Computed, not decorated

What is on the map

• 5,044 stars down to magnitude 6, roughly everything visible to the naked eye under a dark sky, each drawn at its catalogued brightness and tinted by its real colour (B–V index).
• All 88 constellations, with optional names and bright-star labels.
• The Milky Way band.
• The Moon in its true position and phase.
• The naked-eye planets, Mercury, Venus, Mars, Jupiter and Saturn, where they were that night.

How the sky is calculated

1. Your local time becomes a universal instant. The time you enter is read as the clock time at that place, using its time zone and daylight-saving rules for that date.
2. Earth’s rotation. That instant becomes a Julian date and then the local sidereal time: which part of the sky was overhead.
3. Earth’s wobble. Catalogue positions are for the year 2000; they are corrected for precession, the slow drift of Earth’s axis, to your date.
4. Your horizon. Each star’s position is converted to altitude and direction as seen from your latitude and longitude.
5. The poster. The dome of the sky is projected onto a flat disc, with the point straight overhead at the centre and the horizon at the edge.

The Moon and planets are computed with the open-source astronomy-engine library. The calculations are checked by automated tests against known facts. For example, the Pole Star (Polaris) must sit at an altitude equal to the observer’s latitude.

Reading your map

• The centre is the point directly overhead (the zenith); the edge is the horizon.
• With north at the top, east is on the left. That is what you see lying on your back looking up, like a planisphere held overhead.
• The map shows the sky as it was, whatever the weather, and even in daylight: the stars that were above the horizon at that moment.

Data sources

Star catalogue, constellation figures and names and the Milky Way outline are derived from the open-source d3-celestial datasets, which are based on public-domain star catalogues. Solar-system positions come from astronomy-engine, and place search uses Open-Meteo geocoding.