French Ephemeris

FrenchEphemeris is a fast, thread safe ephemeris and astrology calculation library written in Rust, with Python bindings. It computes planetary, lunar and asteroid positions from NASA/JPL DE441/DE442, the same data JPL Horizons serves, together with house cusps, ayanamsas, lunar nodes and Lilith, fixed stars, rise and set times, solar and lunar eclipses, occultations and heliacal events. A pyswisseph style API (swe_compat) lets existing Swiss Ephemeris code switch with one import change.

It is the engine behind every calculation of FreeAstroAPI, and an independent, clean room alternative to the Swiss Ephemeris.

pip install frenchephemeris          # works immediately: years 0 to 2400 are built in
python -m frenchephemeris.packs download   # optional: JPL's exact numbers for 1800 to 2401

• Accurate: within 0.0005″ of JPL Horizons for years 0–1550; every eclipse of the NASA Five Millennium Canons found, years 0–2400.
• Fast: ~500,000 positions/s per call from Python, ~4.5 million/s in batches.
• Thread safe: no global state; settings are per thread and per asyncio task.
• Complete: 24 house systems, 46 ayanamsas, 8,969 fixed stars (every naked-eye star), Chiron, Pholus, main asteroids, any numbered asteroid, years −13198 … +17190.
• No setup: the package ships a compact data set for years 0 to 2400; download JPL's exact numbers when you want them, or a 2 MB set for mobile apps.

Contents

• Installation
• Quickstart
• Data packs
• Features
• Accuracy and validation
• Performance
• FrenchEphemeris and the Swiss Ephemeris
• Documentation
• FAQ
• Licensing · Data sources · Trademarks · Disclaimer · Security

Installation

Prebuilt wheels (CPython 3.9 and later, one abi3 wheel per platform):

pip install frenchephemeris
python -m frenchephemeris.packs download          # exact JPL data for 1800 to 2401, ~69 MB, SHA-256 checked

The engine reads data packs (JPL coefficients in the FEPK format). The wheel includes a compact lossy set for years 0 to 2400 (7.7 MB), so the library works right after pip install. packs download adds JPL's exact numbers from the packs v1 release; they are then used wherever they apply, and the built in set covers the other dates. See Data packs for every set. To build from source you need Rust 1.88+ and maturin: pip install maturin && maturin build --release.

JavaScript (Node.js 18+, Deno, Bun, browsers): the same engine compiled to WebAssembly, with the 2 MB mobile data set for years 1800 to 2400 included.

npm install frenchephemeris

import { load, julday } from "frenchephemeris";
const eph = await load();
eph.chart(julday(1990, 5, 17, 14.5), 48.8566, 2.3522);   // planets with signs, houses

See js/README.md. Build it with tools/build_npm.sh.

Quickstart

Python, pyswisseph style

import frenchephemeris.swe_compat as swe

jd = swe.julday(1990, 5, 17, 14.5)                       # UT

(lon, lat, dist, speed, *_), _ = swe.calc_ut(jd, swe.MOON, swe.FLG_SPEED)
cusps, ascmc = swe.houses(jd, 48.8566, 2.3522, b"P")     # Placidus, Paris; ascmc[0] = Ascendant

swe.set_sid_mode(swe.SIDM_LAHIRI)                         # this thread / async task only
(sid_lon, *_), _ = swe.calc_ut(jd, swe.SUN, swe.FLG_SIDEREAL)

xx, name, _ = swe.fixstar2_ut("Spica", jd, 0)             # fixed stars by name

Most code written for pyswisseph runs after replacing import swisseph as swe with import frenchephemeris.swe_compat as swe. See the migration notes.

Python, native engine and NumPy batches

import numpy as np
import frenchephemeris as fe

eng = fe.open_default()                                   # every installed pack
times = 2448029.1 + np.arange(10_000.0)                   # JD UT
pos = eng.calc_batch_ut(times, list(range(10)))           # Sun .. Pluto, shape (10000, 10, 6)
sid = eng.calc_batch_ut(times, [1], sidereal="lahiri", topo=(2.3522, 48.8566, 35.0))

Batches run without the GIL and return NumPy arrays: longitude, latitude, distance and their speeds.

Rust

[dependencies]
fe-core = { git = "https://github.com/FreeAstroApi/frenchephemeris", tag = "v1.5.0" }

use fe_core::context::{Body, CalcOptions, Engine};

fn main() -> fe_core::Result<()> {
    let eng = Engine::open("packs/fepk-de441-de442-v1.bin".as_ref())?;
    let moon = eng.calc_ut(Body::Moon, 2448029.104166667, &CalcOptions::default())?;
    println!("Moon {:.6}°, {:.4}°/day", moon.lon, moon.lon_speed);
    Ok(())
}

Data packs

python -m frenchephemeris.packs download --set <name> fetches a set and checks every file against published SHA-256 sums. list shows every pack, path prints the data directory.

Packs go to $FRENCHEPHEMERIS_DATA if set, otherwise a per user data directory (~/.local/share/frenchephemeris/packs, ~/Library/Application Support/frenchephemeris/packs or %LOCALAPPDATA%\frenchephemeris\packs). $FRENCHEPHEMERIS_PACK names the main pack directly. Packs are memory mapped, so worker processes share them. Exact packs are always preferred; the built in set answers only for dates or bodies they do not cover (FRENCHEPHEMERIS_NO_FALLBACK=1 turns that off). Outside every loaded pack, calls raise an error instead of returning an approximation.

Features

The complete function list is in the user guide.

Accuracy and validation

Every figure has its method, sample and reproduction command in the engine report and the topic reports listed under Documentation.

Performance

Measured on an Apple M4 with tools/bench_throughput.py --swiss (Python, 10 bodies, random dates 1900–2100, with speeds):

The engine is immutable and Send + Sync; single positions keep the GIL (a few microseconds of work), batches and searches release it.

FrenchEphemeris and the Swiss Ephemeris

FrenchEphemeris is an independent implementation, written from published sources only. Where an independent reference exists (JPL Horizons, NASA eclipse canons, IERS Delta T), FrenchEphemeris is the closer of the two; where none exists (future Earth rotation, Mean Lilith conventions), the reports say so. Data files are a choice: Swiss ships small lossy files, FrenchEphemeris ships JPL's numbers bit identical, plus a lossy mobile set that is smaller than Swiss's files for the same years. The full point by point comparison is in docs/ENGINE_REPORT.md and docs/SWISS_GAP_ANALYSIS.md.

Documentation

FAQ

What is FrenchEphemeris? An open source ephemeris library for Python and Rust that computes the positions of the Sun, Moon, planets, asteroids and stars from NASA/JPL data, with the geometry astrology needs: houses, ayanamsas, nodes, crossings, eclipses and rise/set times.

Is it a replacement for pyswisseph and the Swiss Ephemeris? Yes for most code. frenchephemeris.swe_compat accepts pyswisseph's function names, flags and return shapes (calc_ut, houses, set_sid_mode, fixstar2_ut, rise_trans, eclipse functions and more). Results agree with Swiss Ephemeris 2.10 to under 1″ for modern charts. It is an independent implementation, not affiliated with the Swiss Ephemeris.

How accurate is it? It reads JPL's DE441/DE442 coefficients directly: within 0.0005″ of JPL Horizons for years 0–1550, and within 0.13″ for the Sun, Moon and inner planets after 1550 (where it uses the newer DE442). Charts display 0.001° = 3.6″, far coarser than either figure.

Which data files do I need, and how large are they? None to start: the package includes a compact set for years 0 to 2400 (planets within 0.007″ of JPL, Moon within 0.4″). python -m frenchephemeris.packs download adds JPL's exact numbers for 1800 to 2401 (69 MB), --set full for −2 to 2581 (256 MB), and --set mobile (2 MB) is a lossy set for apps.

Can I use it in a mobile or embedded app? Yes. The mobile set is 2.03 MB for 1800–2400, planets, Moon and minor bodies included, with planets within 0.007″ and the Moon within 0.4″ of JPL. The Rust crate depends only on memmap2 and thiserror.

Is it thread safe? Can I use it in FastAPI, Django or Celery? Yes. There is no global mutable state; the sidereal mode, observer position and Delta T settings in swe_compat are stored per thread and per asyncio task, so concurrent requests never see each other's settings.

Does it support Vedic and KP astrology? Yes: 46 ayanamsas including Lahiri, Raman, Krishnamurti and True Chitra, sidereal houses, nakshatra ready longitudes, the true node, and sunrise times for panchang. FreeAstroAPI runs its Vedic, KP and dasha endpoints on it.

Which date range is covered? Years 0 to 2400 out of the box, −2 to 2581 with --set full, and −13198 to +17190 with the extended packs. Outside the loaded range, calls raise an error.

Is it free for commercial use? Yes under the AGPL-3.0, provided you meet its conditions (including publishing the source of modified versions you offer over a network). For closed source products, the commercial licence costs USD 200 once per company, with every future version and unlimited end users.

Who maintains it, and is it used in production? Gabriel Naulleau (FreeAstroAPI). It has computed every FreeAstroAPI response since October 2026.

How do I report a bug or a security issue? Open an issue for wrong results or crashes; report vulnerabilities privately as described in SECURITY.md.

Licensing

FrenchEphemeris is dual licensed:

• AGPL-3.0-only (LICENSE), free, with the AGPL's obligations, including offering the source of modified versions to users who interact with them over a network (§ 13); or
• a commercial licence, USD 200 one time, per company, for the full term of copyright with all updates, including use behind your own APIs and apps for unlimited end users, for closed source or otherwise non AGPL use; see the commercial licence terms, contact contact@freeastroapi.com.

SPDX: AGPL-3.0-only OR LicenseRef-FrenchEphemeris-Commercial. Copyright (C) 2026 Gabriel Naulleau EI, trading as FreeAstroAPI. Contributions require the contributor licence agreement in CONTRIBUTING.md.

Data sources and attribution

Third party data and their terms are listed in NOTICE: NASA/JPL DE441/DE442 and small body ephemerides (Park et al. 2021), ERFA (BSD-3) coefficient tables, the IAU WGSN star names (CC BY 4.0), Hipparcos (ESA; van Leeuwen 2007), SIMBAD (CDS), IERS EOP and leap seconds, USNO and Stephenson–Morrison–Hohenkerk Delta T, NASA eclipse catalogues (F. Espenak), the ELP 2000-82B and ELP/MPP02 lunar theories (Chapront-Touzé, Chapront, Francou), and the published astronomy and astrology definitions behind each feature. Per constant provenance is in the docs/*REPORT*.md files.

The Swiss Ephemeris was used only as a black box oracle through its public API to measure compatibility. No Swiss code, data files or outputs are part of this repository; the oracle comparison fixtures are kept in a private corpus (FE_ORACLE_DIR), and the tests that need them are skipped without it.

Trademarks

"Swiss Ephemeris" is a trademark of its owner. FrenchEphemeris is an independent implementation; it is not affiliated with, sponsored or endorsed by the owner or the authors of the Swiss Ephemeris. The name is used here only to describe compatibility and to report measured comparisons. The swe_compat Python module is an interface compatibility layer that accepts the same function names and argument shapes; it contains no Swiss Ephemeris code.

"FrenchEphemeris" and "FreeAstroAPI" are trade names of Gabriel Naulleau EI; see TRADEMARKS.md for how forks and derived works may refer to them.

Disclaimer

FrenchEphemeris is provided "as is", without warranty of any kind, as stated in the AGPL-3.0 (sections 15–16) and in the commercial licence. Results are intended for astrology, education and general astronomy. It is not designed or certified for navigation, surveying, spacecraft operations, legal or civil timekeeping, or any safety critical use. Positions far from the present (before about 1600 and after about 2100) inherit the uncertainties of the underlying models (Delta T, minor body orbits), which are documented in the reports.

Security

See SECURITY.md to report a vulnerability. Community rules are in CODE_OF_CONDUCT.md.