The concept of ayanāmśa leads to a lot of unnecessary confusion and heart-burn among people with differing views.
And as with everything else related to Ancient Indian knowledge, there are dozens of differing views. I have one too.
The root cause of this issue is that nothing stays in the same place for all time. So nothing is fixed. However, somethings move slower than others, or move slow enough to be treated as fixed for the purpose of the application.
Indian Geo-Centric Astronomy is used today for Archaeo-Astronomy (using star positions to find out when something happened) by historians and for predictions by the Hindu astrologers. I am interested in both. There may be other applications that I am not aware of at this moment.
4 kinds of new years are celebrated by humans. The winter solstice, the spring equinox, the summer solstice and the autumn equinox. The new year that westernised humans celebrate is a time-shifted winter solstice. So is Christmas. telugu, kannaDa and marAThI hindus celebrate the new moon near the spring equinox as their new year. North Indians celebrate the full moon near the spring equinox as their new year. malayALIs and gujarAtIs celebrate the autumn equinox based on the the śravaNa nakSatra and new moon near the autumn equinox respectively. The Egyptian New Year was connected to the Summer Solstice.
If all you care about is the seasons and full moons and new moons, you are all set.
If you would like to use the stars are a guide for navigation on land or sea for example, then you would like to know how they move. Though I suspect, humans were curious first and found uses later.
Astronomical cultures are all about rising before dawn. Since you can’t see stars during the day, it is in the pre-dawn period that you can see which stars are rising in the east and setting in the west. This is the brāhmi muhurta. My view is that the earliest observations were of real stars and constellations and not of imaginary lines in the sky. So the bright stars and constellations were observed. And it was duly noted that during the course of a year, different stars rose in the eastern pre-dawn sky.
(My thinking is that this was a kRta yuga activity, because some vEda mantrA: of the trEta yuga already start treating nakSatrA: as an area of the sky).
If you just look at the Sun on an equinox day and at nothing else, both equinoxes look the same. But the pre-dawn stars in the east will be different. Using 27 stars and 4 quarters per star, you had 108 nakSatra pAdA: or 108 star-steps. The Sun covers nearly 108 star-steps between two winter solstices.
The word nearly is what causes the problem. Each winter solstice, the Sun has fallen behind it’s previous position by 50.3 seconds of arc per year (or 1 degree every 71.6 years). This may not matter much in the life time of one person, but in about 238 years, the sun is behind by one nakSatra pAda, and in 954 years, the winter solstice occurs in a previous nakSatra. Approximately, every thousand years, the Winter Solstice (and the equinoxes and the Summer Solstice) occur in a previous nakSatra. This is the precession of the solstices and equinoxes.
People who follow the dRk pancAngam or tropical zodiac, solve this problem by declaring that the Sun position on Spring Equinox, is what defines the aświni nakSatram zero point. That is the start point of Aries or mESa. Someone who is born at the moment of the Spring Equinox, counts as someone with the Sun in Aries/mESa. This way of thinking is called the sa-ayana or sāyana way of thinking. Admit that the start point of the zodiac moves. And forget that 50.3 arc second each year.
The other school is the nir-ayana school of thought. This school says that you can’t redefine the definition of a nakSatra each year. So a person born at the moment of the spring equinox this year, could count as someone as born with the Sun in meena rāśi or even earlier. It depends on what the star we started with as the Winter Solstice nakSatram.
If the spring equinox occurs at aświni 0 deg, then the Summer Solstice occurs at 10 degrees ARdra, ie at the 4th pAda of ARdra. Autumn Equinox occurs at citra 6 degrees 40′ ie 3rd pAda of citra and the Winter Solstice occurs at uttaraSADa 2nd pAda, ie 3 degrees 20′ of uttarASADa. (mESa/Aries, karkATaka/cancer, tulA/libra and makara/capricorn).
paitAmaha siddhAnta indicates a dhaniSTa winter solstice.
There are 7 nakSatra pAdA: (3 of uttarASADa and 4 of śravaNa) between uttarASADa 2nd pAda and dhaniSTa 1st pAda. That is 23 degrees and 20 minutes.
If the dhaniSTa winter solstice was the real start point, then our spring equinox occurs not in aświni 0 degrees, but in uttarābhādra 2nd pāda, 7 pādā: earlier. So a person born on spring equinox this year would be counted as someone born with the Sun in meena.
There are many schools of ayanāmśa in use today. Each uses a different “real” start year and different value for the rate of precession.
garga samhita counts kRttika as the first nakSatram.
Many thinkers say that kRttika was the spring equinox nakSatram. That means an 8 pāda or 26 deg 40 min correction.
There is a strong case for moola as the winter solstice star. mUla is the base or first and the star just before it is jyESTha or the oldest, the 27th.
From uttarāşāđa 2nd pādam , this gives a 9 pāda or 30 degree correction. That is a whole rāśi. Your whole jataka cakram (in sāyana) would move back by one rāśi (in nirāyana). The Spring Equinox nakSatram would be poorvābhadra 4th pādam. meena rāśi would be starting. (This means that we have replaced the fish with goat’s head or ram/mESa… like in dakSa story).
From dhaniSTa 1st pAdam to moola 1st pAdam you have 9 + 7 = 16 pādā: . 4 nakSatrāNi worth. A precession 3200 minutes of arc gives about 3817 years ago as a date for the paitāmaha siddhānta.
Many other methods of ayanāmśa give a smaller value than 30 degrees. So as per those methods, a few degrees of moola have elapsed at the winter solstice. moola 3rd to dhaniSTa 1st, 14 pāda difference due to precession.
The Indian Government standardised the ayanāmśa given by lahiri, I have read that this is because nehru thought highly of him and wanted a standard ephemeris to be used through the country. However Astrologers and Archaeo-Astronomers as well as amateurs have published their own ideas of what is right.
I suppose I am an amateur astrologer/astronomer/student. So what is my view? After all I blog because no one lets me speak!
I think that real alignments matter more than imaginary lines. So I think that if you look up in the sky and find that kRttika (pleiades) and śukra are in line, then that’s the fact. Any conclusions you wish to draw based on that observation, you should. Even if you use nakSatrANi as equal areas in the sky, they should stay anchored to real, bright, stars and constellations as was originally intended.
Even in this approach there are those who would debate as to which star/constellation should be identified with which nakSatra and why. I believe we should go with what is bright and likely, fits with the stories and can be seen without a telescope.