How GetBirthChart calculates your chart

The calculation pipeline

Every chart follows the same deterministic sequence, so the same inputs always produce the same chart:

• Birth input — date, time (if known) and place.
• Timezone — the birthplace is resolved to its IANA timezone, with historical daylight-saving rules applied.
• Universal Time — local time is converted to UTC for the exact instant of birth.
• Ephemeris — a dedicated engine computes planetary positions from professional ephemeris data.
• Houses & angles — the Ascendant, Midheaven and house cusps are derived from time and location.
• Aspects — the angular relationships between planets are measured against defined orbs.
• Canonical chart — the result is a structured, versioned chart object.
• Interpretation — grounded, AI-assisted synthesis is written only from that chart's facts.

Why timezone and Universal Time come first

A birth chart is a snapshot of the sky at one instant. To locate that instant on the astronomical clock, local birth time must be converted to Universal Time using the correct timezone — including the daylight-saving rules that were actually in force at that date and place.

GetBirthChart resolves the birthplace to an IANA timezone and applies historical offsets, rather than assuming a single fixed offset. This matters most for the time-sensitive parts of the chart: the rising sign, the angles and the houses.

Positions, houses and aspects

Planetary longitudes come from professional ephemeris data via the calculation engine; the frontend does not approximate them. Each chart records which ephemeris and timezone data versions produced it, so a result is always traceable.

Houses and the four angles are computed from the birth time and geographic coordinates. Aspects are the angles planets make to one another, measured within defined orbs — the allowable distance from an exact angle.

Where interpretation comes from

Astronomy and interpretation are kept separate. The engine produces the facts of the chart. Written interpretation is generated with AI that is restricted to that chart's own evidence — it synthesises meaning from the calculated placements and does not compute or invent planetary positions.

Interpretation is offered as a reflective framework, not a scientific prediction of events. See our data sources and calculation guide for more detail.

Precision and limitations

Chart accuracy depends on input accuracy. A birth time that is off by minutes can shift the rising sign and house cusps. When a birth time is unknown, GetBirthChart omits the rising sign, houses and angles rather than guessing them, and still shows the placements that do not depend on exact time.

We describe the engine's precision only to the extent it is validated, and we do not overstate it.