midnight sun

the sun visible at midnight in mid-summer in arctic and antarctic regions.

[1855-60]

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 the Sun, as seen in the Arctic or Antarctic, where the tilt of the Earth's axis, relative to the plane of its orbit, produces at least one 24-hour period of daylight, and one of night, in every year. At the poles, both day and night are theoretically six months long, though the actual periods of light and dark are modified by the twilight periods. The effect of the tilt of the axis is seen in lower latitudes as a lengthening of daylight hours in the summer and their shortening in the winter.

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