Where the numbers come from.

Planetary positions come from astronomy-engine, an open-source library under the MIT licence.

It is deliberately not a Swiss Ephemeris derivative. Those are published under the AGPL, which would require the source of this site to be handed to anyone who loads the page. astronomy-engine is half the size and holds well under an arcminute across the range a birth chart needs.

Obliquity of the ecliptic is the mean value, IAU 1980. Nutation peaks near 9.2 arcseconds and is left out, because it is smaller than the error in a birth time written down to the nearest minute.

Sidereal positions use the Lahiri, or Chitrapaksha, ayanamsa. It is anchored at J2000 and carried forward on the IAU 2006 figure for general precession in longitude.

Houses are Placidus, computed by dividing each degree's own semi-arc rather than the equator. Above the polar circles the method is genuinely undefined, and the calculation says so instead of guessing.

One chart, computed independently with Swiss Ephemeris through pyswisseph, is committed as a fixture. Every build asserts against it before anything else is allowed to run.

Two layers, and they are checked differently. Seven planetary longitudes, twelve house cusps and three angles are held to a tolerance, because they carry real error bars. The verdicts drawn from them — lord of action, its dignity, its house, sect, which step of the cascade answered, the tenth-division rising sign, its tenth lord and condition, the current period — are checked exactly, with no tolerance at all. A planet is Mars or it is not, and a tolerance on a verdict is a way of not noticing you broke it.

Worst disagreement across the nineteen measured values: 0.86 arcminutes. The fixture is itself rounded to the whole arcminute, so it cannot express better than half of one, which puts the real error under half an arcminute.

Ptolemy, Tetrabiblos IV.4, in the line of Paulus, Rhetorius and Lilly.

The lord of action is found by a cascade. A trade planet making a heliacal phase answers first. Failing that, a trade planet configured with the culminating degree. Failing both, the ruler of the Midheaven.

Which step answered is recorded, and so is every step that failed and why. A chart where the first two steps found nothing is a different claim from one where the first step answered, and the reading is written knowing which it was.

The Dasamsa, the tenth-division chart, read for its rising sign and for the placement and condition of its tenth lord.

Timing is Vimshottari: twenty-seven nakshatras, nine period lords, one hundred and twenty years, entered at the longitude of the Moon at birth.

The Hellenistic cascade and the Dasamsa are computed separately from the same positions and are never averaged. Where they agree, the reading says so. Where they split, the reading shows the split rather than resolving it.

Those two produce the reading you get today. Further traditions are written and unit-tested in the codebase but are switched off, and nothing switched off contributes a word to a reading.

A place name becomes coordinates and a time zone through Open-Meteo's geocoding API.

Local clock time becomes universal time through the platform's own copy of the IANA time zone database, which carries the historical rules rather than today's. Los Angeles in January 1959 resolves to Pacific Standard Time because the database knows the 1959 rules, not because anybody told it.

Twice a year that conversion has no single answer. An hour that never existed is moved forward to the first instant that did. An hour that happened twice is handed back to you as a question, because only you know which one you were born in.

Every number above is computed before a word is written. The reading is written by Claude, a model made by Anthropic, from those numbers.

It interprets a finished chart. It does not calculate one, and nothing it writes can move a position, a cusp or a period.

Next.js and React, MIT. Instrument Serif, Inter and IBM Plex Mono, under the Open Font Licence. The site is served by Cloudflare.

The chart arithmetic runs in your browser. Nothing on this page requires you to take our word for where a number came from.