A high-precision astronomical ephemeris library for Python, powered by NASA JPL DE440/DE441 ephemerides and IAU 2006/2000A standards.
Drop-in replacement for PySwissEph - pure Python, no C extensions, easier deployment.
- NASA JPL DE440/DE441 - modern planetary ephemerides via Skyfield, with full-range DE441 support for deep-history and far-future work
- IAU 2006/2000A standards - precession, nutation, and obliquity via the official ERFA library
- Validated high precision - planetary differences typically measured in fractions of an arcsecond, house cusps < 0.02", benchmarked across 4,400+ comparison rounds (full report)
- Four backends, one API - Skyfield, LEB (~14x speedup), Horizons API, and adaptive auto mode through the same
calc_ut()interface - 25 house systems, 43 ayanamsha modes - independently verified against pyswisseph
- Physical planet centers - outer planets corrected from barycenters using JPL satellite ephemerides
- Thread-safe contexts when you need them - SwissEph-compatible globals for drop-in migration,
EphemerisContextfor concurrent workloads - 15,000+ years of coverage -
base,medium, andextendedprecision tiers from modern use to -13200 / +17191 CE - Pure Python 3.12+ - no C extensions, clean installs across CI, containers, and serverless
Swiss Ephemeris is the industry standard for planetary calculations. But its Python binding (pyswisseph) is a C extension - hard to build, hard to debug, tied to a single computation model.
LibEphemeris provides the same API with a modern foundation:
- NASA JPL ephemerides instead of semi-analytical theory - DE440/DE441 are the latest planetary ephemerides from the Jet Propulsion Laboratory, the same data used for spacecraft navigation.
- IAU 2006/2000A standards - precession and nutation computed via the official ERFA library (the open-source implementation of IAU SOFA), not custom routines.
- Physical planet centers - Jupiter, Saturn, Uranus, Neptune corrected from system barycenters to actual body centers using JPL satellite ephemerides. Most libraries skip this.
- Independently verified - every function cross-validated against pyswisseph, JPL Horizons, and astropy/ERFA. Precision report with full methodology.
- Pure Python - readable source, standard debugging, no build toolchain. Runs on any platform, any CI, any serverless environment.
Switching from pyswisseph? Your existing code works with minimal changes. Migration guide.
import libephemeris as swe
from libephemeris.constants import SE_SUN, SE_MOON, SEFLG_SPEED
jd = swe.julday(2000, 1, 1, 12.0) # J2000.0
sun, _ = swe.calc_ut(jd, SE_SUN, SEFLG_SPEED)
moon, _ = swe.calc_ut(jd, SE_MOON, SEFLG_SPEED)
print(f"Sun: {sun[0]:.4f} deg, speed {sun[3]:.4f} deg/day")
print(f"Moon: {moon[0]:.4f} deg, speed {moon[3]:.4f} deg/day")# House cusps (Placidus, Rome)
cusps, ascmc = swe.houses(swe.julday(2024, 11, 5, 18.0), 41.9028, 12.4964, b"P")
print(f"ASC: {ascmc[0]:.4f}, MC: {ascmc[1]:.4f}")For concurrent or multi-threaded workloads, use EphemerisContext instead of the module-level global state.
More examples: Getting Started
Every number independently measured against pyswisseph across 4,400+ comparison rounds. Full report.
| Category | Typical | Max | Scope |
|---|---|---|---|
| Sun-Pluto | 0.04-0.26" | 1.17" | Pluto max 0.75"; Neptune is the widest observed planetary delta |
| Moon | 0.70" | 3.32" | Different underlying lunar models |
| House cusps | < 0.01" | 0.02" | All 25 systems |
| Fixed stars | < 0.1" | 0.51" | 116 Hipparcos catalog stars |
| Solar eclipses | - | < 6s | Timing accuracy |
| Lunar eclipses | - | < 8s | Timing accuracy |
| Ayanamsha | < 0.001 deg | 0.006 deg | All 43 sidereal modes |
Choose your trade-off between speed, locality, and setup. The same calc_ut() interface works across all four modes, from zero-install Horizons lookups to precomputed LEB throughput.
| Mode | Backend | Speed | Use case |
|---|---|---|---|
"auto" |
LEB -> Horizons -> Skyfield | adaptive | Default. Best onboarding; resolves local or remote data transparently |
"skyfield" |
JPL DE440/DE441 via Skyfield | ~120 us | High-precision local JPL workflow |
"leb" |
Precomputed Chebyshev polynomials | ~5 us | Maximum throughput for repeated calculations |
"horizons" |
NASA JPL Horizons REST API | ~300 ms | No local ephemeris files required |
from libephemeris import set_calc_mode
set_calc_mode("leb") # or via env: LIBEPHEMERIS_MODE=lebpip install libephemerisOut of the box, the wheel includes a bundled LEB2 base-tier core for the 14 core bodies (1850-2150). With the default medium tier, the library can auto-download the additional LEB2 data it needs on first use.
Recommended first-time setup:
libephemeris init # Optional but recommended interactive config
libephemeris download auto # Download exactly what your config needs
libephemeris status # Verify installed data and active setupPrefer to install a tier directly? Use one of these:
libephemeris download base # 1850-2150, lightweight
libephemeris download medium # 1550-2650, ~200 MB (recommended)
libephemeris download extended # -13200 to +17191 CE, full rangeOptional extras: pip install libephemeris[stars] for star-catalog tooling, [nbody] for REBOUND/ASSIST n-body integration, [all] for everything. Details.
- Getting Started - installation, ephemeris tiers, first calculations
- Migration from PySwissEph - API mapping, flag compatibility, known divergences
- Precision Report - full methodology, comparison tables, verification process
- Flag Reference - all supported flags with examples
- House Systems - all 25 systems, verified against pyswisseph
- Ayanamsha Modes - 43 sidereal modes
- LEB Binary Ephemeris - format, generation, LEB2 compression
- Horizons Backend - HTTP client, pipeline, precision
- Architecture - internal design and data flow
- Methodology - planet centers, lunar apsides, pyerfa integration
- CLI Reference - full command reference
- Changelog - release history
git clone https://github.com/g-battaglia/libephemeris.git
cd libephemeris && uv pip install -e ".[dev]"
poe lint # Ruff lint + auto-fix
poe test:leb:fast # Recommended fast unit suite
poe test:compare:skyfield # Cross-validate vs pyswissephAGPL-3.0. See LICENSE.