A leap year is a calendar year that has 366 days instead
of the usual 365 days. This extra day is added to February which typically
only has 28 or 29 days. The insertion of the extra day compensates for the
fact that Earth's orbit around the sun takes 365.24 days, causing seasonal
drift to slowly creep over the course of about one day every four years.
Leap years are necessary to maintain accurate astronomical readings and
calculations which are used in modern society.
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