Reading the day and night map
The shaded half of the Everywhen live map is the part of the Earth where the sun has set. Its edge, the terminator, is the line of sunrise and sunset. Watching how it moves explains a lot about seasons and daylight.
Why the shadow moves
The Earth turns once a day from west to east, so the terminator sweeps from east to west: dawn reaches Tokyo before Seoul, Seoul before Beijing, and so on around the world. At any moment, roughly half the planet is in daylight.
Why its shape changes
The Earth's axis is tilted by about 23.4° relative to its orbit. As the Earth goes around the sun, first one hemisphere and then the other leans towards it.
- Around the equinoxes (about 20 March and 22 September) neither hemisphere leans towards the sun. The terminator runs almost straight from pole to pole, and day and night are close to 12 hours everywhere.
- Around the June solstice (about 21 June) the northern hemisphere leans towards the sun. On a flat map the terminator becomes a deep curve: the Arctic stays lit all day, the midnight sun, while Antarctica stays dark.
- Around the December solstice (about 21 December) it is the other way round: long days in the south, the polar night in the Arctic.
This is also why day length changes so much more in Reykjavík than in Singapore. Near the equator the terminator crosses at almost the same angle all year; near the poles a small change in the sun's angle makes a big difference.
Twilight
The sky does not go dark the moment the sun sets. Civil twilight lasts until the sun is 6° below the horizon, and there is still enough light outside for most activities. That is why the edge of the shadow on the map is soft rather than sharp. Far from the equator, twilight can last a long time; in summer in northern Europe it can last all night.
Flat map or globe?
On a flat map the terminator looks like a wave, because the map stretches the areas near the poles. On the 3D globe it is always a perfect circle around the Earth, the boundary between the half facing the sun and the half facing away. Switching between the two views is a good way to see what the flat map distorts.
Try it
Open the live map, use the time bar to move through the day, and switch to the globe to see the shadow from space. Each city's page on the world clock shows its longest and shortest day of the year.
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