siddhāntic type of ayanāmsa (although Theon said trepidation varied within a rage of ±8° only : Surya Siddhāntic trepidation was deduced by multiplying 90° with 0.3, Theon multiplied 27° again with 0.3 to get 8° ). This oscillating type of ayanāmsa, known as trepidation, was a favourite of Indian, Arab and European astrologers and astronomers till the time of Copernicus. Modern science does not support the idea of trepidation or oscillating ayanāmsa. 490 AD is regarded as the zero date of this type of ayanāmsa according to Surya Siddhānta, Aryabhatiya and other ancient treatises. Thus the present value of traditional ayanāmsa is nearly +22.64°, which is less than modern the value of about +24°.
After 2299 AD, the traditional ayanāmsa will start decreasing from the maximum value of +27°, while modern value will keep on increasing. Equations of sunrise and ascendant (lagna) need accurate value of ayanāmsa, upon which all important components of religious almanac and horoscopes are based in India.
The ayanamsha describes the increasing gap between the tropical and sidereal zodiacs. The ayanamsa, changes continually through the Precession of the Equinoxes at the rate of approximately 50" a year, is currently about 24° (Lahiri). But traditionalists in India put the current value at about 22.6 degrees which is the value according to Suryasiddhanta and Raman's ayanamsha approximates it while Yukteshwar ayanamsha is almost exactly equal to Suryasiddhantic ayanamsha.
Western Astrologers Fagan and Bradley computed it at 24 degrees in 1950; however, there are various values in use in India. While the general consensus among Western siderealists is that the star Alcyon represents the first point of Aries, differences arise because of the indefinite ancient boundaries of the constellation of Aries. Indian definition of astrological signs is not based on constellations but on equal angular division of sky, which makes it difficult to define signs in terms of stars and constellations. This is the source of controversy about ayanamsha.