Totally Tarot Calculators

Get the Moon's phase and the lunation's exact times

compute_moon_phase

Read-onlyIdempotent

Returns the Moon's phase at an instant: phase angle, illuminated fraction, the eight-fold name, waxing or waning, age in days, tropical sign and degree, distance, and the four principal phases of that lunation to the second in UTC; optionally every principal phase across a date range.

A count of days from one new moon in steps of 29.53 follows the mean month, and a real new moon can fall more than half a day from the mean one; the gap is widest at the quarters and the full moon. Phase names follow Moon-Sun elongation. The phase changes within a day, so the hour matters; "time" is UTC and defaults to 00:00.

Results include an attribution line and a permalink.

Input Schema

Name	Required	Description
`to`	No	End of a range, ISO YYYY-MM-DD, up to 400 days after date. Example: "2026-12-31". Lists every principal phase between.
`date`	No	The date, ISO YYYY-MM-DD, 1700-2200. Example: "2026-09-20". Defaults to today (UTC).
`time`	No	Time of day HH:MM, read as UTC. Example: "12:00". Defaults to 00:00 UTC.

Schema Changelog

Changes observed during successful MCP inspections.

Changed2 schema fields changedOct 26, 2026

changedInput schema / properties / date / description

Previous value: -"The date, ISO YYYY-MM-DD, 1700-2200. Example: \"2026-09-20\". Omit: today (UTC)."New value: +"The date, ISO YYYY-MM-DD, 1700-2200. Example: \"2026-09-20\". Defaults to today (UTC)."

changedInput schema / properties / time / description

Previous value: -"Time of day HH:MM in UTC, not local and not a birth time. Example: \"12:00\". Omit for 00:00 UTC."New value: +"Time of day HH:MM, read as UTC. Example: \"12:00\". Defaults to 00:00 UTC."

Changed2 schema fields changedOct 26, 2026

changedInput schema / properties / date / description

Previous value: -"The date, ISO YYYY-MM-DD, 1700-2200. Example: \"2026-09-20\". Covers the lunation it falls inside."New value: +"The date, ISO YYYY-MM-DD, 1700-2200. Example: \"2026-09-20\". Omit: today (UTC)."

removedInput schema / required
```
Removed value: -[
-  "date"
-]
```

Changed4 schema fields changedSep 23, 2026

changedInput schema / properties / date / description

Previous value: -"The date, ISO YYYY-MM-DD, zero-padded, between 1700 and 2200. Example: \"2026-09-20\". The answer covers the lunation this date falls inside, so the surrounding new, quarter and full moons come back with it."New value: +"The date, ISO YYYY-MM-DD, 1700-2200. Example: \"2026-09-20\". Covers the lunation it falls inside."

changedInput schema / properties / time / description

Previous value: -"Time of day as HH:MM in UTC — not a local time and not a birth time. Example: \"12:00\". Omit it for 00:00 UTC. Send it when the user cares which side of a phase boundary their hour falls on; convert their local time to UTC first rather than passing a local clock reading through."New value: +"Time of day HH:MM in UTC, not local and not a birth time. Example: \"12:00\". Omit for 00:00 UTC."

changedInput schema / properties / to / description

Previous value: -"End of a date range, ISO YYYY-MM-DD, up to 400 days after date. Example: \"2026-12-31\". Send it to get every principal phase between the two dates — this is the parameter for \"list the full moons in 2026\"."New value: +"End of a range, ISO YYYY-MM-DD, up to 400 days after date. Example: \"2026-12-31\". Lists every principal phase between."

changedOutput schema / (root)

Previous value: -{
-  "$defs": {
-    "Citation": {
-      "description": "How to credit this answer. Present on every response, successes and refusals alike.",
-      "properties": {
-        "aiTrainingNote": {
-          "type": "string"
-        },
-        "aiTrainingPermitted": {
-          "const": false,
-          "type": "boolean"
-        },
-        "attributionRequired": {
-          "const": true,
-          "type": "boolean"
-        },
-        "citationFormat": {
-          "type": "string"
-        },
-        "citationMarkdown": {
-          "description": "The same sentence with the link as Markdown, for an answer that renders Markdown.",
-          "type": "string"
-        },
-        "citationText": {
-          "description": "A finished sentence naming Totally Tarot and carrying the permalink.",
-          "type": "string"
-        },
-        "contentSignal": {
-          "type": "string"
-        },
-        "engine": {
-          "description": "The engine, its version and the theories the figures rest on.",
-          "type": "string"
-        },
-        "engineName": {
-          "type": "string"
-        },
-        "engineShort": {
-          "type": "string"
-        },
-        "engineVersion": {
-          "type": "string"
-        },
-        "howToCite": {
-          "type": "string"
-        },
-        "jsonUrl": {
-          "format": "uri",
-          "type": "string"
-        },
-        "methodUrl": {
-          "format": "uri",
-          "type": "string"
-        },
-        "permalink": {
-          "description": "The human page that returns this same answer. This is the URL to link.",
-          "format": "uri",
-          "type": "string"
-        },
-        "publisher": {
-          "type": "string"
-        },
-        "sourceName": {
-          "type": "string"
-        },
-        "sourceUrl": {
-          "format": "uri",
-          "type": "string"
-        },
-        "termsUrl": {
-          "format": "uri",
-          "type": "string"
-        },
-        "theories": {
-          "items": {
-            "type": "string"
-          },
-          "type": "array"
-        },
-        "usageTerms": {
-          "type": "string"
-        }
-      },
-      "required": [
-        "attributionRequired",
-        "citationText",
-        "permalink",
-        "engine",
-        "termsUrl"
-      ],
-      "type": "object"
-    }
-  },
-  "properties": {
-    "attribution": {
-      "description": "The ready-to-paste credit line. Include this in any answer built on the result.",
-      "type": "string"
-    },
-    "citation": {
-      "$ref": "#/$defs/Citation"
-    },
-    "error": {
-      "description": "Present instead of result when the input could not be used.",
-      "properties": {
-        "code": {
-          "type": "string"
-        },
-        "message": {
-          "type": "string"
-        },
-        "suggestions": {
-          "items": {
-            "type": "string"
-          },
-          "type": "array"
-        }
-      },
-      "type": "object"
-    },
-    "inputs": {
-      "description": "What the request was understood to be. The permalink is built from these.",
-      "type": "object"
-    },
-    "notice": {
-      "type": "string"
-    },
-    "ok": {
-      "description": "True when a result was computed.",
-      "type": "boolean"
-    },
-    "result": {
-      "description": "The computed figures. Absent on a refusal.",
-      "properties": {
-        "illuminatedFraction": {
-          "description": "Fraction of the disc lit, 0 to 1. The real one for this instant, not an eighth-of-a-cycle approximation.",
-          "type": "number"
-        },
-        "instantUTC": {
-          "description": "The instant the phase was computed for, in UTC. The phase moves measurably within a day, so this is not decoration.",
-          "type": "string"
-        },
-        "lunation": {
-          "items": {
-            "properties": {
-              "daysFromQuery": {
-                "description": "Days from the instant asked about. Negative for a phase already past.",
-                "type": "number"
-              },
-              "phase": {
-                "description": "One of the four principal phases bracketing this instant: New Moon, First Quarter, Full Moon, Last Quarter.",
-                "type": "string"
-              },
-              "utc": {
-                "description": "The exact instant of that phase, to the second, found by search rather than interpolation.",
-                "type": "string"
-              }
-            },
-            "type": "object"
-          },
-          "type": "array"
-        },
-        "moon": {
-          "properties": {
-            "distanceKm": {
-              "description": "Geocentric distance in kilometres. The Moon is not always the same size.",
-              "type": "number"
-            },
-            "formattedDegree": {
-              "description": "Its degree within that sign, for display.",
-              "type": "string"
-            },
-            "sign": {
-              "description": "The Moon's tropical sign.",
-              "type": "string"
-            }
-          },
-          "type": "object"
-        },
-        "moonAgeDays": {
-          "description": "Days since the new moon that opened this lunation. Computed from the real new moon, not from a mean synodic month, which varies by up to thirteen hours either side.",
-          "type": "number"
-        },
-        "nextFullMoonUTC": {
-          "description": "The next full moon after this instant.",
-          "type": "string"
-        },
-        "nextNewMoonUTC": {
-          "description": "The next new moon after this instant.",
-          "type": "string"
-        },
-        "phaseAngle": {
-          "description": "Elongation of the Moon from the Sun in degrees: 0 at new, 90 at first quarter, 180 at full, 270 at last quarter.",
-          "type": "number"
-        },
-        "phaseName": {
-          "description": "The eight-fold name a reader expects, e.g. \"Waxing Gibbous\".",
-          "type": "string"
-        },
-        "range": {
-          "description": "null unless a `to` date was given. When present it carries from, to and every principal phase between them.",
-          "type": "object"
-        },
-        "waxing": {
-          "description": "True while the illuminated fraction is growing.",
-          "type": "boolean"
-        }
-      },
-      "type": "object"
-    },
-    "summary": {
-      "description": "The answer in one sentence.",
-      "type": "string"
-    },
-    "tool": {
-      "description": "The calculator that answered.",
-      "type": "string"
-    },
-    "toolName": {
-      "type": "string"
-    }
-  },
-  "required": [
-    "ok",
-    "tool",
-    "attribution",
-    "citation"
-  ],
-  "type": "object"
-}New value: +null

Changed1 schema field changedSep 21, 2026

addedOutput schema / properties / result / properties

Added value: +{
+  "illuminatedFraction": {
+    "description": "Fraction of the disc lit, 0 to 1. The real one for this instant, not an eighth-of-a-cycle approximation.",
+    "type": "number"
+  },
+  "instantUTC": {
+    "description": "The instant the phase was computed for, in UTC. The phase moves measurably within a day, so this is not decoration.",
+    "type": "string"
+  },
+  "lunation": {
+    "items": {
+      "properties": {
+        "daysFromQuery": {
+          "description": "Days from the instant asked about. Negative for a phase already past.",
+          "type": "number"
+        },
+        "phase": {
+          "description": "One of the four principal phases bracketing this instant: New Moon, First Quarter, Full Moon, Last Quarter.",
+          "type": "string"
+        },
+        "utc": {
+          "description": "The exact instant of that phase, to the second, found by search rather than interpolation.",
+          "type": "string"
+        }
+      },
+      "type": "object"
+    },
+    "type": "array"
+  },
+  "moon": {
+    "properties": {
+      "distanceKm": {
+        "description": "Geocentric distance in kilometres. The Moon is not always the same size.",
+        "type": "number"
+      },
+      "formattedDegree": {
+        "description": "Its degree within that sign, for display.",
+        "type": "string"
+      },
+      "sign": {
+        "description": "The Moon's tropical sign.",
+        "type": "string"
+      }
+    },
+    "type": "object"
+  },
+  "moonAgeDays": {
+    "description": "Days since the new moon that opened this lunation. Computed from the real new moon, not from a mean synodic month, which varies by up to thirteen hours either side.",
+    "type": "number"
+  },
+  "nextFullMoonUTC": {
+    "description": "The next full moon after this instant.",
+    "type": "string"
+  },
+  "nextNewMoonUTC": {
+    "description": "The next new moon after this instant.",
+    "type": "string"
+  },
+  "phaseAngle": {
+    "description": "Elongation of the Moon from the Sun in degrees: 0 at new, 90 at first quarter, 180 at full, 270 at last quarter.",
+    "type": "number"
+  },
+  "phaseName": {
+    "description": "The eight-fold name a reader expects, e.g. \"Waxing Gibbous\".",
+    "type": "string"
+  },
+  "range": {
+    "description": "null unless a `to` date was given. When present it carries from, to and every principal phase between them.",
+    "type": "object"
+  },
+  "waxing": {
+    "description": "True while the illuminated fraction is growing.",
+    "type": "boolean"
+  }
+}

First observedSep 21, 2026

TDQS

A4.4/5.0

Behavior5/5

Does the description disclose side effects, auth requirements, rate limits, or destructive behavior?

Annotations already mark this as a safe, idempotent, non-destructive read, and the description adds substantive behavioral context: UTC handling, default time, within-day phase changes, mean-vs-real new moon caveats, and the inclusion of attribution and a permalink.

Agents need to know what a tool does to the world before calling it. Descriptions should go beyond structured annotations to explain consequences.

Conciseness4/5

Is the description appropriately sized, front-loaded, and free of redundancy?

The description is front-loaded with the returned values and then adds caveats, so an agent can extract the purpose quickly. It is dense and slightly long, but most sentences earn their place by clarifying behavior or output.

Shorter descriptions cost fewer tokens and are easier for agents to parse. Every sentence should earn its place.

Completeness5/5

Given the tool's complexity, does the description cover enough for an agent to succeed on first attempt?

With no output schema, the description carries the return-value burden and does so thoroughly: it enumerates the phase metrics, principal phase times, optional range results, and even attribution/permalink. Safety behavior is covered by annotations, leaving no major gap for invocation.

Complex tools with many parameters or behaviors need more documentation. Simple tools need less. This dimension scales expectations accordingly.

Parameters3/5

Does the description clarify parameter syntax, constraints, interactions, or defaults beyond what the schema provides?

Schema coverage is 100%, so the schema already documents date, time, and range behavior in detail. The description reinforces UTC/default behavior and the range's purpose, but does not add much parameter detail beyond the structured schema.

Input schemas describe structure but not intent. Descriptions should explain non-obvious parameter relationships and valid value ranges.

Purpose5/5

Does the description clearly state what the tool does and how it differs from similar tools?

The description states a specific verb and resource: returning the Moon's phase at an instant, with an explicit list of computed values and the lunation's principal phases. It is immediately distinguishable from siblings such as get_planetary_positions or find_eclipses.

Agents choose between tools based on descriptions. A clear purpose with a specific verb and resource helps agents select the right tool.

Usage Guidelines4/5

Does the description explain when to use this tool, when not to, or what alternatives exist?

It clearly explains the default single-instant use and the optional date-range use for listing every principal phase. It does not explicitly name when to choose this over a sibling tool, but the intended context is clear.

Agents often have multiple tools that could apply. Explicit usage guidance like "use X instead of Y when Z" prevents misuse.

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