How we calculate and write your birth chart

Every position in the report is calculated in code from astronomical models, not written by AI. We convert your birth time to Universal Time with the historical time zone of your city, calculate 10 planets, Chiron, the lunar nodes, Lilith, the Ascendant and the Midheaven to about one arcminute, then an AI model writes the interpretation from those positions only.

Which astronomical data do we use?

• Sun, Moon and planets: Astronomy Engine, an open-source library based on the VSOP87 and NOVAS models, accurate to about one arcminute. Positions are true (apparent) ecliptic longitudes of date.
• Chiron: a daily table from 1900 to 2060 taken from the NASA JPL Horizons ephemeris, interpolated to your birth time. Chiron moves less than 0.1° a day.
• North Node: the true node, from the Moon's osculating orbit. The South Node is exactly opposite.
• Black Moon Lilith: the mean lunar apogee, from the formula in Jean Meeus, Astronomical Algorithms.
• Part of Fortune: Ascendant + Moon − Sun for a day birth, Ascendant + Sun − Moon for a night birth. A birth is by day when the Sun is above the horizon.

How do we find your birth time and place?

Cities come from GeoNames (CC BY 4.0): more than 170,000 places with over 1,000 inhabitants, each with its latitude, longitude and time zone. The local time is converted with the IANA time zone database, which records daylight saving time and past changes of offset. One known limit: before 1970, a few places that IANA merged with a neighbour (for example Amsterdam before 1940) can come out a few minutes off.

We show your birth date in words before payment, because day and month swapped give a different chart. Dates are entered day first, as in the UK.

Which zodiac and house system do we use?

The tropical zodiac, used by most Western astrologers, and Placidus houses. Above 66° latitude Placidus cannot be calculated for part of the year, so we use Whole Sign houses there and the report says so. Without a birth time we calculate positions for noon and leave out the houses, Ascendant, Midheaven and Part of Fortune.

Which aspects and orbs do we use?

Six aspects between planets and angles. Orbs for Chiron are three quarters of those below.

We also list each planet's dignity (domicile, exaltation, detriment or fall, with modern rulers), the balance of elements and modes, and the patterns stellium, grand trine, T-square, grand cross and yod.

Where does the AI come in?

The Sun, Moon and rising sign profiles are fixed texts we wrote for each of the 12 signs. The report matches them to your chart with set rules: the decan of your exact degree, a nearby sign when you are close to a cusp, the element and mode of your Sun and Moon, the aspect between them and where your chart ruler sits.

The chapters on each planet, house and aspect are written by Claude, an AI model made by Anthropic. It receives your calculated positions, houses, aspects and patterns as plain text, and is told to use only those facts. It does not receive your name, birth date or city. The chapter headings are fixed in our code, so every report covers the same topics. The AI does not calculate anything.

What do we check, and what is not reviewed?

The calculation is tested automatically against published charts with well-known birth data. Every position must match within 0.1°, and the tests also check the house cusps and the day and night formula. No astrologer reads each report before it is sent: the interpretation is written by the AI from your chart, within the rules above.

Is astrology scientifically proven?

No. The positions in the report are real astronomy, but their meaning comes from astrological tradition, not from science. The report is for entertainment and self-reflection, not medical, legal or financial advice.

See the natal chart report or try the free birth chart calculator.