Short answer: the Swiss Ephemeris computes planetary positions to better than 0.1 arcsecond against Astrodienst’s reference. AstroWay runs that same engine and verifies the output against three independent sources plus the NASA eclipse catalog. The numbers below are what we actually measure, not marketing claims.
What the Swiss Ephemeris is
Section titled “What the Swiss Ephemeris is”The Swiss Ephemeris is the C library from Astrodienst (the people behind astro.com), derived from NASA JPL’s DE431 ephemeris. It’s the same engine inside professional desktop software - Solar Fire, Kepler, Janus, Astro Gold - that astrologers have trusted for 30+ years. AstroWay runs it on its own servers and serves it over REST, so you get that precision without installing a C extension or shipping ephemeris data files.
Measured accuracy
Section titled “Measured accuracy”| What | Drift | Reference |
|---|---|---|
| Planetary positions | < 0.1″ (arcsecond) | Astrodienst swetest CGI |
| Placidus house cusps | 0.000″ (exact match) | swetest |
| Eclipses | < 1 minute | NASA 5-Millennium Eclipse Catalog |
| Astrocartography (ACG) lines | < 1.5 km at the equator | swetest |
| Sunrise / sunset | < 10 seconds | timeanddate.com |
| Moon void-of-course, ingresses, conjunctions | sub-second | swetest |
How it’s verified
Section titled “How it’s verified”Two layers:
- Triangulation. Core calculations are cross-checked against three
independent implementations - Astrodienst’s official
swetestCGI, Kerykeion, and Prokerala - plus the NASA 5-Millennium Canon of Solar Eclipses. When sources disagree, the divergence is documented rather than hidden. - Regression snapshots. 873 snapshot tests pin the
output of the calculation endpoints. Every pull request re-runs them against
the live
swetestCGI and the NASA catalog, so a drift can’t slip into production unnoticed.
The full methodology and per-endpoint divergences live on the accuracy page.
When “less accurate” is fine
Section titled “When “less accurate” is fine”The Swiss Ephemeris has three precision modes. The full JPL-derived mode is the default. The built-in Moshier mode trades a little precision (still well under an arcsecond for most use) for not needing data files - fine for a horoscope app, overkill to worry about for a research tool that needs the JPL path. AstroWay uses the high-precision path by default, so you don’t have to choose.
Is the Swiss Ephemeris accurate enough for professional astrology? Yes - it’s the same engine in Solar Fire and Kepler, accurate to under 0.1 arcsecond for planetary positions.
How does AstroWay differ from running pyswisseph myself? Same engine, no install. You skip compiling a C extension and shipping ephemeris files; you make an HTTPS call instead. See REST vs library.
Can I see the verification data? Yes - the accuracy page lists the references and per-endpoint divergences, and the regression suite runs on every change.
Next steps
Section titled “Next steps”- Accuracy & validation: full methodology
- Birth Chart API: the natal endpoint
- Natal chart API in Python: build with it
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