What the map shows
| On the map | What it is |
|---|---|
| Stars | every star above the horizon down to magnitude 6, drawn larger the brighter it is; on the dark colors each one is tinted by its measured color, blue-white to orange |
| Lines and names | the figures of all 88 constellations, and the names of the large ones |
| The Moon | where it stood, lit as it was: the bright side faces the Sun |
| Planets | Mercury, Venus, Mars, Jupiter and Saturn when they were up |
| The edge | the horizon all around; the centre is the point straight overhead |
Of the 5,044 stars in the catalogue, about half are above the horizon at any moment, so a map holds some 2,500 of them. Under the map the tool names the highest bright star, the Moon’s phase and the planets, with how high each one stood.
Picking the moment
- Choose the town where it happened, and the date.
- Set the time, or drag the slider under the map and watch the sky turn through the night.
- Write a title and a line of names. The place, the date and time, and the coordinates print under it on their own.
Printing it
The PDF draws every star, line and the Moon as a vector shape, so the poster is as sharp on A3 or Tabloid from a copy shop as on Letter or A4 at home; the lettering is set at 300 dots per inch. Dark posters use a lot of ink: for a home printer the “Paper” colors print the same sky in black on white. Print at 100 percent and the poster fills the sheet with a narrow border, ready for a standard frame of that paper size.
More to make for the season is on the Christmas page, and the photo calendar is another gift that comes out of a printer.
Questions
Is it the real sky of that night?
Yes, computed for the place and the minute you choose. The stars come from a catalogue built on the Hipparcos satellite's measurements, every star down to magnitude 6 (about what the eye sees on a dark night), moved for the slow wobble of the Earth's axis since the year 2000. The Moon's position and phase and the planets Mercury to Saturn are computed for the same minute. Clouds and city light are not on the map; it shows what was above the horizon.
What if it was daytime or cloudy?
The stars were still there. Under the map the tool says when the Sun was up or it was twilight at that minute, so you know the sky on the poster was hidden by daylight. Many people pick the evening of the day instead, or keep the real minute for the meaning.
Why is east on the left?
Because the map shows the sky as you see it lying on your back with your head to the north. Looking up, east is on your left and west on your right, the opposite of a road map. The horizon is the edge of the circle and the point straight overhead is its centre.
How exact do the time and place have to be?
The sky turns about 15 degrees an hour, so the time matters most: an hour changes the picture clearly. The place matters less: 50 kilometers north or south shifts the sky by less than half a degree. The town list has every town of 50,000 people or more; pick the nearest one, or use your own location.
Where does the data come from?
Stars and constellation lines: the star catalogue of the d3-celestial project (Olaf Frohn, BSD license), built on the XHIP compilation of Hipparcos data. Planets: the approximate orbital elements published by NASA's Jet Propulsion Laboratory, most exact between 1800 and 2050. Towns: GeoNames (CC BY 4.0). Dates from 1900 to 2100 work.
Is my location uploaded?
No. The list of towns is downloaded to your browser and searched there, and "Use my location" asks your browser, which keeps the position on your device. The map, the PDF and the pictures are all made in your browser.