What Is Synastry? How to Read Two Charts Together
Sun-sign compatibility is checkers; synastry is chess. The five planet contacts that matter most, the aspects in plain language, and what the map can't decide.
Synastry is the astrological technique of overlaying two birth charts and reading the aspects between one person's planets and the other's to map how a relationship works: attraction, emotional fit, friction and longevity.
Synastry at a glance
| What synastry is | Two natal charts overlaid, with the aspects between one person's planets and the other's read as the relationship's dynamics |
|---|---|
| What you need | Both birth dates, times and places; without times you lose the Moon's accuracy and the house overlays |
| Top contacts | Sun-Moon, Venus-Mars, Moon-Moon, Venus-Venus, Saturn to a personal planet |
| The aspects | Conjunction 0°, sextile 60°, square 90°, trine 120°, opposition 180° |
| Synastry vs composite | Synastry: how you affect each other. Composite: the relationship as its own chart |
| What it cannot do | Tell you whether to stay; it maps the terrain, not the decision |
| Run it | Synastry calculator, then the composite chart calculator |
Sun-sign compatibility is checkers. Synastry is chess. It's the technique astrologers actually use when two people ask "are we compatible?": laying one complete birth chart over another and reading how the planets interact. Here's the beginner's version that still respects the craft.
What synastry is
Synastry compares two birth charts, every planet in yours against every planet in theirs, and reads the geometric angles (aspects) between them. Each contact describes one channel of the relationship: how your emotional needs meet their instincts, how your love style meets their desire, where you stabilize each other and where you grind. The result isn't a score; it's a map of how the relationship works, which is far more useful.
The five contacts that matter most
- Sun-Moon. One person's core identity meeting the other's emotional nature. Harmonious contacts here are the classic marriage signature: you feel understood at the daily level.
- Venus-Mars. The attraction circuit: one person's love style meeting the other's desire. Strong contacts produce the chemistry people can't explain; absent ones produce "great on paper."
- Moon-Moon. Emotional weather compatibility. Same-element Moons grieve, celebrate and decompress in compatible dialects, the quiet foundation of living together (more in moon sign compatibility).
- Venus-Venus. Whether your definitions of love itself align: affection, money, beauty, pleasure. Different Venus languages can coexist, but someone has to be bilingual (start with your own: Venus sign meaning).
- Saturn contacts. The glue, and the lesson. Saturn touching a partner's personal planets adds commitment and longevity, and a strain of seriousness. Relationships without any Saturn contact often feel wonderful and evaporate.
Synastry starts with your own chart
Free reading from your birth date, time and place: your love style, your needs, your timing.
Start my free readingThe aspects, in plain language
Conjunction (same spot): fusion, intensity, for better and worse. Trine (120°): ease, things flow without effort. Sextile (60°): friendly, opportunity that rewards a little work. Square (90°): friction, the contact you grow from or fight about. Opposition (180°): polarity, magnetic attraction plus the seesaw. A good synastry has a mix: all trines is pleasant and inert; all squares is a gym membership; chemistry plus a workable amount of friction is what lasting looks like.
Synastry aspects cheat sheet
| Aspect | Degrees | Orb that counts | How it feels between two people |
|---|---|---|---|
| Conjunction | 0° | Up to 8° (personal planets), 10° for Sun or Moon | Fusion. The two planets act as one; intense, obvious, for better or worse |
| Sextile | 60° | Up to 4° | Friendly, easy to activate, rewards a little effort; often overlooked |
| Square | 90° | Up to 6° | Friction. The contact you fight about or grow from; keeps a relationship awake |
| Trine | 120° | Up to 8° | Ease. Things flow without work; pleasant, and inert if nothing else is present |
| Opposition | 180° | Up to 8° | Polarity. Magnetic attraction plus the seesaw; projection is the risk |
| Quincunx | 150° | Up to 2° | Awkward adjustment; two planets that never quite understand each other |
| Semi-sextile | 30° | Up to 2° | Mild, neighbourly; rarely decisive on its own |
Orbs are the allowance either side of the exact angle. Tighter is stronger: a Venus-Mars conjunction at 1° is felt immediately; at 7° it is a background hum. Most astrologers give the Sun and Moon the widest orbs and the outer planets the narrowest.
What is synastry, a synastry chart, and a synastry calculator?
The three terms get used as if they were the same thing. Synastry is the technique: the comparison of two birth charts. A synastry chart is the picture it produces, usually one chart drawn inside the other as a double wheel, with the aspects between them listed in a grid. A synastry calculator is the tool that builds that chart from two sets of birth data.
The inner wheel is usually person A and the outer wheel person B, so B's planets can be read against A's houses. That house overlay is the second layer of synastry: where a partner's Venus lands in your chart shows where they bring pleasure, where their Saturn lands shows where they bring weight.
What synastry can't decide
Whether to stay. Synastry describes the terrain, where the ease is, where the work is, what the relationship is for, but two mature people can build on hard aspects and two unwilling ones can waste a beautiful chart. Use it the way astrologers do: to understand the dynamic you're already feeling, locate the recurring fight's actual source, and decide with open eyes. For the single-chart starting points, begin with who is my soulmate, and for the full both-charts method, the pillar guide: birth chart compatibility.
Synastry vs composite
Two different tools, often confused. Synastry compares the two charts as separate things, your planets and theirs. A composite chart (build one with the composite chart calculator) merges both into a single third chart representing the relationship itself as an entity. Synastry answers "how do we affect each other"; composite answers "what is this relationship, as its own thing." Serious relationship astrology uses both.
Synastry aspects, explained one by one
Each conjunction below gets its own guide: what it means, who feels it, where it works and where it strains.
- Moon conjunct Ascendant synastry
- Venus conjunct Ascendant synastry
- Sun conjunct Ascendant synastry
- North Node conjunct Moon synastry
- Saturn conjunct Venus synastry
- Mars conjunct Mars synastry
- Moon conjunct Moon synastry
- Sun conjunct Venus synastry
- North Node conjunct Venus synastry
- Venus conjunct Venus synastry
- Moon conjunct Venus synastry
- Sun conjunct Moon synastry
- Venus conjunct Mars synastry
Go further: composite chart meaning for the relationship as its own entity, the pillar guide to birth chart compatibility, and the tools: synastry calculator and composite chart calculator.
Go deeper
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Get my reading ($17)Frequently asked questions
What is synastry in astrology?
The technique of overlaying two birth charts and reading the aspects between them, planet by planet, to map how a relationship functions: attraction, emotional fit, friction and longevity.
What is the strongest synastry aspect for soulmates?
Astrologers most often cite harmonious Sun-Moon and Venus-Mars contacts, with at least one Saturn contact for staying power. One aspect alone never tells the story.
Do I need exact birth times for synastry?
For Moon positions and house overlays, yes, or at least estimates. Sun and Venus contacts usually survive an unknown birth time. (Recovering yours: what time was I born?)
What is the most important aspect in synastry?
Sun-Moon contacts are among the strongest markers of lasting compatibility, while Venus-Mars drives attraction. Most strong relationships show several significant contacts rather than relying on one.
Is synastry accurate?
Synastry is accurate at describing how two people affect each other: where the ease is, where the friction is, what the attraction runs on. It is not a prediction of whether the relationship lasts, because that depends on choices the chart cannot see. Accuracy also depends on both birth times being correct.
What is a good synastry score?
There is no meaningful score; percentage calculators add up aspects with arbitrary weights. A better test is qualitative: at least one Sun-Moon or Moon-Moon harmony, a Venus-Mars contact, one Saturn link, and a square or two for growth. A chart with those has what lasting relationships usually show, whatever number a tool prints.