How accurate is it?

Planet positions

We use astronomy-engine by Don Cross, an open-source (MIT licence) library built on the VSOP87 planetary theory and checked by its author against NOVAS and JPL data. It gives the apparent position of each planet on the ecliptic of date, corrected for light travel time and aberration, which is the frame astrologers use.

To check it, we asked NASA's JPL Horizons system for the same positions at nine moments between 1931 and 2039 and compared every planet. The biggest difference for each:

″ is a second of arc, 1/3600 of a degree. Charts print positions to the minute (60″), so the largest difference is about a third of the smallest step you'll see. These comparisons run as automated tests every time the code changes.

Houses and angles

We compute the Ascendant, Midheaven and house cusps ourselves from the sidereal time, the true obliquity of the ecliptic and your birthplace's latitude, using the standard formulas. Placidus is computed from its definition, by finding each cusp's own semi-arc. We check the results against published Swiss Ephemeris values: every cusp agrees to within a tenth of a second of arc. We also test that the Ascendant degree really sits on the eastern horizon, and the Midheaven on the meridian, for places from Reykjavík to Tierra del Fuego.

Above the polar circles some degrees never rise, so Placidus houses can't exist. There we use Porphyry and say so on the chart, matching what Swiss Ephemeris does.

Time zones and clock changes

The most common reason two calculators disagree is the clock, not the astronomy. We convert your birth time to universal time with the IANA time zone database for your birthplace, the same history your phone uses, including wartime double summer time and local rules that changed over the years. If the clocks skipped your birth time (spring forward) or showed it twice (fall back), the chart says which reading we used.

Birthplaces

Places come from GeoNames: about 170,000 towns and cities with a population of 1,000 or more, each with its coordinates and time zone. If you were born somewhere smaller, choose the nearest town; a few kilometres makes almost no difference.

The nodes

We show the true North Node: the ascending node of the Moon's actual orbit at the moment, as Astrodienst and most modern software do. Our tests check it against Meeus' published series. Read more in the lunar nodes.

What it can't tell you

Accuracy here means the sky is where we say it was. What that means for your life is interpretation, and astrology isn't a science. Use it to think, talk and reflect, not to make medical, legal or money decisions.