Polar Circle

polar circle [′pō·lər ′sər·kəl]

(geodesy)

A parallel of latitude whose distance from the pole is equal to the obliquity of the ecliptic (approximately 23°27′).

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Polar Circle 

one of the two parallels of latitude 66°33′ (angle of inclination of the earth’s axis to the plane of the ecliptic) from the equator. The polar circle in the northern hemisphere is called the arctic circle, and the polar circle in the southern hemisphere, the antarctic circle. On the summer solstice (June 21 or 22) the sun does not set north of the arctic circle, and on the winter solstice (December 21 or 22) it does not rise. The number of days during which the sun does not go below the horizon or does not rise above it increases as one approaches the pole, at which point day and night each last six months. A similar phenomenon is observed in the southern hemisphere. Refraction of light somewhat complicates this phenomenon, increasing the length of the polar day at the expense of the polar night and increasing the number of days on which the sun does not set. The polar circles are considered the boundaries of the frigid zones.