Several vedic works offer further more ancient positions of the equinoxes and solstices as evidenced by the following:
..in several Brahmanas and Sutras the full-moon night in the month of Phalguna has been pronounced to be the first night of the year. The Shatapatha Brahmana (vi. 2. 2. 18) says “the Phalgunl full-moon is the first night of the year”. The Taittiriya (i. 1, 2, 8) and the Sankhyayana (iv. 4 and v. 1) Brahmanas contain similar passages, while the Gopatha Brahmana (i.19) after stating that the Uttara and the Purva PhaIguni are respectively the beginning and the end of the year, adds just as the two ends of a thing meet so these two termini of the year meet together…we must hold that the Phalguni full-moon night was once considered to be actually the first night of the year, or to put it in a modern form, the new year’s night…make the full-moon commence the year, at the winter solstice,…” – The Orion, pp. 67 – 69.
These references have been presented in an astronomical fashion by Bal Gangadhar Thilak as follows:
Taking the data given in the Vedanga Jyotisha as his basis, the late Krishna Shastri Godbole has thus calculated the position of the four cardinal points of the ecliptic, when the winter solstice, as stated in the Brahmanas, occurred on the full-moon day in the month of Phalguna :
(1) The winter solstice in 3 20′ of the divisional tiara Bhadrapada;
(2) The vernal equinox in the beginning of Ardra;
(3) The summer solstice in 10 of Uttara Phalguni ; and
(4) The autumnal equinox in the middle of Mula ;” - The Orion, pp. 72.
Now it is easy (using the interactive model) to locate the positions described above with the year of their last occurrence being 4535 BC:
As the model indicates, we are now peering into the time period of the Mehrgarh civilization (III period), which was a precursor to the Indus Valley civilization! At this time, the vernal equinox moved into Mrugashirsha.
Bal Gangadhar Thilak also discusses a reference in the Bhagavad Gita:
…the statement in the Bhagavad-Gita (x. 35) where Krishna tells Arjuna that he, Krishna, is “Margashirsha of the months (and), Vasanta of the
seasons.” – The Orion, pp. 79.
He points out that if this verse had said “Margashirsha of the nakshatras”, then the analysis would have been much simpler, but since it says “months (masanam)”, that makes Mula or Jyeshta the nakshatra where the vernal equinox or winter solstice resided (remember a month takes its name from the nakshatra where the full moon resides, and a full moon by definition is in opposition to the Sun with respect to the Earth; hence, if Mrugashirsha was the first month, the full moon of that month would have fallen in the Mrugashirsha nakshatra, and the Sun on that day should have been in Jyeshta or Mula.); both these possibilities lead to erroneous astronomical conclusions. Hence he concluded the verse itself was composed following an erroneous tradition of Mrugashirsha month being the first month of the year instead of Mrugashirsha nakshatra being the first nakshatra of the year.