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Open Ephemeris

Open Ephemeris

by openephemeris
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Python
Remote

Moon Phase

ephemeris_moon_phase
Read-onlyIdempotent

Returns the Moon's phase name, illumination percentage, zodiac sign and degree, and next void-of-course window for any given datetime, or for now if omitted.

Instructions

Use this when the Moon's state is wanted as data for a specific moment — 'what phase was the Moon on my birthday', 'what sign was the Moon in on 12 May 1990', 'moon phase and illumination as numbers'. Returns phase name, illumination %, the Moon's sign and degree, and the next void-of-course window; omit datetime for right now.

CREDIT COST: 2 credits.

Input Schema

NameRequiredDescriptionDefault
datetimeNoISO 8601 with a required zone, or local time with `timezone`. Default now.
latitudeNoOptional; for local void-of-course.
timezoneNoIANA zone for a local datetime.
longitudeNoOptional; for local void-of-course.

Schema Changelog

Changes observed during successful MCP inspections.

  1. Changed4 schema fields changedv4.29.0
    • changedInput schema / properties / datetime / description
      Previous value: -"ISO 8601 datetime to query, stating its zone ('2026-03-20T12:00:00Z' or '2026-03-20T08:00:00-04:00'), or a local time together with `timezone`. If omitted, returns the current live moon phase (UTC now)."New value: +"ISO 8601 with a required zone, or local time with `timezone`. Default now."
    • changedInput schema / properties / latitude / description
      Previous value: -"Observer latitude (optional, used for local void-of-course calculations)."New value: +"Optional; for local void-of-course."
    • changedInput schema / properties / longitude / description
      Previous value: -"Observer longitude (optional, used for local void-of-course calculations)."New value: +"Optional; for local void-of-course."
    • changedInput schema / properties / timezone / description
      Previous value: -"IANA timezone (e.g. `America/Chicago`); required when the datetime is naive, ignored otherwise."New value: +"IANA zone for a local datetime."
  2. Changed2 schema fields changedv4.8.0
    • changedInput schema / properties / datetime / description
      Previous value: -"ISO 8601 datetime to query. If omitted, returns the current live moon phase (UTC now)."New value: +"ISO 8601 datetime to query, stating its zone ('2026-03-20T12:00:00Z' or '2026-03-20T08:00:00-04:00'), or a local time together with `timezone`. If omitted, returns the current live moon phase (UTC now)."
    • addedInput schema / properties / timezone
      Added value: +{
+  "description": "IANA timezone (e.g. `America/Chicago`); required when the datetime is naive, ignored otherwise.",
+  "type": "string"
+}
  3. First observedv0.1.0

TDQS

A3.7/5.0
Behavior4/5

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

Annotations already declare readOnly/idempotent/non-destructive, so the safety profile is covered; the description goes beyond them by declaring the return payload (phase name, illumination %, sign and degree, next void-of-course window) and the 2-credit cost. It stops short of 5 by saying nothing about precision, errors, or how the void-of-course window is bounded when latitude/longitude are absent.

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 trigger condition and example phrasings are front-loaded in the first sentence, and the credit-cost line is cleanly separated. The three quoted examples are somewhat redundant with one another, costing a little density but aiding recognition.

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

Completeness4/5

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

With no output schema, the description correctly enumerates the returned fields, so an agent knows what it gets; the 100%-covered input schema carries parameter detail. The remaining gap is tool selection against overlapping lunar-phase siblings, which the description leaves entirely unaddressed.

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 description coverage is 100%, so datetime, timezone, latitude and longitude are already documented in structured form; the description only restates "omit datetime for right now." Baseline 3 is appropriate — it adds no syntax, format, or interaction detail beyond the schema.

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

Purpose4/5

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

The description states a specific verb+resource: returning the Moon's phase/illumination/sign for a given moment, with concrete example queries. It falls short of 5 because it never distinguishes its scope from the near-duplicate siblings `explore_moon_phase` and `ephemeris_next_lunar_phase`, leaving the agent to guess which one to open.

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

Usage Guidelines3/5

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

It gives usable context ("when the Moon's state is wanted as data for a specific moment") and a default ("omit datetime for right now"), which is more than nothing. But there is no when-not guidance and no routing to the obvious alternatives (`explore_moon_phase`, `ephemeris_next_lunar_phase`), which is the exact decision an agent faces here.

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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