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Open Ephemeris

Open Ephemeris

by openephemeris
OverviewSchemaSourceRelated ServersDiscussionsAdmin
Python
Remote

Planet Position

ephemeris_planet_position
Read-onlyIdempotent

Calculate one celestial body's ecliptic longitude, latitude, distance, speed and retrograde status at a given moment. Use it to locate a planet now, check a zodiac sign placement, or find where a body was on a birth date.

Instructions

Use this when the user asks 'where is Mars right now', 'what sign is the Moon in today', 'what's my sun sign / star sign' (planet_id=0; a bare birth date resolves to noon UTC, no location needed), 'where was Jupiter on my birthday', 'where is the North Node' — one body at one moment. Returns ecliptic longitude, latitude, distance, speed and retrograde flag.

PLANET IDS: 0=Sun, 1=Moon, 2=Mercury, 3=Venus, 4=Mars, 5=Jupiter, 6=Saturn, 7=Uranus, 8=Neptune, 9=Pluto, 10=North Node (Mean), 11=North Node (True), 12=Lilith (Mean), 15=Chiron, 17=Ceres, 18=Pallas, 19=Juno, 20=Vesta. NODES: id 10 is the mean North Node (mean-node polynomial); id 11 is the true North Node (osculating, referenced to the true ecliptic of date). There is no South Node id; the South Node longitude is a North Node longitude plus 180° (mod 360).

CREDIT COST: 1 credit per call.

Input Schema

NameRequiredDescriptionDefault
datetimeYesISO 8601; zone required (`Z`/offset, or local time plus `timezone`).
latitudeNoOptional; topocentric if set.
timezoneNoIANA zone for a local datetime.
longitudeNoOptional; topocentric if set.
planet_idYes0 Sun, 1 Moon, 2–9 Mercury–Pluto, 10 mean node, 11 true node, 12 Lilith, 15 Chiron, 17–20 Ceres/Pallas/Juno/Vesta.

Schema Changelog

Changes observed during successful MCP inspections.

  1. Changed5 schema fields changedv4.29.0
    • changedInput schema / properties / datetime / description
      Previous value: -"ISO 8601 datetime. A datetime with no zone is rejected; give a `Z` or ±HH:MM suffix, or a local time with the `timezone` argument. Date-only resolves to 12:00 UTC."New value: +"ISO 8601; zone required (`Z`/offset, or local time plus `timezone`)."
    • changedInput schema / properties / latitude / description
      Previous value: -"Observer latitude for topocentric position (optional). Omit for geocentric."New value: +"Optional; topocentric if set."
    • changedInput schema / properties / longitude / description
      Previous value: -"Observer longitude for topocentric position (optional). Omit for geocentric."New value: +"Optional; topocentric if set."
    • changedInput schema / properties / planet_id / description
      Previous value: -"Planet/body ID (0=Sun, 1=Moon, 2=Mercury, 3=Venus, 4=Mars, 5=Jupiter, 6=Saturn, 7=Uranus, 8=Neptune, 9=Pluto, 10=North Node (Mean), 11=North Node (True), 12=Lilith (Mean), 15=Chiron, 17=Ceres, 18=Pallas, 19=Juno, 20=Vesta)."New value: +"0 Sun, 1 Moon, 2–9 Mercury–Pluto, 10 mean node, 11 true node, 12 Lilith, 15 Chiron, 17–20 Ceres/Pallas/Juno/Vesta."
    • 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 changedv4.24.2
    • changedInput schema / properties / datetime / description
      Previous value: -"ISO 8601 datetime; zone required — a `Z`/±HH:MM suffix or the `timezone` argument. Date-only resolves to 12:00 UTC. Full rule: server `instructions`."New value: +"ISO 8601 datetime. A datetime with no zone is rejected; give a `Z` or ±HH:MM suffix, or a local time with the `timezone` argument. Date-only resolves to 12:00 UTC."
    • changedInput schema / properties / planet_id / description
      Previous value: -"Planet/body ID (0=Sun, 1=Moon, 2=Mercury, 3=Venus, 4=Mars, 5=Jupiter, 6=Saturn, 7=Uranus, 8=Neptune, 9=Pluto, 10=North Node (Mean), 11=North Node (True), 12=Lilith (Mean), 15=Chiron, 17=Ceres, 18=Pallas, 19=Juno, 20=Vesta). There is no South Node id — take the North Node and add 180°."New value: +"Planet/body ID (0=Sun, 1=Moon, 2=Mercury, 3=Venus, 4=Mars, 5=Jupiter, 6=Saturn, 7=Uranus, 8=Neptune, 9=Pluto, 10=North Node (Mean), 11=North Node (True), 12=Lilith (Mean), 15=Chiron, 17=Ceres, 18=Pallas, 19=Juno, 20=Vesta)."
  3. Changed3 schema fields changedv4.8.0
    • changedInput schema / properties / datetime / description
      Previous value: -"ISO 8601 date/time in UTC or with offset (e.g. '2026-03-20T12:00:00Z' or '2026-03-20T12:00:00-05:00')."New value: +"ISO 8601 datetime; zone required — a `Z`/±HH:MM suffix or the `timezone` argument. Date-only resolves to 12:00 UTC. Full rule: server `instructions`."
    • changedInput schema / properties / planet_id / description
      Previous value: -"Planet/body ID (0=Sun, 1=Moon, 2=Mercury, 3=Venus, 4=Mars, 5=Jupiter, 6=Saturn, 7=Uranus, 8=Neptune, 9=Pluto, 10=MeanNode, 11=TrueNode, 12=Lilith, 15=Chiron, 17=Ceres, 18=Pallas, 19=Juno, 20=Vesta)."New value: +"Planet/body ID (0=Sun, 1=Moon, 2=Mercury, 3=Venus, 4=Mars, 5=Jupiter, 6=Saturn, 7=Uranus, 8=Neptune, 9=Pluto, 10=North Node (Mean), 11=North Node (True), 12=Lilith (Mean), 15=Chiron, 17=Ceres, 18=Pallas, 19=Juno, 20=Vesta). There is no South Node id — take the North Node and add 180°."
    • addedInput schema / properties / timezone
      Added value: +{
+  "description": "IANA timezone (e.g. `America/Chicago`); required when the datetime is naive, ignored otherwise.",
+  "type": "string"
+}
  4. First observedv0.1.0

TDQS

A4.3/5.0
Behavior4/5

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

Annotations already cover safety traits (read-only, idempotent, non-destructive), and the description adds useful context such as returned fields and credit cost. It does not contradict annotations, though it could say more about auth/permissions or error behavior.

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 usage examples and then organized into ID, node, and cost sections. It is appropriately sized for the domain, but the PLANET IDS list partially duplicates the schema's planet_id description.

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?

For a no-output-schema tool it explains the return fields, ID mapping, datetime nuance, node behavior, and cost. It still leaves some domain details unstated, such as zodiac/coordinate frame assumptions and error cases.

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

Parameters4/5

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

Schema coverage is 100%, so the baseline is 3, but the description adds meaning beyond the schema: the full planet ID list, node semantics, South Node derivation, and the bare-birth-date-to-noon-UTC rule. Latitude/longitude/timezone behavior is mostly left to the schema, so it is not a 5.

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 resource and scope: one celestial body at one moment, returning ecliptic position data. The examples ('where is Mars right now', 'what sign is the Moon in today') make it distinguishable from sibling chart, transit, and phase tools.

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 gives clear usage context through several concrete example queries and the 'one body at one moment' scope. It does not explicitly name when-not-to-use cases or direct the agent to a specific sibling alternative, so it falls short of a 5.

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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