Calculation Accuracy

TL;DR

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: < 0.1 arcsec vs official swetest CGI at Astrodienst
Placidus house cusps: 0.000" exact match
Eclipses: < 1 minute vs NASA 5-Millennium Eclipse Catalog
ACG lines: < 1.5 km at the equator vs swetest
Sunrise/sunset: < 10 seconds vs timeanddate.com
Moon VOC, ingresses, planet conjunctions: sub-second precision

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

Accuracy is verified through triangulation — comparison against three independent sources:

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)

An independent Python library using pyswisseph bindings instead of our WASM. Confirms that our WASM layer does not distort the data.

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

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

Each of the core calculation endpoints is covered by frozen snapshot tests on 3 reference charts (Monroe / Diana / Einstein) = 561 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

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)

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)

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)

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)

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

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

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

Regression is checked on every PR through CI:

api-calc/tests/endpoints/ — 561 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

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