Methodology & Accuracy
Celestium
Last updated: July 21, 2026
An astrology app is only as honest as its astronomy. This page shows exactly how Celestium computes your chart, which traditions inform the interpretations, and the measured accuracy of every number we show you; verified, not asserted.
The Engine
Celestium computes every position on your device with its own astronomical engine. Nothing is fetched from a horoscope feed and nothing is estimated by an AI. The methods are the published, peer-reviewed standards of positional astronomy:
- Sun, Mercury through Neptune: VSOP87 planetary theory (Bureau des Longitudes, Paris), with nutation and precession-to-date applied so positions sit in the tropical zodiac of the moment.
- Moon: Jean Meeus' 60-term lunar series (Astronomical Algorithms, ch. 47).
- Pluto: Meeus' dedicated periodic series, valid 1885 to 2099.
- Lunar nodes: the true node (mean node plus periodic corrections), the count most working astrologers use.
- Chiron: Keplerian orbital elements calibrated against Swiss Ephemeris J2000 values.
- Houses and angles: sidereal-time-based Ascendant and Midheaven with historical timezone resolution for the birth moment, including DST edge cases.
Verified Accuracy
We diff the engine against Swiss Ephemeris, the reference standard used by professional astrology software, across a 13-date grid spanning 1900 to 2090. The full artifact (every date, every body, every delta) is regenerated with the test suite and ships in the open. Current maximum absolute differences across the entire 190-year grid:
| Body | Max difference | In plain terms |
|---|---|---|
| Sun | 0.0016° | about 6 arcseconds |
| Moon | 0.017° | about 1 arcminute |
| Mercury–Neptune | ≤ 0.015° | under 1 arcminute |
| Pluto | 0.031° | under 2 arcminutes |
| True Node | 0.097° | under 6 arcminutes |
For scale: a chart wheel drawn at poster size would place these differences well inside the width of the glyph's stroke. Chiron is verified separately against astro.com reference charts (within 0.5°).
Your Chart, Your Method
Celestium does not lock you into "our" house system or a single tradition, but its defaults are chosen, not arbitrary. Placidus is the default house system because it is the standard of professional practice and the one most reference charts are drawn in; you can switch to whole sign, Porphyry, or Koch at any time, and your chart re-renders under the method that resonates with your practice.
The tropical zodiac is the default frame for the same kind of reason. The meanings of the signs grew up describing the turning of the seasons, so tropical is the authoritative frame for every interpretation written in that tradition. The sidereal zodiac of Vedic astrology is offered as its own tradition with natively written readings rather than tropical text relabeled, because the two frames each earned their meanings honestly. And the constellational, thirteen-sign view is offered as a labeled astronomical perspective, with no houses or rulers of its own, because it answers a question of where the planets literally sit rather than a question of meaning. The same principle guides where the app is heading: astrology first and deep, with other calendrical wisdoms joining only when we can do them authentically.
The Traditions Behind the Readings
Celestium reads traditional-first. Its backbone is Hellenistic: the sect, dignity, lots, and time-lord methods set down by Ptolemy and Vettius Valens around the turn of the era. That craft was carried east and made exact by the Perso-Arabic astrologers of the Islamic Golden Age, Abu Ma'shar and al-Biruni among them, whose rigor about a planet's condition still shapes every judgment the app makes. Vedic astrology (Jyotisha) is treated as a complete parallel tradition rather than a variant, with its own sidereal frame, nakshatras, and dasha timing. The modern century adds the outer planets and a humane, psychological tone, layered on top of the older discipline rather than replacing it. A fuller walk through each lineage, with a glossary that names the tradition behind every term, is in The Traditions Behind Your Chart.
The Interpretive Layer
Interpretations are composed from a curated text corpus grounded in the tradition (including public-domain professional sources), matched to the exact placements, dignities, and aspects your chart actually has. The composition is deterministic; what personalizes it is the astronomy, not a language model guessing. Optional AI synthesis exists as a separate, consent-gated layer that uses your own API key and always interprets engine-verified positions; it never calculates them.
Honest Limits
- Pluto positions are computed for 1885–2099; outside that range the series degrades and we say so rather than extrapolate silently.
- Houses require a birth time. If yours is unknown, house-dependent readings should be treated with care; first-class support for time-unknown charts is on our roadmap.
- Astrology itself is a symbolic tradition, not a predictive science. We hold the astronomy to scientific standards and the interpretation to the standards of the tradition; the difference matters and we keep it visible.