How we calculate your chart.

Most astrology sites never explain their math. This one does. Here is everything a chart on zodiacs.org claims, and exactly how it is calculated.

The ephemeris

Planetary positions come from Astronomy Engine, an open-source astronomical library accurate to about one arcminute (a sixtieth of a degree). We compute apparent geocentric tropical longitudes of date — the standard frame of Western astrology — including light-time, aberration, and nutation. The Moon uses a dedicated high-accuracy lunar model; the lunar node is the true node, derived from the Moon's instantaneous orbit.

Every change to the engine must pass an automated accuracy gate before it ships: positions are compared against NASA JPL Horizons reference data at multiple epochs (including a 1907 chart), with tolerances of a few hundredths of a degree for planets. The test suite also checks the sky against itself: at sunrise the computed ascendant must match the Sun's position; at solar noon the midheaven must.

Angles and houses

The ascendant and midheaven are computed from Greenwich apparent sidereal time, your birthplace coordinates, and the obliquity of the ecliptic (IAU 2006). We offer whole sign houses (the oldest system — each house is one complete sign) and Placidus (the most common modern system, computed by iterative semi-arc division). Placidus is mathematically undefined near the poles; above 66° latitude we fall back to whole sign and say so on the chart.

Time zones

A birth chart is only as good as its clock conversion. We resolve your birth time against the complete IANA time zone history of your birthplace: daylight saving in all its regional weirdness, wartime clock changes, and pre-standardization local mean time — including offsets with seconds, like Mexico City's −6:36:36 before 1922.

• Skipped times (spring-forward gaps) shift forward across the gap — the standard convention — and the chart says so.
• Repeated times (fall-back folds) use the earlier pass, flagged on the chart.
• Very old dates use the local mean time of your birthplace's reference zone, the same convention professional software applies. For births before civil time standardization, a chart can differ by a few clock minutes from tools that use the exact town meridian — we flag these charts too.

Every result lists its computed UTC instant, so the conversion is yours to check.

Birthplace search

Place lookup runs against a bundled index of ~34,000 cities from GeoNames (CC BY 4.0), each carrying its IANA time zone. Coordinates are stored to ~1 km precision — far finer than a birth chart needs.

Privacy

The entire calculation happens in your browser. Your birth date, time, and place are never transmitted anywhere — there is no server-side chart API, no account requirement, and no analytics attached to birth data. Charts you save live in your browser's local storage, on your device, deletable by you at any time.

Honest limits

• Chiron and the asteroids aren't included yet (the engine's remit is major bodies); they're planned via public-domain JPL data.
• Unknown birth times get a noon chart: planets are exact to the day, but rising sign and houses genuinely require the clock, and we won't pretend otherwise.
• Supported birth years: 1800–2199.

Found a discrepancy? We treat accuracy reports as bugs. The sky is checkable, and so is our arithmetic.