How the calculator works

Planet positions

An open source library called astronomy-engine supplies positions for the Sun, Moon and planets by applying VSOP87 planetary theory alongside a full lunar theory. These positions are geocentric and measured along the ecliptic of date, the one astrologers use.

We downloaded a Chiron position table from NASA JPL Horizons. It covers 1900 to 2100 with entries every two days. The North Node is the mean node, and Lilith is the mean lunar apogee.

The time zones

We convert your birth time into world time using a database built into your browser, the IANA time zone database. For every zone, it covers daylight saving rules and past changes going back to the 1970s, plus earlier records when available. You can see the offset used in the result and check it against your birth certificate.

Houses

We calculate the Ascendant and Midheaven using sidereal time plus the ecliptic's obliquity. We find Placidus cusps through iteration. At latitudes above 66 degrees, Placidus offers no solution. Your chart then uses Porphyry and tells you. You can also choose Whole Sign, Equal, Porphyry or Regiomontanus.

Orbs and aspects

We use the quincunx along with the five major aspects. If the Sun or Moon is involved, the orb is 8 degrees. Between planets, it's 6 degrees. The nodes, Chiron, Lilith and the angles use 3 degrees, and the quincunx uses 2 degrees. An exact aspect scores 100, and the score drops to 0 at the orb's edge.

Confidence in your birth time

We search for a rising sign change within the time window once you tell us how sure you are. If the Ascendant changes sign inside that window, the result gives the exact clock time when it changes.