Astrology method and sources

The positions of the Sun, the Moon, and the planets are calculated by Astronomy Engine, an open-source library (MIT license) based on published astronomical theories. They are apparent longitudes, measured on the true ecliptic and the equinox of date: light travel time, aberration, and nutation are taken into account, as in professional astrology software.

The same calculation runs in your browser and in our checks: each version of the site is compared with the reference engine written in Python (102 charts, 18,152 identical values to within a millionth of a degree). That engine is itself compared with Swiss Ephemeris, the reference for astrology software, on 102 charts from 1890 to 2030. Swiss Ephemeris is used only for these offline comparisons: it is not part of the site.

To place a sky, you first need to know the exact moment you were born. The time written on a birth certificate is a legal time, and it has changed many times: daylight saving time, wartime, time zones. Versana converts it to universal time with the IANA time zone database (version 2026e), from 1800 to 2100, rather than with your browser’s data, which varies from one browser to another for older dates.

France requires particular care. Between June 14, 1940, and November 2, 1942, the free zone and the occupied zone did not have the same time: Paris switched to German time, while the free zone followed French rules. Alsace-Moselle lived on Berlin time until November 11, 1918. Before 1891, France had no single legal time: the local mean time of the place is used, calculated from its longitude. When a time existed twice (return to winter time) or did not exist (switch to summer time), the form asks you which one you mean.

The lunar nodes are the two points where the Moon’s orbit crosses the ecliptic. The true node follows the Moon’s actual orbit at every moment; the mean node smooths out its oscillations using Jean Meeus’s formula. Both are used; the true node is our default setting, and you can switch to the mean node at any time.

Lilith, in our chart, is the mean Black Moon: the point of the lunar orbit farthest from the Earth, on average, projected onto the ecliptic. There are other Liliths, calculated differently; we state which one we show, and it stays hidden until you ask for it.

All calculations happen in your browser. Your date, time, place, and first name are not sent to our servers, not written into the page address, and not stored: what you enter is kept only in the open tab, so you can move from one tool to another, and disappears when you close it.

The place search also happens on your device. The browser downloads public place files, like any other resource on the page; each is named after the first two or three letters of a city name, never your date or time. None of your birth data appears in these requests.

A chart is accurate if the birth time is. A time rounded to the quarter hour moves the rising sign by several degrees, and sometimes by a sign: our tools show the range of times during which your rising sign stays the same. For the French free zone, the dates come from the IANA time zone database, which cites works by historians of timekeeping, and we have not yet cross-checked them against a reference treatise; for towns crossed by the demarcation line, you are asked a question.