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On Friday, it's time to synch our calendar to the universe

By Times staff
Published February 25, 2008


This is a leap year, which means it has 366 days instead of 365. That extra day - Feb. 29 - is called an intercalary (or inserted) day or a leap day, and it comes this Friday.


Why is a leap year necessary?

Leap years are added to the calendar to keep it working properly. The 365 days of our calendar are meant to match up with the solar year, or the time it takes the Earth to orbit the sun. But the trip actually takes about 365 1/4 days - 365 days, 5 hours, 48 minutes and 46 seconds. So the calendar year is a bit shorter than the solar year. After four years the calendar falls behind the solar year by about a day. That difference would increase to 25 days after a century. So every four years a leap day is added to allow the calendar to catch up to the solar year.


Who figured this out?

The Egyptians, but the idea was not formally inaugurated until Julius Caesar reformed the Roman calendar in 46 B.C. The Julian calendar also designated Feb. 29 as the leap day.


Last time I looked, we didn't use the Roman calendar. What happened?

Remember, the solar year is about 365 1/4 days long. It's actually 11 minutes and 14 seconds less than 365 1/4 days. That means even with a leap day, the calendar still slightly overshoots the solar year - by 11 minutes and 14 seconds per year. After 128 years, the Julian calendar gains an entire extra day. That doesn't sound like much, but by the 1500s, the spring equinox was falling around March 11 instead of March 21.

In 1582, Pope Gregory XII adjusted the calendar by moving the date ahead by 10 days and introducing an exception to the four-year rule for leap years. Now, leap years would be omitted three times every 400 years. The calendar would be shortened periodically, getting rid of that annual excess of 11 minutes and 14 seconds. So, a century year is not a leap year unless it is evenly divisible by 400. Thus 1700, 1800 and 1900 are not leap years, but 1600, 2000 and 2400 are.

Although the calendar year and the solar year are still off by 26 seconds, it now takes more than 3,300 years for the difference to amount to one day.


Did everyone agree on the solution?

The Roman Catholic nations of Europe adopted the Gregorian calendar almost immediately. Many Protestant countries ignored the decree. This led to some interesting adjustments as the centuries passed. The British Calendar Act of 1751 declared the day after Wednesday, Sept. 2, 1752, to be Thursday, Sept. 14. That Wednesday evening, millions of British subjects at home and in the colonies went to sleep and woke up 12 days later. This "time travel" was necessary so Great Britain (and its American colonies) could adopt the Gregorian calendar. Germany and the Netherlands came on board in 1698, Turkey in the early 1900s and Russia only after the 1918 revolution. Greece waited until 1923. Today, many Orthodox Christian churches still follow the Julian calendar.


What's the chance of being born on Feb. 29?

About 1 in 1,500.


How many people were born on leap day?

About 4-million people, including 187,000 in the United States, can celebrate their actual birthdays only every four years. If you are among them, happy birthday on Friday.

Sources: Leap Year 101 by Borgna Brunner, Leap Year Explained by Ann Marie Imbornoni, The Curious History of the Gregorian Calendar by Ben Snowden; World Book

[Last modified February 24, 2008, 22:01:24]

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