Trust & methodology
Data sources
A birth chart is only as accurate as the data behind it. GetBirthChart relies on Swiss Ephemeris data for planetary positions, the IANA timezone database for time conversion, and geographic place data for coordinates. Each calculated chart records the ephemeris and timezone data versions used to produce it.
Editorial standards overseen by Luis PhamMethodology
Ephemeris data
Planetary positions are computed from Swiss Ephemeris (opens in a new tab) 2.10.03 (via pyswisseph) by GetBirthChart Core, not approximated in the browser. An ephemeris is a table of where the Sun, Moon and planets are over time. The gbc-astro core records the provider version in calculation metadata.
Every chart stores the ephemeris provider and data version that produced it, and that provenance is shown in the chart's calculation-metadata section so results stay traceable.
Timezone data
Local birth time is converted to Universal Time using the IANA time zone database (opens in a new tab), which encodes historical timezone boundaries and daylight-saving rules. This is what lets a chart use the offset that was actually in force at your birth date and place, rather than a single fixed guess.
The timezone data version is recorded alongside each chart.
Place and coordinate data
When you search for a birthplace, GetBirthChart matches it to a GeoNames `cities5000` place record with latitude, longitude, country/region labels and an IANA timezone. The import uses GeoNames country and administrative reference files; place search lives in the website, while the engine itself takes coordinates and a timezone, not a city name.
The selected coordinates are passed to the calculation engine so the horizon, angles and houses are computed for that point on Earth. A place label is not a substitute for the coordinates and timezone stored with the chart.
Default calculation rules
The product default is the tropical zodiac, Placidus houses, True Node, Standard aspects and Chiron enabled. Major aspects use these orbs: conjunction 8°, sextile 5°, square 7°, trine 7°, opposition 8°. Retrograde is taken from negative ecliptic longitude speed.
The Python implementation of these calculation rules is available in GetBirthChart Core (opens in a new tab) (gbc-astro), an open-source engine licensed under AGPL-3.0-only. Swiss Ephemeris remains a separate upstream dependency with its own terms; see the official licensing information (opens in a new tab).
Celebrity and unknown-time source policy
Celebrity pages distinguish date-only, reported-time and reliable-time records. A date-only record does not support an exact birth time, Rising sign, houses or angle-based narrative. Conflicting or weak source records remain labeled rather than being upgraded by a chart calculation.
For an unknown personal birth time, the engine assessment—not a midnight or end-of-day guess—is authoritative. A stable Moon sign may be shown without an exact degree; uncertain or incomplete assessments are withheld or labeled. See unknown-time guidance.
What we do not claim
We only describe data we actually use. We do not cite sources we do not rely on, and we do not present interpretive astrological tradition as peer-reviewed science.
For how these inputs flow into a finished chart, see our methodology and calculation guide. Editorial standards for how calculation versus interpretation is described are overseen by Luis Pham.
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See the data sources and versions recorded for your own chart.
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