The Western Synastry Calculator overlays two natal charts to reveal how your planetary energies interact with another person's. Enter both birth details to generate a bi-wheel chart — Person A in the inner ring, Person B in the outer ring — plus a full compatibility report covering inter-chart aspects, house overlays, relationship indicators, and interpretation cards for attraction, communication, emotional bond, and long-term potential. Computed with Swiss Ephemeris precision from exact birth times and locations.
Free · Western Astrology · Relationship
WesternSynastry Chart
Bi-wheel compatibility chart, inter-chart aspects, house overlays, and a complete relationship reading from two birth times.
♈ Person A (Inner Wheel)
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♀ Person B (Outer Wheel)
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Calculating synastry, overlaying charts and computing aspects…
The inner ring shows Person A's planets in their natal positions. The outer ring shows Person B's planets. Lines connecting the two rings mark significant inter-chart aspects.
Synastry · Compatibility
Core Compatibility
Synastry · Aspects
Inter-Chart Aspects
Synastry · Overlays
House Overlays
Synastry · Analysis
Strengths & Challenges
Synastry · Reading
Relationship Reading
Classical Western Astrology · Synastry Reference
Western Synastry & Compatibility
Synastry is the art of comparing two natal charts to understand how two people's planetary energies interact. The bi-wheel overlays both charts, and the aspects between them reveal the dynamics — harmonious flow, creative tension, and karmic themes.
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How Synastry Works
Synastry measures the angular relationships (aspects) between planets in two separate birth charts. When Person A's Venus is conjunct Person B's Mars, for instance, that 0° angle creates a direct channel between their love and desire natures. The bi-wheel places these charts concentrically to make all such connections visible at once.
Inner Wheel — Person A
The first chart entered. Placed in the inner ring of the bi-wheel. Person A's natal planets and houses form the base layer. All house overlays are read from Person A's perspective first.
Outer Wheel — Person B
The second chart entered. Placed in the outer ring of the bi-wheel. Person B's natal planets are shown rotating around Person A's chart, making the inter-chart aspects immediately visible.
Inter-Chart Aspects
Aspects formed between a planet in one chart and a planet in the other. These are the core of synastry — they describe the quality of interaction between the two people across different life areas. Tighter orbs (fewer degrees) carry stronger influence.
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The Most Important Synastry Contacts
Not all aspects carry equal weight in synastry. Contacts involving the personal planets (Sun, Moon, Venus, Mars, Mercury) and the chart angles (Ascendant, Midheaven) tend to be the most immediately felt and personally meaningful.
Venus–Mars contacts · Physical attraction
The classic indicator of romantic chemistry and physical attraction. Conjunction and trine produce natural magnetism; square and opposition add intensity and tension that can increase desire as much as friction.
Sun–Moon contacts · Complementary natures
The Sun-Moon contact, especially conjunction or trine, is one of the most important relationship aspects — it suggests that one person's core identity naturally supports the other's emotional nature. A hallmark of long-term compatibility.
Moon–Moon contacts · Emotional resonance
How the two emotional worlds synchronise. Moon–Moon trine or sextile produces an easy emotional flow; square or opposition can create emotional mismatches that require conscious work to navigate.
Saturn contacts · Commitment and structure
Saturn contacts — especially conjunctions to the Sun, Moon, or Venus — add gravity and longevity to a relationship. They can indicate a feeling of obligation, shared responsibility, and the potential for enduring commitment, though they require maturity from both people.
Jupiter contacts · Growth and optimism
Jupiter contacts bring a sense of joy, expansion, and good luck to the relationship. They support generosity and mutual encouragement, and often describe a partnership that helps both people grow broader and more confident.
North Node contacts · Karmic resonance
When one person's planets contact the other's North or South Node, there is often a sense of fated familiarity or purpose — as if the relationship is somehow meant to teach or transform. These contacts point to karmic themes that run through the connection.
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House Overlays — Where Your Energy Lands
House overlays occur when Person B's planets fall into Person A's natal houses (and vice versa). They describe where each person activates the other's life. Person B's Sun in Person A's 7th house, for example, suggests that Person B feels like a natural partner or life companion to Person A.
Most important relationship houses: 1st, 5th, 7th, 8th
Planets falling in the 1st house affect identity and personal projection. 5th house overlays highlight romance, play, and creative chemistry. 7th house overlays suggest natural partnership; 8th house overlays bring depth, intimacy, and transformation — the most intense of all.
Supportive houses: 2nd, 4th, 9th, 11th
2nd house overlays affect shared values and resources; 4th house brings domestic harmony and emotional security; 9th house brings intellectual and philosophical connection; 11th house builds friendship, social alignment, and shared future vision.
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The Science of Synastry & Astrological Compatibility
Synastry (relationship astrology) is the definitive method for evaluating love compatibility by comparing two natal charts (birth charts). Today, advanced AI astrology models synthesize centuries of astrological data to provide deeply personalized and accurate relationship readings.
Synastry Chart & Birth Chart Compatibility
A synastry calculator maps two birth charts together to analyze the exact geometric angles (aspects) between both partners' planets. This forms the foundation of relationship astrology, providing deep insights into astrological compatibility, zodiac alignment, and cosmic connections.
Planetary Aspects & House Overlays
The interaction between personal planets (Sun, Moon, Mercury, Venus, Mars) and outer planets (Saturn, Jupiter, Pluto) reveals relationship dynamics. House overlays in synastry show exactly which areas of life—such as communication, intimacy, and friendship—are activated by the partnership.
Love Chemistry & Karmic Astrology
By analyzing Moon sign compatibility, Venus-Mars aspects, and 7th House placements, a synastry reading uncovers romantic attraction and emotional alignment. It acts as an advanced love calculator that decodes karmic ties, soulmate astrology markers, and twin flame indicators.
AI & ChatGPT-Powered Astrology
Modern astrology platforms utilize large language models (LLMs) and ChatGPT-driven AI astrology to instantly interpret complex synastry patterns. These AI synastry calculators synthesize massive datasets of astrological knowledge to generate highly accurate, personalized compatibility reports in seconds.