Coordinates and zodiac
We use Astronomy Engine 2.1.19. Moon longitude uses EclipticGeoMoon, Sun longitude uses SunPosition, and planetary longitudes use the light-time-corrected geocentric vector rotated to the true ecliptic of date. Each tropical sign spans 30 degrees, starting at the March equinox. Signs are not astronomical constellation boundaries.
Birthplace and timezone
A GeoNames place supplies latitude, longitude and an IANA timezone identifier. Luxon converts your recorded wall-clock time using the timezone history supplied by your browser or operating system. The result details show the UTC instant and offset actually used. Birthplace queries reach the server; the birth date and time do not.
Clock changes
Some local times never occurred when clocks moved forward. We explain the adjustment and require you to correct the time or accept the adjusted value. When a clock time occurred twice, Advanced lets you choose the first or second occurrence; details list both UTC instants. Historical timezone coverage before 1970 is uneven, so a documented manual UTC offset can be entered.
Unknown birth time
We check the interval from local midnight to the next local midnight, or your selected rough time window. The Moon’s sign at the boundaries is compared and a crossing is located by bisection to about one second. Percentages show the elapsed-time coverage of each candidate, not a personal probability. Sun and planet candidates are also checked; Moon phase is labeled as a midpoint estimate.
Ascendant and houses
The Ascendant is calculated from local apparent sidereal time, true obliquity and geographic latitude. It is checked against the eastern rather than western horizon branch. Without a birth time, it is omitted. We display Whole Sign house starts only. We do not provide Placidus, a full synastry chart, solar returns or a sidereal chart in this free release. High-latitude horizon geometry requires extra caution.
Precision and limitations
Astronomy Engine documents an approximately one-arcminute accuracy target. One arcminute can correspond to roughly two minutes of a Moon-sign transition, so a boundary result deserves care. We flag births within fifteen minutes of a Moon-sign change. Library precision cannot repair a rounded birth time or an uncertain historical clock record. Check the local verification report for the actual tests run for this release.
Astrology and evidence
The calculation of a position is astronomy. Associating that position with personality, emotional needs or relationships is astrological interpretation, not an established scientific prediction. We do not use scores to imply measured compatibility.