What is synastry?
Compatibility is not a single number, it is two whole charts in conversation. Here is what synastry reads, and what it can honestly tell you.
Synastry is the practice of laying two birth charts over one another and reading what happens where they meet. Not one chart judged against a type, and not a score pulled from thin air — two complete charts, each its own fixed moment of sky, held together to see how one person's planets land on the other's.
The word means, roughly, a bringing-together of stars. That is the whole method. Your chart does not change and neither does theirs. Synastry simply asks what the two of them do in the same room.
What synastry is
A birth chart is a single moment held still. Synastry takes two of them and reads the relationship between the moments — which is to say, between the people. It is the difference between describing one person and describing a connection, and a connection cannot be read from either chart alone. It lives in the space between them.
This is why compatibility is never a single number. A number can rank a pairing; it cannot show you where two lives actually touch. Synastry works instead by looking at particular contacts — this planet of yours meeting that planet of theirs — and reading each one for what it asks and what it gives.
What it looks at: cross-aspects and house overlays
Synastry reads two things above all, and they answer different questions.
The first is cross-aspects: the angles between a planet in one chart and a planet in the other. When your Venus sits at a close angle to their Mars, or their Moon meets your Saturn, that contact carries a specific flavour — ease, friction, warmth, weight. Astrologers work from the same five Ptolemaic aspects used everywhere else: conjunction, sextile, square, trine, and opposition, each measured within a standard orb. A trine tends to flow; a square tends to catch. Neither is good nor bad on its own — a square is friction, and friction is also where two people grow.
The second is house overlays: where one person's planets fall among the other's twelve houses. Your Sun might land in their seventh house of partnership, warming exactly the area of life that house governs; their Mars might fall in your second house of money and worth, which is a different kind of heat entirely. The planet says what; the house says where in the life it is felt.
The bi-wheel visual
Synastry is usually drawn as a bi-wheel: two rings around a single zodiac. The inner ring holds one chart, the outer ring the other, and the lines struck across the centre are the cross-aspects between them. It is the clearest way to see the shape of a connection at a glance — where the two charts clasp, and where they pull.
Read it the way you would read a single chart, only doubled. The rooms are shared; the question is who brings what into each.
What it can and cannot promise
Here is the honest part. Synastry describes a dynamic — it does not deliver a verdict. It can show you where two people flow easily and where they will have to work, where the attraction lives and where the long patience is asked. It cannot tell you whether to stay, and it cannot reduce a relationship to a percentage that means anything on its own.
A single flowing aspect does not guarantee a happy life together, and a hard one does not doom it. Charts do not choose; people do. What synastry offers is a truthful map of the terrain — the currents you are swimming in — so that the choosing is done with clearer eyes. Read it as description, never as sentence.
From two charts to a shared reading
None of this works without two real charts, computed properly. Each person needs their full birth detail — date, place, and the hour of birth — because the house overlays depend entirely on the exact placements, and the hour is what fixes the houses. A guessed birth time gives a plausible-looking bi-wheel built on the wrong rooms.
So synastry begins where every reading begins: with the single chart. If you have not yet seen your own — the signs, the houses, the planets that make up your half of any connection — start with your birth chart. It is the ground the two-chart reading is drawn on, and it reads more richly once you know your own before you hold it up to someone else's.