Synastry — comparing two charts
Ptolemy, Tetrabiblos IV.5 (“Of Marriage”), Alexandria, 2nd century CE
Reading two birth charts against each other is not a modern idea. Ptolemy already judges a marriage from both nativities at once: unions last, he writes, when the luminaries of the two charts are in harmonious aspect, and more so when the man's Moon agrees with the woman's Sun. Dorotheus, writing in the 1st century and surviving mainly through an Arabic translation, gives similar cross-chart rules. What is modern is the apparatus — the grid of every planet against every planet, with orbs and tallies — and who first drew that grid is not attested anywhere we could find.
What it measures
The angular separations between the points of one chart and the points of another: the same geometry as an aspect within a single chart, measured across two. Nothing moves, so nothing is applying or separating — both charts are frozen instants.
Where practitioners disagree
- Whose orbs apply?
One chart's orb settings for the whole comparison — or an average, or the wider of the two
No traditional rule governs this, because the traditional sources compare specific placements rather than run a full grid.
This engine: Chart A's orb profile, named on the page, applied to every contact. - Which points take part?
Planets only — or planets plus angles, nodes and lots
Including the angles means the comparison depends on both birth times being exact; a rectified or rounded time moves them by degrees.
This engine: Thirteen points including the angles and the north node — and the tables say so, so a reader with a shaky birth time knows which rows to distrust. - Does a compatibility score mean anything?
Weighted scoring engines are common in software — no traditional source computes one
Scoring collapses contradictory contacts into one number, which is where most of the information goes.
This engine: No score. Every contact and its orb, and the reading is left to the reader.
Still open
How much of the modern practice descends from the ancient material is genuinely unclear. The Hellenistic sources compare charts for specific questions — chiefly marriage — rather than as a general method, and the grid-and-orbs apparatus has no attested inventor.
Sources
- Claudius Ptolemy, Tetrabiblos IV.5 (2nd c. CE)
- Dorotheus of Sidon, Carmen Astrologicum V.16 (1st c. CE)