What Is an Ephemeris?
An ephemeris is a table of precise planetary positions calculated for specific dates and times. Modern ephemerides rely on high‑accuracy astronomical models (numerical integrations) to determine the exact location of the Sun, Moon, planets, nodes, and other points. These positions form the foundation for all chart calculations, transits, and timing techniques.
How planetary positions are computed
- Numerical integrations: professional ephemerides use long‑term simulations of planetary motion, including gravity interactions, light‑time delay, aberration, precession, and nutation.
- Astronomical data: many modern ephemerides (including Swiss Ephemeris) are derived from NASA JPL’s high‑precision DE ephemerides, a gold standard in celestial mechanics.
- Output formats: results can be expressed in ecliptic longitude/latitude, right ascension/declination, retrograde state, zodiac sign, and more.
Why accurate ephemerides matter
- They ensure chart calculations match real astronomical positions.
- They produce consistent transit windows and aspect timing.
- They support sidereal, true sidereal, and tropical frameworks.
- They are essential for eclipse mapping and precise lunar motion.