Linda French
What’s in a Year?  Leap Year Proof It’s All Relative

February 12, 2008

BLOOMINGTON, Ill. – Time does not really exist.

Certainly the sun rises and sets, the Earth spins, but time as we know it – chopped into months, hours and milliseconds – is a fabrication of mankind. Never are we more aware of this fact than on that rare day, February 29 of leap year.

“Leap year is one of those ways we keep the clock that we live by in sync,” said Linda French, associate professor of physics at Illinois Wesleyan University. “If we didn’t have leap years, then over decades, we would find the seasons start to drift, and instead of the first day of spring coming in at the end of March, it would come at the beginning of March.”

In other words, leap year works out the kinks in our calendar. The Earth takes about 365 days to go once around the sun – 365.24222 days to be exact.  The idea of adding one more day every four years is to take care of all of those numbers past the decimal. “If you add in one day in the calendar, it catches us up so that we still have the first day of spring around March 21,” said French.

Noticing this slipping of seasons, Pope Gregory XIII decided in 1582 to revamp the old Julian calendar. His Gregorian calendar we follow today set down leap year as every four years. This, of course, included exceptions that sound a bit like the disclaimer portion of pharmaceutical ads:

Every year that is exactly divisible by four is a leap year, except for years that are exactly divisible by 100; the centurial years that are exactly divisible by 400 are still leap years. For example, the year 1900 is not a leap year; the year 2000 is a leap year.

Chalk it up to a stubborn universe that refuses to go on Greenwich time, but in fact none of our closest celestial neighbors tend to roll by our watch. “It’s a big inconvenience, really, that no astronomical events happen commensurate with each other,” said French, who teaches students how time would be different if we judged it by a star other than the sun. “The time it takes the Earth to go around the sun and the time it takes the Earth to rotate on its axis simply don’t divide into each other evenly.”

Even more frustrating is the knowledge that leap years will have to be revamped by future generations because the Earth’s rotation is slowing down.  Friction due to tides is causing the Earth’s rotation to slow by 0.002 seconds per century.  “Yes, the Earth is slowing, but it is happening very slowly.  It’s nothing we, or our children, or their children need to worry about,” said French.

So time rolls on, in leaps and bounds, whether on a stellar scale, a planetary scale, or the clock on the corner of your computer screen.  “Time may be elastic,” said French, “but I wouldn’t advise anyone to use the Earth’s changing rotation rate as an excuse for being late for work.”

Contact: Rachel Hatch (309) 556-3960