This model illustrates how the Procession of the Equinoxes has been used to deduce (some would say speculate upon) the antiquity of the Hindu calendric systems.
This model displays a snapshot of the four equinoxtial and solstitial positions of the Sun in a geocentric model (the dotted circle connecting them indicates the apparent annual orbit of the Sun around the Earth, i.e. the ecliptic). These positions are drawn against a backdrop of the Hindu Rashis or Nakshatra Mandala. It is worth remembering (but not modeled here) that when moving from vernal equinox to autumnal equinox the Sun in over the northern hemisphere; when moving from autumnal equinox to vernal equinox the Sun is over the southern hemisphere.
First select an Epoch you want to work with. The Lahiri epoch is more modern and accurate. The Surya Siddhantha epoch is ancient but less accurate.
This model starts by displaying a starting position (285 AD / 490 AD depending upon which Epoch you select) where the Vernal equinox was aligned with the First Point of Aries (i.e. Mesha). If you change the Julian Year to 2012 (using the input box or slider) you will see that the Vernal Equinox has fallen behind Mesha as of today (the equinoxes precess in a direction opposite to that of the orbital direction of the Sun). As you proceed toward 3000 AD the Vernal Equinox precesses further back and eventually crosses over to Kumbha.
Indian researchers have discovered references in ancient Sanskrit texts such as the Vedas and Vedanga Jyotisha which refer to the Vernal Equinox being in positions other than Mesha. These references have been used to determine the period when such positions last occured and thus provide a pointer to the antiquity of these texts. Specifically, this model illustrates the arguments presented in The Orion, or Research into the Antiquities of the Vedas, Bal Gangadhar Tilak, 1893.
"This work (i.e. the Vedanga Jyotisha) gives the following positions of the solstices and the equinoxes:
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'' a year." - The Orion, pp. 38.
Click on the check-box labeled Vedanga Jyotisha on the model to reach this position. Since this model uses 50'' / Year as the rate of precession, the year shown by this model is 1180 BC. This corresponds to the post-Harappan period (Iron Age - Indo Gangetic tradition).
Going further back in time:
"There are many passages in the Taittirlya Sanhita, the Taittiriya Brahmana and other works where the Krittikas occupy the first place in the list of the Nakshatras...granting that the Krittikas were the mouth of the Nakshatras in the sense that their list always commenced with them, it may be asked what position we are to assign to the Krittikas in the course of the year. We cannot ... place the summer solstice in the Krittikas, unless we are prepared to take back the composition of the Taittiriya Sanhita to about 22,000 B. C...the same thing may be said against placing the Krittikas in the autumnal equinox...it is more natural to presume that the vernal equinox coincided with the Krittikas at the time when the Taittirlya Sanhita was compiled." - The Orion, pp. 39 - 41.
"Supposing the Krittikas to denote the asterism of that name this gives us, according to Prof. Whitney's calculation, 2350 B. C. as the probable time for the compilation of the Taittirya Sanhita...Now as the position, of the asterism of the Krittikas in its zodiacal portion is 10 50' from the beginning..." - The Orion, pp. 55.
Click on the check-box labeled Taittriya Samhitha on the model to reach this position. You will find that this model does indeed place the Vernal Equinox approximately 11 degrees inside Krittika on 2150 B. C. This corresponds to the Mature Harappan period (Harappan 3C) of the Indus Valley civilization.
"the Nakshatras of the Devas begin with the Krittikas and end with Vishtakha, whereas the Nakshatras of Yama begin with the Anuradhas and end with the Apa-Bharanis." - The Orion, pp. 41.
In the above, Nakstaras of the Devas indicates those Nakshatras which are traversed by the Sun in the northern hemisphere and the Nakshatras of Yama are those traversed by the Sun in the southern hemisphere. The accuracy of these definitions too can be observed on the model.
Antiquity - The Age of Mrugashirsha
"..in several Brahmanas and Suttras 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 Phalguni 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.
"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 nakshatra 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.
Click on the check-box labeled Antiquity on the model to reach this position. The corresponding Julian Year works out to 4330 BC. This corresponds to the Mehrgarh III period of the Indus Valley civilization.
Remote Antiquity - The Age of Punarvasu
Going even further back in time:
"The passage from the Taittiriya Sanhita ... states that the Chitra and Phalguni full-moons were the beginnings of the year,...The Sutra-writers, though not so explicit, do however distinctly state that the annual sacrifices 'should be commenced either on the Chaitra or the Phalguni full-moon night,' thus clearly indicating that these were regarded as the beginnings of the year..." -The Orion, pp. 61-67.
"With the Phalguni full-moon, at the winter solstice, the vernal equinox was in Mrigashiras; so with the Chitra full moon at the solstice the vernal equinox would be in Punarvasu..." - The Orion, pp. 200.
Click on the check-box labeled Remote Antiquity on the model to reach this position. The corresponding Julian Year works out to 6000 BC. This corresponds to the Mehrgarh I period of the Indus Valley civilization.
Mapping to the Indus Valley Civilization
It is easy to map these dates to the chronology of the Indus Valley civilization.