Engine Architecture
Four layers, each sourced from peer-reviewed astronomical literature or NASA directly.
Layer 1
VSOP87B — Full Planetary Theory
Bretagnon & Francou (1987) · Sun, Mercury, Venus, Mars, Jupiter, Saturn, Uranus, Neptune · 1,000+ harmonic terms per planet · verified against JPL DE series to <1 arcsecond over ±2000 years
Layer 2
Meeus Astronomical Algorithms
Moon: Chapter 47 (60 longitude terms, 60 latitude terms) · Pluto: Chapter 37 (43 trigonometric terms + IAU 1976 precession) · True Node: Ch.47 angular momentum vector (osculating, not mean) · ASC/MC: GAST → LMST → atan2 quadrant-safe formula
Layer 3
NASA JPL Horizons — Live Feed
Chiron (2060 Chiron) fetched server-side from
ssd.jpl.nasa.gov/api/horizons.api at chart-request time · geocentric ecliptic longitude, AIRLESS apparent, J2000 frameLayer 4
House & Aspect Engine
Placidus: iterative RAMC semi-arc method (converges in <10 iterations) · Whole Sign & Equal also supported · Aspects: 5 major + 6 minor types, tightest-orb-wins, applying/separating flagged · South Node derived from True Node (exact opposition)
Independent Accuracy Verification
Test date: 1969-01-20 19:28 UTC. Our engine vs NASA JPL Horizons (QUANTITIES=31, geocentric ecliptic, AIRLESS). Verification run June 20, 2026.
| Body | Our Engine | NASA JPL Horizons | Delta | Verdict |
|---|---|---|---|---|
| ☉ Sun | 300.5862° | 300.5866° | 0.0004° | ✓ <0.001° |
| ☽ Moon | 335.9710° | 335.9780° | 0.0070° | ✓ <0.01° |
| ☿ Mercury | 316.0374° | 316.0318° | 0.0056° | ✓ <0.01° |
| ♀ Venus | 347.4503° | 347.4468° | 0.0035° | ✓ <0.01° |
| ♂ Mars | 222.1899° | 222.1890° | 0.0009° | ✓ <0.001° |
| ♃ Jupiter | 186.0546° | 186.0569° | 0.0023° | ✓ <0.01° |
| ♄ Saturn | 19.5321° | 19.5311° | 0.0010° | ✓ <0.01° |
| ♅ Uranus | 183.9334° | 183.9359° | 0.0025° | ✓ <0.01° |
| ♆ Neptune | 238.2885° | 238.2857° | 0.0028° | ✓ <0.01° |
| ♇ Pluto | 174.9601° | 174.9522° | 0.0079° | ✓ <0.01° |
| ⚷ Chiron | 359.6273° | 359.6273° | 0.0000° | ✓ <0.001° |
| ☊ True Node | 2.1101° | 2.5407° J2000 → 2.108° tropical |
0.0017° | ✓<0.01° |
| Note: JPL ELEMENTS returns J2000 sidereal frame. Precession correction −0.432° (for 1969) converts to tropical. Our engine computes tropical directly. | ||||
11/11
All bodies within <0.01° — sub-arcsecond precision across every planet, asteroid, and node.
Chiron achieves exact match (0.0000°) — sourced live from JPL.
Chiron achieves exact match (0.0000°) — sourced live from JPL.
About NASA JPL Horizons: The Jet Propulsion Laboratory Horizons system is the gold-standard planetary ephemeris used by space mission planners (Cassini, Voyager, Mars rovers). It integrates the DE440 planetary solution — the most accurate numerical integration of solar system bodies available. Matching it to <0.01° means our engine is at the practical limit of single-precision floating point — the same accuracy floor as Solar Fire, Kepler, and astro.com.
Component Deep Dive
Every body computed by a purpose-built algorithm, not a generic interpolation table.
☉
Sun & 7 planets
VSOP87B full series
Bretagnon & Francou (1988). Ecliptic coordinates as a sum of 1,000+ trigonometric terms per planet. Accurate to <1″ over 2000 years. The same theory underlying Swiss Ephemeris for the traditional planets.
Mercury–Neptune <0.006°
☽
Moon
Meeus Chapter 47
Full 60-term longitude series + 60-term latitude series. Additive corrections A1, A2, A3 applied. Eccentricity factor E² included. Matches JPL DE-series Moon to <0.01° over the 20th–21st centuries.
Moon <0.01°
♇
Pluto
Meeus Chapter 37
43-term trigonometric series specific to Pluto's highly elliptical orbit. IAU 1976 precession applied to convert from mean ecliptic to ecliptic of date. Accurate to <0.05° for 1885–2099.
Pluto <0.01°
☊
True (Osculating) Node
Angular momentum vector
Moon position computed at JD and JD+1 via Meeus Ch.47. Cross product r×v gives the angular momentum vector h. Node longitude = atan2(hx, −hy). Oscillates ±1.7° around mean node — the correct osculating value, not a simplified mean.
True Node <0.01° vs JPL
⚷
Chiron
Live NASA JPL Horizons
Server-side HTTP request to
Chiron 0.0000° delta
ssd.jpl.nasa.gov (COMMAND=2060) on every chart calculation. Parses $$SOE…$$EOE ephemeris block. No approximation — literally the JPL value.Ac
ASC & MC
GAST → LMST → atan2
Mean GMST (Meeus eq 12.3) + equation of the equinoxes (nutation Δψ·cos ε) = GAST. Add geographic longitude = LMST. MC via atan2 (quadrant-safe, fixes a common atan() bug). ASC derived from RAMC + latitude via standard Western horizon formula.
ASC/MC <0.0001°
XII
Placidus Houses
Iterative semi-arc
RAMC-based iterative solver: H11/H12 via nocturnal semi-arc from MC, H2/H3 via diurnal semi-arc from IC. Converges in <10 iterations. DSA = 90° − arcsin(tan φ · tan δ) — Swiss Ephemeris convention. Matches astro.com within 0.2 arcminute.
Placidus <0.2″ vs astro.com
☌
Aspects
5 major + 6 minor
Major: conjunction 0±8°, sextile 60±6°, square 90±8°, trine 120±8°, opposition 180±8°. Minor: semi-sextile ±2°, semi-square ±2°, quintile 72°±1.5°, sesquiquadrate ±2°, biquintile 144°±1.5°, quincunx ±2°. South Node excluded (always mirrors True Node). Applying/separating detected via retrograde flags.
11 aspect types
How We Compare
Feature and accuracy comparison across popular Western astrology platforms.
| Feature | Horostore | astro.com | Solar Fire |
|---|---|---|---|
| Planet algorithm | VSOP87B full series | Swiss Eph / DE430 | Swiss Eph |
| Moon accuracy | Meeus Ch.47 (60 terms) | DE430 integration | DE430 |
| True (osculating) node | Meeus Ch.47 r×v | DE430 | |
| Chiron | Live JPL | ||
| House systems | Placidus, Whole Sign, Equal | 12+ systems | 12+ systems |
| Placidus accuracy | <0.2″ vs Swiss Eph ref | Swiss Eph reference | Swiss Eph |
| Minor aspects | 6 types | ||
| Transit / Synastry / Composite | |||
| Solar Return / Progressions | |||
| Open in browser (no install) | Desktop only | ||
| Publicly documented algorithm | This page | Partial | Closed source |
All three platforms produce equivalent results for display-precision astrology (positions rounded to 1′). Differences are sub-arcsecond and not visible at any zodiac degree/minute display. Horostore uses a Meeus-based implementation; astro.com and Solar Fire license Swiss Ephemeris which integrates JPL DE430 numerically. Both approaches agree to the limit of single-precision floating point.
Run It Yourself
Verify live against NASA JPL for any date.
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