At a glance
| What | Source | Used for |
|---|---|---|
| Positions of the planets, the Moon, Chiron and the main asteroids | Swiss Ephemeris 2.10.03 by Astrodienst, files compressed from NASA JPL DE431 | Every chart, transit, comparison, solar return and sky event |
| Fixed stars | The Swiss Ephemeris star catalogue | Fixed star contacts in the chart data and the API |
| Birthplace search and coordinates | Google Maps Platform: Places API and Geocoding API | The latitude and longitude of the place you type |
| Time zone of a place | timezone-boundary-builder, built from OpenStreetMap data, read with geo-tz | Which zone a birthplace belongs to |
| Clock changes and daylight saving time | IANA Time Zone Database | Turning local birth time into universal time |
| Readings, chat answers and weekly texts | Google Gemini 3.5 Flash, through the Gemini API | All interpretation text |
| Map on a shared city reading | OpenStreetMap | The map on a shared location card |
| Accuracy check | NASA JPL Horizons | The comparison in the accuracy report |
Positions of the planets and points
Every position comes from the Swiss Ephemeris, version 2.10.03, the library Astrodienst maintains. The data files are sepl_18.se1 for the planets, semo_18.se1 for the Moon and seas_18.se1 for Chiron and the main asteroids. They are compressed from NASA JPL’s DE431 ephemeris and cover the years 1800 to 2400.
Astrodienst moved its files to the newer DE441 in May 2026; ours are the DE431 set. Against NASA JPL Horizons, which serves DE441, the largest difference on our test chart is 0.53 arcseconds: see the accuracy report.
The zodiac is tropical, the houses are Placidus, the lunar node is the true node and Lilith is the osculating lunar apogee. Without a birth time the planets are calculated and the ascendant and houses are left undetermined. Fixed stars come from the Swiss Ephemeris star catalogue, which lists the fixed stars brighter than magnitude 5. More on the library on the Swiss Ephemeris page.
Places and time zones
When you type a birthplace, Google’s Places API suggests places and returns their coordinates, as WGS 84 latitude and longitude. Google’s Geocoding API names a place from a point on the map.
The time zone of a place comes from the timezone-boundary-builder maps, which are built from OpenStreetMap data, read with the geo-tz library. The history of that zone, its clock changes and daylight saving time, comes from the IANA Time Zone Database, so a birth in 1985 is converted with the clocks of 1985.
Map data and time zone boundaries © OpenStreetMap contributors, available under the Open Database License.
The AI model
Readings, chat answers, the weekly text and the location and compatibility readings are written by Google’s Gemini 3.5 Flash, through the Gemini API.
The model does not calculate. It receives the calculated chart, the transits and the results of our tools, and our instructions forbid it to state a degree, sign or house that is not in that data. The method is explained in Chart-Grounded AI Astrology and in Does AI astrology make things up?
What the model knows about astrology comes from Google’s training, not from us. Google’s model card says Gemini 3.5 Flash is based on Gemini 3 Flash, and the Gemini 3 Pro model card describes the training data of the family: publicly available web documents, text, code, images, audio and video; licensed data; data from users of Google products under Google’s terms; and synthetic data, followed by instruction tuning, reinforcement learning and human preference data. Google publishes no list of astrology sources.
We do not fine-tune the model and keep no database of interpretations of our own. The meaning in a reading is the model’s, written within our rules for tone, structure and the use of the calculated data. We use the paid Gemini API, under whose terms Google does not use prompts or responses to improve its products. What we send is described in the privacy policy.
Sources
The pages below were read on .
- Swiss Ephemeris: what the library computes, its data basis and its precision against JPL.
- NASA JPL planetary ephemerides: DE431 and its successor DE441.
- NASA JPL Horizons: the service our accuracy report compares against.
- Google Places API: place search and coordinates.
- Google Geocoding API: turning a map point into a place name.
- timezone-boundary-builder: time zone boundaries built from OpenStreetMap.
- geo-tz: the library that reads those boundaries.
- IANA Time Zone Database: historical clock rules for every zone.
- OpenStreetMap copyright and licence: the Open Database License and the attribution it asks for.
- Gemini 3.5 Flash model card: that the model is based on Gemini 3 Flash.
- Gemini 3 Pro model card: the training data of the Gemini 3 family.
- Gemini API terms: that prompts and responses on paid services are not used to improve Google's products.
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