gbc-astro

A Python astrology calculation engine powered by Swiss Ephemeris.

gbc-astro is the installable Python package for the open-source GetBirthChart astrology calculation engine.

The canonical source repository is maintained at github.com/getbirthchart-com/gbc-astro-engine.

• Project: GetBirthChart
• Website: https://getbirthchart.com/
• Maintainer: Luis Pham

It computes natal chart facts: planetary positions, tropical zodiac signs, houses, Ascendant, Midheaven, aspects, lunar nodes, and Chiron. It does not include the website, accounts, payments, or interpretation text.

Package release 1.12.2 uses calculation engine 1.12.1. Natal schema 1.3.0.

Installation

Python 3.12 or newer.

pip install gbc-astro

This installs the pyswisseph binding. Swiss Ephemeris .se1 data files are not included. Provision them yourself and point the library at the directory:

export GBC_SWISS_EPHE_PATH=/path/to/swiss/ephemeris

Required files for the modern-era natal path: sepl_18.se1, semo_18.se1, and seas_18.se1 (Chiron). A helper script in this workspace can fetch them:

./scripts/fetch-ephemeris.sh
export GBC_SWISS_EPHE_PATH="$(pwd)/ephemeris/swiss"

Those files have their own upstream redistribution terms.

If GBC_SWISS_EPHE_PATH is unset or the files are missing, natal calculation raises ProviderDependencyError. The engine requests Swiss files (FLG_SWIEPH) and does not fall back to the Moshier ephemeris.

Quick start

from gbc_astro import calculate_chart

chart = calculate_chart(
    date="1990-05-15",
    time="09:30",
    latitude=51.5074,
    longitude=-0.1278,
    timezone="Europe/London",
    house_system="placidus",
)

print(chart.bodies["sun"].sign)
print(chart.angles["ascendant"].longitude)

timezone is required. Coordinates are geographic degrees, not a place name.

To run the HTTP adapter locally or on the VPS:

pip install "gbc-astro[api]"
uvicorn gbc_astro.api.app:app --host 127.0.0.1 --port 8000

pip install gbc-astro does not install FastAPI or uvicorn.

Timed birth chart

With a known local time, the result includes bodies, angles, twelve house cusps, and aspects:

chart = calculate_chart(
    date="1992-11-03",
    time="14:35",
    latitude=21.0285,
    longitude=105.8542,
    timezone="Asia/Ho_Chi_Minh",
    house_system="placidus",
)

chart.subject.birth_time_known  # True
chart.bodies["sun"].longitude
chart.bodies["moon"].longitude
chart.angles["ascendant"].longitude
chart.angles["mc"].longitude
chart.houses
chart.aspects

A checked sample from the test suite (Hanoi, 1992-11-03 14:35, Asia/Ho_Chi_Minh, Placidus):

• Sun longitude 221.14154838535987 (Scorpio)
• Moon longitude 321.2929834918872 (Aquarius)
• Ascendant longitude 350.1088136374758 (Pisces)

Unknown birth time

If time is omitted or None, the library does not guess a birth time and does not substitute noon.

chart = calculate_chart(
    date="1990-05-15",
    time=None,
    latitude=51.5074,
    longitude=-0.1278,
    timezone="Europe/London",
)

Output	Unknown-time behavior
subject.birth_time_known	false
Ascendant, MC, DSC, IC	omitted ({})
House cusps	omitted (())
bodies.*.house	null
Vertex, Part of Fortune, chart ruler	omitted / empty
Warning	UNKNOWN_BIRTH_TIME

Bodies are still computed at local date start (midnight in the given IANA timezone). That is an explicit approximation, not a claimed birth time.

Moon: there is no separate Moon-uncertainty flag. The Moon can move about 13° in a day, so sign and degree can differ from the unknown true time.

calculate_houses(...) without a time raises MissingBirthTimeError instead of returning fabricated cusps.

Supported calculations

• geocentric ecliptic longitude, latitude, distance, and longitude speed
• tropical zodiac sign and degree in sign
• house assignment when birth time is known
• retrograde from signed longitude speed
• Ascendant, Midheaven, Descendant, IC
• twelve house cusps
• major aspects with orb and applying/separating phase
• true node, mean node, south node, Chiron
• derived points when geometry allows (vertex, Part of Fortune)
• derived natal facts (big three, moon phase, element/modality counts, dignities)

AstrologyEngine also exposes relationship charts, transits, returns, and related surfaces. Those are not part of the small calculate_chart API.

Supported house systems

Ids: placidus, koch, porphyry, campanus, regiomontanus, alcabitius, topocentric, morinus, meridian, whole_sign, equal.

Default: placidus.

Placidus and Koch have no solution beyond the polar circles. The engine raises HouseCalculationUnavailableError there. It does not silently switch systems.

Timezone handling

• date must be a real Gregorian calendar date YYYY-MM-DD
• time is HH:MM or HH:MM:SS when known
• timezone is an IANA identifier (Europe/London, Asia/Ho_Chi_Minh)
• latitude must be in [-90, 90], longitude in [-180, 180]
• DST spring-forward gaps raise NonexistentLocalTimeError
• DST overlaps raise AmbiguousLocalTimeError unless fold=0 or fold=1 is set
• there is no geocoder in this package

Local datetimes are timezone-naive. UTC conversion uses zoneinfo and the IANA database.

Ephemeris setup

Planetary, lunar, node, Chiron, house, and angle calculations use Swiss Ephemeris through pyswisseph. There is no internal planetary formula.

Default natal profile: tropical zodiac, Placidus houses, true node, major aspects (western-modern-v1).

Output model

calculate_chart returns a frozen NatalChart dataclass:

chart.subject.birth_time_known
chart.subject.utc_datetime
chart.bodies["sun"].longitude
chart.bodies["sun"].sign
chart.angles["ascendant"].longitude   # present only when time is known
chart.houses                          # empty when time is unknown
chart.aspects
chart.warnings
chart.meta.engine_version             # "1.12.1"
chart.to_dict()

gbc_astro.__version__ is the package release (1.12.2). ENGINE_VERSION and chart meta.engine_version remain 1.12.1. SCHEMA_VERSION is 1.3.0.

Accuracy and testing

Automated tests include golden Swiss natal values, hostile inputs, DST boundaries, and unknown-time contracts. Independent geometry-parity tolerances used in this engine are on the order of 1e-5 degrees for angles/cusps against an in-repo reference implementation.

This package does not claim identity with Astro.com, Astro-Seek, or other commercial chart services. Those are not committed oracles here. Astrology is not treated as a scientifically validated predictive system.

Limitations

• Swiss Ephemeris .se1 files are not on PyPI and must be provisioned
• unknown birth time omits angles and houses; body positions use local midnight
• altitude is stored but not applied to positions or houses
• FastAPI adapter source is in the wheel; install gbc-astro[api] for the HTTP server
• closed-source distribution of this package is incompatible with AGPL-3.0
• Swiss Ephemeris itself is dual-licensed; this project uses the AGPL path

License

GNU Affero General Public License v3.0 only (AGPL-3.0-only). See LICENSE.

Swiss Ephemeris is copyright Astrodienst AG and is dual-licensed (AGPL or the Swiss Ephemeris Professional License). pyswisseph is distributed on PyPI under AGPL v3. Ephemeris .se1 files are not redistributed by this package. See THIRD_PARTY_NOTICES.md and https://www.astro.com/swisseph/swephinfo_e.htm.

This is not an MIT-licensed project.

Development and testing

Python 3.12+:

python3.12 -m venv .venv
source .venv/bin/activate
python -m pip install -e ".[dev]"
./scripts/fetch-ephemeris.sh
export GBC_SWISS_EPHE_PATH="$(pwd)/ephemeris/swiss"
python -m pytest

Project links

• GetBirthChart: https://getbirthchart.com/
• Source code: https://github.com/getbirthchart-com/gbc-astro-engine
• Issue tracker: https://github.com/getbirthchart-com/gbc-astro-engine/issues
• Maintainer: https://getbirthchart.com/author/luis-pham/