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.
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.
But all orthodox schools of Vedic astrology reject modern astronomy and still base their computations upon traditional texts and treatises, mostly following Surya Siddhānta or treatised based on it. Some of them do it out of habit, while others argue that modern astronomy deals with material objects while Vedic astrology deals with divine agencies which are coindentally called planets but are quite distinct from material planets. They use ayanāmsha according to Surya Siddhānta, in which ayanāmsha 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 ayanāmsha suggests. Ancient Indian astronomer Manjula advocated cyclical concept of ayanāmsha, but it could not gain currency among almanac makers. In West Theon (cir. 4th century AD) was the earliest known advocate of Surya Siddhāntic type of ayanāmsha (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 ayanāmsha, 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 ayanāmsha. 499 AD is regarded as the zero date of this type of ayanāmsha according to Surya Siddhānta, Aryabhatiya and other ancient treatises. Thus present value of traditional ayanāmsha is nearly +22.64°, which is less than modern value of about +24°. After 2299 AD, traditional ayanāmsha 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 ayanāmsha, upon which all important components of religious almanac and horoscopes are based in India.
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. While the general consensus is that the star Alcyon represents the first point of Aries, 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".