How accurate is a star map?

A Sky, That Night map is not a decorative pattern. It is calculated for the date, time and place you enter.

• The stars. Positions come from a catalogue based on the Hipparcos space-telescope measurements — over 41,000 stars. We draw every star down to magnitude 6.8, about the faintest a sharp eye can see under a perfectly dark country sky: roughly 6,000 stars above the horizon on any one night.
• Your date. The catalogue is fixed to the year 2000, so we move every star to where it stood on your date (the slow wobble of Earth's axis, called precession). Dates before 15 October 1582 use the Julian calendar, as people did then.
• Your time and place. The sky turns about 15 degrees an hour, so the time and the place decide which stars were above you. Enter the time as it was on the clock there: we use that place's time zone, including summer time for that date.
• The Moon. Its phase at that moment is shown beneath the map.

• Weather and light pollution — the sky is drawn as if the night were clear and dark.
• Planets — the map shows the stars, the constellations, the Milky Way and the Moon's phase.
• Buildings and hills — the circle is the whole sky, from the horizon to directly overhead.

Constellation lines are the traditional figures, drawn to help you find your way; they are not official boundaries.

Within 24 hours of your payment we email you the finished file, exactly as it will be printed. If a date, a place or a word is wrong, reply within 24 hours and we fix it before anything is printed.