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Home* News and Stories → Polar night, polar day

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