Why do the days get longer in spring and summer and shorter in fall and winter?

I know it has something to do with the sun's amount of time facing earth but I don't know what it actually is and I am 27 and should know. It is not because of the temperature on earth at given times of year is it? What causes it? I assumed the earlier it gets dark the sun is farther from earth and thus... show more I know it has something to do with the sun's amount of time facing earth but I don't know what it actually is and I am 27 and should know.
It is not because of the temperature on earth at given times of year is it? What causes it? I assumed the earlier it gets dark the sun is farther from earth and thus it is colder and the closer the hotter the earth gets and the days are logner.
Please explain
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