# Skyfold Sun Position and Shadow Length

> Sun angle and the length and direction of a shadow for any height, place and time. Computes the Sun’s azimuth and altitude, the length and compass direction of the shadow cast by an object of a given height on level ground, the shadow through the day, sunrise, solar noon, sunset and golden hour, plus a day-and-night map.

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://skyfoldsunshadow.usefulweb.app/ (interactive tool at https://skyfoldsunshadow.usefulweb.app/tool)
- JSON API: POST https://skyfoldsunshadow.usefulweb.app/api/v1/calculate with a JSON object of inputs (schema: https://skyfoldsunshadow.usefulweb.app/api/v1/tool)
- OpenAPI: https://skyfoldsunshadow.usefulweb.app/openapi.json
- MCP (Streamable HTTP): https://skyfoldsunshadow.usefulweb.app/mcp - tool `skyfold_sun_shadow`

## 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
- height (number): Object height
- unit (select): Unit
- stepMinutes (select): Table step

## Method
- Sun position from VSOP87 with nutation and aberration (Meeus chapter 25), topocentric for the place, with standard refraction for 10 °C and 1010 hPa.
- Shadow length = height ÷ tan(altitude) on level ground; the shadow points away from the Sun (azimuth + 180°). Below 0.5° altitude the shadow is treated as unbounded.
- Day-and-night map: sub-solar point from the apparent Sun and sidereal time; boundaries where the Sun is at −0.83°, −6°, −12° and −18°.

## Limits
- Assumes flat, level ground and a thin vertical object; terrain, slopes, neighbouring buildings and atmospheric conditions change real shadows.