then sinks back toward the horizon before sinking below it at the autumnal equinox.
A civil twilight period of about two weeks occurs before sunrise and after sunset, a nautical twilight period of about five weeks occurs before sunrise and after sunset and an astronomical twilight period of about seven weeks occurs before sunrise and after sunset.
These effects are caused by a combination of the Earth’s axial tilt and its revolution around the sun. The direction of the Earth’s axial tilt, as well as its angle relative to the plane of the Earth’s orbit around the sun, remains very nearly constant over the course of a year (both change very slowly over long time periods). At northern midsummer the North Pole is facing towards the sun to its maximum extent. As the year progresses and the Earth moves around the sun, the North Pole gradually turns away from the sun until at midwinter it is facing away from the Sun to its maximum extent. A similar sequence is observed at the South Pole, with a six-month time difference.
Time
In most places on Earth, local time is determined by longitude, such that the time of day is more-or-less synchronised to the position of the sun in the sky (for example, at midday the sun is roughly at its highest). This line of reasoning fails at the North Pole, where the sun rises and sets only once per year, and all lines of longitude, and hence all time zones, converge. There is no permanent human presence at the North Pole, and no particular time zone has been assigned. Polar expeditions may use any time zone that is convenient, such as GMT, or the time zone of the country they departed from.
Climate
Arctic shrinkages of 2007 compared to 2005 and also compared to the 1979-2000 average.