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a day 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 does not drop below the horizon at certain times of the year. This occurs whenever the sun, in its apparent annual motion along the ecliptic, is located north of the parallel of declination δ = 90° — ϕ. At points on the arctic and antarctic circles, the sun will not set once each year. For the arctic circle this occurs on the summer solstice (June 21 or 22), when the sun is at maximum north declination δΘ= 23°27′; for the antarctic circle this occurs on the winter solstice (December 21 or 22), when the sun is at maximum south declination δΘ= – 23°27′. The length of the polar day increases with proximity to the poles, and at the poles themselves it is six months long. This phenomenon is complicated by refraction, which causes the length of the polar day to increase.