How we calculate
Every chart on The Birth Sky is calculated from the positions of the real Sun, Moon and planets for your exact minute and place. Here is exactly how.
Last updated 4 November 2026
From your birth details to a moment in time
You give us a date, a local clock time and a town. We look the town up in the GeoNames database of 34,000 places, which gives its coordinates and its time zone. Your browser then applies the full history of that time zone, including daylight saving and past changes of offset, to turn your local time into Universal Time. A 1975 birth in Chicago gets the rules that applied in Chicago in 1975.
Planet positions
We compute the Sun, Moon, Mercury, Venus, Mars, Jupiter, Saturn, Uranus, Neptune and Pluto with Astronomy Engine, an open-source library built on established astronomical models. Positions are geocentric and corrected for light travel time and aberration, then expressed in the tropical zodiac on the true ecliptic and equinox of the date, the frame used by Western astrology.
The lunar North Node is the mean node, the most widely used form.
Houses and angles
The ascendant and midheaven come from the local sidereal time at your birthplace and the true obliquity of the ecliptic. Houses use the Placidus system, the one most astrologers use. Close to the poles, above about 66° latitude, Placidus has no solution for some times of day; there we switch to Porphyry houses and say so on your chart.
How accurate is it?
We tested our engine against Swiss Ephemeris, the library behind most professional astrology software, on six births across four continents. The largest difference for any planet was 0.3 arcminutes, about one two-hundredth of a degree. The ascendant, midheaven and every house cusp matched to within a hundredth of an arcminute.
| Birthplace | Date | Largest planet difference | Largest house difference |
|---|---|---|---|
| New York | 15 Jun 1990 | 0.21′ | 0.00′ |
| London | 3 Nov 1975 | 0.24′ | 0.00′ |
| Sydney | 28 Feb 2001 | 0.27′ | 0.00′ |
| Tallinn | 20 Aug 1962 | 0.23′ | 0.00′ |
| Tokyo | 24 Dec 1988 | 0.11′ | 0.00′ |
| Reykjavik | 1 Jul 2010 | 0.30′ | 0.01′ |
In practice the limit on accuracy is the birth time. The ascendant moves about one degree every four minutes, so a time rounded to the hour can shift your rising sign or houses. If you only know an approximate time, treat the houses and rising sign as approximate too.
Without a birth time
If you tell us you don't know your time, we calculate for midday and leave out the rising sign and houses, which depend on the exact minute. The Moon moves about 13° a day, so its position may be off by up to about 6° and, near a sign boundary, could fall in the neighbouring sign.
Your privacy
The whole calculation runs in your own browser. Your birth date, time and place are kept only on your device and are never sent to us, except the name of the town you type, which our server uses to look up its coordinates.