Synastry API

Request

A real request for two synthetic people: the demonstration birth in Kyiv and a second birth in New York.

curl -X POST https://api.natalchart.ai/v1/synastry \
  -H "Authorization: Bearer $NATALCHART_API_KEY" \
  -H "Content-Type: application/json" \
  -d '{"personA":{"date":"1991-09-06","time":"18:30","latitude":50.4501,"longitude":30.5234},"personB":{"date":"1989-03-14","time":"07:45","latitude":40.7128,"longitude":-74.006}}'

Response

Part of the answer to that request: the main fields, with every list cut to its first three items.

{
  "aspects": [
    {
      "personA": "Neptune",
      "personB": "Lilith",
      "aspect": "Trine",
      "angle": 120.08,
      "orb": 0.08
    },
    {
      "personA": "Saturn",
      "personB": "Jupiter",
      "aspect": "Trine",
      "angle": 119.66,
      "orb": 0.34
    },
    {
      "personA": "Mercury",
      "personB": "Moon",
      "aspect": "Sextile",
      "angle": 60.38,
      "orb": 0.38
    }
  ],
  "sphereScores": {
    "identity": 89,
    "love": 81,
    "body": 86,
    "money": 76,
    "home": 83,
    "mind": 66
  }
}

What comes back

• aspects: every major aspect between a body of one chart and a body of the other, with its angle and orb.
• overlays: the house each person's planets fall into in the other's chart, both ways. A side is null when the other person's birth time is unknown, because houses depend on it.
• sphereScores: identity, love, body, money, home and mind, each from 0 to 100, built from the inter-chart aspects (60%) and the house overlays (40%).
• scoring: the rule behind the scores, stated in the response. It describes the geometry of two charts, not the future of a relationship.

Limits

• API Pro only. On the free key the endpoint answers 403 with a link to upgrade.
• Births from 1800 to 2399. Tropical zodiac, Placidus houses, the five major aspects.
• No geocoding: send coordinates. The time zone is resolved from them, with the daylight-saving rules of the birth date.
• Birth data is read, used and forgotten: request bodies are never stored.

In an AI assistant

The MCP server at https://www.natalchart.ai/mcp offers the same calculation as the synastry tool, so Claude, ChatGPT or Cursor can compare two charts before they talk about compatibility.

The rest of the API

The same key opens the whole astrology API: natal charts, daily transits, synastry, transits for a period, solar returns, best days and astrocartography. Every endpoint, field and error is in the documentation, and the pricing page says what counts as a request.

Questions