Due to the axial precession of the Earth, the equinoxes and solstices precess approximately 50” / year in a direction opposite to that of the sun’s apparent movement around the earth. This means over 26,000 years the equinoxes and solstices will return their starting point. Watch this video to understand this better.
This celestial phenomenal has interesting calendrical implications.
In the Hindu calendrical system, a solar year is divided into two halves – Uttarayana and Dakshinayana. These ayanas are defined as the time taken by the Sun to move between winter solstice and summer solstice and vice versa respectively, i.e. the duration when the sun moves in a northerly and southerly direction (due to the inclination of the earth’s axis).
[Another division - Devayana and Pitrayana are defines as the time taken by the Sun to move between vernal equinox and autumnal equinox and vice versal respectively, i.e. the duration when the sun is overhead in the northern and southern hemispheres respectively.]
The oldest extant Hindu calendrical treatise – Jotisha Vedanga – has the following to say about this northern and southern movement of the Sun:
When the Moon and the Sun travel in the heavens together with Vasava (i.e. the nakshatra Dhanishta) then is the beginning of the yuga, the shuklapaksha of Magha, and the northern ayana. The Sun and Moon proceed northwards at the beginning of Shravishtha, (i.e Dhanishta) and southwards in the middle of Sarpa (i.e. Ashlesha); (these two events occur) always in the month of Magha and Shravana respectively.
- The Mesopotamian Origin or Early Indian Astronomy, David Pingree
The above passage has been interpreted in an astronomical fashion by Bal Gangadhar Tilak a follows:
1. The winter solstice in the beginning of Shravishtha, (divisional) ;
2. The vernal equinox in 10° of Bharani;
3. The summer solstice in the middle of Ashlesha, and
4. The autumnal equinox in 3° 20′ of Vishakha.
…From these data astronomers have calculated that the solstitial colure occupied the position above mentioned between 1269 B. C. to 1181 B. C., according as we take the mean rate of the precession of the equinoxes 50* or 48*-6 a year.
- The Orion, or Researches into the Antiquity of the Vedas, Bal Gangadhar Thilak, 1893, pp. 38
Clearly the vernal equinox as located above is very different from where we find the vernal equinox today (the vernal equinox of today of today falls approximately 24° on the other side of Ashwini, i.e. it falls in Uttara Bhadrapada), the cause being the precession of the equinoxes. Since this procession is a regular phenomena that occurs at a rate of 50” / year, it is an easy matter to calculate when in the past the vernal equinox would have been at 10 degrees of Bharani.
The following figure illustrates the positions of the equinoctial and solstitial points indicated in the Vedanga Jothisha, and also indicates 1180 BC as being the year when these positions last occurred (I had to use the Surya Siddhantha epoch of J490 AD to arrive at this year; the Lahiri epoch pushes this date even further back by 200 years to 1382 AD).
The above figure is a snapshot of an interactive model of the precession of the equinoxes which indicates various milestones derived from references in the ancient Indian texts (such as the one mentioned above). This interactive model provides supporting illustrations for the analysis carried out by Bal Gangadhar Thilak and published in 1893 as “Orion, or a Research into the Antiquity of the Vedas“.
This model also indicates that the year 1382 BC falls in the Late Harappan period of the Indus Valley civilization. This period pre-dates Greek astronomers such as Hipparcus and Appolonius of Perga by a full millenium, this contradicting the popular western belief that Indian astronomy derived from Greek source. This period was also contemporaneous with Babylonian civilization thus contradicting another popular western belief that Indian astronomy derived from Babylonian astronomy; this seems to support a bi-directional flow of information, if at all.