Synastry vs Composite Chart: How Relationship Astrology Works
Synastry shows how two people affect each other. A composite chart shows what the relationship itself is. Here is when to use which.
Synastry compares two birth charts by placing one on top of the other, to see how each person's planets affect the other person. A composite chart combines two birth charts into a single new chart, calculated from the midpoints between both people's planets, that describes the relationship itself as if it were a third "person". Synastry answers "what do we trigger in each other?"; the composite answers "what is this bond here to become?"
Both work for any relationship: partners, friends, family, business partners or a parent and child.
Synastry vs composite at a glance
Synastry | Composite chart | |
|---|---|---|
What it is | Two charts overlaid | One chart made of midpoints |
Focus | Each person's experience of the other | The relationship as its own entity |
Key question | What do we activate in each other? | What is the purpose and nature of "us"? |
Read first | Aspects between personal planets | Composite Sun, Moon, Ascendant and Venus |
Shows | Attraction, friction, chemistry | Shared identity, the couple's path |
Needs birth times? | Ideally, for houses and angles | Ideally, for the Ascendant and houses |
How synastry works
In synastry you look at two kinds of contact:
Inter-aspects: angles between person A's planets and person B's planets. A's Venus trine B's Mars, for example, suggests easy attraction and mutual desire.
House overlays: where A's planets fall in B's houses. If A's Moon falls in B's 4th house, B may feel "at home" with A.
Synastry contacts to look at first
Sun–Moon: a classic sign of mutual understanding; one person's identity meets the other's needs.
Venus–Mars: romantic and physical attraction.
Moon–Moon: emotional rhythm; how easily you comfort each other.
Mercury–Mercury: how you talk, argue and make plans.
Saturn contacts: commitment and loyalty, but also heaviness or a sense of duty.
Pluto contacts: intensity, magnetism and power dynamics.
Chiron contacts: you touch each other's old wounds, which can hurt or heal.
North Node contacts: a feeling of fate or "we were meant to meet".
No single aspect makes or breaks a relationship. Look for repeated themes: if three different contacts all point to strong communication, that is meaningful.
How a composite chart works
A composite chart takes the midpoint between each pair of planets. If your Sun is at 10° Aries and your partner's Sun is at 10° Gemini, the composite Sun sits at 10° Taurus. This is done for every planet and point, creating a new chart that neither of you owns alone.
(Some astrologers use the Davison chart instead, which uses the midpoint in time and space between both births. It is a different method with a similar purpose.)
What to read in a composite chart
Composite Sun: the core purpose of the relationship.
Composite Moon: the emotional climate between you.
Composite Ascendant: how the relationship looks to the outside world.
Composite Venus: how the relationship loves and what it values.
The house with the most planets: the area of life this bond focuses on. A heavy 10th house can point to a couple that builds something public together; a heavy 4th house to a bond centred on home and family.
Which one should you use?
Use synastry to understand chemistry, attraction and recurring friction between two people.
Use a composite to understand the relationship as a living thing: its strengths, its purpose and its typical challenges.
Use both for the full picture. Synastry is "you and me"; the composite is "us".
What relationship astrology cannot tell you
Astrology does not predict whether a relationship will last, and no chart combination is "doomed". Charts describe dynamics and potential. The choices two people make, and how they communicate, always matter more than any aspect.
Frequently asked questions
Do I need both birth times for synastry? For planet-to-planet aspects, dates are often enough. For house overlays, the Ascendant and the composite houses, you need both birth times.
Is a composite chart the same as a compatibility score? No. A score reduces a relationship to a number. A composite chart describes the relationship's character and purpose.
Can I do synastry for friendships or family? Yes. Synastry and composite charts work for any bond between two people.
Explore your relationship
Synastry and composite charts are coming to AstrologyBird. Until then, start with your own birth chart and try the Light & Black Light spread in the AstrologyBird Oracle with a relationship question in mind.
