Mainly due to the curved shape of the earth. The places closest to the equator have to longest light, the farther away the shorter. This is why Northern Norway/Canada etc have some very dark days around December 21st, and virtually no night around 21st of July.
Due to the axis en the rotation, every day the time of rising and setting of the sun adjusts a little. Growing longer between December and July, and shorter between July and December.
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