From birth moment to chart: our methodology

What Gaist actually calculates

1. Time and place: first make the moment unambiguous

You enter the local clock time at the birthplace. AstroAPI resolves the relevant IANA time zone from latitude and longitude and converts that local moment to Universal Time (UT). It then derives the Julian day used by Swiss Ephemeris. This also accounts for historical daylight-saving rules: technically and astrologically, a clock time without its place is incomplete.

Place does more than identify the time zone. It defines the local horizon and meridian, and therefore the Ascendant, Midheaven and house cusps. A few minutes can matter near a sign or house boundary, which is why angles and houses become more dependable with an accurate birth time.

2. Planetary positions from Swiss Ephemeris

Our open-source AstroAPI backend uses Swiss Ephemeris directly. For the UT moment it calls swe_calc_ut with the Swiss Ephemeris data files and reads the geocentric ecliptic longitude. Speed data is also used to determine retrograde motion. No language model participates in this calculation.

Longitude runs from 0° to just under 360°. The tropical zodiac divides that circle into twelve 30° sections. A longitude of 188.4°, for example, is 8.4° Libra: Aries starts at 0° and Libra at 180°. At this stage it is a precise coordinate on the zodiac, not yet a statement about a person.

3. Houses and angles: where a theme enters life

In astrological grammar, planets describe the force or function involved and signs colour how it tends to express itself. Houses answer a third question: in which area of life is it most likely to be experienced? “Mars in Virgo” gains a more specific setting when placed, for example, in the sixth house of routines and work or the seventh house of relationships. The chart angles orient this personal map: the Ascendant marks the eastern horizon and the Midheaven the upper meridian.

There is no single house system shared across the whole history of astrology. Placidus and Koch are time-based, unequal systems, so some houses can be much larger than others. Whole Sign assigns one complete sign to each house. Equal Houses uses twelve equal 30° sectors beginning at the Ascendant. Campanus, Regiomontanus, Porphyry, Topocentric, Alcabitius, Meridian and Domical divide space according to still other geometrical ideas. A change of system can therefore change cusps and house placements while leaving planetary zodiac positions untouched.

Gaist passes your choice to swe_houses and reports both the requested and effective system in the result. Legacy saved charts with no recorded choice are treated as Placidus. If Swiss Ephemeris cannot calculate Placidus or Koch at an extreme geographic latitude, AstroAPI transparently falls back to Porphyry. This is not a hidden interpretive switch: the result flags the technical exception.

4. Aspects: the geometry between chart factors

Aspects describe the shortest angle between two positions. They matter astrologically because a chart is not merely a list of isolated placements: a close Sun–Moon relationship tells a different story from the same two factors without a strong angle. Conjunction, sextile, square, trine and opposition form the familiar core; Gaist also calculates quincunx, semisextile, sesquiquadrate, quintile, biquintile and novile.

An aspect need not be mathematically exact. Its permitted deviation is the orb. AstroAPI currently uses 8° for conjunctions, squares and oppositions; 6° for sextiles and trines; 4° for sesquiquadrates, quintiles and biquintiles; 2° for quincunxes and semisextiles; and 1° for noviles. Two positions 92° apart therefore form a square with a 2° orb. The geometry is calculated; how strongly or constructively it is weighted belongs to interpretation.

5. What the public site shows—and what the app interprets

The public birth-chart calculator on this website returns the calculated positions, houses, angles and aspects and supports them with educational guides. It deliberately does not generate a personal AI interpretation here. You can inspect the underlying chart before adding a narrative layer.

The GAIST app does offer personal astrological interpretations. There, AI can bring the calculated factors together, explain their interplay and formulate a reading around your chart. The astronomical foundation remains the same: AI writes the interpretation; it does not invent the planetary positions. Astrological interpretation is a symbolic tradition, not a scientifically validated diagnosis or guaranteed prediction.

6. Privacy, reproducibility and limits

The public guest calculator sends your inputs for the requested calculation but does not save them as a personal profile. The same moment, coordinates and house system can be calculated again. Rounding, a software update or a different ephemeris release can produce small display differences.

We are enthusiastic about astrology precisely because it opens complex images and useful questions. It still does not replace medical, psychological, legal or financial advice. Whenever a decision needs verifiable facts, verifiable facts come first.

Three worked examples

Technical sources and source code

Swiss Ephemeris provides the astronomical calculation. OpenAstroAPI exposes the Java calculation path used by Gaist, including time-zone resolution, houses, aspects and fallbacks.

• Swiss Ephemeris
• OpenAstroAPI