CycleCalcs Astronomy

Astronomy calculations for any place and date: sun and moon rise/set times, planet positions and visibility, Mercury and Venus apparition events, solar and…

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What it can do

  • Astro Sky Today: One-call snapshot of the whole sky for a place and moment: moon phase and illumination, which planets are up and worth looking at, the next eclipse, and (with a location) sun times. R
  • Astro Sun: The complete solar day for one location: sunrise, sunset, solar noon, day length, civil, nautical and astronomical twilight boundaries, and explicit polar day/night status at high latitudes
  • Astro Moon: The Moon at an instant or as a daily series: phase name and angle, illuminated fraction, distance, apparent size, libration, bright limb, and the next quarter phases. A location adds rise/

What data it sees

Do you need an account

An API key from the service settings is required

Astronomy calculations for any place and date: sun and moon rise/set times, planet positions and visibility, Mercury and Venus apparition events, solar and lunar eclipses, lunar phases, dark-sky observing windows, and place lookup. Eleven tools over a 29-endpoint REST API, covering 1700-2200. Streamable HTTP, no install. Tool calls need a CycleCalcs API key from RapidAPI (the Basic plan is free); browsing the tool list needs nothing. Astronomy, not astrology.

How to connect

How to connect CycleCalcs Astronomy to Claude

The server address to paste into Claude settings:

https://astronomy--cyclecalcs.run.tools

  1. Open Claude (claude.ai or the desktop app).
  2. Go to Settings → Connectors.
  3. Click Add custom connector.
  4. Paste the server address copied below into Remote MCP server URL and click Add.
  5. In a chat, click + → Connectors and switch the new connector on.

Custom connectors are available on Free, Pro, Max, Team and Enterprise plans (Free is limited to one). On Team and Enterprise an organization Owner adds the connector first under Organization settings → Connectors.

Authorization

After you click Add, a sign-in window for the service opens. Sign in with your own account and approve access. Claude never sees your password.

How to connect CycleCalcs Astronomy to ChatGPT

The server address to paste into ChatGPT settings:

https://astronomy--cyclecalcs.run.tools

  1. Open ChatGPT in a browser (chatgpt.com). A Plus, Pro, Business, Enterprise or Edu plan is required.
  2. Turn on developer mode once: Settings → Apps → Advanced settings → Developer mode.
  3. Open Settings → Connectors and click Create.
  4. Fill in the form: Name (anything), Description (one line about what the service does), MCP server URL (copy it below).
  5. Under Authentication choose OAuth if the service requires sign-in, otherwise None. Click Create.
  6. In a new chat open + → Apps/Connectors and enable the connector.

OpenAI has renamed this section before (Connectors → Apps/Plugins). If the label differs, search settings for "developer mode". On Business/Enterprise workspaces an admin must allow custom connectors first.

Authorization

On first use ChatGPT opens the service's sign-in window. Sign in and approve access.

How to connect CycleCalcs Astronomy to Cursor

The server address to paste into Cursor settings:

https://astronomy--cyclecalcs.run.tools

Fastest: click Open in Cursor below and confirm the prompt.

Manually:

  1. In Cursor open Settings → Cursor Settings → MCP and click Add new global MCP server.
  2. Paste the JSON copied below into ~/.cursor/mcp.json (per project: .cursor/mcp.json in the repo root).
  3. Save the file. The server appears in the MCP list; authorize it there if asked.

Authorization

If the service needs sign-in, an authorize button appears next to the server in the MCP list.

Server tool list (11)

Raw names from tools/list. Only developers need these.

astro_sky_todayOne-call snapshot of the whole sky for a place and moment: moon phase and illumination, which planets are up and worth looking at, the next eclipse, and (with a location) sun times. Reach for this first when the question is broad, like "what is in the sky tonight". For solar-day detail use astro_sun; for choosing an observing night use astro_dark_window; for one planet's exact position use astro_positions.
astro_sunThe complete solar day for one location: sunrise, sunset, solar noon, day length, civil, nautical and astronomical twilight boundaries, and explicit polar day/night status at high latitudes. Location required. For a series, send start and end (step is whole days, e.g. "1d" or "7d"). For "is it dark enough to observe" prefer astro_dark_window; for a broad snapshot prefer astro_sky_today.
astro_moonThe Moon at an instant or as a daily series: phase name and angle, illuminated fraction, distance, apparent size, libration, bright limb, and the next quarter phases. A location adds rise/set and altitude. For a calendar of new and full moons use astro_moon_phases; for the Moon's exact coordinates use astro_positions.
astro_moon_phasesEvery new moon, quarter and full moon in a window (or the next few from a date): each with its exact instant, distance, apparent size, supermoon classification under both competing definitions, traditional full-moon name, and any eclipse falling on it. Use for "when is the next full moon" and phase calendars. For the Moon's state right now use astro_moon.
astro_positionsExact positions for up to 20 bodies at an instant or over a time grid: right ascension and declination in both J2000 and of-date frames, ecliptic longitude and latitude, distance, and, with a location, altitude and azimuth with refraction stated per field. Use for "where exactly is X". Do not pass earth. For rise and set TIMES use astro_rise_set; for a visibility overview of all planets use astro_planet_board.
astro_rise_setRise, upper transit, set and lower transit for one body at one location, with an explicit status at extreme latitudes (circumpolar, never rises) instead of missing values. Accepts fixed radec targets. For the Sun specifically, astro_sun returns richer twilight structure; for positions between events use astro_positions.
astro_eclipsesSolar and lunar eclipses: the next or previous from a date, or all in a range, with type, magnitude, obscuration, Saros series and global geometry. With a location it adds local circumstances, contact times, and an explicit visible-from-here answer; set visible_only to true to keep only eclipses visible there. NOTE: count applies per type, so count=3 with type "both" can return six events.
astro_dark_windowThe genuinely dark, moonless observing window for a night: astronomical night intersected with the Moon being down, ranked across up to 62 nights with a trend. The right tool for "when should I stargaze / photograph the Milky Way / observe deep-sky objects". Location required. For plain twilight times use astro_sun.
astro_planet_boardAll eight planets in one call for a date and optional location: constellation, magnitude, apparent size, elongation from the Sun, morning or evening sky, retrograde state with the next station, rise/set, and a worth-looking-tonight assessment. The right tool for "which planets are visible tonight". For exact coordinates of specific bodies use astro_positions.
astro_planet_eventsThe apparition cycle of the inferior planets as dated events: inferior and superior conjunctions, greatest eastern and western elongations, peak brightness (a Venus-only event: Mercury's brightness peaks behind the Sun where it cannot be seen), and the rare transits across the Sun. With no dates it also reports where each body is in its cycle right now: morning star or evening star, the conjunctions bounding the current apparition, and the live elongation, phase, magnitude and apparent size. The right tool for "when does Venus become the morning star", "when is Venus brightest", or "Mercury's next greatest elongation". For tonight's visibility of all eight planets use astro_planet_board. Conjunction instants use the classical heliocentric convention, named on each event.
astro_find_placeResolve a place name to coordinates, region, country, IANA timezone and a stable place_id, or reverse-look-up the nearest places to a lat/lon. Results are GeoNames data (CC BY 4.0); the response carries the required credit in its attribution field, which must be preserved when results are shown. Note the other tools accept a place argument directly, so this is only needed to disambiguate a name, filter by country, or reverse-geocode.