A topic which brings much doubt and tests the faith of newcomers in Jyotish is the topic of Ayanamsa. Where our western counterparts who use the tropical zodiac aren’t affected by this problem, those who use the sidereal zodiac, or a zodiac based on star positions are cast into a deep dilemma as to which ayanamsa to use. Where western sidereal astrologers are almost forced to take the plunge into learning the intricacies of the ayanamsa and thereby choose the ayanamsa they like the best, vedic astrologers are presented with a peculiar situation where faith tends to be the compelling factor in choosing ayanamsa.
This writeup doesn’t claim to ascertain a correct ayanamsa, but tries to shed light on the many different facets of the ayanamsa, and some possibilities within the same.
i. What is ayanamsa
To explain ayanamsa it becomes necessary to describe two different zodiacs, the Niryana chakra and the Sayana chakra. These are two different zodiacs in a sense.
The Niryana is based on the stars and the Sayana is based on the equinoctial points, i.e. the seasons. The Niryana or star-based zodiac is moving in comparison to the Sayana chakra, which is seen by the stars being in different positions during the same time every year. This difference between Niryana and Sayana chakra is called Ayanamsa.
ii. Two different theories on precession
In an attempt to explain the cause of the ayanamsa two major theories have arisen: luni-solar theory and what some are calling the binary theory.
The luni-solar theory was initially referred to by Copernicus and later Newton, and has been modified several times ever since. The theory is based on the idea that the reason the stars are changing with reference to the earth, is because earths angle to them is changing. The reason for the many modifications of the theory, is because its based on the idea that the Sun and Moons gravity is the cause of the earths changing angle. Here whilst scientists could prove that the Sun and Moon did indeed affect the earths angle, it wasn’t enough to explain the change in ayanamsa. Further the theory was based on a number of calculation principles which didn’t deal well with predicting future positions of the ayanamsa.
Here we come to the binary theory. This theory is much closer to home and coincides with some of the Vedic concepts of the universe, namely that the Sun is moving around another point, i.e. Dhruva or some other fixed point. See, the Moon revolves around the Earth in about 27 days. This is a lunar return. Yet, a full moon occurs only after ~29 days. So the Moon has to spend 2 days extra catching up with the Sun to complete the full moon phase.
Similarly, the earth revolves around the Sun in ~365 days. Yet, based on the binary theory, the Sun is simultaneously moving around another point/binary star, and therefore the earth, just like the moon, must spend more time to catch up with the star-placements in the heavens.
Because the earth has to spend more time moving forwards around the Sun it appears as if the stars are moving backwards every year.
iii. Rate of precession
This brings us to the entire predicament of the ayanamsa… the rate at which it is moving! The rate at which its moving is called precession. The ayanamsa moves based on the star positions with regards to the equinoctial points, and it has been found that the precession speed is changing. Where it initially was 46 seconds of arc (”) in 150 AD , it is now ~50 seconds of ark and increasing. Therefore for a complete cycle around the zodiac, it would take about 25 thousand years with present information. Yet with data only spanning over a period of 2000 years, we only have a 1/12th fraction of the knowledge of the zodiac movement.
These days most makers of astrological software do not take into account the change in precession rate, and calculate a fixed rate of precession from a given date. This is a completely wrong approach.
Some intelligent programmers have instead based their ayanamsa calculations on actual ephemeris data of star positions. Though this is the best approach, it is limited to empirical/observed data. Therefore the goal of astronomers and astrologers today is to define and make a model which can imitate the precession.
iv. Clues from the seers
We are not left without clues from the seers of the vedas. According to Surya Siddhanta, the circumference of the nakshatras wherein the stars exist are sixty times larger than the circumference of the earths movement around the Sun. By this is implied that it takes sixty times longer for the Sun to progress one degree, than it takes the earth. Therefore it takes 60 years before the Sun has progressed one degree in the stars, and 120 years before it progresses two degrees. This makes the average rate of precession of the ayanamsa one minute per year. This is ten arc seconds faster than the current precession (50″).
Assuming this data to be accurate, it would give a cycle lasting 21600 years before the Sayana and Niryana chakras aligned again.
v. Ayanamsa choices
Today the most common choice among the vedic astrologers is the Lahiri ayanamsa. The reason being that the ayanamsa was selected by a committee after intense scrutiny. The Lahiri ayanamsa is based on the Chitra Paksha Ayanamsa. There are two main ayanamsas given in the vedic scripture namely Revati-paksha and Chitra-paksha. The word paksha refers to 180 degrees or an exact opposite position from a fixed star. Here Revati and Chitra do not refer to a large constellation of nakshatras spanning ~13 degrees, but to the original stars which make up the base of the nakshatra positions. The star Chitra corresponds to the star Spica, whilst Revati corresponds to the star Zeta Piscium. 180 degrees opposite the star Chitra is said to be 0 degrees of Aries, whilst 180 degrees opposite Revati is 0 degrees of Libra.
Today the tropical degree of Spica is 23 degrees 57 minutes 2.03 arc seconds. These degrees are the actual ayanamsa at present. This corresponds to the sidereal degree of 0 degrees Libra and opposite this is the sidereal 0 degrees of Aries according to the Chitra Paksha/Lahiri Ayanamsa. The reason for choosing stars at the beginning of these two points is that they correspond to the Deva Ayana, i.e. the day of the Deva begins in Aries, and ends in Libra, where the day of the Asuras begin. Preferably any ayanamsa which depicts the movement of the stars should be based on these starting/ending points as according to the Rg Veda, the stars are the abode of the Deva.
There also exists the Raman Ayanamsa and the Krishnamurty Paddhati Ayanamsa. Where the former was not openly announced by its author, the latter is very close to Lahiri and could be an approximation made to compensate for the inaccuracies in the Lahiri ayanamsa at the given time. This is because astrologers had relied on a mean motion of the ayanamsa which caused inaccuracies in the calculation with time. One more ayanamsa called Yuktesvaras ayanamsa exists but it wasn’t used to predict planetary positions but to justify yuga calculations, hence its unclear as to whether the author of it actually meant to use it for any other purpose.
What is common for these three mentioned ayanamsas is that their authors haven’t linked them to any fixed-star positions.
vi. How to choose?
Unfortunately, the astronomical backing is lacking in many of the ‘new’ ayanamsas today, and mainly because astrologers are too quick to ignore the star positions before ascribing to an ayanamsa. And if it isn’t the star position, then the precession rate itself doesn’t correspond to the actual precession of the stars. This leaves ‘new’ ayanamsas as being imaginary or arbitrary positions in space.
The fixed stars are the basis of the Niryana chakra, without which there is no basis for fixing the zodiacs starting points. Therefore good advise to beginners on choosing ayanamsa is to analyze these following points:
1. Stars initiating the ayanamsa.
2. Actual precession rate of the stars.
3. The signs which initiate the ayanamsa.
So far the only software today, to this authors knowledge, which depicts the Ayanamsa based on actual star positions is Jagannath Hora, by PVR Narasimha Rao. The software uses an ephemeris with the actual star positions to fix the ayanamsa. Yet, it is this authors hope that one day astrologers will be able to calculate the ayanamsa for any given time using the knowledge given to us by the seers.
 Hipparchos and Ptolemys observations.
 Source from Binary Research Institute (http://www.binaryresearchinstitute.org)
 Surya Siddhanta, Chapter XII, sloka 80.