Why day and night are not of equal duration?

The day and night are not of equal length exactly at the time of the March and September equinoxes (also known as spring and autumnal equinoxes respectively). The dates on which day and night are each 12 hours occur only for a few days before and after the equinoxes. The specific dates for this occurrence are different for different latitudes.

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http://www.wonderwhizkids.com/index.php/seasons