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. 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).

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 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.

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 Sankranti), 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.

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