Skip to content

JPL DE-series ephemeris kernels — what they are, when to use which

antikythera-spectral validates the encoder against JPL's DE-series ephemeris files. Different eras of validation need different kernels.

Kernel summary

Kernel Coverage Size on disk Use when
de421 1899-07-29 to 2053-10-09 ~16 MB Modern control only (E-H1a). Cannot reach Hellenistic dates.
de422 3001 BCE to 3000 CE ~660 MB Antikythera-era queries OK. Older long-coverage option.
de440 1849 to 2150 ~32 MB Modern queries with current best dynamics.
de441 13201 BCE to 17191 CE ~3.1 GB JPL current best (Park et al. 2021). Default for Hellenistic.
de441_part1 13201 BCE to 1550 CE ~1.5 GB Hellenistic-only subset of DE441. Recommended for Antikythera work.
de441_part2 1550 CE to 17191 CE ~1.5 GB Modern + future subset of DE441.

How to download

Per ADR 0001 / 0003 we never auto-download. Explicit user opt-in:

antikythera-spectral kernel download de441_part1   # prompts y/N, ~1.5 GB

Or programmatically:

from antikythera_spectral.ephemeris import download_kernel
download_kernel("de441_part1")    # interactive y/N prompt + size warning

The download lands in ~/.antikythera_spectral_data/ (or wherever skyfield's Loader resolves to). Subsequent load_ephemeris('de441_part1') calls find it on disk.

Allowlist

The package only accepts kernel names from a frozen tuple, validated before any URL or filesystem path is constructed:

ALLOWED_KERNELS = (
    "de421", "de422", "de440", "de441", "de441_part1", "de441_part2",
)

Adding a kernel requires a PR review (and an ADR if the new kernel comes from a non-JPL source). See ADR 0003 for the discipline.

Why so many

JPL releases successive ephemerides as their dynamic models improve. DE441 is the current best long-coverage kernel; DE422 is its predecessor; DE421 is the pre-2008 modern-only standard. We retain DE421 for two reasons:

  1. Modern-control test (E-H1a) — pin the encoder against the modern era using a kernel small enough to ship in CI cache.
  2. Cross-validation comparator — compare_ephemerides('de421', 'de441') shows users how much the modern best-estimate differs from older models within their overlap range.

ΔT — see DELTA_T_MODEL.md

Hellenistic-era queries inherit ~3 hours of Earth-rotation drift uncertainty from the ΔT model the kernel uses. The cross-kernel differences a user sees at -200 BCE are mostly ΔT, not real kernel disagreements.

References

  • Park, R. S. et al. (2021). The JPL Planetary and Lunar Ephemerides DE440 and DE441. Astron. J. 161:105.
  • Folkner, W. M. et al. (2014). The Planetary and Lunar Ephemerides DE430 and DE431. IPN Progress Report 42-196.
  • Skyfield documentation: https://rhodesmill.org/skyfield/