See the day length for world capitals on a rotating 3D globe. Pick a date and see how day length varies from north to south.
The rotatable globe shows day length for the world's capitals at once. Pick a date, spin the globe, and the difference between north and south becomes immediately visible — instead of a table of numbers you see a pattern that settles into bands from pole to pole.
The boundary between the lit and dark halves is called the terminator. It always splits the globe into two equal halves, but it only stands perpendicular to the equator twice a year, at the equinoxes. The rest of the year it leans, because the Earth's axis is tilted 23.4° relative to its orbit around the Sun. That tilt is exactly what gives us seasons.
The lean also explains why day length changes so differently depending on where you are. Near the equator the terminator cuts almost straight across the lines of latitude and the day stays close to 12 hours all year. Further north or south it cuts at an angle, and the swing grows.
North of the Arctic Circle at 66.6°, the terminator does not come round at all for part of the year. In summer the region sits in constant light — midnight sun — and in winter in constant shadow, the polar night. The closer to the pole, the longer those stretches last: at the pole itself they run to roughly half a year each.
The southern hemisphere mirrors the northern. When the UK has its shortest day in December, South Africa and Australia have their longest, so a solstice is only a "summer" or "winter" solstice as seen from one hemisphere at a time. Spin the globe from north to south on a date in December and you can watch day length grow the whole way.
Every flat map of the world is wrong in some way — that is a mathematical certainty, not a criticism of any particular projection. A sphere cannot be flattened without stretching something, and projections differ only in what they choose to sacrifice.
For daylight this matters more than usual, because the distortion is worst exactly where the daylight story is most interesting. The Mercator projection, the one most web maps use, inflates area enormously towards the poles: Greenland appears roughly the size of Africa when it is about a fourteenth of it. So the polar regions — where day length does the extreme things, running to 24 hours or zero — are also the regions a flat map represents least honestly.
On a globe the day–night boundary is what it actually is: a great circle, cutting the sphere into two equal halves, tilted with respect to the axis. Flattened onto a projection that same circle becomes a curve whose shape depends entirely on the projection, and the fact that it is dividing the world into two equal halves stops being visible at all.
Astronomically the Arctic and Antarctic are mirror images: whatever the north is doing in June, the south is doing in December. In every other respect they are nothing alike, and spinning the globe makes the reason plain.
The Arctic is an ocean surrounded by land; the Antarctic is a continent surrounded by ocean. Water stores and releases heat far more slowly than land or ice, so the Arctic's seasons are buffered while the Antarctic's are extreme — and Antarctica is additionally a high plateau, most of it two kilometres or more above sea level, which makes it colder still. The result is that four million people live within the Arctic Circle, in cities such as Murmansk and Tromsø, while Antarctica has no permanent population at all.
So identical day length curves produce completely different places. It is a useful reminder that daylight sets the rhythm of a climate but does not, on its own, determine it.
The Arctic Circle is usually quoted as 66.5° or 66.6° north, and it is normally described as though it were painted on. It is not. It is defined as the latitude at which the sun fails to rise for at least one day a year, and that depends on the Earth's axial tilt — which is not constant.
The tilt oscillates slowly between about 22.1° and 24.5° over a cycle of roughly 41,000 years, and it is currently decreasing. The Arctic Circle is therefore drifting north at something like 14 to 15 metres a year. Iceland's island of Grímsey, the one piece of Icelandic territory the line crosses, marks it with a large concrete sphere that has to be physically moved every year to keep up.
The same slow oscillation is one of the drivers of long-term climate cycles, because it changes how strongly the seasons are expressed. On the timescale of a human life it changes nothing you would notice — but it is a good demonstration that the numbers on this page describe the present configuration of the solar system rather than a permanent law.
To go further, see the same data as a curve on the year chart, city by city on the daylight map, or side by side on the comparison page. The underlying geometry is explained in what day length is.
The terminator is the boundary between the lit and dark halves of the Earth. It always splits the globe into two equal halves, but it only stands perpendicular to the equator at the equinoxes — the rest of the year it leans, because the Earth's axis is tilted 23.4°.
Because the Earth's axis is tilted. When the northern hemisphere leans towards the Sun, the southern leans away, and the other way round. So when the UK has its shortest day in December, Australia has its longest.
At the Arctic and Antarctic Circles, 66.6° north and south. North of the Arctic Circle the sun does not set around the June solstice and does not rise around the December one. The closer to the pole, the longer those stretches last.