Tropical zodiac
HellenisticModern WesternEditorial synthesisAstronomical factTraditional doctrine
Definition
The zodiac convention that fixes 0° Aries to the March equinox point, anchoring the signs to the seasons rather than to the stars. It is the standard convention of Western astrology.
Also called: Seasonal zodiac
In more depth
Ptolemy's Tetrabiblos already argues for beginning the zodiac at the equinox point, tying the signs to the solar year. Under this convention the signs are defined by the Sun-Earth geometry of the seasons.[1][2]
Because Earth's axis wobbles over a cycle of roughly 25,800 years (axial precession, an astronomical fact), the equinox point slowly drifts against the background constellations. The tropical signs therefore no longer align with the like-named constellations, which is a definitional consequence, not an error in either system.[3][4]
References
Numbered citations in the text link to the records below; each record links to the full source registry.
- Claudius Ptolemy. Ptolemy's Tetrabiblos, or Quadripartite: Being Four Books of the Influence of the Stars. J. M. Ashmand (translator). W. Foulsham & Co. Ltd, London. 1900. Historical primary source View source record
- Nicholas Campion. A History of Western Astrology, Volume I: The Ancient and Classical Worlds. Continuum. 2009. Historical scholarship View source record
- NASA Science. Milankovitch (Orbital) Cycles and Their Role in Earth's Climate. 2020 (last updated 2024). Astronomical fact View source record
- International Astronomical Union. The Constellations (official list of the 88 IAU constellations). current (living reference page). Astronomical fact View source record