How a progressed composite is formed
First, calculate each person's secondary progressed planets for the same target date. This page uses one ephemeris day after birth for every 365.24219 days of elapsed life. The target date is represented by noon UTC, so a comparison made a few hours apart within one date has the same target. It calculates the two symbolic ephemeris instants separately because the two people were born at different times.
For each progressed chart, the natal sidereal orientation advances by the mean solar Naibod key of 0.98564733° per tropical year. The progressed MC follows from that directed orientation and the obliquity on the symbolic date. We then take the shorter-arc midpoint of the two progressed planetary longitudes and the two progressed MCs. The latter fixes the composite reference-place orientation; a chosen latitude and house system produce the Ascendant and house cusps. The virtual clock used to solve this orientation is a computational device. Its planetary positions are discarded.
This is a composite of two progressed charts. Progressing a precomputed natal midpoint composite would perform operations in a different order and can produce different positions.
Reading limits and uncertain inputs
A birth time is required for each natal sidereal orientation. A wrong historical UTC offset changes both natal and progressed angles. The two equal midway points of an exact 180° opposition cannot be distinguished by short-arc geometry, so both are shown and houses are withheld where necessary. A near opposition can flip its midpoint with a small birth-data change. Planet positions are apparent geocentric tropical longitudes from the site's independent ephemeris and rounded symbolic UTC minute. Placidus, Campanus, and Regiomontanus may fall back to Porphyry at difficult latitudes; the result labels that fallback.
Astrological interpretation is symbolic. This calculation supplies positions and method provenance, not a compatibility score or prediction.