# Sun Windows

> Find approximate direct-sun windows from your location and measured obstacles. Explore direct-sun windows, an optional window facing direction and a monthly comparison from your location and measured obstacle angles.

Part of usefulweb.app (https://usefulweb.app/). Part of [Hearth](https://hearth.usefulweb.app/) (ownerApp: hearth). 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://hearthsunwindows.usefulweb.app/ (interactive tool at https://hearthsunwindows.usefulweb.app/tool)
- JSON API: POST https://hearthsunwindows.usefulweb.app/api/v1/calculate with a JSON object of inputs (schema: https://hearthsunwindows.usefulweb.app/api/v1/tool)
- OpenAPI: https://hearthsunwindows.usefulweb.app/openapi.json
- MCP (Streamable HTTP): https://hearthsunwindows.usefulweb.app/mcp - tool `hearth_sun_windows`

## Inputs
- day (date): Date
- latitude (number): Latitude
- longitude (number): Longitude
- utcOffset (number): UTC offset on selected date
- facing (number): Window facing (degrees, -1 = open sky)
- stepMinutes (select): Sampling interval (minutes)
- obstacles (table): Obstacles from the observation point

## Method
- NOAA fractional-year solar position equations with 1, 5 or 15 minute midpoint sampling. Supply the UTC offset including daylight saving for the selected date.
- Obstacles are angular sectors with elevation atan(height/distance). Facing restricts light to the front half-plane of a vertical window. No weather, terrain or building download, refraction or PV-yield model.
- Monthly comparison uses the 21st of each month at 15-minute resolution and the same fixed UTC offset; it is a geometric comparison, not a local-clock daylight-saving calendar.