Why does India use a sidereal zodiac, even though their source texts define the zodiac tropically? I will present an answer to this question, explained in the context of the historical evolution of astrology and the zodiac over 6,000 years and about half-a-dozen cultures.
Historical Evolution of the Zodiac
The ancient cultures of Kali-yuga – Mesopotamian, Egyptian, Chinese, and the remnants of Vedic cultures – primarily used the stars as reference points for their astrological observations and divinations.
Yet they all also had calendars based on a 360-day solar cycle divided into 12 lunar cycles of 30 days each. So, in addition to their stellar observations, they also divided the ecliptic into 12 equal segments, as we do with our modern zodiac. Yet they didn’t initially use these 12 segments much for divination.
Because they couldn’t.
They could roughly keep track of the Sun’s position through the 12 divisions, and so could use the 12 divisions calendrically, but they lacked the mathematical systems and observational tools to calculate where the other planets were in reference to it, so they couldn’t use these divisions effectively for astrological observations and divination.
Instead, they relied on easily observable phenomena in the night sky. “Omens.”
ex: comet, halo, eclipse
Improved use of Sidereal Divisions
Thus, the initial astrological systems that evolved during this ancient age (roughly beginning 5 or 6,000 years ago) were omens in reference to the stars in the night sky. As these systems became more sophisticated, the night sky became divided into symbolic constellations giving rise to various sidereal systems, including, in India, the 28 Vedic nakṣatras.
Improved use of Tropical Divisions
As mathematical and observational skills improved (coming towards roughly 2000 years ago), it became possible to fairly accurately measure where planets were in reference to the 12-fold equal divisions of the Sun’s path. Thus around this time, the astrological cultures began to do so. We have concrete evidence of it happening in Mesopotamia.
Measurement of the Sun’s path with its 12 divisions was always done by measuring the amount of sunlight vs. shadow, confirmed also by measuring the amount of daylight vs. night. The astronomical statements well preserved in Indian tradition are unequivocal examples of this (SS 14.7, VP, BP 5.21.3). These differences are caused by the Sun’s movement northward and southward of the equator, and so are known as “tropical” calculations.
Astrological divination, however, had evolved for centuries around observation of phenomena in the night sky, against the stars. In Latin, such stellar observations are called sīdereus, which today we called “sidereal.”
So, when astrologers first began extensively using the 12-fold divisions about 2000 years ago, they correlated them to the sidereal divisions.
In some cases, the sidereal divisions dissolved into the 12. This seems to be the case with the 18 Babylonian divisions morphing to fit the 12 division. In other cases, the sidereal divisions were kept and integrated into the 12. The Vedic culture seems to have dropped their 28th constellation and standardized the constellation borders in a manner that allowed their sidereal system to interface cleanly and compellingly with the 12-fold tropical system.
Discovering the Flaw of Sidereal Projection
An important discovery was made at this time, a little more than 2,000 years ago, usually credited to the Greek astronomer and mathematician, Hipparchus. He realized that the 12-fold system based on the Sun’s tropical movement did not stay fixed in reference to the sidereal divisions of the stars. We call this discovery the “precession of the equinoxes.”
There was some reluctance to accept a purely tropical method of divination because up until that point, almost all astrological divination had been based on stellar (sidereal) observations. But since the 12-fold division was inherently measured in reference to the Sun’s tropical movement, most of the astrological world quickly adopted a purely tropical calculation of the 12 divisions as they became aware of Hipparchus’ discovery. Some resisted but almost all divinatory use of sidereal 12-fold divisions had ceased by about 1600 years ago.
The one exception was India, which never accepted a purely tropical zodiac, and continued to conceive of the 12 divisions in terms of stellar, sidereal, constellations.
It is not possible they were unaware of the precession of equinoxes, for it is impossible to even calculate a horoscope accurately without that knowledge, and the Indian’s measured the disparity between the tropical zodiac and its sidereal counterpart by using a value called ayanāṁśa. The Indian’s use of 12 sidereal divisions for divination was intentional and deliberate.
It is not because they were just dumb, because they were a genius culture, and it is obvious that they knew about precession.
I don’t think it was simply a blunder either. I think the decision was deliberate and intentional.
Ancient India had developed an extensive mundane and electional sidereal astrology system, probably more mathematically advanced and elaborate than the systems developed in Mesopotamia at the time, and which was intimately, inexorably tied to their larger, more important spiritual and religious culture, because it was used for to establish the timings of the rituals and ceremonies at the foundation of Vedic life. Because their sidereal system of 28 nakṣatras was so essential to the spiritual fabric of their culture, they did not abandon them as the 12-fold solar divisions became more usable and popular. Instead, they modified them very slightly and made them fit well with the 12-fold solar divisions.
After accomplishing a very organized and compelling integration of their stellar constellations with the 12 divisions, when Hipparchus brought the flaw of sidereal projection to light, Indian’s, though acknowledging it (or perhaps even already being aware of it, or separately discovering it), were unwilling to waste the rich overlay of rāśī and nakṣatra they had created by sidereal projection, for that would ruin much of their recent development in natal astrological techniques. So they refused to liberate the 12 divisions from sidereal space and allowing them to exist in their own space, as purely tropical entities. They had probably already begun developing interpretative systems relying on the interplay of rāśī/navāṁśa with the nakṣatra and their (probably newly-minted) four quarters. They were not willing to let these developments go to waste, so they resisted the call to calculate the 12-divisions as purely tropical phenomena.
Several hundred years after Hipparchus, about 1500 years ago, the great Indian mathematician and astrologer Varāha Mihira noted the problem. In Bṛhatsaṁhita (3.1-3) he wrote that the northern solstice used to be in Āśleṣā, but now it isn’t, so we have to make observations of the locations of solstices and equinoxes relative to the nakshatras if we are to know where Cancer and Capricorn begin.
And in Pañcasiddhāntikā (3.21 & 32) he explained that the equinoxes and solstices gradually move through the nakshatras, making the correlation between them require constant correction. But, he said, people were not doing so, and instead just following tradition.
It Was Theoretical Then
In the centuries between Hipparchus and Varāha Mihira, the difference between the 12-fold Zodiac and its sidereal projection was not much more than a fancy theoretical debate, hardly more significant than the common margin of error in calculations of the time. By Varāha Mihira’s time, the difference had become noticeable. 2000 years later, the difference is now extreme (about 24º) and beginning to become very problematic.
Over the millennia astrologers in India have certainly managed to remain at least as accurate and relevant as astrologers elsewhere (which unfortunately isn’t saying much), mainly because of the continuous interest in and support of the art. As the divergence between sidereal and tropical systems continues to increase, however, to maintain reasonable accuracy, the definitions of the fundamental symbols in a sidereal system must mutate further and further from their natural and rational origins.
In the 20th and 21st centuries, a growing number of ethnic and non-ethnic astrologers practicing Indian astrology have adopted purely tropical calculations for the 12-fold zodiac, while maintaining sidereal calculations for the 27 nakṣatras. This is my own stance, as fixing a problem (even a beautiful problem) seems a far better approach than continuously adjusting to accommodate it.