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How it works
How synastry compatibility is calculated
This tool compares the actual planetary positions from two natal charts and identifies every cross-chart aspect — a geometric angle between a planet from Chart A and a planet from Chart B. The five category scores are derived from which planet pairs form which aspects, using the same astronomical algorithms that power professional astrology software.
Driven by Sun–Moon and Moon–Moon aspects. Harmonious contacts suggest deep attunement; tense aspects indicate differing emotional styles.
Powered by Venus–Mars cross-aspects. Describes physical chemistry, desire compatibility and how each person's values meet the other's drive.
Based on Mercury aspects. Describes how naturally ideas flow and whether conversations energise or require conscious effort.
Weighted toward Saturn contacts. Favourable angles add loyalty and structure; tense angles can indicate power struggles or feeling restricted.
Accumulates from Mars–Mars, Pluto–Moon and other high-tension pairs. Low means fewer friction points; high signals intensity and transformative pressure.
What to look for
Key connections in a synastry chart
While every aspect matters, four cross-chart connections carry the most weight in traditional synastry interpretation.
Emotional attunement
When one person's Sun aligns with the other's Moon, there is a natural complementary rhythm — the Sun person shines; the Moon person reflects and nurtures. Frequently found in long-term partnerships.
Chemistry & desire
The classic attraction aspect. Venus represents what we find beautiful; Mars represents how we pursue. Harmonious contacts spark immediate chemistry and keep desire alive over time.
Commitment & longevity
Saturn contacts bind two people across time. Supportive angles build loyalty and shared responsibility. Challenging angles can feel restricting but often create the most enduring and growth-oriented bonds.
Communication flow
Mercury cross-aspects determine whether two minds click or clash. Harmonious contacts make conversation feel effortless; tense aspects may require conscious patience to bridge different thinking styles.
Common questions
Frequently asked questions
What is a synastry chart?
A synastry chart overlays two birth charts to reveal how two people interact astrologically. It identifies aspects — angular relationships between planets from different charts — that show areas of natural harmony, chemistry, tension and opportunity for growth in the relationship.
What does the compatibility score mean?
The overall score is a weighted average across five dimensions: Emotional Bond (30%), Attraction (25%), Communication (20%), Long-term Stability (15%) and Conflict Potential (10%). A higher score indicates more natural ease and alignment — but a challenging score often signals a deeply transformative relationship rather than an incompatible one.
Do I need my birth time for synastry?
Birth time is optional but recommended. Without it, all planetary aspects are still calculated accurately using date alone. Adding a birth time and city for either person unlocks house overlay analysis — this shows which life areas (career, romance, family, finances) each person's planets activate in the other chart.
What is the most important synastry aspect?
Sun-Moon and Venus-Mars cross-aspects carry the most weight. A Sun-Moon conjunction or trine between two charts creates a complementary rhythm — the Sun person illuminates; the Moon person nurtures and reflects. Venus-Mars contacts drive physical attraction. Saturn aspects add commitment and longevity. Most long-term couples have at least one strong Sun-Moon or Venus-Mars connection.
What is a house overlay in synastry?
A house overlay occurs when one person's planet falls inside a specific house of the other person's chart, directing that planet's energy toward that life area. If your Venus falls in your partner's 5th house (romance and creativity), you activate romantic energy in their life. If their Saturn falls in your 7th house (partnerships), they may feel like a serious, stabilising influence on how you approach relationships.
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