Polar night explained: where the polar night can be seen, and the types of darkness it actually means

The definition is narrower than the phrase suggests. Polar night is the period when the sun stays below the horizon for more than 24 hours, and it can only happen inside the polar circles, at roughly 66.5° north and south. One sunless day counts. So does eleven weeks.

Its opposite is polar day, the midnight sun. The two are not quite mirror images, because the atmosphere bends sunlight over the horizon: refraction lengthens polar day and shortens polar night, so the area of the planet that goes dark is slightly smaller than the area that gets a midnight sun.

What is polar night, and what it is not

The most common misunderstanding is that it means darkness. Ask what is polar night and the answer people expect is a black sky at noon. In most places where it happens, it is not.

Because sunlight refracts over the horizon, an affected location will usually be in one of the phases of polar twilight instead, and midday is still the brightest part of the day. In Vadsø in northern Norway, a typical day during civil polar twilight runs through night, astronomical twilight, nautical twilight and civil twilight in that order, then back down through the same phases in reverse. It has a shape. It just never reaches daylight.

There is one context where the everyday reading is right. The psychological benefit of daylight depends on ambient light levels of up to 10,000 lux, and no stage of twilight reaches that. So for the purposes of seasonal affective disorder and light therapy, every midday twilight inside the polar circles counts as night, whatever it looks like out of the window. That is the sense in which asking what is polar night has a medical answer as well as an astronomical one.

Where can the polar night be seen?

Where can the polar night be seen depends on how much darkness you are asking for. The plain version, at least one day with no sunrise, begins immediately inside the polar circles and lengthens as you go poleward: 24 hours just inside the circle, and 179 days at the poles themselves.

Put more precisely, where can the polar night be seen at all has a single boundary answer, the polar circles, while where you can see each of the deeper grades is a separate question with four answers, set out below.

Named places help more than latitudes. Longyearbyen on Svalbard sits deep enough north that the sun stays well below the horizon from about 11 November until 30 January. Dikson in Russia gets a shorter version, roughly 6 December to 6 January. Pond Inlet in Nunavut gets about ten days, from around 16 December to 26 December. Alert in Nunavut, the northernmost settlement in the world, is in deeper darkness still, roughly 19 November to 22 January.

The four types of polar night, explained

The four types are defined by how far below the horizon the sun sits at solar noon, which in practice means by latitude. Each step inward from the polar circle removes one grade of twilight, and once the sequence is explained that way the boundaries stop looking arbitrary. The types are named after the grade of twilight that survives at midday, not after the one that has been lost, which is the detail most summaries get wrong and the reason the labels are so often explained backwards.

Civil polar twilight

Roughly 67°24’ to 72°34’. On the winter solstice the sun is below the horizon all day, but by more than 50 minutes of arc and less than 6° at solar noon. There is no true daylight at midday, only civil twilight, and there is still enough of it for most ordinary outdoor activity. Under thick cloud that changes: on the Finnmark coast, at about 70°, street lamps may stay on through the middle of the day, and someone standing in a brightly lit room can still see their reflection in the window, because it is darker outside than in.

Nautical polar twilight

Roughly 72°34’ to 78°34’, which is exactly 6° to 12° inside the polar circles. No civil twilight at solar noon. At midday the eye can make out the general outlines of things on the ground but cannot do detailed work outdoors. Nowhere on mainland Europe reaches this; Longyearbyen does.

Astronomical polar twilight

Roughly 78°34’ to 84°34’, exactly 12° to 18° inside the polar circles. No nautical twilight at solar noon. The midday sky is dark enough for astronomical observation of point sources such as stars, unless skyglow, moonlight or auroras interfere, though faint diffuse targets like nebulae and galaxies may still need darker conditions. Ny-Ålesund on Svalbard gets this from about 12 to 30 December; the Eureka base in Nunavut from about 2 December to 8 January; Franz Josef Land from about 27 November to 15 January.

True polar night

Above roughly 84°34’, exactly 18° inside the polar circles and about five and a half degrees from the pole. No astronomical twilight at solar noon at all. Sixth-magnitude stars, the faintest the naked eye can pick up, stay visible through the whole 24 hours. At solar noon the sun sits between exactly 18° and about 23°26’ below the horizon. At the poles these conditions last about eleven weeks: roughly 11 May to 1 August at the South Pole, and 12 November to 28 January at the North Pole.

The only permanent settlement anywhere in that band is the Amundsen–Scott station at the South Pole, whose winter crew are completely cut off from mid-February to late October.

What lives through it

The biological consequence is the part that gets least attention. If light organises behaviour, months without a sunrise ought to switch that behaviour off, and in the Arctic Ocean it does not. Work by Kim Last at the Scottish Association for Marine Science and colleagues found that the daily vertical migration of small marine animals keeps running through the sunless winter, timed to the lunar cycle instead of the solar one. Moonlight in midwinter is faint by any normal measure, and it is still enough to be worth hiding from. The animals doing the hiding are the subject of our zooplankton explainer.

That is the useful way to hold the whole idea. It is not an absence of light. It is a long stretch where the light that remains comes from somewhere other than a sun above the horizon, and both instruments and animals turn out to be sensitive enough to use it.