How our astrology map works

The method

Astro Map draws classic astrocartography lines in mundo, the convention Jim Lewis used: for each planet, the places on Earth where it was on one of the four angles at your birth moment.

• Positions: apparent geocentric right ascension and declination of date (aberration, light-time, nutation), from astronomy-engine (opens in a new tab) 2.1.19, plus Greenwich apparent sidereal time (GAST). Geocentric, not topocentric.
• MC line: longitude = RA − GAST, a meridian from pole to pole. IC line: the same plus 180°.
• ASC and DSC lines: the places where the planet sits on the geometric horizon (altitude 0, the planet's centre, no refraction, no parallax). By hour angle H between 0° and 180°: latitude = atan(−cos H / tan δ), ASC longitude = RA − GAST − H, DSC longitude = RA − GAST + H. Together they form one great circle 90° from the point where the planet is overhead.
• Signs use the tropical zodiac. We don't draw Chiron, the lunar nodes, parans or local space lines.
• The rising sign is hidden for birthplaces above the polar circles (latitude beyond 90° minus the obliquity), where the ecliptic Ascendant is ambiguous.

Accuracy checks

We tested the engine against two independent references before shipping (re-run October 10, 2026):

Swiss Ephemeris is the engine astro.com and astro-seek run; we used it only as a local check and don't ship it. The real-world limit is your birth time: one minute moves every line by a quarter of a degree, far more than any of the numbers above. Our unit tests repeat these comparisons on every build.

How places are scored

For each city we measure the great-circle distance to every line. A line counts fully within 150 km (100 miles) and fades with a cosine curve to zero at 1,000 km (620 miles). Each planet and angle has an editorial weight from 0 to 3 per lens (for example Jupiter MC is 3 for Career, Venus DSC is 3 for Love, Saturn IC is 3 for Go gently). A city's lens score is its best weighted line plus half of its second best, so the maximum is 4.5.

Lists keep cities scoring at least 1.0, within 70° of the equator, sorted by score, then by the nearest line, then by population, and skip any city within 100 km of one already listed. Labels: 3 or more is a strong match, 2 a clear match, 1 a light match.

When the birth time is approximate ("Within an hour", "Rough guess" or a time of day), each line becomes a band that sweeps 15° of longitude per hour. We score a city by how consistently a line stays within 400 km of it across the whole window, not by one close pass, and we compare that with what a band can give a place at its own latitude. That matters because a band is narrower in kilometers far from the equator, so a plain average would favor far-northern towns; above 60° we also scale the score down by the band's width there. Among equally good places, larger and better-known cities come first (a mild population preference). With no time at all, a day of uncertainty sweeps every line around the globe, so we don't rank.

"Your top places" shows four different cities: if one city leads several lenses, it appears under its strongest one with "also strong for…", and the other lenses show their next city. The full lists keep their order.

The weights are our own model, written from widely shared traditional symbolism. They are not a published standard, and astrologers disagree about them, which is why every list shows the distance and the line behind it.

Time zones

• After 1970 we use your browser's time-zone database. Before 1970, for the 89 zones where the browser's history is simplified, we use a table built from the IANA tz database 2026a with its "backzone" history (Amsterdam on July 1, 1935 was UTC+1:19:32; Stockholm in summer 1947 was UTC+1:00).
• Before a town adopted standard time, clocks followed local mean time. We use your birthplace's own longitude divided by 15 (Ulm in 1879 was about UTC+0:40), not the zone's reference city.
• If your time didn't exist (clocks jumped forward) we read it with the earlier offset and tell you; if it happened twice (clocks fell back) we ask which one and default to the first.
• Known gap: before 1967 some US towns kept their own daylight-saving rules that no database fully records. Check the offset we show, and set your own if your certificate says otherwise.

Privacy

On your device All calculations run in a Web Worker in your browser. No request carries your birth date, time or place, nothing goes in the link (no query string, no #fragment), there is no account and no AI. The birthplace search loads same-origin files by first letter, so a request reveals only that someone typed a word starting with that letter. "Remember my birth details" stores them in this browser only when you tick it; "Forget me" deletes them. Our site analytics count visits and button clicks without any of your inputs. Full policy: legal.plusbeauxjours.com/privacy (opens in a new tab).

Data and licences

• City data © GeoNames (opens in a new tab), licensed under CC BY 4.0 (opens in a new tab), modified (a subset of cities15000 and cities5000 with our own region tags). Downloaded from download.geonames.org/export/dump/ (opens in a new tab) on 2026-10-10.
• Map land: Natural Earth (opens in a new tab) (public domain) via world-atlas (opens in a new tab) 110m.
• Time zones: IANA tz database 2026a (public domain).
• Fonts: Atkinson Hyperlegible Next and Mono (Braille Institute), Bricolage Grotesque; SIL Open Font License.
• Planet positions: astronomy-engine by Don Cross, MIT licence:

MIT License

Copyright (c) 2019-2023 Don Cross <cosinekitty@gmail.com>

Permission is hereby granted, free of charge, to any person obtaining a copy of this software and associated documentation files (the "Software"), to deal in the Software without restriction, including without limitation the rights to use, copy, modify, merge, publish, distribute, sublicense, and/or sell copies of the Software, and to permit persons to whom the Software is furnished to do so, subject to the following conditions:

The above copyright notice and this permission notice shall be included in all copies or substantial portions of the Software.

THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.

Names and trademarks

"astrocartography" is used here only as the everyday, descriptive name for drawing a birth chart on a map. Astro Map is a descriptive tool name by plusbeauxjours. We are not affiliated with, endorsed by or connected to any astrology service, software or trademark owner.

Changelog

• October 10, 2026: first release. 40 lines in mundo, five lenses, 1,229 ranked cities, precision bands, pre-1970 time zones, local mean time, 10 planet guides.

Back to your map.