Polar darkness lasts longer the closer one gets to the North Pole. At the pole itself, the sun never rises above the horizon for six months. Those areas which experience 24-hour darkness in winter enjoy the Midnight Sun – round-the-clock daylight – for a similar period in the summer. These times are a product of the Earth’s axis of rotation, which is tilted 23.4° from the vertical on its path around the Sun. That affects the amount of light reaching the surface.
But the Polar darkness is not complete throughout the day. Sunlight is spread and scattered in the upper air, which in turn illuminates the areas below. Had the Earth lacked an atmosphere, the dividing line for the Polar darkness would have been clearly marked at the Arctic Circle. But sunlight passing through the air is split and bent downwards, creating various forms of twilight in the Arctic.
These fall into three categories.
From the time when the centre of the sun is less than 6° below the horizon until it rises in the morning, and vice versa in the evening. This is the limit for light being adequate, under good weather conditions, to see objects on the ground clearly. Some outdoor activities can be pursued without artificial light.
From the time when the centre of the sun is between 6° to 12° below the horizon. Under good atmospheric conditions, the outline of individual objects can be detected on the ground. Artificial lighting is normally needed to continue outdoor activities.
When the centre of the sun is between 12° and 18° below the horizon, and contributes no natural light. Most observers would regard the heavens as quite dark. Once the centre of the sun is more than 18° beneath the horizon, night has fallen.
In some places, the sun will fail to rise higher than 6° beneath the horizon for a number of days during the Polar darkness. No twilight will be experienced, and Polar night will prevail.
At Longyearbyen in Svalbard, for example, this period of unrelieved dark lasts from 11 November to 30 January.