Sidereal Zodiac

The sidereal zodiac is the second of the two major zodiac systems used in modern astrology. Where the tropical zodiac is anchored to the equinoxes, the sidereal zodiac is anchored to the fixed stars. It tries to keep the signs aligned with the actual constellations, even though those constellations slowly drift relative to the seasons due to the precession of the equinoxes.

The offset between the two systems is called the ayanamsa. As of 2026, the most commonly used ayanamsa (Lahiri) is approximately 24 degrees, which means a planet at 14 degrees tropical Aries would be at roughly 20 degrees sidereal Pisces. The exact offset depends on which ayanamsa value is used; there are several competing definitions.

Sidereal astrology is the dominant tradition in Vedic (Indian) astrology, where it has been in continuous use for over two thousand years. Vedic astrology also uses different house systems, different timing techniques, and a different interpretive framework — sidereal calculation is not the only difference between Western and Vedic approaches.

ZODIA uses the tropical zodiac, consistent with Western astrological practice. The choice between tropical and sidereal is not a question of which is more accurate; both are internally consistent coordinate systems, and both produce coherent results when interpreted within their own tradition. They simply measure different things: tropical measures the season, sidereal measures the visible star background.