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Leap Year Babies Celebrate Birthdays Today

The science behind keeping time in synch

New York, Feb 29, 2008

Tillie Iverson of Chippewa Falls, Wisconsin, is observing her 24th birthday today - at the age of 96. Born in 1912, Iverson’s February 29th birth date only comes around once every four years.

Ms. Iverson has marked her birthday on either February 28 or on March 1 for the better part of a century due to a slight variance between the length of a solar year (365.24219 days or 525,948.7536 minutes) and the 525,600 minutes (365 days) that are measured on the Gregorian Calendar, the internationally accepted civil calendar.

“Leap years” are years with 366 days while “common years” have the usual 365 days. A solar year is roughly 348 minutes – 5.8 hours – longer than the year marked on a Gregorian calendar.

To keep the calendars year synchronized with the astronomical and seasonal calendars, an extra day is added to the calendar at the end of every fourth February. Years that are evenly divisible by four (2008, for example) have 366 days.

But this adjustment causes the solar year to be about 11 minutes longer than the calendar year; after 128 years, those 11-minute increments add up to one whole day. For this reason, only one out of every four century years is considered as a leap year. Century years are only considered as leap years if they are evenly divisible by 400. Therefore, 1700, 1800, 1900 were not leap years, and 2100 will not be a leap year. But 1600 and 2000 were leap years, because those year numbers are evenly divisible by 400.

Geoff Chester, a 20-year veteran of the Smithsonian Institution's Albert Einstein Planetarium in Washington, D.C., and current public affairs officer for the U.S. Naval Observatory, points to an excerpt from the Explanatory Supplement to the Astronomical Almanac as a comprehensive explanation of calendars and time measurement.

Chester also explains the division of time management responsibilities between the U.S. Commerce Department’s National Institute of Standards and Technology and the Defense Department’s U.S. Naval Observatory (USNO).

“It’s like comparing a stopwatch and a clock,” said Chester. “NIST defines the unit of time while the USNO determines precise time and maintains the master clock for the United States.”

Ninety atomic clocks, including 19 hydrogen masers and 39 cesiums, are synchronized once every 100 seconds to determine USNO time data for the United States.

“We operate more atomic clocks than any other organization in the world,” confirmed Chester.

Of all the world’s timing centers, the USNO also submits the highest volume of data – 40% of the total – to the International Bureau of Weights and Measures. This clock measurement data is then analyzed to compute Coordinated Universal Time (UTC) and plan for the addition or subtraction of the leap seconds that need to be introduced periodically to keep time scales in synch with the Earth’s rotation. [see related article]

The efforts of the experts at NIST and USNO are especially important to Tillie Iverson, who is counting down the seconds to tonight’s birthday party. And a Happy Birthday to Tillie from all of us at ANSI . . . we hope you have a great "time" at your party!

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