Cross-chart aspects
western.relations.synastry_aspectsEvery cross-chart aspect between the two charts, with exact orb and which partner casts it.
Returns: aspect list with orb, applying/separating and both directions.
Western
Synastry
Relationship and compatibility astrology
Synastry is the comparison of two natal charts to analyze compatibility and relationship dynamics between people.
What we calculate
Synastry studies aspects between partners' planets, planet positions in each other's houses, and composite chart creation.
Cross-aspects
All aspects between partners' planets (Sun, Moon, Mercury, Venus, Mars and outer planets). Reveals attraction, harmony and tension points.
Sun–Moon: deep emotional bond. Venus–Mars: passion and attraction. Saturn–Moon: stability and responsibility.
House Overlay
Where one partner's planets fall in the other's houses. Shows how you influence each other's life areas.
Partner's Venus in your 7th house: strong romantic connection. Partner's Saturn in 4th house: family responsibility.
Composite Chart
Chart built from midpoints of both partners' planets. Shows the relationship itself as a living, independent entity.
Composite Sun in 7th house: partnership-centred relationship. Venus conjunct Jupiter: joy and abundance together.
Davison Chart
Chart for the average date and place between partners' births. An alternative analytical lens on the relationship.
Reveals the core themes, mission and karmic purpose of the relationship from a different angle.
Key Areas
Romantic Compatibility
Partners' Venus and Mars, 5th and 7th houses. Moon-Venus, Sun-Moon aspects show emotional and romantic connection.
Emotional Connection
Partners' Moons, aspects to personal planets. Shows emotional understanding and support.
Intellectual Connection
Partners' Mercury, 3rd and 9th houses. How partners communicate, understand each other, intellectual bond.
Physical Attraction
Mars and Venus, 8th house. Physical attraction, passion, sexual compatibility.
Spiritual Connection
Neptune, 12th house, Lunar Nodes. Spiritual unity, karmic bonds, transcendent understanding.
Practical Compatibility
Saturn, 2nd and 10th houses. Alignment of goals, values, plans, relationship stability.
✦
AI analyses your synastry
Ask the AI anything about your compatibility — it has full access to all calculated data.
Key Concepts
Interpersonal Aspects
Aspects between partners' planets show relationship dynamics. Harmonious give ease, challenging ones bring growth through challenges.
House Overlay
One partner's planets fall in other's houses, activating those life areas and showing mutual influence.
Composite Chart
Chart built from midpoints of partners' planets. Shows relationship as separate entity.
Compatibility Score
Analysis of element harmony, planetary qualities, Moon and Venus aspects. Saturn (stability) and Lunar Nodes (karma) are also key.
Relationship Timing
Transits and progressions show favorable times for starting relationships, important decisions, and periods of challenge.
Davison Chart
Chart for average date and place between partners' births. Alternative view of relationship.
Step by step
How to read a synastry in AstroCodex
Two birth charts in, the full relationship picture out — cross-aspects, overlays and both midpoint charts, computed rather than generalised from sun signs.
- 1
Save both birth charts
Date, exact time and place for each person. Without birth times the houses and the Moon are unreliable, and house overlays are half the reading.
- 2
Open Compatibility
Choose the two charts. Cross-aspects, house overlays, the composite and the Davison are computed in one pass.
- 3
Read chemistry and container separately
Synastry says how you affect each other. The composite says what the relationship becomes. They routinely disagree, and the disagreement is the useful part.
- 4
Take the hard aspects to the AI
A Saturn square is not a verdict. The assistant reads the actual orb, direction and houses involved and explains what it costs and what it builds.
Under the hood
What the engine computes for a pair
The real calculators behind the page — each is also callable from the API and from an AI assistant over MCP.
House overlays
western.relations.house_overlaysWhere each person's planets land in the other's houses — the life areas the partner switches on.
Returns: planet-in-house placements, both ways round.
Composite chart
western.relations.composite_chartThe midpoint composite: a chart built from the midpoints of both charts, treated as the relationship itself.
Returns: a full chart — planets, houses and aspects of the pairing.
Davison chart
western.relations.davison_chartThe Davison relationship chart, cast for the midpoint in time and space between the two births.
Returns: a real, astronomically valid chart for that midpoint moment.
Progressed synastry
western.relations.progressed_synastryProgressed synastry — the two charts advanced to now, showing how the dynamic has moved since you met.
Returns: progressed cross-aspects with timing.
Relationship score
western.relations.relationship_scoreA weighted relationship score across the classical relationship indicators.
Returns: a numeric score with the factors that drove it.
Declination contacts
western.relations.synastry_declinationsParallels and contraparallels by declination — out-of-sign contacts most calculators skip entirely.
Returns: declination contacts with orb.
Full comparison
western.relations.synastry_fullOne call that runs the whole set and assembles it into a single reading.
Returns: the complete comparison, every section above.
Synastry, composite or Davison — which one answers your question
Three different charts, three different questions. Most disagreements about relationship astrology come from asking one of them what another one answers.
Synastry
"What happens between us?"
- How it is built
- Two separate charts laid over each other; every planet of one measured against every planet and house of the other.
- What it shows
- Chemistry, friction, attraction, and which areas of life each partner activates in the other.
- Use it when
- Always first. It is the only one of the three that keeps both people visible as individuals.
Composite
"What is the relationship itself like?"
- How it is built
- Midpoints between the two charts — a third chart that belongs to neither person.
- What it shows
- The character and purpose of the partnership as an entity, and how it is perceived from outside.
- Use it when
- Once the pairing exists and you want to know what it is becoming rather than how it feels.
Davison
"What sky does this relationship belong to?"
- How it is built
- The midpoint in time and space between the two births — a genuine moment with a genuine sky.
- What it shows
- Much the same ground as the composite, but from a chart that is astronomically real and can be progressed.
- Use it when
- As a second opinion on the composite, and when you want to time the relationship by progression.
Experienced practice does not pick one. Read the synastry for the chemistry, then the composite for the container, then check the Davison where the two disagree.
The contacts that carry the reading
Not all cross-aspects weigh the same. These are the ones that decide whether a relationship sparks, settles or stalls.
1
Venus – Mars
The attraction axis. This is the aspect that answers "is there a spark", and its absence explains relationships that are warm but never ignite.
2
Sun – Moon
The classical marriage contact. One person's identity meeting the other's emotional nature — the most reliable single indicator of long-term ease.
3
Moon – Moon
Whether you are soothed or unsettled by each other at rest. Decides what daily life together actually feels like.
4
Saturn contacts
Duration and weight. Saturn to a personal planet binds a relationship and also restricts it; nothing lasts without Saturn, and nothing feels light with too much of it.
5
Ascendant contacts
Physical recognition — the "I knew immediately" contact. Planets on the other's Ascendant are seen before they are understood.
6
Outer-planet contacts
Uranus disrupts, Neptune idealises, Pluto intensifies. Powerful, rarely comfortable, and worth reading before mistaking intensity for compatibility.
House overlays — where the partner lands in your life
The house their planets fall into in your chart tells you which part of your life they activate. This is the half of synastry that aspect lists miss.
What to ask the AI once both charts are in
The assistant reads the computed comparison — these questions get specific, chart-backed answers instead of horoscope generalities.
About chemistry
- «What are our tightest cross-aspects, and which of them actually matter?»
- «Do we have Venus-Mars contacts, and in which direction do they run?»
- «Which of our aspects are applying rather than separating?»
About the relationship as a whole
- «What does our composite chart say that the synastry does not?»
- «Compare our composite and Davison charts and explain where they disagree.»
- «What is our relationship score built from — which factors carried it?»
About friction and time
- «Where does Saturn sit between our charts, and what does it cost us?»
- «Which of my houses do their planets fall into, and what does that activate?»
- «How has our progressed synastry changed since we met?»
- «Do we have any declination parallels the aspect list missed?»
Frequently asked
What is the difference between synastry and a composite chart?
Synastry compares two charts against each other and describes what happens between two people — attraction, friction, which areas each one activates. A composite is a single new chart built from the midpoints of both, and describes the relationship as an entity. Synastry is the chemistry; the composite is the container.
Composite or Davison — which is more accurate?
Neither supersedes the other. The composite is a mathematical construct from midpoints; the Davison is cast for a real moment in time and space, so it corresponds to an actual sky and can be progressed. Most practitioners read both and pay attention to where they disagree.
Which synastry aspects matter most for a long relationship?
Sun-Moon contacts for ease, Venus-Mars for attraction, Moon-Moon for daily life, and Saturn for durability. Saturn is the one people misread: it restricts, but relationships without it rarely last.
Can synastry be done without birth times?
Only partly. Planet-to-planet aspects between slower bodies still hold, but the Moon can move up to 13° in a day, and the Ascendant and all house overlays are unavailable. Since overlays carry roughly half the reading, treat a no-time synastry as provisional.
Does a hard aspect mean the relationship will not work?
No. Squares and oppositions create tension, and tension is also what holds attention — many long marriages run on them. What matters is which planets, how tight the orb, and whether there is enough easy contact elsewhere to absorb the friction.
Does AstroCodex use tropical or sidereal positions here?
Tropical, computed from Swiss Ephemeris, as Western practice requires. The sidereal reading with kutas and nakshatras is a separate page — Vedic compatibility — and the two are never mixed in one verdict.