What a synastry chart is
A synastry chart (from the Greek synastria, "a meeting of stars") is the technique astrologers use to compare two natal charts and read the relationship between two people. Instead of looking at one chart in isolation, you place chart A and chart B side by side and study how the planets of one person land on the planets and angles of the other.
The everyday phrase for this is simply "compatibility chart" or "relationship chart." But synastry is more specific than a vague "are we compatible" question. It produces a concrete map: her Venus sits exactly on his Mars; his Saturn squares her Moon; her Sun falls into his seventh house. Each of those contacts has a recognisable meaning, and together they describe the texture of the bond — where it pulls, where it sticks, where it grows.
Two things worth separating early:
- Synastry compares two separate charts and reads the aspects between them. This article is about synastry.
- A composite chart is a different tool: it mathematically merges two charts into a single "chart of the relationship itself." Useful, but a separate technique — don't confuse the two.
Synastry answers "how do these two people affect each other?" That is usually the question people are really asking.
How the chart-to-chart comparison works
A synastry reading is built in layers. You don't read a single aspect and call it a verdict — you read the whole picture and weigh which contacts dominate.
The personal points that matter most
Five points carry most of the weight in relationship work:
- Sun — the core identity, the sense of "who I am." Sun contacts show whether two people recognise each other.
- Moon — emotional needs, the feeling of safety. Moon contacts decide whether you feel at home together.
- Venus — how each person loves, attracts and enjoys. The "affection" signal.
- Mars — drive, desire, the spark and the friction. The "charge" signal.
- Ascendant (and its ruler) — the instant first impression, physical chemistry, how you read each other at the door.
Cross-aspects
The heart of synastry is the cross-aspect: an angle between one person's planet and the other person's planet. The classics:
- Conjunction (same degree) — fusion. The two energies merge; very strong, can be sweet or overwhelming.
- Trine / sextile (harmonious) — easy flow, things "just work" in that area.
- Square / opposition (hard) — tension, attraction-through-friction, the parts that ask for conscious work.
A relationship made only of soft aspects can feel pleasant but flat. A relationship made only of hard aspects can feel electric but exhausting. Most lasting bonds are a mix: enough harmony to feel safe, enough tension to stay interested.
House overlays
The second layer is house overlays: where one person's planets fall in the other person's chart. Your partner's Sun landing in your seventh house (partnership) feels marriage-flavoured; in your fifth (romance, play) it feels like fun and flirtation; in your eighth (intimacy, depth, shared resources) it feels intense and transformative. The house tells you which area of life the other person lights up in you — which is why you need real birth times, not just dates.
What synastry actually reveals
Three broad questions, three sets of markers.
Attraction
What makes two people want each other:
- Venus–Mars contacts (either direction) — the single most reliable chemistry marker. One person's Venus on the other's Mars is the textbook "spark." There's a whole separate piece on this: Venus–Mars synastry.
- Ascendant contacts — a planet on the other's ascendant creates instant physical recognition.
- Sun–Moon contacts — the deep "I see you" pull, less about heat, more about belonging.
Friction
Where the work lives:
- Hard Saturn contacts — one person's Saturn on the other's Moon or Venus brings duty, restraint, sometimes a "you hold me back" feeling. Not fatal — often the backbone of long marriages — but heavy.
- Hard Mars contacts — passion that tips into argument; the same energy that attracts can ignite.
- Outer-planet hits (Uranus, Neptune, Pluto on personal points) — instability, idealisation or power struggles. Intense, hard to handle unconsciously.
Longevity
What helps a bond last:
- Sun–Moon harmony between the charts — the most cited "stayer" signal.
- Saturn contacts, the constructive ones — commitment and staying power.
- Strong house overlays in the relationship houses (7th, and the personal 4th/5th) — the relationship has somewhere to live.
Karmic markers — the lunar nodes and Saturn read as long-term "lesson" themes — add another layer. That's covered in karmic compatibility.
Synastry vs sun-sign compatibility
You've seen the magazine version: "Aries and Libra — opposites attract!" or a compatibility grid that scores you by sun sign alone. That is sun-sign compatibility, and it compares exactly one point in each chart — the Sun — out of the dozens synastry uses.
Why that falls short:
- Two "Scorpios" can be wildly different people. One has Moon in Aquarius and Venus in Libra; the other has Moon in Cancer and Venus in Scorpio. Their charts barely resemble each other.
- Sun-sign tables ignore the Moon, which decides emotional fit — arguably the most important factor in whether two people feel at home together.
- They ignore Venus and Mars entirely, so they say nothing about real chemistry.
- They ignore house overlays, so they can't tell you which area of life you activate in each other.
A sun-sign table is a fun starting point and genuinely useful as a first sketch — there's a zodiac compatibility table for exactly that. But a synastry chart is the difference between a horoscope headline and an actual conversation about two specific people. If you only know each other's birth dates, you can already do a compatibility-by-birth-date sketch; for the full picture you'll want both birth times too.
How to read your synastry chart
A practical order of operations:
- Gather both sets of data. Date, exact time and place of birth for both people. Without times you lose the ascendants and all house overlays — a big chunk of the reading.
- Cast both natal charts, then bring them together in a bi-wheel (one chart inside, one outside).
- Find the Sun, Moon, Venus, Mars and Ascendant contacts first. These five carry most of the meaning.
- Map the house overlays — whose planets land in whose 1st, 5th, 7th and 8th houses.
- Weigh harmony against friction. Count the soft and hard contacts, and notice which planets are involved. Hard Mars is different from hard Saturn.
- Read the whole, not the parts. One scary square doesn't doom a relationship; one beautiful trine doesn't save it. The pattern is the point.
The maths is fiddly to do by hand — you need both charts cast accurately and every cross-aspect checked. An automated reading does the calculation from real ephemeris data and surfaces the contacts that actually matter, so you spend your time on meaning rather than arithmetic.
Common mistakes when reading synastry
- Reading a single aspect as the whole story. "We have Venus square Saturn, we're doomed." No — that's one contact in a chart full of others. Weigh the pattern.
- Skipping birth times. Without times there are no ascendants and no house overlays, and you lose half the reading. Two birth dates alone is a sketch, not a synastry chart.
- Treating hard aspects as "bad." Friction is often what keeps a relationship alive and growing. Many long marriages run on Saturn and Mars contacts.
- Confusing synastry with a composite chart. They answer different questions — synastry is the interaction between two people, the composite is the "third entity" of the relationship.
- Expecting a percentage. A relationship isn't 87% compatible. Synastry describes texture — where it's easy, where it's work — not a score.
FAQ
Frequently asked
What is a synastry chart in simple terms?
It's a comparison of two people's natal charts. You overlay one chart on the other and read the cross-aspects — how one person's planets relate to the other's — plus the house overlays, which show where one person's planets fall in the other's chart. Together they describe the dynamic between the two people: where there's attraction, where there's friction, where there's staying power.
What do I need to build a synastry chart?
Two full sets of birth data: date, exact time and place of birth for both people. The time matters because it sets each person's ascendant and houses — without it you lose the house overlays and a large part of the reading. Two birth dates alone only give you a rough sketch.
Which aspects matter most in synastry?
The contacts between the five personal points: Sun, Moon, Venus, Mars and the Ascendant. Sun–Moon contacts show deep recognition and longevity; Venus–Mars contacts show chemistry; ascendant contacts show physical first-impression pull; Saturn contacts show commitment and weight. Outer-planet contacts (Uranus, Neptune, Pluto) add intensity but are harder to handle.
What's the difference between synastry and a composite chart?
Synastry compares two separate charts and reads the aspects between them — it answers "how do these two people affect each other?" A composite chart mathematically merges the two into a single chart of "the relationship itself." They're complementary tools, but they're not the same thing and shouldn't be read interchangeably.
Is synastry more accurate than sun-sign compatibility?
Yes, substantially. Sun-sign compatibility compares just one point — the Sun — in each chart. Synastry uses the Sun, Moon, Venus, Mars, Ascendant, Saturn and house overlays — dozens of factors. Two people with the same sun signs can have completely different synastry depending on the rest of their charts.
Can a synastry chart tell me if a relationship will last?
No. It maps the tendencies and dynamics between two people — where things flow easily, where they'll need conscious work. It doesn't predict the outcome. Relationships with plenty of friction often endure when both people are aware; "easy" charts can still drift apart. Synastry is a tool for understanding, not a guarantee of the future.