VSOP87

VSOP87 (Variations Séculaires des Orbites Planétaires) is an analytical planetary theory published by P. Bretagnon and G. Francou (Bureau des Longitudes, 1988). It expresses heliocentric positions as trigonometric series in time and is widely used in open-source ephemeris code.

AstroRust’s VsopEphemeris (astrorust_ephemerides) evaluates this family of series, transforms to geocentric ecliptic coordinates, and feeds core.chart and vsop.ephemerides.

Why VSOP here

MIT-compatible — no Swiss Ephemeris (AGPL) dependency in the workspace.
Self-contained — positions at arbitrary Julian dates without shipping giant ephemeris files.
Good natal accuracy — sufficient for sign/house work when combined with a documented Moon model.

Precision (literature)

Bretagnon & Francou quote typical uncertainties for 1900–2100 at sub-arcsecond to tens of milliarcsecond levels for inner planets and Earth–Moon barycenter; Saturn is looser (0.1″ in some tables). Over longer spans around J2000, VSOP87 is often quoted within **1″** for inner planets over thousands of years (see VSOP model).

That is far better than a degree for Sun–Pluto sign placement, but not identical to JPL DE440/Swiss Ephemeris at mas level.

Implementation notes in AstroRust

Integration tests anchor Sun/Moon to fixture longitudes with documented tolerances (Sun ~2°, Moon ~5° in longitudes.json) — implementation regression bounds, not a claim of VSOP error.
The Moon uses a separate Meeus-style path in this crate; lunar longitude is the usual weak point vs DE ephemerides.
vsop.ephemerides exposes raw planet_position and house_cusps; core.chart wraps the same backend into NatalChartOutput.

Versions and time scale

VSOP87 has variants (A–E: heliocentric vs barycentric, J2000 vs date). Implementations pick one consistent set of files. Time argument is ideally TT; natal APIs accept UTC and convert to Julian Day — see ephemeris-uncertainty, julian-day-ephemeris.

References

Bretagnon, P. & Francou, G. 1988, A&A 202, 309 — original paper
VSOP model — Wikipedia
Bureau des Longitudes VSOP87 data — VizieR catalog VI/81