A synastry chart overlays two people’s birth charts to see how their planets interact. Where one person’s Venus lands on the other’s Mars, whether their Moons are in harmony or tension, how each person’s Sun sits in the other’s chart — these “aspects” are read as the dynamics of the relationship: the attraction, the friction, the parts that feel like home and the parts you’ll always have to work at.
Here’s what a synastry chart shows and how to read yours.
What You Need
Two full birth charts, which means both people’s birth date, exact birth time, and birth place. Birth time matters more here than in a solo reading — without it, the house placements and the Moon can be off, and those are central to synastry. A rounded-to-the-hour birth time is workable; a completely unknown one limits the reading to Sun, Venus, Mars and the outer planets.
A synastry calculator (free ones exist) overlays the two charts and lists the aspects between them.
The Planets That Matter Most
| Planet contact | What it governs in the relationship |
|---|---|
| Sun–Moon | Core compatibility; whether you “get” each other’s basic nature. The classic marriage aspect. |
| Venus–Mars | Physical chemistry and romantic attraction. Strong Venus–Mars = magnetism. |
| Moon–Moon | Emotional rhythm; how you comfort and irritate each other day to day |
| Venus–Venus | Shared values around love, beauty, money, affection |
| Saturn contacts | Commitment, longevity, and also restriction — Saturn makes a bond serious and heavy |
| Sun–Ascendant | Instant recognition; “I saw you across the room” |
| Moon–Venus | Tenderness and emotional generosity; one of the sweetest contacts to have |
Astrologers usually look at the whole picture rather than one aspect — a chart with hard Saturn contacts but strong Sun–Moon harmony reads very differently than one with neither.
Harmonious vs. Challenging Aspects
- Conjunctions, trines, sextiles — flow, ease, mutual understanding. Too many and the relationship can lack spark.
- Squares and oppositions — tension, attraction, growth. The most passionate relationships usually have several; the work is not letting them turn into a stalemate.
- The best charts have both — enough harmony to feel safe, enough tension to stay interested.
There’s no such thing as a “perfect” synastry chart, and a difficult one doesn’t doom a relationship — it describes where the effort goes. A chart heavy on squares between the two Moons, for example, doesn’t mean “don’t marry this person”; it means “you’ll rub each other the wrong way emotionally on ordinary days, so build habits for repairing that quickly.”
A worked example
A couple runs their synastry and finds a gorgeous Venus–Mars conjunction (strong attraction), a supportive Sun–Moon trine (they genuinely get each other), and a Saturn square between one person’s Saturn and the other’s Moon. The Venus–Mars and Sun–Moon are why the relationship feels both hot and easy. The Saturn–Moon square is the recurring friction: the Saturn person can come across as cold or critical exactly when the Moon person needs warmth. Knowing that in advance turns a mystifying pattern (“why do I feel small when you go quiet?”) into a named dynamic they can talk about directly.
Synastry vs. Composite Chart
- Synastry = your planets vs. their planets. “How do we affect each other?”
- Composite = a single new chart made from the midpoints of both. “What is the relationship itself like, as its own entity?”
Astrologers often read both for couples considering marriage — synastry tells you about the interaction, the composite tells you about the thing the two of you become together.
Get a quick compatibility read to send to your partner with the free Soulmate Score — a 60-second destiny-and-compatibility score built to be shared, no birth time required.
Frequently Asked Questions
What is a synastry chart?
An overlay of two birth charts that shows how each person’s planets aspect the other’s — used to read the attraction, harmony, and friction in a relationship.
How do you read a synastry chart?
Look at the major planet contacts first (Sun–Moon, Venus–Mars, Moon–Moon, Saturn), note which are harmonious and which are challenging, and read the balance rather than any single aspect.
What makes two people astrologically compatible?
Strong Sun–Moon and Moon–Moon contacts for understanding, Venus–Mars for attraction, and some Saturn for staying power — ideally with a mix of easy and challenging aspects, not all one or the other.
Do you need birth times for synastry?
Ideally yes. Without exact birth times, the Moon positions and house placements can be inaccurate, and those are central to a synastry reading.
What’s the difference between synastry and a composite chart?
Synastry compares your two charts (“how we affect each other”). A composite chart blends them into one (“what the relationship itself is like”). Couples often look at both.
Is a “hard” synastry chart a reason not to marry someone?
No. Every long, happy marriage has hard aspects in its synastry — they’re where the growth and the passion live. A chart tells you where the work is, not whether to do the work. The couples who use synastry well treat the difficult contacts as a heads-up, not a verdict.
What aspect is best for marriage specifically?
Sun–Moon contacts (especially conjunction or trine) are the classic “marriage” aspect — they indicate two people who fundamentally understand each other’s nature. Supportive Saturn contacts add staying power. But astrologers read the whole chart; one great aspect doesn’t carry a relationship and one hard one doesn’t sink it.
Where the Effort Goes
A synastry chart doesn’t tell you whether to be together — it tells you where the relationship will feel effortless and where it’ll always take work. Both are useful to know before you marry.
Run a quick compatibility read with the free Soulmate Score.
