Synastry — two natal charts

The inputs mirror the birth-chart call, twice. House system can be set; the public single-chart form uses PLACIDUS. Inter-chart aspects, when computed, use the same angles as a single chart: 0° orb 8°, 60° orb 6°, 90° orb 8°, 120° orb 8°, 180° orb 8°. The geometry tables on the sign pages are the special case where both “planets” are sign centers 15° into each sector. Real synastry uses the longitudes people were actually born under.

That is why this site does not publish 66 compatibility essays. Two sun signs determine one row of that center-to-center table, which you can already read on each sign page, and they leave out both Moons, both Ascendants, and every other body. Publishing a verdict from the sun pair alone would be an entity-swapped template.

If you want a two-person reading, run two birth charts. The aspect vocabulary you need is already documented on the conjunction, sextile, square, trine, and opposition pages.

Two noon births on 21 March and 21 September, both in Paris, put the Suns near 0° and near 180°. The opposition on the Aries and Libra pages already says that. It does not say where either Moon is, and those Moons are the part most sun-sign compatibility pages invent. Add birth times of 04:10 and 22:40 and the rising signs diverge as well. The synastry call is those extra numbers plus the rest of the bodies. The sign table is not a substitute dataset.