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Polar night, polar day

Thursday (February 20) the return of the sun at the end of the polar night is celebrated in Barentsburg. Longyearbyen has to wait until March 08, because there are more mountains towards the southern horizon.

A good opportunity for some general thoughs about polar night and polar day. The basic celestial mechanics are generally known: the tilt of the axis of the Earth, which is the reason for polar night and day. Also the so-called Novaya Zemlya effect: the refraction of sunlight in the cold atmosphere, which makes the polar day seemingly (!) longer than it should be, astronomically: the sun is visible even though it is actually just under the horizon, because the sun rays are refracted in the cold atmosphere. This makes a difference of several days and depends on the weather situation. It is called Novaya Zemlya effect because of an early observation of this phenomenon on Novaya Zemlya by Willem Barentsz’ expedition, 1596-97, the same voyage during which Spitsbergen was discovered.

So far so good. But still, the polar night should have the same duration in Arctic and Antarctic on any given latitude, one would expect. But this is not the case. This is a quotation from the South Polar Times, the famous expedition gazette published for the first time during Scott’s first expedition on the Discovery in April 1902. The editor, by the way, was a certain Ernest Shackleton. The following quotation is on page 18: “The South Polar Winter is nearly eight days longer than the North Polar Winter. This is because in the former case, the Earth is farther away from the Sun (aphelion) ,and therefore moves more slowly in its orbit. In the Northern Winter the Earth is closer to the Sun (perihelion) ,and therefore moves more rapidly.” (punctuation as in the South Polar Times, which was typed manually under rather enduring conditions).

The reason is the second of Kepler’s laws of planetary motion: “A line joining a planet and the Sun sweeps out equal areas during equal intervals of time.” In other words: The Earth is moving faster when it is closer to the sun. Simple, isn’t it?

This again means that the Earth spends less time in a position that creates the polar night in the Arctic. On the contrary, the Earth is moving more slowly when further away from the sun, and accordingly spends more time in the position that results in the polar night in the Antarctic.

How big is the difference now? The duration of the polar night is

on 80 degrees north: 122 days (21 Oct – 20 Feb)
on 80 degrees south: 128 days (18 Apr – 24 Aug)

meaning that the difference amounts to six days! The calculations can quite easily be made on a website provided by the US Naval Observatory.

Hut Point, where the South Polar Times was written in 1902 for the first time, is at 77°47’S, 133 nautical miles north of the 80th parallel. The difference of eight days claimed there is accordingly a little bit too much. The value is seven days at the poles.

This is why the polar night is longer in the Antarctic than it is on a corresponding latitude in the Arctic. The answer is Kepler’s second law of planetary motion.

Thanks to Andreas Kaufer for specialist advise and reference to the US Naval Observatory.

The last light of the sun in Barentsburg at the beginning of the polar night, October 22nd.

Polar night, Barentsburg

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last modification: 2014-07-01 · copyright: Rolf Stange
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