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Ayanamsa is the sanskrit term for the longitudinal difference between the tropical or Sayana and sidereal or Nirayana zodiacs. It is defined as the angle by which the sidereal ecliptic longitude of a celestial body is less than its tropical ecliptic longitude.
- 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 50".29 per year 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.
Today's astronomical calculations always use tropical longitudes, but certain schools of astrology, notably the Vedic school of astrology use sidereal longitude. Hence, when the proponents 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 ayanamsha.
From the Hindu word ayana which means the arc it translates literally as "yearly degree" and more precisely as precession. It 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°.
Western Astrologers Fagan and Bradley computed it at 24 degrees in 1950 but there are various values in use in India. Differences arise because of the indefinite ancient boundaries of the constellation of Aries.
Ayanamsha is a Sanskrit word and is to be pronounced with the third "a" long, so: "Ayanaamsha". It is a compound word composed of the words "ayana" and "amsha" where "ayana" means "precession" and "amsha" means "component".