How we calculate

A birth chart is a map of the sky for one minute, seen from one place. It records where the Sun, the Moon and the planets stood along the zodiac when you were born, which degree was rising on the eastern horizon, and which stood highest in the sky. So far nothing in it is interpretation: it is positional astronomy, and it can be checked.

For every chart we calculate the positions of the Sun, the Moon and the eight planets from Mercury to Pluto, together with the Moon’s north node and Lilith. The positions are geocentric, as the sky appeared from the Earth, measured in the tropical zodiac. They come from a published astronomical ephemeris library.

From the exact minute and place of birth we calculate the Ascendant, the Midheaven and the twelve houses in the Placidus system. Inside the polar circles, where Placidus is not defined, we use whole-sign houses. Then we measure the angle between every pair of points and keep the five major aspects: conjunction, sextile, square, trine and opposition. Place names, coordinates and time zones come from the GeoNames database.

A chart is calculated for a moment in universal time, but you were born by a local clock. Turning one into the other takes the time-zone and daylight-saving rules in force on that day, in that place, and those have changed more often than most people expect. We apply the historical rules of the IANA time-zone database.

One example: between 1968 and 1971 the United Kingdom kept its clocks an hour ahead of Greenwich all year round. Someone born in London at 7:30 on a January morning in 1970 was born at 6:30 universal time, not at 7:30. An hour’s error moves the Ascendant by about fifteen degrees on average, half a sign, and can carry planets from one house into the next.

We test our calculation against an independent reference ephemeris, on ten charts with birth dates from 1933 to 2024. These are the largest differences we have measured:

Planetary positions

within 17 arc-seconds

Ascendant, Midheaven and Placidus house cusps

within 0.2 arc-seconds

For scale: one degree is 3,600 arc-seconds, and the full Moon is about 1,800 across. Seventeen arc-seconds is less than a hundredth of the Moon’s width. In practice the limit on a chart is the birth time, not the arithmetic: the Ascendant moves about one degree every four minutes, so a time rounded to the quarter hour weighs far more than any rounding of ours. We state no accuracy on this page beyond what these tests measure.

Not everyone knows their birth time, or the exact place. We calculate what can be calculated, and say plainly what cannot. Without a birth time we cast the chart for noon. The signs of the Sun and the planets hold through the day, but the Ascendant, the Midheaven and the houses depend on the hour, so we leave them out rather than guess. The Moon covers about thirteen degrees in a day: if it changed signs on your birthday, we say so, and name both signs.

Without a birthplace there are no houses and no Ascendant either, since both are measured against the local horizon. The signs of the planets do not need the place, only the time zone. A chart without houses is a smaller chart, not a wrong one, and a reading made from it stays with what is known.

Our readings follow the classical tradition of Western astrology: the meanings that centuries of practice have attached to the planets, the signs, the houses and the aspects, applied to the chart we calculated for you. A reading names the positions it works from, so you can always see where an observation comes from.

A reading describes tendencies: what comes easily to you, what takes effort, how the different sides of your nature talk to each other. It does not foretell events, and it does not tell you what to do. We offer it for self-reflection and entertainment; it is not a substitute for medical, financial or legal advice.