Natal Chart Lab MCP server

An MCP server that gives AI assistants accurate astrology and sky data: where the Sun, Moon and planets are on the tropical zodiac, the signs in a birth chart, retrograde periods with shadow dates, and Moon phases.

It is a thin client for the free, keyless Natal Chart Lab astrology API. Positions come from Astronomy Engine and are checked against NASA JPL Horizons (largest difference at 50 test points is 19.5 arcseconds, well under a hundredth of a degree). No API key, no sign-up, no personal data stored.

Tools

Tool	What it returns
`get_current_sky`	Sun, Moon and planet signs and degrees now; the Moon's phase, illumination, next sign change and void-of-course windows for seven days; which planets are retrograde or in shadow
`get_planet_positions`	Longitude, sign, degree, daily speed and retrograde flag for every body at any UTC time, 1800–2200
`get_birth_placements`	Sun, Moon and planet signs for a local birth date, clock time and UTC offset (no rising sign or houses; see the birth chart calculator for those)
`get_retrogrades`	Retrogrades of Mercury–Pluto in a date range with station times, signs, degrees and pre/post-shadow dates (1950–2050)
`get_moon_phases`	New Moons, quarters and Full Moons in a date range with sign, degree, distance and eclipses (1950–2050)

Example questions it answers: "Is Mercury retrograde right now, and when is the next one?", "What sign is the Moon in, and is it void of course?", "I was born 4 July 1990 at 9:30 in New York. What are my Sun, Moon and Venus signs?", "List the full moons in 2027 and their signs."

Install

Requires Node.js 18 or later.

Claude Desktop

Add to claude_desktop_config.json:

{
  "mcpServers": {
    "natalchartlab": {
      "command": "npx",
      "args": ["-y", "natalchartlab-mcp"]
    }
  }
}

Claude Code

claude mcp add natalchartlab -- npx -y natalchartlab-mcp

Cursor, Windsurf, VS Code and others

Use the same command, npx -y natalchartlab-mcp, as a stdio server.

Data and accuracy

Tropical zodiac; apparent geocentric positions on the true ecliptic of date.
Mean North Node and mean Black Moon Lilith.
Retrograde and Moon phase tables are CC BY 4.0 and also published as datasets (retrogrades, Moon phases).
Full method: natalchartlab.com/methodology.

Set NATALCHARTLAB_URL to point the server at another deployment of the same API.

Development

npm install
npm test
npm run build

Licence

MIT. Data from Natal Chart Lab.

natalchartlab

Documentation