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Synastry: How to Compare Two Charts, Step by Step

Astrology · ~8 min read

Synastry is the art of overlaying two birth charts to see how they answer one another. It is neither a compatibility test nor a verdict: it is a map of the areas where two people understand each other quickly, and of those where effort will be more conscious. Here is how to read it, step by step.

What is synastry?

The word comes from the Greek synastria, "a bringing together of stars." In practice, you calculate two natal charts — one per person — then place them one over the other, usually as a double wheel: chart A on the inside, chart B on the outside. You then observe two things:

• the inter-aspects: the angles that person A's planets form with those of person B;
• the house overlay: which areas of A's life person B's planets fall into, and vice versa.

Synastry differs from the composite chart, another technique that builds a third chart from the midpoints of the two charts. The composite speaks about the relationship as an entity; synastry speaks about the dialogue between two people. The two complement each other, but it is best to master the second first.

A useful clarification: synastry is not just about couples. It applies equally well to a friendship, a parent-child relationship, or a professional pairing.

What you need before you begin

For each person, you need the date, the most exact time possible, and the place of birth. The time is especially decisive here: without it, the Rising sign, the Moon (which changes sign about every two and a half days), and above all the houses become uncertain — and the house overlay loses its reliability. If you don't know the other person's time, stick to the inter-aspects between planets, keeping in mind that the Moon may be imprecise. To brush up on the basics, our guide to reading your natal chart lays out the vocabulary.

The method in five steps

1. Look at the luminaries. Sun–Moon contacts between the two charts are traditionally read as a foundation: a sense of familiarity, the feeling of "speaking the same language" without having to explain it.
2. Spot Venus and Mars. Venus describes what we love and how we show affection; Mars, desire and the way we act. Their cross-contacts (one person's Venus with the other's Mars) are classically associated with attraction and the tempo of desire.
3. Examine Mercury. This is the planet of conversation. Two Mercuries on good terms, and you understand each other with barely a word; in tension, you will probably need to spell things out more — which is far from insurmountable.
4. Note the contacts to the angles. A planet of B placed on A's Rising sign or Midheaven often signals a very visible influence, on self-image or public trajectory. If the topic of the angles escapes you, see how to calculate your rising sign.
5. Do the house overlay. Person B's planets falling into A's 7th house (partnerships) or 5th house (play, creativity, romance) color those areas. This is often the most telling step, and the most overlooked.

Aspects: what each angle evokes

The aspects used are the same as in a natal chart — our article on astrological aspects details them. In synastry, they are read as follows:

• Conjunction (0°) — a merging of the two functions. Intense, for better or for busier, depending on the planets involved.
• Trine (120°) and sextile (60°) — fluidity. What flows without friction… and what you risk not even noticing.
• Square (90°) and opposition (180°) — tension. Not "incompatibility," but friction: what forces you to adjust. Many lasting bonds rest on a few well-accepted squares.

Two useful technical markers. First, tighten the orbs: in synastry, you generally keep 2 to 5° depending on the planets, otherwise the chart fills up with flat, meaningless aspects. Second, an aspect from Saturn to a personal planet is traditionally associated with duration and responsibility — sometimes experienced as a reassuring framework, sometimes as a constraint. It is a symbolic pointer, not a prediction.

What it sheds light on day to day

The value of synastry is not in knowing whether a relationship "will work." It is in naming mechanisms that were lived without words. A few examples of what the reading can make visible:

• Rhythms. A fast Mars facing a Moon that needs time: the question is no longer "who is wrong," but "how do we sync our tempos."
• Recurring misunderstandings. A Mercury–Neptune tension can symbolize a communication where the implicit takes up too much room; the remedy is prosaic: rephrase.
• Comfort zones. Fluid aspects show where to lean when everything else wobbles.
• Activated areas. Depending on the houses touched, the same relationship may mobilize mainly the home, work, or the urge to learn together.

Used this way, synastry is less an oracle than a support for conversation. Many people find in it above all a permission: that of acknowledging a difference without turning it into a flaw.

Points of caution

• No chart decides a relationship. There is no "doomed" combination nor a guaranteed one. Two charts very harmonious with each other can fade out; two tense charts can last for decades. People count more than the chart.
• Don't look for a verdict. If you approach a synastry hoping it will settle a decision for you, it will mostly tell you what you want to hear.
• Beware of using it as a label. Reducing someone to "their Mars in a square" confines rather than illuminates. The reading should open questions, not close portraits.
• It is not a clinical tool. A synastry replaces neither couples therapy, nor psychological support, nor legal advice in the event of a separation. For these matters, turn to a qualified professional.
• Consent matters. Analyzing someone's chart without their knowledge to "understand" their intentions is rarely a good idea — and never a reliable source of information about their actions.

Frequently asked questions

Do you need the birth time of both people?

Ideally, yes. Without a time, the inter-aspects between slow and personal planets remain readable, but the Rising sign, the Midheaven, and the house overlay become unusable, and the Moon's position may be approximate. An approximate time is better than nothing, provided you keep that margin of uncertainty in mind.

Which aspects announce "good" compatibility?

None, strictly speaking. Tradition associates Sun–Moon and Venus–Mars contacts and aspects to the Rising sign with a sense of closeness, and Saturn with duration. But these are symbolic tonalities: they describe possible dynamics, never a certain outcome. A chart predicts neither a breakup nor a marriage.

Synastry or composite chart: which to choose?

Start with synastry: it describes the exchange between two people, which is more concrete and more actionable. The composite, built on midpoints, then adds a light on the "atmosphere" specific to the relationship. The two are read as complements, not in competition.