What is synastry?

A synastry reading takes every planet in one chart and compares it with every planet in the other. The Sun, Moon, Venus and Mars get the most attention because they describe identity, need, attraction and drive. But Mercury contacts shape how a couple argues, Saturn contacts shape the sense of duty, and the outer planets often describe a generational backdrop rather than a personal dynamic.

The aspects are the same major aspects used in a natal chart: conjunction, sextile, square, trine and opposition. A conjunction fuses, a trine flows, a square creates friction, and an opposition stretches the pair across a divide. The orb matters: a contact within one degree is felt more consistently than one at the edge of the orb.

A synastry score cannot tell you whether a relationship will work. It can tell you which contacts are likely to repeat, which needs are easy to meet and which ones will require a conversation. Two people with a lot of squares can build a strong relationship if they are willing to work with the friction. Two people with a lot of trines can drift if they never say the difficult thing.

The useful question is not "are we compatible" but "which pattern keeps showing up, and what does it ask of each of us". That is a more honest use of the technique, and it keeps the agency where it belongs.

Synastry compares two charts. A composite chart takes the midpoint of each pair of planets and reads the relationship as its own chart. Synastry is about the contact between two people; the composite is about the shared field they create. Most astrologers use both, because each answers a different question.

This site calculates both from the same birth data. You can start with the synastry calculator, then read the composite for the relationship's own weather.