How accurate is a birth chart?
The planets are the easy part: for anyone born since 1900, every position in our charts is within one arcsecond of NASA’s own figures. What actually makes a chart wrong is the birth time, and the time zone it was recorded in.
How exact the planets are
Our positions come from the Swiss Ephemeris, the astronomical engine professional astrology software runs on, and we check them against NASA’s JPL ephemeris, the one used to navigate spacecraft. Since 1900, no position differs by as much as one arcsecond. A chart is drawn to the arcminute, which is sixty of those, so the planets are settled well past anything a chart can show.
Where the error really comes from
- The birth time. The rising sign moves about a degree every four minutes, so a time that’s an hour off moves it by around fifteen degrees, half a sign, which is often enough to change the rising sign and shift every house. No time at all?
- The time zone, as it was that day. Countries have moved their clocks often: daylight saving starting and stopping, wartime time, zones redrawn. A chart has to use the rules in force at that place on that date, not today’s. Before towns adopted standard time, many kept their own local time, a few minutes off their neighbours’, and we use the birthplace’s own longitude for those births.
- The hour that happened twice. On the night clocks go back, 1:30 happens twice, and on the night they go forward, 2:30 never happens at all. Either way it’s two moments an hour apart, so we ask which.
That’s why you pick your birthplace from a list instead of typing it, since we need the exact spot to look up the clock it ran on, which is a more capricious thing than people expect.
What isn’t a question of accuracy
Two correct charts can still disagree, because traditions make different choices, such as which zodiac to count from and how to divide the houses. Those are conventions, not errors. The zodiac question and the houses question.
For comparison, the most capable general AI model we tested could compute a Western chart correctly when asked outright, although it took minutes per chart and estimated in its readings. The test.