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What is precession of equinoxes?

Axial precession is the movement of the rotational axis of an astronomical body, whereby the axis slowly traces out a cone. In the case of Earth, this type of precession is also known as the "precession of the equinoxes", "Luni-solar precession", or "precession of the equator". Earth goes through one such complete precession cycle in a period of approximately 26,000 years or 1 degree every 72 years, during which the positions of stars will slowly change in both equatorial coordinates and ecliptic longitude.

Over this cycle, Earth's North axial pole moves from where it is now, within 1 degree of Polaris, in a circle around the ecliptic pole with an angular radius of about 23.5 degrees. The direction of the Earth's axis of rotation gradually changes over time in a process known as the precession of the equinoxes.

A complete precession cycle requires 25,770 years, during which time the pole of the Earth's rotation follows a circular path across the celestial sphere that passes near several prominent stars. At present the pole star is Polaris, but around 12,000 BC the pole was pointed only five degrees away from Vega. Through precession, the pole will again pass near Vega around AD 14,000. It is the brightest of the successive Northern Pole stars.

Comments:
(1) Vartak's observation: Abhijit. Falling steadily, assumed the position of the Celestial Pole at 12,000 BC is misleading. This seems to give the impression that Abhijit moved from its position and occupied the place meant for Pole Star.
(2) More precisely: it is because of the precession of equinoxes, that Abhijit seemed to have moved from its elliptic position to the position meant for Pole Star. As the precession of equinoxes continues its merry way, it will appear from Earth, as if Abhijit has moved back to its old position. I am afraid this only shows that whosoever wrote Mahaabhaarat, (or whosoever interpolated the episode of the Fall of Abhijit) was aware of the phenomena of Precession of Equinox.
(3) As Vartak has himself pointed out, this shifting of Abhijit, appears in the Mahaabhaarat in the form of a dialog between Indra and Skand. Now if the same information has appeared in the Mahaabhaarat in the form of a dialog of say "Dharm and Krishn" or "Dharm and Karn" or say "Karn and Duryodhan", one could have said, since these persons are referring to an incident that happened in 12000 BC they must have lived during that period. Unfortunately that is not the case. Both Indra and Skand, are to the best of my knowledge, neither major players nor minor players in the Mahaabhaarat war.
(4) In simple words, this episode of "Fall of Abhijit" is an interpolation in Mahaabhaarat and is of no help in fixing the date of Mahaabhaarat war.
(5) Since Abhijit (Vega) was the Pole Star in 12000 BC and will again become Pole Star in 10000 AD, it shows that the whosoever wrote this paragraph in Mahaabhaarat was aware that precession cycle was at least more than 26,000 years. In other words there is no oscillation of equinoxes*. Isn't that a good enough reason to take Vernal Equinox as the starting point?

Polaris has not always been, nor will it always be, the Pole Star. This is because the direction that Earth's axis points slowly changes with time. Like a spinning top whose axis slowly wobbles, the Earth exhibits a similar wobble (called precession), caused by the combined gravitational influences of the Moon and Sun.

Polaris has not always been the Pole Star (it only became so in about 500 AD) and it won't always be the Pole Star (it hands over that job to Gemma Cephei in about 3000 AD).

Over a 26.000 year cycle, the precession of the Earth's axis means that a number of stars take it in turns to be the star that is nearest to the celestial North Pole and thus get designated as the Pole Star, before being superseded by another one moving closer. It is a job-share not a job for life.

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