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This Leap Keeps Calendar Up to Date With the Sun

Time: The extra 24 hours every four years makes up for the fact that the Earth takes about 365.25 days to make one revolution. We can thank Pope Gregory XIII for the quadrennial quirk.


In "Alice's Adventures in Wonderland," Lewis Carroll's story of twisted time, space and logic, the adventures begin with the hapless White Rabbit peering at a watch pulled from his waistcoat pocket and mumbling to himself, "Oh dear! Oh dear! I shall be late!"

His musings could be the rallying cry for the urban world of the late 20th century, where everyone, it seems, could use more time.

Well, mark your calendar, because today is Leap Day, that extra 24 hours added every four years to bring it more in line with the Earth's yearly traverse around the sun.

And though it won't literally add any extra time to our lives, Leap Day does help keep our timekeeping system more on track.

Humans have been obsessed with the passage of time since, well, the beginning of time. And we have invented a number of different ways to mark the sun's apparent movement through the heavens.


But the Earth refuses to cooperate with our need for nice, neat, evenly divided days and instead takes its time to get around the sun-- 365 days, 5 hours, 48 minutes and 46 seconds--or about 365.25 days.

If there were no Leap Days, the calendars would get out of sync rather swiftly, by at least one day every four years, the disparity quickly adding up over time.

Calendars over time accumulate error, and without adjustments, spring might come in December and skiers could be telemarking in July.

In about 46 BC, Julius Caesar ordered a major calendar revision to bring order to a political society.

The Julian calendar added an extra day every four years. His one-time addition of an extra two months and 23 days to the year made up for a shortage that had accumulated in the Egyptian calendar in use up to that time.

In 1582, Pope Gregory XIII ordered another major calendar revision. He hoped to introduce a consistent date for the vernal equinox with which to mark the date for the Easter holiday, according to astronomer John Mosley of the Griffith Observatory.


Relying on increasingly precise measurements of the Earth's orbit and rotation, an astronomer commissioned by the pope devised the Gregorian calendar, which is still used today.

It adds a Leap Day every four years and every centenary year that is divisible by 400. Thus the years 1800 and 1900 were not leap years, but 2000 will be.

"The hallmark of a civilized society is an organized calendar," Mosely said. "Today we hardly think about it, because it's all worked out in advance."

But for those of us who can always use more time, those extra two months that Caesar decreed could come in handy.

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