Synastry is the astrological practice of comparing two birth charts to see how one person's planets interact with the other's. Astrologers look at the angles formed between the two sets of planets, called inter-aspects, and at where each person's planets fall in the other's houses. It's the main technique behind chart-based relationship compatibility, and it goes much further than comparing sun signs.
The word comes from Greek roots meaning "stars together." The idea is straightforward: if a birth chart is a map of one person, synastry lays two maps over each other and notes where they meet.
How synastry works
To do synastry, an astrologer needs a birth chart for each person, ideally with accurate birth times. The two are often drawn as a bi-wheel: one chart in the inner circle, the other person's planets placed around the outside. Then the comparison runs in two directions.
- Inter-aspects. Do any of your planets form a conjunction, sextile, square, trine or opposition with any of theirs? These angles describe how your drives and needs meet. Synastry usually uses tight orbs, often only a few degrees, so that the strongest contacts stand out.
- House overlays. Which of your houses do their planets land in, and which of theirs do yours land in? This describes which areas of each person's life the other one stirs up.
Because the comparison runs both ways, synastry is rarely symmetrical. Your Venus might sit on their Ascendant while their Venus sits quietly in your twelfth house. That asymmetry is often the most revealing part, because it describes what each person experiences in the relationship, which isn't always the same thing.
Without birth times you can still compare planets and most aspects, but house overlays and Ascendant contacts drop out, and the Moon's position may be uncertain. Drawing up each person's birth chart first shows you what you're working with.
The contacts astrologers look at first
With ten bodies in each chart there are a hundred possible pairings before you even count the angles of the chart. Astrologers tend to start with a short list.
- Sun and Moon. One person's Sun conjunct, trine or opposite the other's Moon has long been treated as a classic sign of compatibility: one person's sense of self fits the other's emotional needs.
- Moon and Moon. Harmonious moon signs and aspects suggest you find comfort in similar ways. We cover this in your moon sign in relationships.
- Venus and Mars. These describe attraction and chemistry. One person's Venus in aspect to the other's Mars is a traditional marker of romantic and physical spark.
- Venus with the Sun, Moon or Ascendant. Affection and simple liking: you enjoy each other's company.
- Mercury contacts. How easily you talk, plan and argue, from texting style to how you settle a disagreement about the holidays.
- Saturn contacts. Saturn touching the other's Sun, Moon or Venus can feel serious, steadying or restrictive. Astrologers often link it with commitment and staying power as well as friction.
- The angles. Planets on each other's Ascendant or Descendant tend to be noticed immediately and felt strongly.
Some astrologers also weigh the lunar nodes, or Jupiter contacts for generosity and goodwill. The short list above covers most of what matters for a first look.
House overlays: where you land in each other's lives
House overlays answer a different question. Not "how do we get along?" but "what does this person bring up in me?"
If your partner's Sun falls in your fourth house, you might associate them with home and family, or feel as if you've known them for years. Their Venus in your fifth house suggests romance and fun. Their Saturn in your tenth might come with a sense that they take your career seriously, or that they're critical of it. Planets falling in the seventh house, the house of partnership, are traditionally significant for long-term bonds; our explainer on the seventh house covers why.
Overlays depend on an accurate birth time for the person whose houses are being used. A time off by an hour can move a planet into the next house and change the reading.
Synastry, composite charts and sun-sign compatibility
Three techniques often get mixed up.
- Sun-sign compatibility compares just two signs, such as a Virgo and a Scorpio. It's a quick shorthand and the starting point of our zodiac compatibility pages.
- Synastry compares two full charts, planet by planet. It describes how two individuals affect each other.
- A composite chart merges two charts into one, usually by taking the midpoint between each pair of matching planets. The result is read as a chart of the relationship itself, as though the couple were a third party with its own character.
Many astrologers use synastry and a composite together: synastry for the chemistry between two people, the composite for what the bond is about. The Love Compatibility Report works through synastry between two charts, and a composite chart report calculates the combined chart.
How to read synastry without scaring yourself
A few principles keep synastry useful rather than alarming.
- No single aspect decides a relationship. A square between your Mars and their Sun doesn't doom anything. Look for patterns that repeat across several contacts.
- Hard aspects aren't bad aspects. Squares and oppositions often bring attraction and growth along with friction.
- Easy charts aren't automatically good ones. A comparison full of trines can feel comfortable and still lack spark.
- Charts describe; people decide. How two people talk, keep promises and treat each other matters more than any inter-aspect.
Astrology is a lens for reflection, not a measure of whether a relationship will last. If someone treats you unkindly, consistently, a lovely synastry chart doesn't change that. And if the chart looks difficult but the relationship is kind and honest, believe the relationship.
Questions readers ask
Can synastry tell me if we're soulmates?
No chart technique can prove that. Synastry can describe areas of ease and challenge between two people, which many couples find useful for understanding each other. Whether a bond feels meant to be is something people discover through experience, not geometry.
Do I need both people's birth times for synastry?
For a full reading, yes, because house overlays and Ascendant contacts depend on them. Without times you can still compare planets by sign and most aspects. The Moon's position will be uncertain by several degrees, and occasionally by a whole sign.
Is synastry only for romantic relationships?
No. Astrologers use it for friends, family, business partners and parents and children. The same contacts apply, though the emphasis shifts: Venus and Mars matter more for romance, Mercury and Saturn more for a working partnership.
What's the most important aspect in synastry?
There's no single answer, but Sun and Moon contacts, Venus and Mars contacts, and planets on each other's Ascendant are widely treated as the strongest. Tighter orbs generally count for more than wide ones.
