Our birth chart calculation method

The coordinate choices

We use the Western tropical zodiac and geocentric apparent positions. The ten bodies are Sun, Moon, Mercury, Venus, Mars, Jupiter, Saturn, Uranus, Neptune, and Pluto. The tropical signs are twelve equal longitude sectors; they are distinct from the unequal modern astronomical constellations.

Whole Sign houses count signs from the rising sign. The Ascendant and Midheaven are calculated separately. The MC’s Whole Sign house is reported, rather than assumed to be ten. A negative longitude speed marks retrograde motion.

Aspects and selection

Conjunction and opposition allow 8° from exactness, square and trine 7°, and sextile 5°. The same orb applies to every point. The Ascendant–Midheaven pair is excluded. Classifications use full-precision values before display rounding.

All accepted aspects appear in the paid reference tables. The wheel shows twelve of the highest-ranked aspects. Interpretations prioritize personal points, aspect type, and closeness, with stable tie-breaking. Relationships solely among Jupiter through Pluto are listed as slower shared context.

Time and supported details

The form accepts birth dates from January 1, 1900 through today. Historical IANA time zone rules resolve the local clock reading; a place provider’s current offset is never substituted for the offset on a birth date. Before 1970, the confirmation identifies historical rules and the offset used. Repeated and nonexistent clock times require an explicit choice.

Known times use the stated minute. Approximate times use a selected window of plus or minus 15, 30, 60, or 120 minutes. We certify stability throughout that window for signs, houses, angles, aspect membership, and retrograde flags. If stability cannot be established, the complete product is unavailable. Unknown times use the whole local day, without angles or houses.

Complete readings are supported only at latitudes up to 66° north or south. Outside that range, the preview shows supported Sun and Moon information. We never silently switch house systems.

Reproducible calculation and reading

Our calculation engine is astronomy-engine, an MIT-licensed JavaScript library. We check its apparent positions against NASA/JPL Horizons reference data. Each saved chart records the engine, numerical conventions, and time-zone versions. The reference section names the exact calculation settings used for that saved result, so the reading remains inspectable.

Interpretations are assembled from versioned written content and deterministic selection rules. No live generative AI writes a purchased reading. Saved purchases retain their original chart and content; correcting birth details creates a new revision. Each major reading section identifies the actual features it is based on.

Geometry and meaning are different sources

Astronomical calculation establishes positions and angles. Astrological traditions supply the symbolic vocabulary used for reflection. A successful astronomical comparison does not validate a personality claim or prediction.

Sources and calculation facts

Read the versioned calculation conventions, including coordinate handling, time resolution, and the exact aspect policy.

• Astronomical calculation: NASA/JPL Horizons coordinate and ephemeris documentation.
• Historical civil time: IANA Time Zone Database.
• Astrological tradition: House division history, The Astrology Podcast.