CelestialCalendar Python Package

celestial-calendar provides Python bindings for CelestialCalendar: astronomical calculations, Gregorian / Chinese Lunar date conversion, and Jieqi (节气, the 24 solar terms) moments. Each wheel contains its target platform's native library, so using it requires no compiler or separate CelestialCalendar installation. Python 3.11 or newer is supported.

中文

This guide describes the current source, including unreleased changes. The install command gets the latest published version; use its matching documentation. To run current-source examples, build a local wheel. For upgrades from 0.x, see Migrating to 1.0.

Install

Install the wheel for your platform from PyPI into your application's Python environment:

python -m pip install celestial-calendar

Each Python block below is a standalone program for example.py; run it with python example.py in that environment. Apart from this package, the examples use only the Python standard library.

Examples

Today's lunar date

Choose today's civil date at a fixed UTC+8 offset first, then convert that date label with ALGO3. "Today" therefore does not depend on the host timezone. This does not involve UT1, and the clock's UTC is not treated as UT1.

from datetime import datetime, timedelta, timezone

import celestial_calendar as celestial

east_eight = timezone(timedelta(hours=8))
today = datetime.now(east_eight).date()
gregorian = celestial.GregorianDate.from_date(today)
lunar = celestial.gregorian_to_lunar(celestial.LunarAlgorithm.ALGO3, gregorian)
print(today, lunar)

GregorianDate.from_date() accepts a standard-library date, not a datetime. Calling .date() after selecting the offset is deliberate: the library does not silently discard a time or timezone. The lunar algorithms use different date bases and year windows; see Lunar algorithms.

UTC and JDE(TT) conversions

utc_to_jde() and jde_to_utc() are available from 1.1.0. Before 1972 they treat UTC civil fields as UT1 using Algo5, the fixed ΔT model; chart queries still reject pre-1972 UTC. Leap-second boundaries are not generally invertible. After the leap table's last entry, these conversions retain its final TT-minus-UTC offset.

import celestial_calendar as celestial

utc = celestial.CivilDateTime(2026, 10, 28, (19 * 3600 + 35 * 60 + 37) / 86400)
jde_tt = celestial.utc_to_jde(utc)
print(celestial.jde_to_utc(jde_tt))

fraction is a fraction of a day (0.5 is noon). JDE is a floating-point day count, so the converted civil fields can differ slightly from the input.

For UTC input → next Jieqi → UTC+8 display, see the guide with runnable Python and JavaScript programs. It also gives the separate forward/inverse input domains and noninvertible boundaries.

Next Jieqi and remaining days

The reference instant is explicitly supplied as 2026-12-31 00:00 UT1, not taken from the current clock. Query every Jieqi in both 2026 and 2027, sort by actual UT1 moment, and select the first strictly later event. The enum starts at Lichun (立春), not the first event in a Gregorian year.

import celestial_calendar as celestial

cutoff_ut1 = celestial.CivilDateTime(2026, 12, 31, 0)
cutoff_jd = celestial.ut1_to_jd(cutoff_ut1)
events = [celestial.jieqi_moment(year, jieqi) for year in (2026, 2027) for jieqi in celestial.Jieqi]
ordered = sorted(events, key=lambda event: celestial.ut1_to_jd(event.moment_ut1))
upcoming = next(event for event in ordered if celestial.ut1_to_jd(event.moment_ut1) > cutoff_jd)
remaining_days = celestial.ut1_to_jd(upcoming.moment_ut1) - cutoff_jd
print(celestial.jieqi_name(upcoming.jieqi))
print("UT1:", upcoming.moment_ut1)
print("Remaining UT1 days:", remaining_days)

The difference between two UT1 Julian Days retains fractional days; subtracting calendar dates would lose the time of day. This is a fixed contemporary query across one year boundary, not a search algorithm for the entire API domain. The Jieqi year domain and failure rules are under API contract.

In the UT1 examples above, CivilDateTime is explicitly UT1, and its last field is the day fraction: 0.5 means half a day. ut1_to_jde() converts the same UT1 input to a TT-based JDE. moment_ut1 is not UTC or a UTC+8 wall clock. Establish the time-scale conversion before displaying either.

Complete chart snapshot of one instant

The input is explicitly 2026-01-01 12:00 UTC: the host timezone is not read, and UTC is not treated as UT1.

import celestial_calendar as celestial

snapshot = celestial.chart_snapshot(
  celestial.CivilDateTime(2026, 1, 1, 0.5),
  celestial.CivilScale.UTC,
  celestial.GeoLocation(51.5, 0.0),
  celestial.HouseSystem.PLACIDUS,
)
print(snapshot.times.jd_ut1, snapshot.times.jde_tt)
for body in snapshot.bodies:
  print(body.target.name, body.longitude_deg, body.longitude_rate_deg_per_tt_day, body.distance_au)
print(snapshot.houses.cusps_deg)

The result is an owned, frozen dataclass; the body and cusp collections are tuples. The fourteen bodies are, in order, the Sun, the Moon, Mercury through Pluto, the mean ascending/descending nodes, and the true ascending/descending nodes; each has apparent geocentric longitude, latitude, and a signed longitude rate. Positions use the true ecliptic and equinox of date, and angles are degrees. Distances are AU for every body, including the Moon; the standalone moon_apparent_geocentric_coordinate() gives the Moon's distance in kilometres, as MoonCoordinate.distance_km. The longitude rate is instantaneous, in degrees per TT day: a negative value means the longitude is decreasing at that instant (retrograde motion). Nodes have known-zero latitude and None distance. The four axes and twelve cusps are all in [0, 360) degrees, and the cusp tuple starts at house 1. Longitude is east-positive and latitude north-positive; location affects houses only. See the shared replay example for full values.

Capabilities and model identities

capabilities() describes native-owned TT target domains for POSITION, LONGITUDE_RATE, EVENTS and EQUATORIAL. It rejects HOUSES, CHART and DELTA_T with ValueError. NaN endpoints mean no declared hard bound; FINITE_INPUT_EVALUATION permits evaluation at finite epochs, which may still fail. Neither finite evaluation nor coordinate-validity checks establish accuracy or physical plausibility at remote epochs. house_capability(delta_t_model=...) instead returns a CivilHouseCapability: civil years, allowed scales, UTC start date, resolved Delta T key and an optional exclusive derived-TT cutoff. It does not promise geometric solvability. Civil-house policy is queried only through house_capability(), not capabilities(). capability_text() requires a concrete Target for target operations, and subject=None for HOUSES, CHART and DELTA_T descriptions. An inapplicable Target is rejected rather than ignored.

import celestial_calendar as celestial

records = celestial.capabilities(celestial.CapabilityOperation.EVENTS)
pluto = next(r for r in records if r.subject is celestial.Target.PLUTO)
print(pluto.domain_start_jde_tt, pluto.domain_end_jde_tt, pluto.maximum_span_tt_days)
print(celestial.capability_text(celestial.CapabilityOperation.EVENTS, celestial.Target.PLUTO))
identities = celestial.model_identities()
algo5 = next(i for i in identities if i.key == celestial.ModelKey(celestial.ModelKind.DELTA_T, 5))
print(celestial.model_identity_text(algo5.key))

Independent position and house queries

positions() evaluates only the targets you select at one JDE(TT) instant — no rate stencil, no snapshot joint window — so a subset succeeds where a complete snapshot rejects an excluded target's window. house_query() runs the snapshot's civil conversion without the joint window and records which models it resolved. Both fail whole on any error; there is no partial result.

import celestial_calendar as celestial

selected = celestial.positions(
  [celestial.Target.PLUTO, celestial.Target.SUN],
  2409543.5,  # Pluto's window start
)
for position in selected:
  print(position.target, position.longitude_deg, position.latitude_deg, position.distance_au, position.source)

location = celestial.GeoLocation(latitude_deg=51.5, longitude_deg=0.0)
civil = celestial.CivilDateTime(year=1885, month=1, day=1, fraction=0.0)  # before the snapshot window
houses = celestial.house_query(civil, celestial.CivilScale.UT1, location, celestial.HouseSystem.PLACIDUS)
print(houses.times, houses.houses.ascendant_deg, houses.delta_t_model, houses.nutation_model)
again = celestial.house_query(
  civil,
  celestial.CivilScale.UT1,
  location,
  celestial.HouseSystem.PLACIDUS,
  delta_t_model=houses.delta_t_model,
  nutation_model=houses.nutation_model,
)
equatorial = celestial.equatorial_positions([celestial.Target.SUN, celestial.Target.TRUE_ASCENDING], 2451545.0)
for position in equatorial:
  print(position.right_ascension_deg, position.declination_deg, position.distance_au, position.source)

Target is shared by positions, events, capabilities and chart bodies. Ecliptic and equatorial results are distinct frozen values, preserve selection order, and reject empty or duplicate selections. RA is [0, 360) degrees and declination [-90, 90] degrees, geocentric and of date; the projection uses fixed IAU1980 longitude nutation and true obliquity. Neither query computes rates, civil times or houses. Physical distances are AU; mathematical nodes have no distance.

ModelKey(kind, identity) is immutable and kind-qualified; zero is never a resolved identity. Registry entries expose .key, position/chart bodies expose .source, and house/chart results expose .delta_t_model and .nutation_model. Matching keys can replace selector enums without converting registry ordinals. DEFAULT is input-only and native-resolved; use operation text to inspect defaults. Reusing keys preserves model selection, while exact coefficients also require the library/source version. delta_t(..., model=...) still returns a float and validates the native resolved key. Source fields use ModelKey. SunCoordinate.distance_au is in AU; MoonCoordinate.distance_km is in kilometres.

Interval longitude events

longitude_events(), longitude_stations() and longitude_difference_events() expose the C++ interval searches for supplied longitudes, stationary instants and directed two-body longitude differences. Angles carry no names or interpretations — map exact target angles to your own terminology.

import celestial_calendar as celestial

crossings = celestial.longitude_events(celestial.Target.MERCURY, 0.0, 2460735.5, 2460785.5)
for event in crossings:
  print(event.jde_tt, event.longitude_deg, event.direction)

stations = celestial.longitude_stations(celestial.Target.MERCURY, 2460735.5, 2460785.5)
for station in stations:
  print(station.jde_tt, station.motion_before.value, "->", station.motion_after.value)

differences = celestial.longitude_difference_events(
  celestial.Target.MOON, celestial.Target.SUN, -60.0, 2460676.5, 2460707.5
)

Times are JDE(TT) on the published 2^-30-day reporting lattice; rates are degrees per TT day from the published seven-point operator. The interval [start, end) must be nonempty, at most 3660 TT days, and inside the target's longitude-rate domain; an empty tuple is a legal no-event outcome, while numerical ambiguity or work-limit exhaustion raises CelestialError rather than returning a partial list.

Platforms and local builds

Official releases use wheels for the four platforms below; PyPI and the matching GitHub Release carry the same four wheels, byte for byte:

Linux wheel filenames may include other compatible manylinux tags. Each wheel is a py3 wheel for Python 3.11 or newer on its platform. No wheels are published for other platforms, such as Intel macOS, Windows on ARM, or musl-based Linux.

Build a local wheel

Use a full source checkout; copying only bindings/python does not work. The Linux shell example below needs Python 3.11 or newer, system CMake 3.22 or newer, and an installed C/C++23 toolchain. It selects clang-22 and clang++-22; if your compilers have other names, set CC and CXX accordingly. The existing hash-locked requirements supply the build frontend, the backend, and Ninja. Run it from the repository root, with build, virtual-environment, and output directories that are not already in use:

python3 -m venv build/local-wheel-env
. build/local-wheel-env/bin/activate
python -m pip install --require-hashes -r bindings/python/requirements-build.txt
CC=clang-22 CXX=clang++-22 python -m build \
  --wheel --no-isolation \
  --outdir build/local-wheelhouse \
  --config-setting=build-dir="$PWD/build/local-wheel-build" \
  bindings/python
deactivate

Install the .whl produced in build/local-wheelhouse into your application environment. This builds only a wheel, not an sdist. That wheel uses the host's native library; it is not an official repaired wheel or a portable release wheel, and it does not extend the supported platforms in the table above.

API contract

The flat public API uses frozen dataclasses, enums, plain scalars, and tuples; it does not expose the ctypes protocol. CivilDateTime has four fields: year, month, day, and fraction, a finite fraction of the day in [0, 1); there are no separate hour, minute, or second fields. A function name, a field name, or CivilScale states the time scale; civil moments are never implicitly converted to Python datetime values. The docstrings in __init__.py give the per-function contracts.

Wrong input types, including members of the wrong enum, raise TypeError. Values that fail finiteness, range, or domain checks raise ValueError, and so does an object of the expected enum class that is not one of its declared members. Native invalid-input errors also raise ValueError. Computation, resource and internal failures raise CelestialError, as do corrupt successful payloads. Its operation names the public call; recorded is true when native reported a recognized error kind, even if it could not allocate a message. A legitimate absence remains None or (). Errors are not automatically printed.

local_apparent_sidereal_time(jd_ut1, longitude_deg) accepts a finite JD on UT1 whose Gregorian year is in [401, 32766], and a finite, east-positive geographic longitude in [-180, 180] degrees. The result is in [0, 360) degrees. A JD outside the native year range raises ValueError. apparent_solar_time() also accepts an east-positive geographic longitude in [-180, 180] degrees, but its civil input is UTC.

jieqi_moment(year, jieqi) returns JieqiMoment(jieqi, moment_ut1). Its CivilDateTime is UT1, not UTC or a UTC+8 wall clock; shown at UTC+8, the same instant can fall on a different date. Establish the time-scale conversion before displaying UTC or a fixed-offset time. The returned UT1 year can differ from the requested year. The input year domain is [401, 32766], but even an in-range call raises CelestialError when the native calculation cannot produce a unique moment.

moon_phase_moments(year, phase), sun_longitude_crossings(year, longitude_deg), and new_moons_in_year(year) accept Gregorian years in [1, 32766].

sun_longitude_crossings() returns a tuple of JDEs on TT at which the Sun reaches the given apparent geocentric longitude. longitude_deg must be finite and in [0, 360) degrees. Native failures use CelestialError.operation == "sun_longitude_crossings".

new_moons_after(jde, count) accepts count in [0, 4096]; zero returns (). The cap keeps a single native output buffer within 32 KiB. Variable-length result and text buffers have a separate binding resource cap of 64 MiB per buffer; exceeding it raises CelestialError before allocation. Inconsistent native results also raise CelestialError.

Chart inputs and failure boundaries

chart_snapshot(civil_dt, scale, location, system, model=DeltaTModel.DEFAULT) accepts a CivilDateTime, CivilScale.UTC / UT1, a GeoLocation, and HouseSystem.EQUAL / WHOLE_SIGN / PLACIDUS. Native resolves DEFAULT (currently ALGO5); select models explicitly with DeltaTModel or a matching ModelKey. It does not accept a standard-library datetime: the caller must first convert local wall-clock time to the declared input scale, not drop the timezone and pass the rest.

Python checks record/enum/scalar types, finiteness and ABI integer width before native. Wrong types raise TypeError, including a plain string or another enum's member for scale, system, or model, and an enum member in a numeric field. ValueError covers an object of the expected enum class that is not one of its declared members, invalid Gregorian dates, non-finite numbers, and raw values out of range: civil years [1885, 2100), day fraction [0, 1), UTC from 1972-01-01 (select CivilScale.UT1 for earlier input), latitude (-90, 90), and longitude [-180, 180]. The native core owns the shared converted-TT domain JDE [2409543.875, 2488069.125), Algo4's additional JDE < 2464328.5 limit, Ascendant directions unresolved at round-off in every house system, and date-dependent Placidus polar limits. Native domain/geometry rejection raises ValueError; computation failures raise CelestialError, with no partial result or substitute house system. Civil-house queries share the same exclusive derived-TT Algo4 cutoff; this also rejects UT1 inputs near the end of 2034 whose derived TT crosses the cutoff. A valid raw civil year does not guarantee that the converted snapshot can be calculated.

For UTC input, the reported UT1 is modelled UT1, not observed DUT1; Time scales explains how UTC reaches TT and UT1. An inserted leap second is outside the civil day-fraction input. The Python result is copied from the C protocol and keeps no native buffer; incompatible extensions require an explicitly different shape or major.

Date-only bridges and Lunar round trip

GregorianDate(year, month, day) is a proleptic Gregorian date, separate from CivilDateTime and LunarDate. Calculation calls check that Gregorian date and civil-moment inputs have a year in [1, 32767] and a valid month and day. The project's date types still have a wider range than the standard library's: the constructors do not cap the year at 9999.

from datetime import date

import celestial_calendar as celestial

day = date(2026, 8, 15)
gregorian = celestial.GregorianDate.from_date(day)
print(gregorian.to_date())

algorithm = celestial.LunarAlgorithm.ALGO3
lunar = celestial.gregorian_to_lunar(algorithm, day)
print(celestial.lunar_to_gregorian(algorithm, lunar).to_date())

GregorianDate.from_date(value) is a static factory: it always returns exactly GregorianDate, even when called through a subclass. It accepts datetime.date instances and subclasses in the standard library's year range [1, 9999], but rejects every datetime.datetime instance or subclass, with or without a timezone, by raising TypeError. gregorian_to_lunar(algorithm, date) accepts a GregorianDate or the same standard-library date input; it rejects datetime rather than dropping its time or offset.

GregorianDate.to_date() checks the fields and returns a standard-library datetime.date. A field of the wrong type raises TypeError; a year outside [1, 9999] or an invalid Gregorian month or day raises ValueError. Both bridges preserve the year/month/day label. In Lunar conversion, that label belongs to the date basis of the algorithm selected below; neither bridge converts timezones or UT1/UTC/TT.

Lunar algorithms

Select LunarAlgorithm.ALGO1, ALGO2 or ALGO3. Gregorian inputs and outputs are calendar-date labels, not instants. For sources, date bases, Lunar-year domains and selection roles, see Lunar algorithms.

supported_lunar_year_range(algorithm) queries the native algorithm's inclusive Lunar-year range and returns LunarYearRange(start, end). Gregorian coverage runs from the first day of Lunar year start to the last day of Lunar year end, not from January to December of those Gregorian years. lunar_year_info(algorithm, year) returns the Gregorian first_day, the traditional leap_month number or None, and month_lengths in calendar order, with a leap month directly after the regular month of the same name. lunar_to_gregorian() accepts a LunarDate with a traditional month number in [1, 12] and an explicit is_leap boolean, not a positional month index.

lunar_year_info() and lunar_to_gregorian() leave date/year admission to their native calculation. The Python package keeps no separate range table or cache and makes no range query at import or before those calls.

A valid Gregorian label outside the selected algorithm's coverage, or a Lunar leap-month/day combination that does not exist, raises ValueError, as does a Lunar year outside the selected algorithm's domain.

Delta T models

delta_t(year, model=DeltaTModel.DEFAULT) returns TT minus UT1 in seconds for a finite decimal Gregorian year. Select DeltaTModel.ALGO1 through ALGO5, or a matching ModelKey; the default resolves to Algo5. Sources, maintenance status, hard domains and selection roles are in Delta T models.

License

Project-authored package material is licensed under MIT. Third-party components retain their own terms in the included THIRD_PARTY_NOTICES.txt; each section states where it applies. Source and issue tracking are at https://github.com/0xf3cd/celestial-calendar.