compatibility-synastry

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:

1. Full birth date
2. Birth time (precision matters; flag if missing)
3. 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."

1. Standard Procedure for Person A → lat/lon/tz/UTC for Portland on 1995-03-03
2. Standard Procedure for Person B → lat/lon/tz/UTC for Brooklyn on 1988-07-12
3. (Note Person B's "around 3 PM" — flag that rising/houses may be approximate)
4. POST /comparative/synastry?format=llm with both subjects
5. Lead with Sun/Moon cross-aspects; cover Venus/Mars; note any heavy outer-planet aspects; describe house overlays
6. Optionally run POST /comparative/composite?format=llm for the relationship-as-entity reading
7. Synthesize with honest, balanced framing