While engaging with (watching) the astrology community on social media, I sometimes see people arguing passionately about which astrology system works best. I haven’t been into it long enough to pick a side but I understand the technical differences between sidereal and tropical astrology well enough to, hopefully, help you to understand them too.
There are two well known ecliptic coordinate systems used in astrology today, Sidereal and tropical. Both divide the ecliptic into 12 zodiac signs named after the zodiac constellations. Each sign is 30 degrees in longitude. The main difference between the two systems is their longitudinal origins or their axises.
In the tropical system, the origin is the vernal equinox. The vernal equinox is the intersection of the ecliptic and the celestial equator where the sun passes the equator as it is apparently moving northward. This point is also called the sun’s north node.
The vernal equinox is currently moving westward by about 50.3 arc seconds per year due to the earth’s precession. Astrologers sometimes call this the “precession of the equinox”. (We will talk more about this in another post that I will link here after it’s written) The idea is that the coordinate system moves with the earth.
The longitudinal origin in the sidereal system is defined relative to the fixed stars. To find the origin, a correction called ayanamsa is used. Ayanamsa is the distance between a star and the vernal equinox.
There is debate about which star should be referenced in the sidereal coordinate system and how it’s longitude should be calculated. For this reason, the sidereal system has many perceived origins.
Which system is correct?
Both systems are tried and true. The system that works for what you are doing is the system you should use.
one argument I see too often is that the sidereal system aligns with the real zodiac constellations. This is not true. The zodiac, as it is used in astrology, is a concept that was used in math and astronomy waaaay back in the day. I found a book on archive called A Compleat System of Astronomy, written in 1742 that explains this. The book acknowledges the confusion between the zodiac signs and constellations. Maybe this confusion eventually led us to stop using the signs?