Two skies, laid one over the other.

Synastry is the comparison of two birth charts by laying one over the other and measuring the angles between their planets. An angle inside a set margin is called an aspect, and the aspects are the whole of it. It describes where two charts touch, which is geometry, and nothing more than that.

The birth time fixes the horizon, which gives you the Ascendant, the Midheaven and the houses. Without it there is no horizon to measure from, so those are left out rather than guessed, and the Moon is cast for sunrise and can be up to half a sign from where it belongs. Everything else on this page is exact either way.

Five rows are scored from the aspects between the two charts, each contact weighted by how close to exact it is, and the five are combined with published weights. That figure is then ranked against twenty thousand random pairs scored by the same rule, and the rank is what you see. Every weight is listed under "How these numbers are worked out".

No. The number measures how much two charts touch, not whether two people do well, and fifty is the middle of everybody rather than a pass mark. A low number means these two charts leave each other largely alone, which plenty of long relationships do. Nothing here can tell you whether to stay or go.

The printed chart is a single birth chart rather than a pair, drawn from the same engine and the same figures as this page. You can draw yours on the birth chart page and order it there. A printed version of the two-chart comparison does not exist yet.