Shri Prabhakar Phadnis wrote, on his blog:
I have written detailed comments on the concept of fall of Abhijit here earlier. One key line in the shlokas about this topic is ‘धनिष्ठादि तदा कालो ब्रह्मणा परिनिर्मितः.’ Dr. P. V. Vartak had interpreted this to mean that year started from the time when Sun was in Dhanishtha and the appropriate time for start of year should be Summer Solstice as that marks the end of summer and beginning of rainy season in Northern India. Start of year from start of rains is quite logical. Shri. Oak has accepted this theory as far as I know. Summer Solstice was occurring when Sun was in Dhanishtha around 14,500 BCE and therefore the conclusion is that Brahma started his system of marking the end of old year and start of new year around 14,500 BCE. I was also in agreement with this conclusion. Recently, out of curiosity I tried to read ‘The Orion’ by Lokmanya Bal Gangadhar Tilak. I followed his writing only for the first chapter! Later I felt completely out of my depth and gave up. However from what little I followed I gathered two premises he has elucidated. 1. Year was defined by commencement and completion of a set of sacrifices called ‘Satra’, which ran for a full year. Start of the Satra, and thus of the year, was initially from Vernal Equinox. The period from Vernal Equinox to Autumnal Equinox was called the ‘Deva Ayana’ Devas were all supposed to be residing at the Meru or Celestial North Pole and this period was called ‘Deva Ayana’ because Sun would be above the equator and visible to the Devas at Meru. Deva Ayana was the original Uttarayana as Sun was to the north of equator during the period from Vernal to Autumnal equinox. When Sun reached Autumnal equinox, half the year was over. The day of AE was Vishuva Din and it was the middle day of the year. The second half of the year was Dakshinayana or Pitrayana as Sun was below equator, the region of Pitaras. Tilak has quoted many references from Vedas and other vaidik literature in support of this position. 2. At a latter period, start of year was shifted to Winter Solstice. Uttarayana was now defined as the period during which Sun moved towards North upto Summer Solstice. The other half of the year was the Dakshinayana when the Sun moved southwards from SS to WS. In present times, that is how we understand Uttarayana and Dakshinayana. He further says that this change was for general civil purposes but the old system of starting year from Vernal Equinox also continued for conducting Satras. He has of course quoted references for this too! 3. Krittika was the first nakshatra according to him based on many references. I did not find any mention of Dhanishtha as first nakshatra in his book. There is a curious mention that ‘If Sun turns back before reaching Dhanishtha, it would be a bad omen’ according to some Rishi. Now Sun can ‘turn back’ before reaching Dhanishtha only if SS or WS was at Dhanishtha, some 100 years previously. 4. Tilak has not made any reference to the Shloka from Mahabharat, ‘Dhanishthadi …’ Now the question arises – How to reconcile what Tilak says with ‘Dhanishthadi Tada Kaalo …’ from Mahabharata? Dhanishtha was at Vernal Equinox in 21000BCE.If Year began initially from Vernal Equinox as Tilak states, did Brahma set the start of year from Dhanishtha in 21000 BCE? Tilak however has NOT claimed such Large Antiquity for Vedas etc. I refer the question to Shri. Nilesh oak and other learned readers.
The link to this article at his blog site is here: https://mymahabharat.blogspot.com/2016/10/dhanishthaadi-again.html
(I am copying what I wrote at the blog site of Shri Phadnis. Time permitting, I will add specific references from ancient Indian literature that refer to these ‘Ayana’ with varied definitions for their beginning and the end. For now, my response below is what my compressed schedule allowed)
I have explored this issue, from a slightly different angle, a while ago. I like to think I have also written a blog on it. I will have to search.
By different angle, I mean…I did not have ‘fall of Abhijit’ references as my context, but was rather looking at few from Tai. Samhita, Tai. Brahmana and Shatapatha brahamana where two different definitions of Deva-ayana & Pitru-Ayana vs Uttara-ayana & Dakshina-ayana are alluded to.
(The points of AE,WS, SS show nakhsatra positions at the time of Mahabharata War)
My theory is that ..while I do not know which system existed first (Deva and Pitru Ayana or Uttara & Dakshina Ayana)….one became synonymous with the other as identification of one (nakshatra, lunar month) etc. merged with the other…over a period of time, due to ‘precession of the equinoxes’. Over time, the confusion set in. Two diametrically opposite or at least 90 degree apart systems were identified together as if a single phenomenon and hence the confusion in our times (i.e. either that of Lokamanya Tilak or those of others).
When analyzed meticulously, these instances provide further evidence for the ‘mindboggling-ly’ ancient tradition of astronomy observations in India.
It can be shown that, as far as Mahabharata is concerned, Uttarayana and Dakshinayan system (WS & SS), as opposed to Deva & Pitru ayana (VE &AE), was in vogue during Mahabharata times…thanks in part to the decisive chronological aspect of AV observation.
Hope this helps,