North of the Arctic Circle the day stops being a day. In summer the sun does not set at all — the midnight sun — and in winter it does not rise, which is called polar night. Both follow directly from the Earth's 23.4° axial tilt.
The Arctic Circle sits at 66.6° north — 90° minus the axial tilt. Exactly on the circle the sun in theory fails to set on one day a year, at the June solstice, and fails to rise at the December solstice.
In practice you get the midnight sun some way south of the line. The atmosphere bends light, so the sun appears about half a degree higher than it geometrically is, and sunrise and sunset are measured from the edge of the disc rather than its centre. Together that shifts the boundary roughly 80 kilometres south.
The same mechanism has a consequence that catches people out: polar night is always shorter than the midnight sun at the same place. Refraction lifts the sun over the horizon at both ends of the year, lengthening summer and shortening winter. On the Arctic Circle itself you get one day of midnight sun and effectively no polar night at all.
The duration climbs steeply with latitude — it is not a gentle gradient:
| Place | Latitude | Midnight sun | Polar night |
|---|---|---|---|
| Nuuk, Greenland | 64.2°N | none | none |
| Ilulissat, Greenland | 69.2°N | 65 days (20 May – 23 July) | 44 days |
| Tromsø, Norway | 69.6°N | 68 days (19 May – 25 July) | 48 days |
| Ittoqqortoormiit, Greenland | 70.5°N | 76 days (15 May – 29 July) | 57 days |
| Qaanaaq, Greenland | 77.5°N | 124 days (21 April – 22 August) | 107 days |
| Longyearbyen, Svalbard | 78.2°N | 128 days (19 April – 24 August) | 112 days |
Nuuk lies below the Arctic Circle and gets neither — but its day length still swings from about four hours to a little over twenty. At the poles themselves the pattern reaches its limit: roughly six months of continuous daylight followed by six months of night.
"The sun does not rise" does not mean pitch black around the clock. The sun can sit only a few degrees below the horizon at midday and produce a long blue twilight that looks, from a distance, like a very early morning.
So polar night is divided the same way twilight is. Civil polar night, where the sun stays down but it is still light enough at midday to be outside without a lamp. Nautical, where only a faint glow marks noon. And astronomical, where it is fully dark for all 24 hours — which happens only very close to the poles.
Ilulissat, for instance, has 44 days without a sunrise, but the darkest of them still give several hours of usable twilight around midday. Snow, moonlight and the aurora together make the winter landscape far brighter on clear days than the numbers suggest.
You do not need to be inside the Arctic Circle to lose the night. North of roughly 48–49° — which takes in all of Britain and Denmark — there is a stretch of summer when the sun never reaches 18° below the horizon, so it never becomes astronomically dark.
In London that runs to 59 days a year, in Copenhagen 95 and in Edinburgh 96 — the jump between London and Edinburgh is larger than the four degrees of latitude between them suggests, because the effect accelerates as you go north. Push a little further and the same starts happening to nautical twilight: Edinburgh already has 41 nights a year when the sky never gets dark enough to lose the horizon, and Copenhagen 36, while London has none at all. That is what St Petersburg means by its white nights, and what makes deep-sky astronomy at these latitudes a winter activity by necessity. See when it is lightest for how the year runs.
At the Arctic Circle, 66.6° north and south. Because of atmospheric refraction you can in fact see the midnight sun about 80 kilometres south of the line itself.
It depends steeply on latitude: 65 days at Ilulissat (69.2°N), 76 at Ittoqqortoormiit (70.5°N) and 124 at Qaanaaq (77.5°N). At the pole it runs for roughly six months.
Because the atmosphere bends light so the sun appears about half a degree higher than it geometrically is, and because sunrise is measured from the edge of the disc. Both lift the sun over the horizon, lengthening summer at winter's expense.