Synastry

What synastry compares

Synastry lays two birth charts over each other and looks at the angles between one person's planets and the other's. It is the technical version of the question sun-sign compatibility answers approximately — and it gives noticeably different answers, because it uses ten bodies instead of one.

Two people whose Sun signs score badly can have an easy synastry, and frequently do. The Sun is one placement out of ten.

The contacts that carry most weight

• Sun to Moon — the classic marriage contact. One person's core identity meets the other's emotional needs. Traditionally read as the pairing most likely to feel like home.
• Venus to Mars — attraction, and the least subtle contact in synastry. Present in most relationships that started fast.
• Moon to Moon — how similarly you process being upset. Harmonious contacts here make ordinary weeks easy; difficult ones show up in the same argument recurring for years.
• Saturn to anything personal — commitment and weight, with a cost. Saturn contacts are why some relationships last through things that should have ended them, and also why some feel like work.
• Mercury to Mercury — whether conversation is effortless or requires translation.

Reading the aspects

The same aspects mean roughly the same things they do in a natal chart, but with an important difference: in synastry, squares and oppositions are not simply bad. A relationship with no hard aspects at all tends to be pleasant and low-voltage. Most long, engaged partnerships have several, and what distinguishes the ones that work is whether both people can name what the friction is about.

Trines and sextiles are where a relationship is easy. Squares are where it grows or breaks.

What synastry does not tell you

It does not tell you whether to be with someone. A chart comparison describes the shape of the dynamic between two people — where it flows, where it snags — and it is genuinely useful for putting words to a pattern you had already noticed. It is not a compatibility score, and treating it as a verdict is the fastest way to misuse it.

What you need

Two sets of birth data: date, an approximate time, and a city for each. Times matter for the Moon and for house placements. Both charts are computed in your browser; neither person's data is sent anywhere.