Day Length Atlas: How Long Is the Day, Anywhere, Any Day

I wanted to see day length as a map. Pick a place, move a slider to a day of the year, and watch the colors shift.

So I built it, as a new part of GribGrab, my weather app: gribgrab.com/day-length.

What it shows

The map has three modes:

• Day length: the time from sunrise to sunset.
• Sunrise: the local clock time when the sun comes up.
• Sunset: the local clock time when it goes down.

Everything is rounded to 10 minutes. Each full hour has its own color, with a dark line where one hour ends and the next begins. Hover over any point to see the exact numbers, the time zone and how fast the day is changing.

If you use GribGrab, the places you saved there are marked on the map and listed under it, with their own numbers for the day you picked.

Below the map is a slider with all 365 days. Press Play the year to watch the whole year go by in about 20 seconds. You can drag and zoom the map, or jump to a region: Europe, Poland, Scandinavia, and a few more.

Things I noticed

Day length is just stripes. It depends almost only on latitude, so the first mode looks like horizontal bands. On 21 June, Warsaw gets 16 h 50 min of daylight and Vigo in Spain 15 h 20 min. On 21 December it flips: 7 h 40 min against 9 h 10 min.

Sunrise is where it gets interesting. Time zones are political, not solar. Spain uses the same time as Poland, but it lies about 30 degrees further west. So on 21 June the sun rises in Warsaw at 4:10 and in Vigo at 7:00. In the sunrise mode you can see the time-zone borders right on the map.

The equator doesn’t get exactly 12 hours. At the equinox it’s about 12 h 07 min. The atmosphere bends sunlight, so the sun shows up a little before it’s geometrically above the horizon.

The far north is its own world. In Tromsø, the sun doesn’t rise for 48 days in winter and doesn’t set for 69 days in summer. The map marks those areas with grey stripes.

How it works

• The sun’s position is calculated in your browser with standard astronomy formulas. No API, no server.
• Time zones and daylight saving come from your browser’s own time-zone data.
• In countries with several time zones (the US, Russia, Brazil), each point gets the zone of the nearest major city. Near zone borders inside those countries, the map can be slightly off.

Try it

• Set the slider to your birthday and check your town.
• Switch to Sunset and compare the east and west ends of one time zone.
• Hit Play the year on Scandinavia.

Open the Day Length Atlas →

Enjoy the light!