Sam Geppi argues that the tropical zodiac is defined only in Surya Siddhanta, and is therefore not all that important because Surya Siddhanta is primarily astronomical, and is heavily heavily influenced by the Greeks.
Please think about this…
A) “Primarily Astronomical”
It’s absurd to say that astronomy is irrelevant to astrology. It’s the same as saying a birth chart is irrelevant to your astrological reading. Astronomy is what we use to calculate your birth chart.
B) “Heavily Influenced by the Greeks”
So what? All horoscopic astrology is a melting pot of pan-cultural concepts and techniques. All the siddhāntas are heavily influenced by pan-cultural research and development.
If you want an astrology/astronomy that’s not influenced by other cultures, what option is there? Maybe Vedaṅga Jyotish? In which case, say goodbye to your signs altogether, along with your houses and most of the planets planets, because the Vedāṅga Jyotish astronomy/astrology doesn’t involve them.
C) “Only in Sūrya Siddhānta”
It’s simply incorrect to say Sūrya Siddhānta is the only place we find the zodiac defined as a tropical thing. The truth is that the zodiac is defined as a tropical thing in all Indian Texts that define it.
For example, the Purāṇas usually contain sections on astronomy. Every Purāṇa that defines the zodiac (ex Bhāgavat, Padma, and Viṣṇu Purāṇas that I know of offhand) define it tropically – as a measure based on solstices and equinoxes.
I’ve heard Sam say the Purāṇas are irrelevant, too. This is funny. First he says Sūrya Siddhānta is irrelevant, then the Purāṇas? OK, so what IS relevant? If the Purāṇas AND the Siddhāntas are irrelevent, which part of your astrology is “Vedic”? The Purāṇas are anything but irrelevant. They are the most verbose and explanatory part of the Vedic library, and they provide the most extensive record of ancient and classic Indian culture and thought.
We have found about a half dozen tropical definitions of the zodiac in the vedas and classic Indian astronomy texts. How many sidereal definitions have we found? Zero.
But that’s probably “irrelevant” too because [insert nonsequitor diversion, intentionally complex-ificated jargon, or promise of a counter argument from śāstra without actually giving a counter-argument from śāstra]
Vic DiCara's Astrology 