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Ayanaansh is an angular arc between two points namely initial point of sidereal Aries (Ashwinee) and the first point of tropical Aries.

First point of tropical Aries is called VE Ppoint. This is the point of intersection of ecliptic and equator of date. Incidentally both these planes are continuously vibrating due to gravitational and other forces exerted on Earth mainly by Moon and Sun. Thus mean position of equinoxes go on varying slightly. This is corrected by addition or subtraction  of Nutation of the day. Thus :---

The True position of equinox = Mean position of equinox - or + value of nutation of the day.

Modern Astronomers calculate Nutation and Aberration based on new constants and there is a big advancement in this direction. This is done based on 1979 IAU Theory of Nutation.
(You may refer 'Indian Astronomical Ephemeries' which is published every year by Indian Metrological Department).

The second question arises as to how fix the initial point of sidereal Aries. This has to be fixed with the help of some fixed star. The followers of Chitraa Paksha Ayanaansh recommend that Star Chitraa (Spica 16) should act as reference point and its sidereal TRUE longitude should be assumed as 180 deg on ecliptic. The following aspects are to be shorted out:--

(1) Every star has its proper motion. The longitude of Spica along the ecliptic has diminished by 60 sec during the period 1724 years from VE DAY falling in the year 285 AD.

(2) The latest Sixth Fundamental Catalogue (FK6) indicates Mean tropical positions of the stars. On the other hand ,the apparent geocentric longitudes and latitudes are referred to the true equinox and ecliptic of date and corrected for planetary aberration.

(3)Obuously, first point of Aries is to be fixed after taking into account the displacement in star position due to above factors. So True position of the star = Mean position of star -/ + Nutation of the day = 180 deg.

Needless to say that modern astronomy has already measured these values, only astrologers are required to adopt them properly.
Precession rate in longitude per tropical year = 50".29   and
                                         per sidereal year = 50".26

Lahiri never called the Ayanaansh used in his Ephemeris as Lahiri Ayanaansh. As his ephemeris had become very popular, astrologer community started calling it Lahiri's Ayanaansh for the sake of their convenience. Basically it was Chitraa Paksh Ayanaansh.

Let us define Ayanaansh :
Sidereal Zodiac commences from a fixed point on ecliptic, from where the initial point of Ashwinee (Mathematical Ashwinee Naksahtra of 13⁰ 20' for astrological purposes) commences. From this initial point sidereal zodiac sign Aries also commences. This initial point always remain opposite to Star Chitraa (Spica 13).

This is how initial point of Indian sidereal astrology is defined.
When Apollo flight was launched, the catalog positions of star were not matching with physical position of stars. The equations were revised and new star catalogs were introduced which give more precise positions of stars.
VE Point is receding in the back ground of stars with the mean motion of 50".30 per year. Tropical Aries commences from this point and all longitudes of stars are measured with respect to this point.

Thus the definition of Ayanaansh is simple and as under:
Ayanaansh of any epoch = Tropical longitude of Star Spica 13 (Chitraa - which is placed in the middle of Lunar Nakshatra Chitraa even
according to "SSS") of same epoch - 180 degree"

Epoch means -
(1) an arbitrarily fixed instant of time or date, usually the beginning of a century or half century, used as a reference in giving the elements of a planetary orbit or the like.
(2) the mean longitude of a planet as seen from the Sun at such an instant or date.

The initial point will therefore be self adjusting according to proper motion of this star. Star Spica was in the constellation of Virgo along with Saturn and Mars at the time of Vehicle on Mars on Aug 6, 2012, according to the announcement of NASA I am referring to same Spica 13 (Chitraa).

Let accept this definition and 90% Panchaang makers in India had accepted this definition. They follow Chitra Paksh Ayanaansh

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