How to make a star map poster
- Pick the moment. Enter the date and the local time at that place: the evening you met, the hour a child was born, a wedding night. The page works out the time zone and daylight saving for that place and date.
- Pick the place. Type a city or town and choose it from the list, or type its latitude and longitude (for example
40.7128, -74.0060). The list of about 18,000 places is searched on your device. - Write the words. A title, an optional message of up to three lines, and the place, date and coordinates lines, which fill themselves in and can be edited or switched off.
- Choose the look. Midnight navy, black and gold, a navy sky on white, or ink on white (kind to a home printer). Add constellation lines and names, the Moon and planets, a sky grid and a thin border.
- Download. The PDF is vector and exactly the paper size you chose, so it prints sharp even at 18 × 24 inches. Print at 100% ("actual size"), not "fit to page".
How the sky is worked out
The stars are the 9,096 stars of the Yale Bright Star Catalogue (5th revised edition, Hoffleit and Warren, 1991; NASA Astrophysics Data Center and CDS), which lists essentially every star you can see with the naked eye, down to about magnitude 6.5. Their catalogue positions are for the year 2000. For your date, each star is moved by its own proper motion and by precession, the slow 26,000-year wobble of the Earth's axis that shifts the whole sky by about a degree every 72 years. Then the sky is turned to your place: from your latitude and the sidereal time at your longitude and moment, each star gets its altitude above the horizon and its compass direction.
The Moon's position comes from the lunar theory in Jean Meeus's Astronomical Algorithms (its largest terms, within a few hundredths of a degree), corrected for where you stand on the Earth, which can move the Moon by up to a degree near the horizon. Its phase and the direction of its lit side are drawn as they were. Mercury, Venus, Mars, Jupiter and Saturn come from NASA JPL's published orbital elements for approximate planet positions; checked against JPL Horizons, they land within about 0.1° (Jupiter and Saturn) and a few hundredths of a degree (the others), far less than the width of the Moon. Uranus and Neptune are left out because you can't see them without a telescope.
The constellation figures are Marc van der Sluys's ConstellationLines (CC BY 4.0), drawn between catalogue stars. Constellation names are the 88 official ones of the International Astronomical Union. Places and time zones come from GeoNames (CC BY 4.0).
How to read the map
The circle is your whole sky. Its edge is the horizon and its centre is the point straight overhead. North is at the top and east is on the left: a sky chart is meant to be held above your head while you face north, and then left and right match what you see. Bigger dots are brighter stars. The Moon is drawn larger than life so its phase shows, with the lit side facing where the Sun was.
Below the map the page tells you how many stars were above the horizon, whether the Moon was up and how much of it was lit, and which planets were in the sky. If the Sun was up at your moment, or it was still twilight, the page says so: the map still shows the stars, but nobody could have seen them then.
Printing tips
- For a framed gift, pick the size of the frame you have: 8 × 10, 11 × 14 and 12 × 16 inches fit standard US frames, A4 and A3 the rest of the world, and 18 × 24 inches is a classic poster size. Print shops take the PDF as it is.
- Dark styles use a lot of ink. On a home printer, try "Ink on white", or print the navy style on matte photo paper. Check your printer's margins with the printer test page first: most home printers can't print to the very edge, so a borderless dark poster is a job for a print shop.
- The fonts cover Latin, Vietnamese and Cyrillic letters. If your words use other scripts or emoji, the page names the characters the PDF can't draw; the PNG uses your device's fonts for them.
More to print
Pair the poster with a family tree chart for an anniversary, or mark the date on a printable calendar. To see how a total solar eclipse looks from your own town, try the eclipse simulator.