Calculation Accuracy
AstroWay API uses Swiss Ephemeris (the official C code by Astrodienst, creators of astro.com) — the same engine professional astrologers have trusted for 30+ years inside Solar Fire ($495), Kepler ($995), Astro Gold ($29.99/mo), and Janus. We compile it to WebAssembly and expose it as a REST API, without the middleman markup. Every core calculation endpoint is covered by regression snapshot tests; engine precision is verified through triangulation against three independent sources, plus NASA 5-Millennium Eclipse Catalog:
- Planet positions: median
0.0013", maximum0.561"vs the official swetest CGI at Astrodienst (measured 2026-07-29 over 130 samples). Per-body breakdown below - Placidus house cusps:
0.000"exact match - Eclipses:
< 1 minutevs NASA 5-Millennium Eclipse Catalog - ACG lines:
< 1.5 kmat the equator vs swetest - Sunrise/sunset:
< 10 secondsvs timeanddate.com - Moon VOC, ingresses, planet conjunctions: sub-second precision
Methodology: how this was measured
Section titled “Methodology: how this was measured”These are measured numbers, not estimates. The script takes 10 charts spread from 1900 to 2050, asks the live swetest CGI at Astrodienst for the same instants, and compares all 13 bodies we return. 130 samples in total.
| Body | Maximum drift |
|---|---|
| Sun, Venus, Saturn | 0.001" |
| Mercury | 0.002" |
| true Node | 0.004" |
| Mars | 0.007" |
| Jupiter | 0.009" |
| Moon | 0.022" |
| Uranus | 0.083" |
| Pluto | 0.097" |
| Neptune | 0.223" |
| Chiron | 0.561" |
| mean Apogee (Lilith) | 0.000" |
Median across all 130 samples: 0.0013".
This page previously carried a blanket ”< 0.1 arcsec” promise. The 2026-07-29 measurement showed Neptune, Pluto and Chiron do not hold it, so the promise was replaced with the measured figures. Chiron and the outer planets drift further because their positions come from the compressed ephemeris set in the WASM build.
You can reproduce this: api-calc/scripts/accuracy-vs-swetest.mjs makes exactly
these requests and prints the same table.
Check it yourself. The script behind the numbers above is a public MIT repository: astroway/astrology-accuracy-benchmark. It is not ours-only: an adapter for any other API is about thirty lines, so you can run the same 130 samples against a competitor and compare. A sandbox key is enough.
Engine
Section titled “Engine”| Component | Value |
|---|---|
| Library | Swiss Ephemeris C code (aloistr/swisseph) |
| Version | upstream Astrodienst C code |
| Bindings | swisseph-wasm (WebAssembly, Node.js) |
| Ephemeris | DE431 JPL ephemeris (via .se1 files) |
| Fallback | Moshier analytical (for dates outside 1800-2399) |
| Code sharing | Shared @/core with app.astroway.info — one engine, two transports |
Methodology
Section titled “Methodology”Accuracy is verified through triangulation — comparison against three independent sources:
1. swetest CGI (authoritative)
Section titled “1. swetest CGI (authoritative)”The official Swiss Ephemeris reference implementation hosted by Astrodienst — the team behind Astro.com, which serves millions of astrologers worldwide. The most authoritative public reference. Our engine is identical (0.00–0.07 arcsec drift).
2. Kerykeion (Python)
Section titled “2. Kerykeion (Python)”An independent Python library using pyswisseph bindings instead of our
WASM. Confirms that our WASM layer does not distort the data.
3. Prokerala API
Section titled “3. Prokerala API”A remote Swiss Ephemeris API (sidereal Lahiri). Systemic drift of 8–17 arcsec is caused by different Lahiri ayanamsa formula versions — not by engine accuracy.
Triangulation results
Section titled “Triangulation results”| Chart | vs swetest (Astrodienst) | vs Kerykeion (Python) | vs Prokerala API |
|---|---|---|---|
| Monroe 1926 | 0.00” | 0.19” | 16.95” (systemic) |
| Diana 1961 | 0.00” | 0.69” | 8.18” (systemic) |
| Einstein 1879 | 0.07” | Kerykeion LMT artifact | 14.96” (systemic) |
Endpoint-level regression suite
Section titled “Endpoint-level regression suite”Each of the core calculation endpoints is covered by frozen snapshot tests on 3 reference charts (Monroe / Diana / Einstein) = 853 snapshots.
The snapshot suite catches:
- Input-mapping bugs — wrong planet id, UT, house system
- Post-processing — rounding, unit conversion, sign loss
- Defaults divergence — mean vs true node, geocentric vs topocentric
- Schema drift — validation let through a wrong shape
- Stale deploy — prod dist out of sync with code
Default tolerance: 5e-5° (≈0.18 arcsec) for all numeric fields.
Detailed benchmarks
Section titled “Detailed benchmarks”Planet positions (tropical, vs swetest)
Section titled “Planet positions (tropical, vs swetest)”| Chart | Date | Max drift |
|---|---|---|
| Marilyn Monroe | 1926-06-01 | 0.00” |
| Princess Diana | 1961-07-01 | 0.00” |
| Albert Einstein | 1879-03-14 | 0.07” |
House cusps Placidus (vs swetest)
Section titled “House cusps Placidus (vs swetest)”| Chart | ASC drift | MC drift | Max cusp drift |
|---|---|---|---|
| Monroe | 0.000” | 0.000” | 0.000” |
| Diana | 0.000” | 0.000” | 0.000” |
Eclipses (vs NASA 5-Millennium Catalog)
Section titled “Eclipses (vs NASA 5-Millennium Catalog)”| Event | NASA max | Ours max | Drift |
|---|---|---|---|
| 2025-03-14 Lunar Total | 06:58 UT | 06:58 UT | 0.8 min |
| 2025-03-29 Solar Partial | 10:47 UT | 10:47 UT | 0.5 min |
| 2025-09-07 Lunar Total | 18:11 UT | 18:11 UT | 0.8 min |
| 2025-09-21 Solar Partial | 19:41 UT | 19:42 UT | 1.0 min |
Astrocartography (vs swetest RA formula)
Section titled “Astrocartography (vs swetest RA formula)”All MC/IC/ASC/DSC lines use the correct longitude = RA - GMST formula
(Kenneth Bowser standard). Drift from authoritative: < 1.5 km at the
equator for all planets.
Sunrise/Sunset (vs timeanddate.com)
Section titled “Sunrise/Sunset (vs timeanddate.com)”| Location | Date | Parameter | Drift |
|---|---|---|---|
| London | 2026-04-15 | Sunrise | 0.6 sec |
| London | 2026-04-15 | Sunset | 9 sec |
Polar locations (|lat| > 66.5°) automatically return polarState +
a warning that regular planetary hours are undefined.
Aspect orbs
Section titled “Aspect orbs”AstroWay uses variable per-planet orbs (MIN-of-two-planets rule), as in ZET9 and astro.com. Default orbs (natal chart):
| Aspect | Sun | Moon | Inner | Jupiter | Outer |
|---|---|---|---|---|---|
| Conjunction | 12° | 10° | 5° | 8° | 5° |
| Sextile | 6.5° | 6° | 5° | 5° | 5° |
| Square | 10° | 8° | 5° | 7° | 5° |
| Trine | 12° | 8° | 5° | 5° | 5° |
| Opposition | 12° | 10° | 5° | 8° | 5° |
Minor aspects (36°, 40°, 45°, 72°, 108°, 135°, 144°) are off by default.
Enable explicitly via ALL_ASPECTS.
Polar latitudes
Section titled “Polar latitudes”For |lat| > 66.5° the Placidus/Koch/Regiomontanus systems are mathematically undefined. In that case Swiss Ephemeris automatically falls back to Porphyry, and our API adds a warning:
{ "system": "P", "warning": "Placidus system is undefined for lat=68.96° (> 66.5°). Swiss Ephemeris fell back to Porphyry..."}Continuous verification
Section titled “Continuous verification”Regression is checked on every PR through CI:
api-calc/tests/endpoints/— 853 snapshots against reference charts (Monroe / Diana / Einstein) + synastry / composite / davison.github/workflows/api-accuracy.yml— auto-run on PR- Triangulation against swetest CGI + Kerykeion — weekly
- Upstream Swiss Ephemeris monitoring via Dependabot
Known limitations
Section titled “Known limitations”- Extreme dates (before 1800 and after 2399): uses Moshier analytical, precision ~0.1” (vs < 0.01” for SWIEPH with DE431 files)
- True Lilith (id=13) vs Mean Lilith (id=12): up to 12° difference — the
default is Mean Lilith (stable behavior), True Lilith is available via
planetIds: [13] - Topocentric vs geocentric: geocentric by default
References
Section titled “References”- Swiss Ephemeris: https://www.astro.com/swisseph/
- swetest CGI: https://www.astro.com/swisseph/swetest.htm
- NASA Eclipse Catalog: https://eclipse.gsfc.nasa.gov/
- Kerykeion: https://github.com/g-battaglia/kerykeion
- Astrodienst: https://www.astro.com/
Contact
Section titled “Contact”Accuracy issues: report via support@astroway.info with chart data + expected reference (from astro.com or similar authoritative source).