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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