The text from the 7th-10th verses of Chapter 14 of Surya Siddhanta becomes extremely relevant to establishing the tropical zodiac as correct in an Indian context. Here is some of that discussion:
Surya Siddhanta
Chapter 14
1: There are nine ways of measuring time: the creator’s time, the god’s time, the underworld’s time, the progenitor’s time, Jupiter’s time, tropical time, conventional time, lunar time, and sidereal time.
2: We have already defined four of these: tropical, lunar, sidereal and conventional. Jupiter time is important for understanding the sixty-year cycles. The others are not practically important.
3: Tropical time determines the length of the day and night, the 86s, solstices, equinoxes, and the months [sankranti, movement of the Sun into each of the 12 tropical divisions].
4: “86s” are four periods of 86 days starting from the beginning of Libra and extending into the dual signs.
5: They occur when the Sun is at 26 Sagittarius, 22 Pisces, 18 Gemini and 14 Virgo.
6: From 14 Virgo back to the starting point of 0 Libra there are 16 days. These are meant for sacrifice. Whatever is offered to the spirits now has unlimited reward
7: It is well-known that the circle of signs is split by two diameters. One is the line from equinox to equinox. The other is the line from solstice to solstice.
This establishes that the signs are anchored to solstices and equinoxes (i.e. it is tropical, not sidereal). It says that the prime division of the ecliptic is into four sections, where each section begins with either an equinox or solstice.
8: Between each solstice and equinox are two other entrances. These are the three steps of Vishnu.
An equinox marks the first division, and then at equal distances two further divisions are made. These three divisions of the ecliptic per tropical event represent three steps of Vishnu, “Tri-vikrama.”
9: The Sun has entered Capricorn when it begins moving north for six months. It has entered Cancer when it begins moving south for six months.
Now the names of the divisions are given. The Sun begins moving north at the winter solstice. Thus the winter solstice marks the beginning of Capricorn. The Sun begins to move south at the summer solstice, which therefore marks the beginning of Cancer.
10: Seasons last for two signs each, beginning from Capricorn with the frozen season. The twelve signs named Aries, etc. are the months which altogether comprise the year.
Text 11 then tells when it is good to do religious rituals at the commencement of each month. It should be done when the actual disc of the Sun is upon the border between the signs. The text gives the formula to calculate this.
From Text 12 the text moves on to other topics such as lunar time, common time, sidereal time, and then closes the book.
Thank you,
Vic DiCara
Vic DiCara's Astrology 