Polar Night


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polar night

[′pō·lər ′nīt]
(astronomy)
The period of winter darkness in the polar regions, both northern and southern.

Polar Night

 

a night lasting more than 24 hours, occurring in polar regions north of the arctic circle and south of the antarctic circle. At points in the northern hemisphere with a geographic latitude ϕ, the sun will not rise above the horizon at certain times of the year. This occurs whenever the sun, in its apparent annual motion along the ecliptic, enters an area of the sky that is not visible at that given latitude. At such times, the

Table 1
Geographic latitude (degrees)Polar night (24-hour periods)Polar day (24-hour periods)
6700
682340
705570
727286
7486100
7699114
78111126
80123137
82134148
84144158
90176189

sun is located south of the parallel of declination δ = —(90° — ϕ). At points along the arctic circle the sun will not rise once each year; this occurs on the winter solstice (December 21 or 22), when the sun is at maximum south declination δ Θ = –23°27’. As ϕ increases, the arc of the ecliptic in the region of the sky that is not visible also increases. The polar night lengthens, and at the pole itself, it is six months long, lasting from the autumnal equinox to the vernal equinox. At points along the antarctic circle the sun will not rise on the summer solstice (June 21 or 22), and at the south pole the polar night lasts from the vernal equinox to the autumnal equinox.

The refraction of light complicates this phenomenon, and as a consequence the polar night becomes somewhat shorter. Table 1 gives lengths of the polar night and polar day at various geographic latitudes of the northern hemisphere (with refraction taken into account).

N. P. ERPYLEV

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