How we draw your sky
Every dot is a real star in the place it really was. Here is how, and what we leave out.
The stars
The stars come from the HYG Database, version 4.0, compiled by David Nash from the Hipparcos, Yale Bright Star and Gliese catalogues. We keep the 3,231 stars brighter than magnitude 5.6. That is about what the eye can see from a dark site on a clear night, so the map is not cluttered with stars you could never have seen.
Each star's position is given for the year 2000. We rotate the whole sky to your date, allowing for the slow wobble of the Earth's axis (precession and nutation). Stars also drift very slightly against each other over the years, but for any date between 1900 and 2100 the change is too small to show on a print.
The projection
The map is a stereographic projection centred on the point straight above your head. The edge of the circle is the horizon, and the centre is the zenith, so you see the whole sky at once.
North is at the top and east is on the left. That looks mirrored beside an ordinary map, but it is how the sky looks when you lie on your back with your head towards the north and look up: you are viewing it from underneath, not from above.
The time
You give the local time at the place, the way you would read it off a clock or a certificate. We convert it to a single universal time using the IANA time zone database, the same one phones and computers use. That means British Summer Time, and every other change of clocks, is handled for you, including the years when the rules were different.
Two awkward cases: if you pick a time that never happened, because the clocks jumped forward over it, we use the first valid minute after it. If you pick a time that happened twice, because the clocks went back, we use the first of the two.
The Moon and planets
The Moon, the Sun and the planets are calculated with astronomy-engine, an open-source library by Don Cross. Their positions are worked out as seen from your exact place on the Earth, for the minute you chose. The author reports agreement with NASA's Horizons system to about one arcminute, which is far finer than a print can show.
The Moon's phase is calculated, not looked up. On a Moon print, the Moon is drawn the way you would see it from your latitude, so from Sydney it appears the other way up from London. Its surface is NASA imagery.
What the map leaves out
- Light pollution and clouds. The map shows the sky as a dark, clear site would have seen it.
- Stars fainter than magnitude 5.6, and anything that needs a telescope.
- Atmospheric refraction, which lifts objects near the horizon by a fraction of a degree.
- Satellites, comets, meteors and anything else that is not a star, the Moon or a planet.
If the sun was up
If the time you choose is in daylight, the stars are still drawn where they were, even though the Sun's glare hid them. The map is the sky behind the day.
A keepsake, not a navigation tool
The print is made to be looked at and framed. Please do not use it for navigation or for research. If you want to double-check it, free planetarium software such as Stellarium will show the same sky for the same place and time.