Typically today for all Jyotish calculation a correction is added to Sayana positions of planets to arrive at the Nirayana position of planets to account for precession etc. This correction is called Ayanamsa. Now as on date there are many types of Ayanamsa being used in India. Is there any mention of the word Ayanamsa in any of the ancient texts or is it an invention of modern times having lost the ancient method of calculations?
Astrologer P.V.R Narasimha Rao has given some details on Ayanamsa in his book Vedic Astrology: An Integrated Approach.
As earth’s precession changes every year, the difference between the tropical zodiac and the sidereal zodiac changes. This difference is called “ayanamsa” (sidereal difference). There are many opinions on the correct value of ayanamsa, because nobody knows exactly which star is supposed to be the starting point of the real zodiac.
Precession was known to ancient astrologers as well and the concept of Ayanamsa existed before as well which is why it is in use but details on some topics mentioned is not exactly straight to the point which opens the door for interpretation by different astrologers which creates issues in correct calculation.
Astrologers don’t agree even on the basics – such as the zodiac to be used and the ayanamsa. There are just too many things that are ambiguous and not completely clarified in classics. To complicate things further, some modern savants are making their own ill-considered departures from the teachings of Sages and foolishly popularizing them. That only adds to the corruption of this knowledge.
Interestingly, there are many different Ayanamshas today. But only one of the Ayanamshas is based at once on all Indian ancient astronomical Siddhantas, without contradicting them — Pushya Paksha Ayanamsa.