# Skyfold Star Map Poster

> Print the real night sky of any place and moment as an SVG or PDF poster. Draws the stars to magnitude 6.5, constellation figures, the Moon and planets exactly as they stood above a place at a local date and time, in five print styles and seven paper sizes, as a vector SVG and PDF.

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

## 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
- style (select): Style
- size (select): Paper size
- magLimit (number): Faintest stars (magnitude) - Larger numbers show fainter stars; 6 to 6.5 is the naked-eye limit under a dark sky.
- title (text): Title
- subtitle (text): Subtitle
- placeLabel (text): Place line
- dateLabel (text): Date line - Leave blank to print the local date and time.
- constellations (boolean): Constellation lines
- constellationNames (boolean): Constellation names
- starNames (boolean): Names of the brightest stars
- planets (boolean): Moon and planets
- colorStars (boolean): Star colours from the B−V index

## Method
- Stars: every HYG v4.1 star to magnitude 6.5 (8 920 stars), precessed from J2000 to the date with nutation and annual aberration, then converted to azimuth and altitude with refraction.
- Projection: zenithal stereographic, zenith in the centre and the horizon as the circle, north up and east on the left as seen when you look up.
- Constellation figures from d3-celestial (BSD licence); Moon and planets from the Skyfold ephemeris. Dot sizes grow by a factor 1.38 per magnitude.
- Vector output: the SVG and a one-page PDF at the paper size (standard PDF fonts).

## Limits
- Shows the geometric sky without clouds, light pollution or the Milky Way band; proper motions of stars are not applied. Texts outside the Latin-1 character set are replaced in the PDF (use the SVG instead).