Synastry meaning: the astrology of two charts in relationship

What does synastry mean?

Synastry is the branch of astrology that studies relationships by comparing two people's birth charts. The word comes from the Greek synastria, roughly "a joining of stars." The core idea is simple: each person has their own natal chart, and synastry looks at how those two charts interact — how one person's Venus meets the other's Mars, where their planets land in each other's houses, and which contacts feel easy versus challenging.

Synastry works for any pairing — romantic partners of any gender, friends, family, business partners — because it is really about the chemistry between two sets of planetary wiring, not about any assumption of who should be with whom. And it is a lens, not a verdict: a chart contact describes a tendency the two of you might feel, never a ruling that a relationship is fated to work or fail. People with "difficult" synastry build beautiful, lasting bonds all the time; the aspects simply name what the work and the ease are likely to be.

How astrologers compare two birth charts

Synastry is done by overlaying the two charts and reading the connections between them. There are two main layers:

Inter-aspects

An inter-aspect is an aspect — a specific angle like a conjunction, trine, square or opposition — between one person's planet and the other person's planet. A soft aspect such as a trine tends to flow; a hard aspect such as a square tends to generate friction, heat and growth. Astrologers weight these by which planets are involved and how tight the angle is.

House overlays

A house overlay looks at which of your houses the other person's planets fall into. If their Sun lands in your 7th house of partnership, they can feel like a natural partner; if it lands in your 10th, they may feel tied to your goals and public life. The house colours where in life the connection is felt.

The synastry contacts that matter most

Some pairings carry extra weight in relationship astrology:

• Sun–Moon contacts blend one person's core identity with the other's emotional nature — often a marker of deep, home-building compatibility.
• Venus–Mars contacts are the classic signature of attraction and physical chemistry, mixing what one person loves with what the other pursues.
• Moon–Moon and Moon–Venus contacts speak to emotional safety and affection.
• Ascendant contacts shape the instant, gut-level draw when you first meet.
• Saturn contacts add weight, commitment and sometimes a sense of duty or lesson; they show up in many long-term bonds, for better and for harder.

The Cosmic Dating and compatibility features in AstraLink read exactly these contacts and turn them into a plain-language synastry summary with a score — so you get the why behind the number, not just the number. Try the interactive synastry tool or the quick compatibility calculator to see two charts side by side.

Synastry vs composite chart

Synastry keeps the two charts separate and reads the lines between them — it is a conversation between two individuals. A composite chart does something different: it mathematically merges the two charts into a single new one (usually by taking the midpoints between each pair of planets) to describe the relationship itself as its own entity. Synastry answers "how do we affect each other?"; the composite answers "what is the character of the thing we create together?" Many astrologers read both, then weigh them against ongoing transits for timing. Curious about your own match? Join early access to compare charts and meet people by cosmic compatibility.