Libephemeris

libephemeris

libephemeris is a pure-Python astronomical ephemeris library powered by NASA JPL DE440 / DE441. It is a drop-in replacement for and superset of pyswisseph: same function names, parameter order, tuple shapes, flag encoding, and exception surface, plus its own performance/offline extensions. This skill targets an agent writing Python against the library (not developing it).

Key facts to anchor on (verified against the installed package, 3.1.0):

• The public API uses canonical bare names — calc_ut, SUN, FLG_SPEED, SIDM_LAHIRI — never the swe_ / SE_ / SEFLG_ prefixes. (The lone documented back-compat alias is SE_FNAME_DE431.)
• Import convention: import libephemeris as swe — most pyswisseph code then runs unchanged.
• All numeric returns are native Python float (never numpy scalars).

Quick start

pip install libephemeris         # 3.1.0 stable. The wheel already bundles
                                  # the base-tier core: the 14 core bodies,
                                  # JD range 1850–2150. No download needed
                                  # for that slice.

import libephemeris as swe
from libephemeris import SUN, MOON, MARS, FLG_SPEED

jd = swe.julday(2000, 1, 1, 12.0)        # UT1 Julian Day (J2000.0)

# Positions. calc_ut takes UT1; calc takes TT (Terrestrial/Ephemeris Time).
pos, retflag = swe.calc_ut(jd, SUN, FLG_SPEED)
lon, lat, dist, slon, slat, sdist = pos  # deg, deg, AU, deg/day, deg/day, AU/day

# House cusps — NOTE argument order: (jd_ut, lat, lon, hsys). LAT BEFORE LON.
# hsys accepts an int code (ord('P')) OR a bytes char (b'P'); both work.
cusps, ascmc = swe.houses(jd, 41.9028, 12.4964, b"P")   # Placidus, Rome
asc, mc = ascmc[0], ascmc[1]

# Extended houses: cusps, angles, and their daily speeds (always computed).
cusps, ascmc, cusps_speed, ascmc_speed = swe.houses_ex2(jd, 41.9028, 12.4964, ord("P"))

Return shapes (memorize — they match pyswisseph):

• calc_ut(tjdut, body, flags) / calc(tjdet, body, flags) → ((lon, lat, dist, speed_lon, speed_lat, speed_dist), retflag).
• houses(...) → (cusps[12], ascmc[8]). ascmc = [Asc, MC, ARMC, Vertex, EquAsc, CoAsc, CoAscKoch, PolarAsc].
• houses_ex2(...) → (cusps[12], ascmc[8], cusps_speed[12], ascmc_speed[8]).
• fixstar_ut(name, tjdut, flags) → (pos[6], resolved_name, retflag).
• nod_aps_ut(tjdut, body, method=NODBIT_MEAN, flags) → (asc_node, desc_node, perihelion, aphelion), each a 6-float tuple.

Setting where data lives

• LIBEPHEMERIS_DATA_DIR (env var) relocates the downloaded data root (default ~/.libephemeris). This is where tiers/SPKs/IERS land.
• set_ephe_path(dir) — pyswisseph-compatible: point at a directory of ephemeris files; .bsp SPKs found there are auto-registered (no network).
• set_leb_file(path) or LIBEPHEMERIS_LEB — pin one specific .leb binary ephemeris for the fast path.

The four calculation modes

Select with set_calc_mode(mode) or the LIBEPHEMERIS_MODE env var (case-insensitive). Read the current one with get_calc_mode(). The only valid values are:

There is no "jpl" mode — passing "jpl" raises ValueError. "Local JPL" is the "skyfield" mode; "remote JPL" is the "horizons" mode.

mode="skyfield" and mode="horizons" never select LEB implicitly, and mode="leb" never opens a JPL/BSP source. See references/calc-modes.md for the sealed-mode contract, the network policy (auto / allow / sealed), and computation tracing.

Sealed LEB mode (offline, deterministic) — rc14 boundary

mode="leb" is a real source boundary. Persisted states come only from the active LEB files. A missing or corrupt canonical group is a provisioning error; a core request outside every available LEB interval raises EphemerisRangeError. rc14's headline change: no silent substitution to a lower-precision or different source on a miss — it fails loudly.

Network policy is orthogonal and enforced at the socket choke point:

• Policies: "auto" (default), "allow", "sealed". Set via set_network_policy(...), LIBEPHEMERIS_NETWORK_POLICY, or TOML network_policy.
• Under "auto": leb mode is sealed, every other mode is allowed.
• When sealed, any network attempt raises NetworkSealedError (re-exported at the package root; subclass of ConfigurationError and RuntimeError).

Data tiers and per-body coverage

Three cumulative precision tiers (set_precision_tier(...) / LIBEPHEMERIS_PRECISION):

Resolution is best-by-date, per body and per date, in fixed priority base → medium → extended: a narrower, higher-priority artifact is used where it covers the request; a broader tier is consulted only outside that stored interval. Per-body ranges recorded in the companion files are authoritative.

Ask the library rather than assuming coverage:

from libephemeris import get_body_coverage, CHIRON
cov = get_body_coverage(CHIRON, jd)     # BodyCoverage | None
# None => the active LEB reader does NOT contain this body. It does NOT imply
#         that any analytical or online fallback exists.
if cov:
    cov.jd_start, cov.jd_end, cov.data_file, cov.group
    cov.precision_class            # e.g. "ephemeris", "analytical", "numerical-model"
    cov.reviewed                   # manifest-verified?
    cov.contains(jd)               # bool

get_leb_inventory() returns active files, per-body ranges, mode, and effective network policy — the fastest self-diagnostic. See references/tiers-and-coverage.md for the data-v3 artifact set, provisioning, and the full typed error contract.

Provisioning

libephemeris init          # optional interactive config -> libephemeris-config.toml
libephemeris download auto # what the active mode/tier needs (see the caveat below)
libephemeris download leb2-base|leb2-medium|leb2-extended
                           # the SHA-256-pinned LEB2 groups for one tier,
                           # cumulative: 5 files (base), 10 (medium), 15 (extended)
libephemeris status        # verify installed data + active config (add --json)

download auto is mode-dependent. Under mode = "leb" it fetches only the 5/10/15 LEB2 files for the configured tier. Under the default mode = "auto" it also downloads the DE kernel, planet_centers.bsp and the minor-body SPKs — multi-GB at extended. Set the mode before running it in a sealed deployment.

Do not use libephemeris download base|medium|extended to provision LEB2. Those subcommands download DE kernels + SPKs and install zero LEB2 groups, so a sealed-leb runtime fetches hundreds of MB it will never open and still fails provisioning. The LEB2 commands are the leb2- prefixed ones above.

In leb mode you do not need planet_centers.bsp: outer-planet channels store system barycentres directly; only a non-sealed runtime may apply a JPL centre offset on top.

Traps to avoid

• houses arg order is (jd, lat, lon, hsys) — latitude before longitude. Swapping them silently produces a wrong-but-plausible chart.
• calc vs calc_ut: calc_ut takes UT1, calc takes TT. Astrology work almost always wants calc_ut. Mixing them shifts positions by ΔT.
• A body absent from your custom .leb (or outside its stored range) raises a typed error (UnknownBodyError for an unknown id; EphemerisRangeError for out-of-range) — it is not silently zero-filled or downgraded. Check first with get_body_coverage.
• FLG_MOSEPH / FLG_SWIEPH / FLG_JPLEPH are accepted for compatibility; the ephemeris source is governed by the calc mode, not these flags (FLG_MOSEPH is effectively ignored — calculations always use JPL data).
• LEBCorruptionError (a ValueError subclass) signals a truncated/corrupt LEB file; it is treated as fatal provisioning failure and never triggers a source fallback. It is not re-exported at the package root — import it from libephemeris.exceptions if you must catch it distinctly, else it surfaces as ValueError.
• Never seek Swiss Ephemeris files or data as a source; libephemeris is self-contained on JPL/IAU/ERFA. Just install and provision as above.

References

• references/calc-modes.md — the four modes in depth, sealed-mode contract, network policy, and computation tracing.
• references/tiers-and-coverage.md — tiers, best-by-date routing, data-v3 artifact set, get_body_coverage / BodyCoverage, get_leb_inventory, provisioning, and the full error contract.
• references/reference-api-compat.md — drop-in migration, naming, argument-order gotchas, return shapes, flags, and the documented intentional divergences.