How your chart is
calculated.
The astronomical computation behind every chart on Coeluna — and our commitment to doing it in your browser, with your data staying on your device.
How the sky is read
Planet positions use compact VSOP87B tables derived from the full planetary theory. We compare our shipped tables with the full versions separately from independent astronomical references. The Moon and Pluto use their own calculation methods.
The full pipeline: birth date + time + city → UTC → Julian Day → VSOP87 planetary longitudes → apparent geocentric ecliptic longitudes (including nutation, aberration and precession) → sign and degree. The Moon uses its own dedicated high-precision theory. Pluto uses the astrometric solution from Meeus.
Houses are computed in the Placidus system, the most widely-used system in Western astrology. The Ascendant and Midheaven are calculated from Local Apparent Sidereal Time at the birth location.
How accurate is it?
Our published benchmark compares ten charts with independently captured NASA/JPL positions. Across 100 planet comparisons, the largest difference was under 0.03° (about 1.3 arcminutes). It is a sample, not a guarantee for every date in 1900–2100.
Within 0.03° of NASA/JPL in tests
Equal-house fallback near the poles
Accounts for Earth's variable rotation
Birth time matters. Rising-sign intervals vary with latitude and the sign; two hours is only a rough average. Without a time, we use local noon for estimated planet positions, check Moon-sign uncertainty, and hide Rising, Midheaven and houses.
Reproducing the benchmark
The benchmark compares the production engine with a frozen set of NASA/JPL Horizons observer-table results: Earth centre, apparent ecliptic longitude of date, quantity 31, airless, Universal Time. Original headers, rows, precision and request URLs are retained with the reference data. Normal builds and tests make no reference-service calls. The separate full-versus-shipped VSOP87 comparison measures trimming only.
Your data stays with you
Every calculation runs entirely in your browser. Your birth date, time and city never leave your device — there is no server receiving your birth data and no account, and your birth details are never used to track you.
Chart sharing works by packing the birth data directly into the link itself. The shared link is the chart — no database, no stored records. Only the recipient of the link can see the data, and only for as long as they hold the URL.
See the method in action with your own chart.