The sky that night

Pick a date and a place and the poster turns into that sky right away. If you don't know the time, leave it: we draw 9 PM and print no time. Your line is optional. Without one, the date takes its place.

Search any city, or use the map if yours isn't listed. The map is a drawing inside this page, so where you tap goes nowhere.

There are three styles. Picture-book night suits a child's room, ink and paper uses the least ink at home, and starlight gold works as a gift. You can save a 2400×3200 poster, a 1200×2600 phone wallpaper and a 1080×1350 card.

First your local time becomes Universal Time, and this is where most star maps go wrong. 9 PM in Seoul on July 15, 1988 was summer time, UTC+10. Read as plain UTC+9 it would be a different sky, an hour later. So we follow the rules that place actually used on that date.

Then astronomy-engine moves each star to its altitude and direction for that moment, with precession and nutation included. The catalogue is the Yale Bright Star Catalogue: 6,331 stars down to magnitude 6.2, near the naked-eye limit. The poster draws down to 5.5. The Moon shows its true lit fraction with the bright side toward the Sun.

A sky map is the sky seen from below, so with north up, east is on the left. That is the reverse of a street map, and it is correct.

The small line in the corner is the number of stars you could have seen with the naked eye from that spot that night. We add up dark-sky brightness, twilight and moonlight at each point of the sky, then work out the faintest star a person could find there, allowing for the extra air near the horizon. That's why Seoul shows 1,999 on a moonless night and 596 under a full Moon.

In daylight the line says 0. Just after sunset it gives no number, because the model is out of its measured range. An honest gap beats an invented figure.

It runs to a single star, but it is model output, not a measurement. Clouds and city lights are unknown, so they are left out. Use it to compare two nights.

The IAU fixed 88 constellations and their borders in 1922, but no stick figures are official. The lines are convention. We use the ones from d3-celestial.

The Milky Way band is drawn along the galactic equator: its position is calculated, its width and softness are drawing choices. Star sizes show brightness steps, not real sizes. The paper grain, foil and share art were made with AI, then edited.