The winter solstice occurs exactly when the axial tilt of Earth is farthest away from the sun, depending on the polar of reference. The axial tilt of Earth is the cause of seasons like summer and winter. A hemisphere's winter solstice occurs on the shortest day and longest night of the year, when the sun's daily maximum position in the sky is the lowest. Since the winter solstice lasts only a moment in time, another term often used for the day on which it occurs is the longest night.
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