The Question of Ayanamsha
Unlike Western astrology that is based on the Tropical zodiac, the Vedic system of astrology (jyotish) employs the Sidereal (or true) position of the planets. The difference between the two, due to the precession of equinox, is called the "Ayanamsha".
It's a natural response for those not familiar with either system to shake their head in disbelief and refuse to accept the fact that their Cancer sun sign (for instance) is now a Gemini sun sign. They protest that there can only be one zodiac, and of course they're correct ~ there is only one zodiac!
However the apparent position of the planets (including Sun) in relation to the constellations is subject to this precession of the equinox which, basically, is the earth's slight backward movement through the constellations (aprox. 50.25 seconds per year) as it wobbles on its axis.
There are several different Ayanamsha in use today but the one most widely used by traditional astrologers is the Lahiri ayanamsha which is sanctioned by the Indian government. This puts the degree of precession for (Oct) 2002 at approximately 23:53:26.
One of the greatest controversy's within the jyotish community is the question of which is the correct Ayanamsha. Many well-known and knowledgeable astrologers use different Ayanamsha values and all swear by their results. I believe the main reason for this is the tendency to correct the Ayanamsha rather than the T.O.B. which is very rarely 100% reliable ~ even with hospital records!
The date of coincidence of the Tropical and the Sidereal zodiacs was found to be 285 AD and the Ayanamsa (precessional distance) was called Lahiri's Ayanamsa. Different scholars gave different dates but the Indian Govt in order to standardise the Ayanamsa value took the initial point of the zodiac as the point in the ecliptic opposite the star Chitra (Alpha Virginis) which was the vernal equinoctial point on the Vernal Equinox day of 285 AD. The rate of precession of the Vernal Equinox was taken as 50.3 seconds per sidereal year.
In Vedic Astrology, Ayanamsa value is deducted from the Tropical longitudes of planets to get the Sidereal longitudes. Hence the longitudes of planets in Western astrology (Tropical) and Vedic astrology differ by (aprox) 50.25 seconds per year. Obviously Natal charts cast in these 2 systems will not be the same!
The Tropicalists (Western astrologers) maintain that the Vernal Point is presently in the 6th degree of Pisces whilst Siderealists (Vedic astrologers) maintain that the Sidereal starting point remains to be 0 degrees Beta Arietis (Aswini) and that the Sidereal zodiac doesn't change ~ What IS changing is simply the degree of precession.
The important thing to remember is that the majority of traditional jyotish pundits use Lahiri, the ayanamsha that has been sanctioned by the Calendars Reforms Committee (Indian government) since 1954. Lahiri being the name of one of the major signatories.
It's an undeniable fact that the intellect will invariably find a way to fit a square peg into a round hole. If the peg has to be altered to make it fit, then so be it ~ and so it is with the Ayanamsha! If the events of life don't fit, usually due to wrong birth details or the inexperience of the astrologer, then why not change the Ayanamsha to make it fit.
Lahiri is the Ayanamsha I use, it's approved by the government of India and is the preferred Ayanamsha for the majority of traditional astrologers.