| name | compatibility-synastry |
| description | Analyze relationship compatibility between two people using synastry, composite, and Davison/midpoint charts. Use when someone asks about romantic compatibility, relationship dynamics, friendship chemistry, business partnership fit, or how two people's charts interact. |
| version | 1.0.0 |
| updated | "2026-05-23T00:00:00.000Z" |
Compatibility & Synastry
Use this skill whenever someone asks how two people work together — romantically, in friendship, or in partnership. Three complementary tools:
| Tool | Question it answers |
|---|
| Synastry | "How do our energies interact?" (each person's planets aspecting the other's) |
| Composite | "What is the relationship itself like?" (a third chart for the bond) |
| Davison / Midpoint | "What's the time-space midpoint of our two charts?" (an alternative composite based on real averaged ephemeris time + location) |
Required Information
Two complete subjects — for EACH person:
- Full birth date
- Birth time (precision matters; flag if missing)
- Birth location
Standard Procedure for Birth-Data Endpoints
Any endpoint that takes a birth chart (natal, synastry, composite, transits, returns, progressions, Human Design, Vedic, ACG) MUST follow this sequence. Skipping any step produces a wrong chart — usually wrong by hours of arc.
Step 1 — Resolve location to coordinates
If you don't already have precise lat/lon, call:
GET /location/autocomplete?query=<city, country>&limit=5
Confirm the right hit with the user if there's ambiguity (e.g. "Portland, OR" vs "Portland, ME"). Record the latitude and longitude of the chosen result.
Step 2 — Resolve timezone for that location AT THE BIRTH DATE
Historical timezone rules matter. A city's UTC offset in 1962 may differ from today (DST rules, war time, jurisdiction changes). Always look up the timezone for the specific birth date.
POST /timezone/lookup
{ "latitude": <lat>, "longitude": <lon> }
Response gives an IANA timezone string like "America/Los_Angeles". The BirthLocation field below REQUIRES this — it is not optional.
Step 3 — Build the request body
The SubjectRequest schema requires birth_datetime, birth_location, and name. The IANA timezone goes into birth_location.timezone:
{
"subject": {
"name": "Subject",
"birth_datetime": { "iso": "1990-04-15T14:30:00" },
"birth_location": {
"latitude": 41.8781,
"longitude": -87.6298,
"timezone": "America/Chicago",
"city": "Chicago",
"country": "United States"
}
}
}
The iso field is the local civil time at the birth location. The API handles the conversion to UTC using the supplied IANA timezone. Do not pre-convert to UTC and do not pass an offset suffix on the iso string unless the user explicitly knows it.
Step 4 — Choose house system (ask if appropriate)
The natal chart endpoint defaults to Placidus. Before computing, briefly ask:
"I'll use Placidus houses by default (the most common modern Western system). If you prefer a different system — Whole Sign (Hellenistic/traditional), Koch, Equal, Campanus, Regiomontanus, or Porphyry — just say so."
Once chosen, pass it through configuration.house_system on the request.
Step 5 — Always request format=llm unless raw JSON is requested
This reduces token usage by ~73% and gives you a readable, interpretable response. Add ?format=llm to the URL.
Time-unknown handling
If the user doesn't know their birth time:
- Sun sign, outer planets, North/South Node sign — reliable.
- Moon sign — only reliable to within ~12 hours. If the user knows roughly morning/afternoon/evening, it usually narrows enough.
- Rising sign, all house placements, Midheaven — unreliable. Do not interpret these without a time.
- Default fallback: use 12:00 noon local. Disclose: "Without a birth time, I'm using noon as a placeholder. Your rising sign, house placements, and Moon sign may not be accurate. The rest of the chart is still meaningful."
For Human Design, type/strategy/authority/profile depend on precise time — flag any time uncertainty and suggest the user rectify with a Human Design analyst or rectification service before relying on these results.
Run Steps 1–3 of the Standard Procedure for both people independently — geocode and timezone lookup are per-person.
Compute
Synastry
POST /comparative/synastry?format=llm
Body:
{
"subject_a": { "name": "...", "birth_datetime": {...}, "birth_location": {...} },
"subject_b": { "name": "...", "birth_datetime": {...}, "birth_location": {...} },
"configuration": { "house_system": "placidus" }
}
Composite
POST /comparative/composite?format=llm
(same two-subject body shape)
Midpoint Composite (Davison-style)
POST /comparative/composite/midpoint?format=llm
Bi-wheel Overlay (for visual)
POST /comparative/overlay?format=llm
— wheel rendering of one chart overlaid on the other.
How to Read Synastry
Walk through these in order, leading with the most impactful.
1. Sun / Moon / Rising Cross-Aspects
The "Big Three to Big Three" check:
- Sun ↔ Moon (either direction) — classic emotional-conscious compatibility. Conjunction, trine, sextile = warmth. Square, opposition = creative friction.
- Sun ↔ Rising — the way one shows up validates the other's identity.
- Moon ↔ Moon — emotional rhythm alignment. Same element = native fluency. Opposite element = attraction through difference.
- Sun in 7th of the other — magnetic, "destined partner" feeling.
- Moon in 4th of the other — feels like home.
2. Venus / Mars
The romantic / sexual / desire layer:
- Venus ↔ Mars = chemistry. Cross-conjunctions are textbook attraction.
- Venus ↔ Venus = shared aesthetic, love language fit.
- Mars ↔ Mars = how you fight and how you move through the world together.
3. Mercury
Communication style and friendship layer:
- Mercury ↔ Mercury — conversational rhythm. Same element = effortless. Different element = constant interpretation.
- Mercury ↔ Sun / Moon — your mind validates their identity/feelings.
4. The Outer Planets (Heavy Aspects)
These create the deepest binds — and the deepest difficulties:
- Saturn aspects — durability, lessons, sometimes coldness. Saturn ↔ Venus/Moon needs maturity to thrive.
- Pluto aspects — power dynamics, transformation, obsession. Pluto ↔ Venus/Moon is the classic intense bond.
- Uranus aspects — excitement, unpredictability, on-off patterns.
- Neptune aspects — idealization, romance, but also projection and illusion.
5. Nodes
- One person's planet conjunct the other's North Node — soul-pull, "we have work together"
- South Node connections — past-life sense, comfortable but can stagnate
6. House Overlays
Where each person's planets fall in the OTHER's houses tells you what life areas activate between you:
- Person A's Sun in Person B's 10th = Person A energizes B's career
- Person A's Moon in Person B's 4th = Person A activates B's home/emotions
- Heavy stacking in B's 5th = romance/creativity emphasis
- Heavy stacking in B's 12th = deep, sometimes hidden, often unspoken
How to Read the Composite Chart
The composite is the relationship-as-a-being. Read it like a natal chart, but every placement describes the relationship, not either person:
- Composite Sun — what the relationship is for; its life purpose
- Composite Moon — the relationship's emotional climate
- Composite Ascendant — how the relationship presents to the world
- Composite Venus / Mars — its romantic and sexual character
- Composite Saturn — what tests the bond
- House emphasis — where the relationship spends its energy
Synthesis
After walking through the layers, give a short overall narrative:
- Dominant theme — Is this a soulmate-feeling Sun/Moon-heavy bond? A passion-driven Mars/Venus bond? A purposeful Saturn/Pluto destiny bond? A creative Jupiter/Uranus bond?
- Where you'll grow each other — Hard aspects and difficult overlays.
- Where it'll feel easy — Soft aspects and supportive overlays.
- The risk — Every relationship has one. Name it gently. (Saturn-heavy = duty over joy. Pluto-heavy = power struggles. Neptune-heavy = idealization.)
Honest Framing
- Never predict outcomes. Say "this dynamic tends to create…" not "this relationship will…"
- Compatibility is a starting condition, not a destiny. Two people with rough synastry can build something beautiful with awareness; two with easy synastry can let it go stale.
- Both charts must be willing. A reading is for what's there to work with, not a forecast.
- For unknown birth times — synastry without houses is still meaningful (planet-to-planet aspects work fine), but the house overlays are unreliable. Disclose.
How to Frame Astrological Readings
- Symbolic, not deterministic. Astrology maps patterns and tendencies; it does not predict fixed outcomes. Every placement has light and shadow expressions, and conscious effort changes how a chart plays out.
- Not a substitute for professional advice. Astrology is not medical, legal, financial, or psychotherapeutic guidance. If a user is in crisis or asking about real medical / legal / financial decisions, recommend they see a qualified professional in that field.
- Honor the whole chart. No single placement defines a person. Synthesize across signs, houses, aspects, and dignities before offering interpretation.
- Be specific, not generic. Cite the actual placement and explain its meaning in context, rather than reciting trait lists.
- Be balanced. Don't flatter or doom-cast. Hard aspects are growth-producing; soft aspects can become laziness.
- Stay curious. If a user pushes back on a reading, that pushback is data — chart interpretations are dialogues, not verdicts.
Example Flow
User: "Are my partner and I compatible? I'm March 3, 1995, 7:22 AM Portland Oregon. They're July 12, 1988, around 3 PM in Brooklyn NY."
- Standard Procedure for Person A → lat/lon/tz/UTC for Portland on 1995-03-03
- Standard Procedure for Person B → lat/lon/tz/UTC for Brooklyn on 1988-07-12
- (Note Person B's "around 3 PM" — flag that rising/houses may be approximate)
POST /comparative/synastry?format=llm with both subjects- Lead with Sun/Moon cross-aspects; cover Venus/Mars; note any heavy outer-planet aspects; describe house overlays
- Optionally run
POST /comparative/composite?format=llm for the relationship-as-entity reading
- Synthesize with honest, balanced framing
