A polar night occurs over a time period of more than twenty-four hours when there is only darkness and no natural sunlight, the opposite of the midnight sun phenomena. This natural phenomenon takes place during the winter season in areas located both south of the Antarctic Circle as well as north of the Arctic Circle. There are several types of polar nights which are classified according to the nature of the twilight. These types include civil, nautical, and astronomical polar nights.
This natural occurrence only takes place in regions located within the border of the northern and southern polar circles. At this time, the sun is situated less than six degrees below the horizon and no actual daylight is experienced during the midday hours with the exception of something known as civil twilight. During the morning this particular type of twilight occurs when the sun is positioned at six degrees below the horizon and lasts until sunrise. Evening civil twilight takes place during the period between the sun setting and then moving to six degrees beneath the horizon.
Civil Polar Nights
Civil polar nights are characterized by having only a faint amount of light visible during midday. This phenomenon occurs when the sun is situated between zero and six degrees below the horizon and can only be experienced in locations six degrees inside the polar circle and in latitudes above 72° 34'. During civil polar nights, there is no civil twilight, only nautical twilight. In the morning hours, this type of twilight takes place during what’s known as nautical dawn when the sun moves from twelve to six degrees below the horizon. In the evening nautical twilight occurs at nautical dusk when the sun’s position sinks from six degrees below the horizon to twelve degrees. In the Norwegian archipelago of Svalbard, civil polar nights usually last from about November 11th until January 30th.
Nautical Polar Nights
Nautical polar nights are characterized by the absence of natural sunlight with the exception of the time in the middle of the day when, due to refraction, more light is emitted. At the time of a nautical polar night, the sun is situated at less than twelve degrees under the horizon. This type of polar night can only take place in regions located twelve degrees within the polar circle or eleven and a half degrees away from the pole (in latitudes above 78° 34'). In the research base located at Eureka, Nunavut, nautical polar nights are typically observed from approximately December 2nd until January 8th.
Astronomical Polar Nights
This type of polar night can be identified by the complete absence of astronomical twilight. During this type of polar night, continual darkness is experienced throughout the day. Regions which experience astronomical polar nights are located in areas eighteen degrees away from the polar circle and five and a half degrees away from the pole with a latitude above 84° 34’. During astronomical polar nights, the sun is situated between eighteen and 23.5 degrees below the horizon. Earth’s North and South Poles experience this type of polar night for a time period of approximately eleven weeks. In the north astronomical polar nights can be experienced from about November 14th until January 29th. In the south, astronomical polar nights usually occur from mid-May until late July.