Engines of Our Ingenuity

No. 2038:
LEAP SECOND

by John H. Lienhard

Today, we add a second to our lives. The University of Houston's College of Engineering presents this series about the machines that make our civilization run, and the people whose ingenuity created them.

I write this as 2005 draws to a close, and I look forward to New Year's Eve. We'll all observe it by adding a free second to our clocks. The last tick of 2005 will last two seconds.

Actually, since 1972 we've had to do that 22 times, because Earth is almost imperceptibly slowing down. The tides steadily dissipate just a whisper of our rotational energy. Fifty-five years ago we defined the second by dividing Earth's orbit around the Sun into equal pieces. With over 30 million seconds in a year already, one more second goes pretty well unnoticed.

Back in 1895, the Director of the U.S. Naval Observatory reviewed 200 years of records. He found that the length of years had not been uniform. We needed a standard second and a standard year. He averaged the variations and he defined standard time intervals.

His second was an average day divided by 86,400. That worked pretty well for decades, but clock accuracies kept improving. The straw that broke the camel's back was the invention of the cesium clock in the 1950s.

Cesium clocks are timed by natural oscillation of a cesium atom's energy states. On that basis, the second was redefined as the time the cesium atom took to make 9-point-however-many billion oscillations. Suddenly we'd specified the year to 10-decimal-place accuracy, and we could watch the length of solar years increasing. Now we could measure the length of a year within a 300th of a second, while its actual variation was hundreds of times larger.

So we began adding seconds to our years. We made the first adjustment in 1972, after we'd had atomic clocks for 17 years. We added a full ten seconds then, and we've patched in one more every few years ever since.

Calendar adjustments aren't new. The older calendars rounded the year off to 365 days. But that was still eleven minutes, fifteen seconds too short. For centuries we've made adjustments to pick up the slack, without realizing that Earth was slowing down at the same time.

Eighth-century Christians faced the problem of calendar adjustment when they reset their calendars to the birth of Christ. They did pretty well, considering the science of their day. Our best estimates put the birth of Christ in around 4 BC. They didn't miss it by much.

So good listener, unless you are very young, you're probably a half a minute older than you thought you were. But don't worry, we haven't lost anything since we got to live that extra time.

And, if we failed to use that second profitably in the past, I hope we at least enjoyed it as it flew by. Now another second is upon us; but this time, we know ahead of time. Perhaps it is a good sign that one second will be just the time that you and I need to say Auld lange syne to one another.

I'm John Lienhard, at the University of Houston, where we're interested in the way inventive minds work.

(Theme music)


Marshall, E., A Matter of Time. Science, Dec. 18, 1987.

The leap second is fully explained on this Naval Observatory page
http://tycho.usno.navy.mil/leapsec.html

And here is a discussion of more ramifications of the added second:
http://www.ucolick.org/~sla/leapsecs/onlinebib.html

This is a substantially revised version of Episode 295.

12:61 PM
12:61 PM, Dec. 31, 2005 (clipart)


The Engines of Our Ingenuity is Copyright © 1988-2005 by John H. Lienhard.