To see how ancient Vedic astrology utilized tropical measurements, arranged their year around them, and divided the tropical course of the Sun in various ways, including 12 segments, let’s look at the oldest surviving astrological text: the Vedanga Jyotish of Rg Veda.
Rg Vedanga Jyotisha 5 says that a year begins when the Sun is in Shravishtha (an older name for Dhanishtha). Now if you rewind the equinoxes you will find that just about 4,000 years ago (the estimated date of Rg Veda’s compilation / and Vedanga Jyotisha) the winter solstice would occur when the Sun enters Shravishtha. The 5th text states this explicitly by stating that this is the beginning of the year because it is the location of “uttarayana” – the point at which the Sun begins moving northward and the days begin getting longer. The Winter Solstice.
Rg Vedanga Jyotish 6 tells us that the shortest day and the beginning of the northward course of the Sun (and Moon) begins at Shravishtha, and that the beginning of the sourthward course begins in Aslesha (the opposite star from Shravishtha).
Rg Vedanga Jyotish 7 tells how much (on average) the day increases each day from the winter solstice, and visa versa from the summer solstice.
These three texts show beyond any reasonable doubt that ancient Vedic astrologers used tropical calculations (solstices and equinoxes) as the foundation for their measurement of the year. The solstices and equinoxes were translated to stellar locations either because (a) they were unaware that the equinoxes moved in relation to the stars, or (b) because all of the math for their calculations are based on stellar observations, the tropical points need to be transposed. I personally see no need to assume (a) and regardless if it is accepted or not, (b) is evident.
Rg Vedanga Jyotish 31-33 tell of “Vishuda” (equinoxes) – and explain how to calculate them for each year in the 5 year yuga cycle.
Rg Vedanga Jyotish 9 defines each of the six Vedic “seasons” as being equivalent to the Sun’s movement through 4 and a half nakshatra segments (interestingly, the text also gives seasons to the Moon!). This is another example of tropical / seasonal measurements being translated back and forth to sidereal equivalents.
Earlier, in Rg Vedanga Jyotish 5, mention of different months for lunar and solar calendars was made. And the name of the first tropical / solar month of the year was declared to be “Tapas”
Vaajasaneyi Samhita 13.14 gives the names of the 12 solar months:
“Tapas (Austerity) and Tapasya (Performer of Austerity) are the two months of the Frozen Season.
“Madhu (Sweetness) and Maadhava (Enjoyer of Sweetness) are the two months of the Blossoming Season (Spring).
“Shukra (Bright and Clear) and Shuci (Pure and Clean) are the two months of the Hot Season (Summer).
“Nabha (Bursting) and Nabhasya (Fogged) are the two months of the Rainy Season.
“Isha (Fertile) and Oorja (Invigorating) are the two months of the Mature Season (Autumn).
“Saha (Overcoming) and Sahasya (Strong) are the two months of the Frost Season.”
It is not in the Rg Veda version, but in the Yajur Vedanga Jyotish 28 says that the solar year has 366 days! (not an idealized 360, an accurate tropical 366). It says that there are 12 months in 6 seasons in 2 halves in this year, and that five such years constitute a “yuga.”
So we see that the ancient Vedic Astrologers calculated a year as the movement of the Sun from one winter solstice to the next. This movement of the Sun through time and space was divided into various segments, primarily into halves, 6ths and 12ths. The later of these is identical (or at least fundamentally similar) to how the 12 tropical signs of non-Indian origin were calculated. Thus we see that the ancient Vedic Astrologers did have a concept of dividing the tropical movement of the Sun into 12 segments, although we have no evidence that they connected them with rams, bulls and so forth, or asigned elemental, modal and planetary rulership to these, nor that they used them in any interpretive or natal format.
As a footnote, it is rationally consistent with the Vedic paradigm that the year should begin in connection with the winter solstice. The rationale: Vedanga Jyotish and all Vedic and supplimentary texts divide all significant periods of time (days, months and years, for example) into two halves: one half representing a “daytime” and the other representing a “night”. For example, every lunar month has two halves: the bright half (waxing moon) is the “daytime” for the hemigods, and the dark half (waning moon) is their “nighttime.” Thus one of our months is equivalent to one of their days. And further the year has two halves, one when the Sun is moving northward (“uttara-ayana”), the other when the Sun is moving southward (“dakshina-ayana”). The northern course of the Sun is the “daytime” for the main class of demigods, and the southern course is their night. Thus one of our years is equivalent to one of their days.
Now, it is rationale that things start at daybreak, when we wake up and get started. Thus it makes sense to start the month with the new moon – the beginning of the waxing parva – because it begins the day for many hemigods; and it makes most sense to start the year with the Sun beginning to move northward – because it begins the day for the demigods.
The Sun begins moving northward at the winter solstice, which 4,000 years ago was when the Sun entered Shravishtha.
There is an alternative “beginning” of the new year which is the spring equinox. The original name of Mrgashirsha was Agrahaayana – which means “new year.” And this would be from an even earlier period (about 4k B.C.) when the vernal equinox corresponded to Mrgashirsha. Thus from this older period many customs of starting years from the equinox would have persisted. Vedic culture spanned a very long period of time, it appears at least from 4k B.C. to about 1k B.C. During this time the equinox seems to have been NOTED to drift against the stars, and thus the stars associated with the “first” position and the solstices and equinoxes were updated, up till the relative decline of knowledge in Kali-Yuga around 0B.C. at which point we became locked into thinking that the tropical measurements and the stars were identical, and Asvini seems to have been forever deemed the “first star” and the center point of Mula (beginning of sidereal Capricorn) is forever deemed (erroneously) to be the beginning of the Sun moving northward (uttarayana). Thus it seems that ancient Vedic Indians were aware of equinotical precession, but later lost their knowledge of it with the general decline of culture that occurred entering the “A.D.” historical era.