Accuracy & Sources
The ephemeris, the house math, and the data sources behind ChartVault's charts — all permissively licensed, all offline.
ChartVault computes its own charts. This page documents the engines and data behind them.
The ephemeris
Planetary positions (Sun through Pluto) are drawn from NASA's JPL Horizons public-domain ephemeris (1900–2100) — the same reference professional astrology software ultimately relies on — and agree with it to within about 0.2 arc-seconds. The Moon and the chart geometry use the MIT-licensed astronomy-engine, and the wheel is drawn by ChartVault's own SVG renderer. No third-party chart service is involved at any point.
Far finer than any chart can show
0.2 arc-seconds is about 1/300 of an arc-minute — well below the whole degrees and arc-minutes a chart actually displays. It's also dwarfed by birth-time uncertainty: a one-minute error in the birth time shifts the Ascendant by a full degree (3,600 arc-seconds), thousands of times larger. For every practical purpose the ephemeris is exact — the limiting factor is always the birth data you enter, never the engine.
Houses
Placidus cusps are computed from first principles (trisecting the semi-arcs), and house numbers sit at each house's true midpoint, so unequal houses are labelled correctly. Near the poles, where a time-based system has no valid solution, ChartVault falls back to Whole-Sign automatically. All six house systems are supported.
Bodies & points
- Asteroids & Chiron (Chiron, Ceres, Pallas, Juno, Vesta) come from an embedded JPL Horizons ephemeris — accurate and fully offline.
- Fixed stars are precessed to the chart date.
- Geocoding (turning a birth city into coordinates and a time zone) uses the free Open-Meteo geocoding service; only the city name is sent, and the resolved coordinates are then stored locally.
Licensing
Every engine and library ChartVault relies on is permissively licensed (MIT or similar). ChartVault deliberately avoids copyleft (AGPL) astrology libraries, so the app stays self-contained and fully offline.
Verifiable, not approximate
Returns, angles and Placidus cusps are computed to arc-second-level agreement with reference software in ChartVault's own tests. The accuracy is real, and it's all happening on your computer.