The true concept of Ayana and Nirayana

It is commonly understood that the nirayana is synonymous to the modern sidereal. But if that had been the case what's the reference circle that is defined in the old astronomical texts, the Indian siddhantas?

Obviously, any fixed epoch ecliptic cannot serve all the time to the observations, neither a siddhantic epoch nor any modern epoch. This leads us to the concept of a fixed sidereal and the moving sidereal. The fixed sidereal in the Indian astrology circle is popular as the nirayana and it is the moving sidereal which is the sidereal as observed or the modern sidereal.

Since it has to be without any ayana motion in consideration, Nirayana is a fixed framework that has certain characteristics with respect to the circle size and ayana. As detailed in 'The Unanswered Part I', the ayana motion is associated with the obliquity and hence the nirayana rests at a particular ayana, fixed for ever.

A varying precession rate cannot satisfy the nirayana framework, for the nirayana is supposed to be fixed in size. This is the reason why the nirayana precession rate is fixed and higher than the modern sidereal precession rate. The 58.5 sec rate that I've discovered and tamed in my books is the nirayana rate and I've found only this rate can satisfy a yuga figure of 4320K years based on just the mean Sun and mean Moon employing the surya siddhantic framework.

The siddhantic circles are not the sidereal circles as observed but correspond to a nirayana framework. The moving sidereal is derived from it. That’s why the nirayana is not so simple and one needs to go through my book ‘Taming the Untamed’ to understand the finer aspects of it.

Further, the nuances of whether any such fixed sidereal is possible or not needs a wider explanation and cannot be explained in just a few sentences. The subject has been exhaustively discussed in ‘Taming the Untamed’ and can be referred to, if desired.

But, of course, the concept of ayana can be explained in short and you may grasp it visiting the following page:

Understanding the Ayana in depth