Today, we stretch your year by a second. And we shorten your life by a minute. 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 doubt you felt the difference, but 1987 was a second longer than 1986. We added a second to 1987 because the earth had slowed down the year before. Forty years ago we defined the second by dividing Earth's revolution around the Sun into equal pieces. With more than 30 million seconds in a year already, one second more or less 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 average second worked pretty well for several decades, but the technology of clock-making was getting more and more accurate. The straw that broke the camel's back was the invention of the cesium clock in the 1950s.

Cesium clocks are timed by the 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. But the length of actual years was wandering away from that specification. Now we could measure the length of a year within a 300th of a second, while the variation of real years was hundreds of times greater.

So we started adding seconds to our years. We made the first adjustment in 1972 -- after atomic clocks had been in use for 17 years. We began by adding a full ten seconds to 1972. We've patched in 17 seconds since then.

Of course, calendar adjustments aren't new. The older calendars rounded the year off to 365¼ days. But that was still eleven minutes and fifteen seconds too short. For centuries we've made adjustments to pick up those minutes and seconds, without knowing that Earth was slowing down at the same time.

The problem of calendar adjustment faced 8th-century Christians when they reset the calendar to the birth of Christ. They did pretty well, considering the science of their day. Our best estimates presently put the birth of Christ in 4 BC. They missed it by only a few years.

So you -- the average Public Radio listener -- should know you're a half-minute older than you thought you were. But don't worry, you haven't really lost anything, because you actually lived that extra half minute. I only hope that, if you didn't use it profitably, you were at least able to enjoy it -- as it flew by.

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

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

This episode has been substantially revised as Episode 2038.