# Skyfold Planet and Moon Positions

> Where the Sun, Moon and planets are, and when they rise and set, for any place and time. Apparent right ascension, declination, azimuth, altitude, distance, magnitude and phase of the Sun, Moon and seven planets for a place and local time, with rise, transit and set times, twilight and the next Moon phases.

Part of usefulweb.app (https://usefulweb.app/). Part of [Skyfold](https://skyfold.usefulweb.app/) (ownerApp: skyfold). Results are deterministic calculations from the inputs and the listed assumptions; the API returns exactly what the web page shows.

## Use it
- Human interface: https://skyfoldplanetpositions.usefulweb.app/ (interactive tool at https://skyfoldplanetpositions.usefulweb.app/tool)
- JSON API: POST https://skyfoldplanetpositions.usefulweb.app/api/v1/calculate with a JSON object of inputs (schema: https://skyfoldplanetpositions.usefulweb.app/api/v1/tool)
- OpenAPI: https://skyfoldplanetpositions.usefulweb.app/openapi.json
- MCP (Streamable HTTP): https://skyfoldplanetpositions.usefulweb.app/mcp - tool `skyfold_planet_positions`

## Inputs
- location (text): Location (latitude, longitude) - Decimal degrees such as 52.09, 5.12 (north and east positive), or DMS, UTM, MGRS, Maidenhead or a plus code.
- elevation (number): Elevation above sea level
- timeZone (text): Time zone - IANA name such as Europe/Berlin or America/New_York, or a fixed offset such as UTC+2.
- date (date): Local date
- time (time): Local time

## Method
- Sun and planets: truncated VSOP87D series with light-time iteration, conversion to FK5, nutation (IAU 1980, low-precision terms) and annual aberration (Meeus chapters 25, 32, 33).
- Moon: the truncated ELP-2000/82 series of Meeus chapter 47 plus nutation; topocentric parallax for altitude and rise/set.
- Altitudes include standard refraction (Sæmundsson, 10 °C, 1010 hPa). Rise and set use the USNO convention: upper limb on an ideal sea-level horizon with 34′ refraction.
- Time scales: UTC ≈ UT1; ΔT from observed values 2000–2026 and the Espenak–Meeus polynomials outside.

## Limits
- Accuracy is about an arcsecond or a few arcseconds for the Sun and planets and about 10″ for the Moon against JPL Horizons; rise and set times agree with the U.S. Naval Observatory to the minute in the tests. Local horizon obstructions, weather and unusual refraction are not modelled.