How the sky is worked out

The real sky for that minute

Your date and time are turned into universal time with the time zone of the place, daylight saving included. From that come the sidereal time and the position of every star above the horizon, corrected for the slow drift of the Earth's axis since the year 2000. Polaris stands as high above the north horizon as the place is north of the equator, as it should.

Reading the map

The circle is the horizon and the centre is straight overhead. It is the sky seen from below, as if you lay on your back with your head to the north, so east is on the left and west on the right. Hold it above your head facing south and it matches the real sky.

The Moon and the planets

The Moon is drawn where it stood, with the part the Sun lit that night; its phase is also shown in the lettering. Mercury, Venus, Mars, Jupiter and Saturn appear when they were above the horizon. Positions are good to a small fraction of a degree, far finer than a printed dot.

Printing it

The raised plaque prints in two colours with a single filament swap at the top of the base, or in more on a multi-colour printer using the 3MF, where the base, stars, lines and lettering are separate parts. The lithophane is one thin white plate: hold it to a window or a lamp and the stars light up. Print the place side separately and glue it to the back for a two-sided keepsake.

Stars, constellations and names from d3-celestial by Olaf Frohn (BSD-3-Clause), from the Hipparcos and Yale Bright Star catalogues. The Milky Way is drawn from galactic coordinates. Map data © OpenStreetMap contributors. Elevation © Mapzen / AWS Terrain Tiles. More ideas: 3D map gifts, map art posters and plaques, or a full 3D model in the TerraPrinter app.