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Ayanamsha

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Ayanamsha is a term used in horoscopic astrology.

It is defined as the angle by which the sidereal ecliptic longitude of a celestial body is lesser 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 point.

Since the vernal equinox point precesses westwards at a variable 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 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 tradition of astrology and the smaller much more reccent tradtion of western sidreal 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.

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".

Different methods of determining the ayanamsha

Now determining the ayanamsha is a matter of determining the sidereal longitude and tropical longitude of any given body. This can of course be simplified to calculating the angular distance between the respective zero points of the sidereal and tropical reference frame. Since the zero point of the tropical reference frame is the vernal equinox point as said above, we only need to find out the position of the zero point of the sidereal reference frame. And this is where we meet with difficulties.

The difficulties are of the nature of varying definitions for the zero point of the sidereal frame. Since astrological traditions define twelve rashis or zodiacal signs as the sidereal divisions of the ecliptic, and the first of them is Aries, the zero point of the sidereal frame is the same as what is called the "first point of Aries".

Today, the vernal equinox point itself is called the "first point of Aries" but this is only because in ancient times the vernal equinox point used to be at the boundary of the constellations Pisces and Aries. The name "first point of Aries" is more properly applied to the point which is considered to actually lie on the boundary of the constellation of Aries, and opinions vary as to where exact this boundary of Aries on the ecliptic lies.

One school, the most prevalent one in Vedic astrology, says that the fixed star Spica (Chitraa in Sanskrit) is to be fixed at 180° sidereal longitude, its diametrically opposite longitude thus functioning as the zero point and the first point of Aries. Nicolas Copernicus opined that the star Gamma Arietis or Mesarthim is to be taken as lying on the zero longitude line.

Other such traditions exist, and thus any astrological calculation must specify the particular ayanamsha that was used, whether the ayanamsha determined based on Chitraa, Mesarthim, or any other clearly definable or inferable point on the ecliptic.