Imagine picking up leaves on Christmas, or shoveling (铲起) snow on Memorial Day. What about going for a swim on Thanksgiving? Without Leap Day, which takes place every four years, that could happen. "If we didn't have leap years, our calendar would be totally mixed," says Geoff Chester of the U.S. Naval Observatory in Washington, D.C, the nation's official timekeepers.
Our calendar is normally 365 days long. It was created to match the cycles of the seasons. But Roman Dictator Julius Caesar (See-zer) noticed a problem: The Earth doesn’t circle the sun in exactly 365 days. It actually takes 365 and one-quarter days. He figured out that the extra part of a day would cause the calendar to grow apart from the seasons over time. Over 100 years, the seasons would shift about 24 days. Spring would start on April 13 instead of March 20.
Caesar used math to figure out a way to stop the calendar from shifting. He decided to add an extra day to the month of February every four years. His idea helped keep the seasons and calendar matched up. Even so, it still wasn’t perfect—his calendar was adding too many days.
In 1582, Pope Gregory XIII worked on Caesar’s idea. His calendar, called the Gregorian calendar, shows that every year that is evenly divided by 400 is a leap year. Turn-of-the-century years, or years ending in “00,” would not be leap years unless they could be divided evenly by 400. These complicated equations(方程) help keep the calendar in balance with the orbit of the Earth. Today, we still use the Gregorian calendar. In about 3,000 years, the calendar will be only one day out of step with the seasons. It’s still not perfect, but mathematicians decided it was as close as we could get.
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