Midnight sun
The midnight sun is a phenomenon, occurring in latitudes above
the polar circle, where the sun is visible during all 24 hours of the day. It is mostly observed in the Arctic,
because there are no permanent human settlements south
of the Antarctic Circle. Thus, the countries and
territories whose population experiences it are limited to
Alaska, northern Canada, Greenland, Iceland, northern
Norway, Sweden and Finland, and extremities of Russia such as
Novaya Zemlya.
Since the earth's axis is tilted with respect to the ecliptic
by approximately 23 degrees 37 minutes (commonly rounded to 23 degrees
and a half), the sun does not set at high latitudes in (local)
summer. The duration of the midnight sun increases from
one day during the summer solstice at the polar circle to
six months at the poles. At extreme latitudes, it is usually
referred to as polar day.
A corresponding phenomenon in winter is that the sun does
not rise for some time, though lingering dusk may occur
if it is 12 degrees or less below the horizon.
Due to refraction, the midnight sun may be experienced
at latitudes slightly below the polar circle, though
not exceeding a degree (depending on local conditions).
Locations at latitudes up to 60 degrees experience midnight
twilight instead. The sun is just below the horizon, so
that daytime activities, such as reading, are still possible
without resorting to artificial light. These are the "white nights" experienced, for example, in
St. Petersburg, Russia from about 11 June to 11 July.
The period of midnight sun is reportedly very taxing
on the human body. Peaks in the suicide figures and increased
severity of mental disorders have been demonstrated to occur
in summer months. The period in the local winter when there is almost no sunlight also has drastic consequences on people and can trigger depression in many people. People who suffer from seasonal affective disorder are particularly susceptible.
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