Planet positions
Positions of the Sun, Moon and planets come from astronomy-engine, an open-source (MIT) library that computes where bodies appear from Earth for any moment. We use the tropical zodiac, measured from the March equinox, as most Western astrologers do. Charts are geocentric, and the Ascendant and Midheaven are calculated from the local sidereal time at the birth place.
Birth time and place
Birth places come from the GeoNames database (about 70,000 towns and cities with a population of 5,000 or more), which gives latitude, longitude and the time zone. Local birth time is converted to universal time with the time zone rules that applied on that date, including daylight saving time. Without a birth time we use noon: the Sun and planets stay accurate to within a degree or so, the Moon can be up to about 7° off, and the Ascendant and houses are left out.
Accuracy check
We test the engine against a published, birth-certificate-rated chart calculated by Astrodienst (astro.com), which uses the Swiss Ephemeris. The largest difference across all ten planets was 0.2 arcminutes, and the engine’s Placidus house cusps agreed within 0.8 arcminutes. That is far finer than any interpretation depends on.
Aspects and orbs
An aspect is counted when two planets are within the following distance (the orb) of an exact angle. Orbs are widened by 10% when the Sun or Moon is involved.
| Aspect | Angle | Orb |
|---|---|---|
| ☌ Conjunction | 0° | 8° |
| ⚹ Sextile | 60° | 4.5° |
| □ Square | 90° | 7° |
| △ Trine | 120° | 7° |
| ☍ Opposition | 180° | 8° |
Houses
House overlays, on the free result and in the written reading, use Whole Sign houses: each sign is one house, counted from the sign on the Ascendant. Whole Sign houses work at every latitude and stay stable when a birth time is a few minutes off.
The compatibility score
Each aspect gets a strength from how tight it is and how personal the two planets are: the Moon, Sun and Venus weigh most; Uranus, Neptune and Pluto least. Flowing aspects (trines and sextiles) count in favor, tense ones (squares and oppositions) against, and conjunctions depend on the planets involved. Five areas are scored from the aspects that concern them: emotional (Moon, Sun, Venus), attraction (Venus, Mars, Ascendant), communication (Mercury, Sun, Jupiter), staying power (Saturn with the Sun and Moon) and ease (everything). The overall score is a weighted average, with emotional and staying power counting a little more. Scores are kept between 12 and 96, because no chart is all good or all bad.
Composite charts
The composite chart uses the midpoint method: each planet is placed at the nearer midpoint between its positions in the two charts, and the composite Ascendant is the midpoint of the two Ascendants.
Written readings
The paid reading is written by an AI model from structured chart data: both charts, every aspect with its orb, the house overlays in both directions, and the year’s transits to each chart, found by checking each day for exact aspects from Jupiter through Pluto. The model is instructed to use only that data and never to invent placements. Before a reading is shown, automatic checks confirm that it is complete; if it isn’t, it is rewritten. Nothing in a reading is reviewed by a human astrologer.