EOY

Midnight sun and polar night

North of the Arctic Circle and south of the Antarctic Circle, summer brings a Sun that never sets and winter a Sun that never rises. The clock and the calendar carry on regardless.

The cause: a tilted axis

The Earth's axis is tilted about 23.4° from the plane of its orbit. Around the June solstice the North Pole leans towards the Sun, so the whole region within about 23.4° of the pole — everything north of roughly 66.6° latitude, the Arctic Circle — stays in sunlight for a full rotation. Six months later the same region leans away and stays in shadow. The Southern Hemisphere runs the same cycle half a year out of phase.

Midnight sun

The midnight sun is the Sun remaining visible at local midnight. At the Arctic Circle itself it happens on one day a year; the further north, the longer it lasts. Tromsø in Norway (69°N) has it from late May to mid-July; Svalbard has no sunset from about 19 April to 23 August; at the North Pole the Sun is up for about six months, from roughly 18 March to 24 September.

Polar night

Polar night is the mirror image: the Sun stays below the horizon for more than 24 hours. Tromsø and Utqiaġvik in Alaska (71°N) have it from about 1 December to 10 January; Longyearbyen on Svalbard (78°N) from about 27 October to 14 February; the North Pole about 179 days, the South Pole about 186. "Night" is a generous word for much of it: below about 72.5° there is still civil twilight around midday, and only above about 84.5° is the sky fully dark.

Days still change at midnight

None of this touches the calendar. Civil time is kept by clocks set to a zone's offset from UTC, not by sunrise, so a day in Longyearbyen in December begins at midnight on the clock exactly as it does in Rome. New Year arrives on schedule under a dark sky. That is why the countdown never looks at the Sun: the year ends at midnight in the selected zone, whatever the horizon is doing.

Why the equator barely notices

At the equator the tilt hardly changes day length at all — days and nights stay close to twelve hours all year. The variation grows with latitude, from a couple of hours at Mediterranean latitudes to the full six-month extremes at the poles. The mechanism, and the dates of the longest and shortest days, are in solstices and equinoxes.

Sources

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