What is synastry?

Synastry is the practice of laying two birth charts over one another and reading what happens where they meet. You are not judging one chart against a type, and you are not pulling a score from a pair of Sun signs. You are holding two complete charts, each its own fixed moment of sky, and watching how one person's planets land on the other's.

The word comes from the Greek for stars brought together, and that is the whole method in a phrase. Your chart does not change and neither does theirs. Synastry asks what the two of them do in the same room: where they settle each other, where they strike sparks, and where one of them will keep asking the other for something.

What synastry compares

A birth chart is a map of the sky for one moment and one place. It holds the Sun, the Moon and the planets in their signs, and, when the birth time is known, the Rising sign and the twelve houses. If that picture is new to you, our primer on what a birth chart is walks through it.

Synastry takes two of those maps and reads the relationship between them. A connection cannot be read from either chart alone, because it lives in the contacts between them. Your chart can describe how you tend to love. It cannot describe how you love this particular person, whose Moon happens to sit where your Saturn does.

That is why compatibility is never honestly a single number. A number can rank a pairing against other pairings. It cannot show you where two lives actually touch, and the touching is the information. Synastry works by looking at particular contacts, this planet of yours meeting that planet of theirs, and reading each one for what it offers and what it costs.

Cross-aspects: where two charts touch

The first thing synastry reads is the set of cross-aspects: the angles between a planet in one chart and a planet in the other. Astrologers use the same aspects here as in a single chart. A conjunction puts two planets together, a sextile and a trine join them easily, and a square and an opposition set them against each other. Each is counted only when the angle is close, and the closer it is, the louder it reads. Our guide to astrological aspects explains each one in more depth.

The aspect tells you the kind of contact. The planets tell you what is in contact. A few pairings carry most of the weight in a relationship reading:

Sun and Moon. One person's Sun meeting the other's Moon is the classic sign of two people who recognise each other. The Sun person tends to feel seen, and the Moon person tends to feel at home. A hard angle between them does not cancel the recognition, though it can make daily life a little more effortful.

Venus and Mars. These describe attraction and desire. Your Venus in contact with their Mars is often where the chemistry is, and a square between them can be as magnetic as a trine, only less restful.

Moon and Moon. Two Moons in easy contact usually means two people who need similar things to feel safe. Two Moons at odds can mean that one person's comfort is the other's restlessness, which is workable once both of you can name it.

Saturn to a personal planet. Saturn contacts are the ones people worry about, and they are also the contacts that hold relationships together over years. Saturn on your Venus can feel like weight or like commitment, depending on how the two of you carry it.

Mercury contacts. These describe how you talk, argue and make plans together. A pair with warm Venus contacts and clumsy Mercury contacts often loves easily and misunderstands often.

A square is friction and a trine is ease, but neither is good or bad on its own. Friction is where two people push each other to grow, and too much ease can leave a relationship pleasant without ever asking anything of it.

House overlays: whose room a planet walks into

The second thing synastry reads is the house overlay: where one person's planets fall among the other person's twelve houses. The houses are the areas of a life, such as money, home, partnership and work, and our guide to the houses tours all twelve.

An overlay answers a question the aspects cannot. The planet says what the other person brings, and the house says where in your life you feel it. If their Sun falls in your seventh house of partnership, their presence lights up exactly the part of your life that looks for a committed other. If their Mars falls in your second house of money and self-worth, they may push you to earn, spend or value yourself differently, and that can be energising or abrasive. If their Moon falls in your fourth house, you may find they feel like family very quickly.

Overlays run in both directions, and the two directions are rarely the same. Their Sun may fall in your seventh house while your Sun falls in their twelfth, the house of what is private and hidden. In that case the relationship may feel open and central to you and quieter, more inward, to them. Neither account is wrong. They are two people standing in different rooms of the same connection.

A worked example

Picture two people, whom we will call Ana and Ben. Ana's Moon is in Cancer and Ben's Sun is in Cancer, close together. That conjunction is the first thing an astrologer would notice: Ben's way of being himself lands directly on Ana's emotional needs, and she is likely to feel at ease with him early.

Ben's Venus squares Ana's Mars. There is attraction in that, with some friction in it. They may want each other at different paces, or show affection in ways the other reads as too much or too little.

Ana's Saturn sits on Ben's Mercury. Ben may find that Ana takes his words seriously, sometimes more seriously than he meant them, and Ana may find Ben's ideas worth building on. Over time this is the kind of contact that turns talk into plans, provided Ben does not start to feel judged every time he thinks aloud.

Finally, Ben's Sun falls in Ana's fourth house. For Ana, Ben belongs to the home part of life, and she may picture a household with him before she pictures anything else.

None of this says whether Ana and Ben should be together. It describes what will come easily to them, what will need care, and where each of them is likely to feel the other most strongly. That description is what synastry is for.

Synastry and the composite chart

People sometimes confuse synastry with the composite chart, and the two do different jobs. Synastry keeps both charts whole and reads the contacts between them, so it describes how each person experiences the other. A composite chart builds a single third chart from the midpoints of each pair of planets, and astrologers read it as a portrait of the relationship itself, as if the partnership were a person with its own chart.

Both are legitimate techniques. Synastry is the older and more direct of the two, and it is usually where a relationship reading begins, because it shows the two individuals before it shows what they make together.

What synastry can and cannot tell you

A few beliefs about compatibility are worth setting down.

Sun signs alone do not decide compatibility. Tables that pair signs by element are a starting point at best. Two people with supposedly clashing Sun signs can have Moons, Venuses and Rising signs that fit together very well, and a textbook match can share nothing beneath the surface. Our primer on Sun, Moon and Rising shows how much of a person lies outside the Sun sign.

One hard aspect does not doom a relationship, and one easy aspect does not save it. A synastry reading weighs the whole pattern. Most real relationships contain both kinds of contact, and the hard ones often explain why the connection matters.

Synastry does not tell you what to do. It can show where two people flow and where they will have to work, where the attraction sits and where patience will be needed. It cannot decide whether you stay, and it cannot reduce a relationship to a percentage that means anything on its own. Charts do not choose. People do, and a clearer picture of the ground helps them choose well.

It is not only for romance. The same method reads friendships, business partnerships, and the bond between a parent and a child. The questions change and the technique stays the same.

What you need to begin

Synastry depends on two real charts, computed properly. Each person needs a date and a place of birth, and ideally a time. Without a birth time there are no houses and no Rising sign, so house overlays drop out entirely. The Moon moves quickly enough to change sign in the course of a day, so for some birthdays even the Moon's sign is uncertain without a time. A guessed birth time produces a bi-wheel that looks convincing and puts the planets in the wrong rooms.

A synastry chart is usually drawn as a bi-wheel: one chart on the inner ring, the other on the outer ring, and lines across the centre for the cross-aspects. It is the clearest way to see at a glance where two charts clasp and where they pull.

It helps to know your own half first. If you have not yet seen your chart, with its signs, houses and planets, start with your birth chart. Every connection you read later is measured against it, and the other person's chart will make much more sense once you know your own.