Precession of the Equinoxes
Earth’s rotation axis performs a slow, long‑term wobble — similar to a spinning top. This astronomical motion is called axial precession, and it gradually shifts the March equinox along the ecliptic. Over centuries, this causes the tropical zodiac (season‑based) and the sidereal zodiac (star‑based) to diverge by many degrees.
Why the zodiacs slowly drift apart
- Tropical zodiac: fixes 0° Aries to the March equinox each year.
- Sidereal & True Sidereal: fix 0° Aries to the stars or constellations.
- Precession rate: ~1° every ~72 years ⇒ ~24–25° difference today.
This is why the Sun sign in the sidereal framework is often one sign earlier than in the tropical system.
How ephemerides correct for precession
High‑precision ephemeris engines apply corrections for:
- precession (long‑term equinox drift)
- nutation (short‑term oscillation of Earth’s axis)
- aberration & light‑time delay
- gravitational interactions
Swiss Ephemeris uses long‑range numerical integrations (derived from NASA JPL data) to ensure accurate tropical, sidereal, and true‑sidereal computations.