A Validated, Fully Offline Planetary Ephemeris Engine from Public-Domain JPL Data

We describe and validate the positional engine of ChartVault, a desktop application that computes geocentric ecliptic longitudes entirely offline. Planetary positions (Sun through Pluto) are read from public-domain JPL Horizons samples, tabulated daily from 1900 to 2100, and evaluated at runtime by four-point Catmull-Rom cubic interpolation; the Moon is computed from an independent analytic model. Against live JPL Horizons over 10,953 comparisons on a 60-day grid across 1900-2100, the planetary longitudes agree to a maximum of 0.27 arcsec with a root-mean-square error of 0.105 arcsec and no detectable bias. The Moon agrees to about 2.4 arcsec for dates through the present; its residual grows for future dates because the analytic model's Delta-T prediction diverges from that adopted by Horizons, a difference the Moon's rapid motion amplifies. We then argue that this precision already exceeds the practical requirement of natal astrology: a one-minute uncertainty in birth time shifts the Ascendant by roughly 900 arcsec, four orders of magnitude larger than the planetary ephemeris error. The engine is offered as a case study in reaching reference-grade positions on-device, from public data, without a licensed ephemeris library.