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We like our summers in July and our snow in December. Leap Year ensures it remains that way, explains Duke physics professor in this short video. Plesser breaks down the math behind adding a day to the calendar every four years so that we remain in sync with the Earth’s rotation around the sun.

The annual gap between the calendar year and the Earth's rotation “accumulates at the rate of one day every four years, and after a hundred years, you’re 25 days or a month out of sync. That’s a problem,” Plesser said. “What we do is we add a day every four years to make the average year the same as the average time from summer to summer, that way our seasons don’t get out of whack with the calendar.”