How we calculate a chart
A reading is only as good as the chart under it. This is what happens between the form and the wheel.
1. Birth time to universal time
The clock time on a birth certificate is local time. We convert it to universal time using the full historical time zone database: the zone that applied in that town in that year, including daylight-saving rules that have since changed, and local mean time for births before standard time existed. If the time given fell in a skipped or repeated hour on a clock-change day, we tell you which reading of it we used.
2. Planet positions
We compute the apparent position of the Sun, Moon and planets as seen from Earth at that instant, corrected for the travel time of light and the motion of the Earth, and express it along the ecliptic of that date. Our engine is tested against Swiss Ephemeris, the reference used by professional astrology software, across 48 charts from 1879 to 2030. The largest difference for any planet is under 20 arcseconds, which is about a two-hundredth of a degree. The lunar North Node is the true (osculating) node.
3. Rising sign and houses
From the exact time and the latitude and longitude of the birthplace we compute the Ascendant and Midheaven, then the twelve house cusps using the Placidus system, the Western default. Inside the polar circles Placidus cannot be calculated, so we use Whole Sign houses there and say so. Angles and cusps match the reference to within a fraction of an arcsecond.
4. Aspects and patterns
We measure the angle between every pair of points and keep conjunctions, sextiles, squares, trines and oppositions within standard orbs (wider for the Sun and Moon, tighter for the angles). We also detect stelliums, grand trines, T-squares, grand crosses and yods.
5. The reading
The reading is assembled from a library of about 49,000 words written placement by placement: every planet in every sign and house, every pair of planets in aspect, every rising sign and Midheaven. Your chart selects which entries appear, and each section is tagged with the placement that selected it, so you can check any sentence against the wheel. Nothing is generated at random and two different charts never receive the same reading.
What we do when we cannot know
- No birth time: there is no rising sign and there are no houses, so we leave those out. We also check whether the Moon changed sign that day and tell you if it did.
- Approximate birth time: the rising sign moves about one degree every four minutes. We test times around yours and tell you how far off it would have to be to change the result.
What this is, and is not
The astronomy is exact. The interpretation is a symbolic tradition thousands of years old, not a science, and we do not claim it predicts events. Use it the way you would use a good question: as a prompt for honest reflection.