How We Calculate Charts

1. From clock time to astronomical time

What you enter is local clock time. Before calculating, we convert it to Universal Time (UTC) using the time zone of your birthplace. Time zones come from the IANA time zone database, the international standard, which records the historical time zone and daylight saving changes for places around the world.

For example, China observed daylight saving time from 1986 to 1991. For people born in those summers, clock time ran one hour ahead of standard time, and the chart page notes that it was "converted at UTC+9." Get the conversion wrong by an hour, and the rising sign and houses can come out completely different.

If you don't know your birth time, planet positions are calculated for noon that day. Most planets move very little in a day, so those results hold up. The Moon moves about 13° a day, so we can only show the signs it may have been in. The Ascendant and houses can't be determined, and the page says so plainly.

2. Planet positions

The positions of the Sun, the Moon and the eight planets are calculated with astronomy-engine, an open-source astronomy library. It first finds where each body appears from Earth at that moment, then converts that to ecliptic coordinates for the date and takes the ecliptic longitude. That is what astrology calls a sign position in the tropical zodiac: Aries begins at the vernal equinox point, and each sign spans 30°.

Whether a planet is retrograde depends on whether its ecliptic longitude is increasing or decreasing at that moment. Retrograde is apparent motion as seen from Earth; the planet itself isn't moving backward.

3. Ascendant, Midheaven and houses

The Ascendant is where the eastern horizon meets the ecliptic at the moment of birth; the Midheaven is where the meridian meets it. Both depend on birth time, longitude and latitude together, and they move about 1° every 4 minutes. So the more accurate your birth time, the more reliable your rising sign and houses.

Houses use the Placidus system, the most widely used house system in modern Western astrology. Near the Arctic and Antarctic Circles (latitudes above about 66°), Placidus breaks down mathematically, so we switch to Whole Sign houses: the whole sign holding the Ascendant becomes the 1st House, and so on around the chart. The chart page notes which system was used.

4. Aspects

Two planets form an aspect when the angle between them is close to one of the values below. How far it may stray from the exact angle is called the orb. We use a common, fairly conservative set, since we'd rather list fewer aspects than count loose ones:

The chart shows the actual orb of each aspect. The smaller the orb, the more noticeable the aspect usually is.

5. How we check it

The Swiss Ephemeris is the calculation engine behind most professional astrology software. We picked 14 representative charts and compared them with it point by point, including births during China's daylight saving years, in the Southern Hemisphere, at western longitudes, at high latitudes, in 1905, and with an unknown birth time.

For the ten planets, the Ascendant, the Midheaven and the Placidus house cusps, every position is within 1 arcminute of the Swiss Ephemeris. The largest difference we measured was 17 arcseconds, less than 0.005°. That is far finer than the precision shown on the chart, and it never changes the sign, house or aspects of any planet. These comparisons are part of our automated tests and run again every time the calculation code changes.

6. What the AI does

The AI doesn't calculate any positions. When you ask a question, our server first computes your chart using the method above, then writes out the planets, houses, aspects and orbs as text for the AI. The AI is told to answer only from that data: no making things up, no calculating on its own, and no predicting specific events.

So a chart's accuracy comes from the calculation, while the quality of a reading depends on whether it's grounded in your specific placements. Every answer names the placements it's based on, so you can check them against your chart and judge for yourself.