Keeping Time: Is there Room for Uncertainty? (Herbst)

By William Herbst
John Monroe Van Vleck Professor of Astronomy, Wesleyan University

Oddly enough, just as the show Passing Time is being exhibited at Wesleyan, the world organization in charge of keeping time  — the International Earth Rotation and Reference Systems Service or IERS — is considering a fundamental change in how we do it. The question before this little known agency is whether, in a society increasingly dominated by complex machinery, the time has come to adjust our conventions to the exacting needs of the computer. The alternative is to stick with what we have used since the dawn of civilization, a method designed for the convenience of humans that unfortunately includes an element machines don’t like — uncertainty.

Since Day One, which was Jan. 1, 4713 BC in the widely used (by astronomers!) Julian calendar, humans have kept time by counting days. The sun comes up, the sun goes down, a day goes by …. count it. Passing Time opens on Julian Date 2455953 (Jan. 27, 2012) so the sun has come up and gone down more than two million times between Day One and the show opening. Time keeping has always been nothing more, really, than counting days — and I must say we astronomers are good at it. The rising and setting of the Sun is of course driven by the spin of the Earth on its axis, which is why the IERS is our timekeeper.

In the 1950’s, as we entered the Atomic Age, we found out that the Earth is not a perfect timekeeper. It does not spin on its axis in a completely steady way due to the action of tides, earthquakes, solar wind and even weather. Oscillations of atoms, the metronomes of atomic clocks, are at least a million times steadier. The atomic second was created as the most reliable time unit on Earth and set equal to 1/86,400 of the length of an average day in 1960.¹

Half a century later we have found that, indeed, old Earth is falling behind the beat set by the atoms. The Earth has gradually been slowing its spin due mostly to tidal friction with the Moon, a process that has been going on for billions of years. The result is that this year, 2012, will have more atomic seconds in it than the year 1960 had. On average, we have to add about one second every two years to keep the slowing Earth in pace with the steady drumbeat of the atomic clocks. The IERS keeps track of Earth’s rotation and informs the world when and if a second will be needed. The seconds usually get inserted just before midnight on New Year’s Eve and are called leap seconds. The last one was in 2008. The IERS makes the call about six months before the second is added, i.e. around June 30.

The problem for some is that we cannot predict more than six months in advance if a leap second will be needed, because the Earth’s rotation is unpredictable. (Most) human beings have no real problem with skipping or adding a beat occasionally — we hardly notice. Computers, on the other hand, don’t like it (remember the Y2K scares?). The IERS is being lobbied by groups, including air traffic controllers, who see danger in the possibility that computers will get out of synch if we keep changing our time on such short notice. Out-of-synch computers could, of course, go dangerously haywire. The lobbyists argue that we should abandon the Earth and Sun as our clock and fully embrace the regularity and certainty of the atom.

Leap seconds keep people happy because they guarantee in the long run that the Sun will be overhead at noon and it will be dark at midnight. They make the wonderfully human time interval of the day dominant over the atomic second. Computers don’t care about day and night and dislike the leap second because it is unpredictable. The computer lobby argues that we only need the precision of the atomic clocks and the certainty of their time keeping. They believe the IERS should eliminate the leap second.

As you pass time with Passing Time I hope you may consider how closely our sense of time is connected to a motion we cannot feel and doubted for millennia, the spin of our planet. On Dec. 31, 2012, keep an eye out for the leap second. If we get one, chalk up a small victory for humans in the battle with machines for the future of our planet. As a thoroughly human astronomer I think a little uncertainty in life is a good thing.