OpenEphemeris Is Now Available as a Claude Connector

You can now add OpenEphemeris to Claude as a connector.

That means real astronomy — natal charts, transits, Human Design, astrocartography — available inside any Claude conversation, calculated the moment you ask. Not recalled from training data. Not approximated. Computed against the same NASA JPL DE440 ephemeris files used for deep-space mission planning.

The connector is live in the Anthropic MCP directory, and connecting it takes about two minutes. No config file to edit. No code to write. You authorize it once in your browser, and Claude gains a calculation engine for the sky.

Why an Astronomy Connector Matters

Ask any language model where Mars is right now and it will answer with full confidence. It will frequently be wrong.

This is not a flaw to be prompted away. Language models predict text — they do not track orbital mechanics. Planetary position is deterministic, meaning it can be computed exactly from a date, a time, and a location, the same way every time. A model reaching into its training data for a position is doing the wrong kind of work for the question. We wrote about this failure mode in detail in Why AI Gets Astrology Wrong — the short version is that a single approximated position cascades into a wrong Moon sign, a wrong house, a wrong aspect, and an interpretation built on a sky that never existed.

The connector closes that gap. When a conversation needs a planetary position, Claude stops guessing and calls a tool — which means the degree it reports back to you was calculated by an ephemeris engine, not pattern-matched from forum posts.

The calculation is deterministic. The interpretation is Claude's. That division of labor is the whole point.

How to Add It in claude.ai

The connector runs as a remote MCP server at https://mcp.openephemeris.com/mcp. Adding it to claude.ai is a browser flow — here is the full setup:

1. Open Settings → Connectors in claude.ai.
2. Choose Add custom connector.
3. Paste the URL: https://mcp.openephemeris.com/mcp
4. Leave the OAuth Client ID and Secret blank.
5. Click connect. A browser popup handles sign-in, and you authorize access to your OpenEphemeris account.

That last step is OAuth 2.1 with PKCE and Dynamic Client Registration — which means Claude negotiates credentials with the server automatically and you never paste an API key into a config file. You sign in the way you sign in to any website, and the connection is scoped to your account.

Once it connects, the tools appear. Ask "What is the current moon phase?" — if Claude answers with a specific phase name and illumination percentage instead of a general explanation, you are live.

What You Can Ask

The connector exposes 64 tools spanning the full range of practice — Western, Vedic, Human Design, BaZi, electional, and astrocartography. A few things that become a single sentence:

• "Calculate my full natal chart. I was born July 15, 1987 at 9:01 AM in Dallas, Texas."
• "What transits am I in right now, and which ones matter most?"
• "Generate my Human Design bodygraph and explain my type, strategy, and authority."
• "Where does my Venus line fall in Europe?"
• "Find a good electional window to sign a contract in the next 30 days."
• "Build a synastry chart between me and someone born March 3, 1990 in London."

Every one of those answers is backed by a calculation, not a recollection.

The Charts Render in the Conversation

This is the part that surprises people. The connector does not just return numbers — it draws.

When you ask for a chart, Claude can render it as an interactive app right there in the conversation. A full natal chart wheel with house cusps and aspect lines. A bi-wheel layering transits or a second person's chart over your natal positions. A Human Design bodygraph with defined centers, channels, and gates. A moon dial showing the current phase and what is coming next.

These are live, in-conversation visuals built on the calculated positions — which means the wheel you see is drawn from the same arcminute-precise data Claude is interpreting, not a stock image. You read the chart and talk to it in the same window.

What It Costs

Nothing, to start. The free Explorer tier includes a one-time grant of 150 calculation credits — enough to run charts for yourself, your friends, and your family, and to see the difference accuracy makes. A natal chart is one credit. The current moon phase is one credit. Most people get plenty of exploring done before they top up — and credits never expire.

When you need more — production volume, higher rate limits, the full predictive and relational tool set — the paid tiers lift the ceiling. The same connector, the same engine, more headroom.

Beyond claude.ai

The remote connector is the fastest path, but it is not the only one. The same server backs the @openephemeris/mcp-server npm package, which connects Claude Desktop, Cursor, Windsurf, VS Code, and the Gemini CLI through a local command. If you want a step-by-step desktop walkthrough, Turn Claude into a Master Astrologer covers it end to end. Developers building their own agents will want the MCP server overview and the docs.

The directory listing is the front door. Behind it is a Go calculation engine that has been computing sub-arcsecond positions across a date range from 1550 to 2650 CE since long before the connector existed.

Add it once. Ask Claude for your chart. Compare the degrees to a chart you trust. One of them was computed — and now it is the one inside Claude.