here is a simplified version of my recent paper
Problem: the international community of astrologers does not agree on how to define the 12 signs of the zodiac. There is a minority but significant number of astrologers, most prevalent in India, who define the 12 signs in one way (“sidereal”) while everyone else defines them in another (“tropical”).
What is the Difference?
Less than you might think. The only difference between sidereal and tropical sign systems is the point from which they start. Sidereal signs start from a certain distance west of a specific star. Tropical signs start from the position of the Sun when it crosses the earth’s equator headed north (the “vernal equinox”).
What’s not different?
Both systems define the 12 signs mathematically, as the space traversed by the sun during the course of a single lunar cycle. Since there are 12 lunar cycles in a year there are twelve equal signs in the zodiac, sidereal and tropical
The tropical signs are not constellations, and neither are the sidereal signs. It’s just that sidereal signs start from a point determined by a star. Tropical signs start from a point determined by the relation of the Sun and Earth (the vernal equinox).
Which System is Correct?
Since its stronghold is in India, and since that is also my own astrological and spiritual background lets first answer the question with by examining Indian astronomical and astrological authority.
The source of authority for Indian astrological calculation is the Surya Siddhanta. The first chapter of this book defines the 12 rasis as seasonal (tropical) entities. The second chapter defines 28 stars as sidereal entities. It gives all the math for calculating planetary positions sudereay and then gives a principle called “ayanamsa” to convert those calculations to the tropical signs for finding the ascendant, the anchor of a horoscope.
The Srimad Bhagavatam is the culmination of Vedic knowledge. The 3rd chapter of the 5th division defines the 12 signs as tropical entities. The 5th defines 28 sidereal stars.
Indian authority agrees that there are two coexisting factors: tropical signs and sidereal stars. Perhaps because this is rather sophisticated and unique, Indian practitioners have fumbled this knowledge over the long centuries.
What about logic?
Even ignoring authority logic dictates that the 12 signs are tropical. All the traits of elements, modes, rulers and house order which define the character of each sign derive their meaning from the logic of tropical symbology.
Can you be more detailed?
Yes. A lot more: read my paper
And what does this mean to how we practice astrology?
It means we use tropical coordinates in reference to signs, period. And we use sidereal coordinates in reference to the 28 stars. This applies to everyone, western, Indian, or from the south-Bronx.
– Vic DiCara