Science: Leap Second

Please Pass the Science - the Mysterious Leap Second
by Scott Berk

"Your attention please. At the recommendation of the International Time Bureau, commencing at 23 hours, 59 minutes, 60 seconds UTC, an extra second will be inserted into the NBS time scale. This adjustment is required to maintain internationally coordinated universal time, as broadcast from these stations, in close agreement with UT1, or astronomical time..."
These words appear at the beginning of the song "Time Zones" on Negativland's album "Escape From Noise," and are in fact culled from a short-wave radio broadcast originating from the Time & Frequency Division of the National Institute of Standards and Technology (formerly the National Bureau of Standards, formerly Yodolo(tm)) in Boulder, Colorado (2.000, 5.000, 10.000, 15.000 and 20.000 MHz - tune in!). Aside from being an excellent introduction to a song which more or less addresses the issues of power, borders, global struggle, and an uninformed American public, the existence of the so called "leap second" is a poignant reminder of humanity's mastery over nature, a result of technology so advanced that it has forced us to reconsider and redefine the very notion of time itself! But why is it necessary to add a second into our national time scale? Well, after a bit of WEB-SURFING on the INTERNET (AKA, the information STUPORhighway), I was able to JACK IN to the NIST, where I ACCESSED the Time and Frequency FAQ list. Ah, the INTERNET - just as technology brings the world closer together, we develop a system which adds a layer of electronic filters between people. Why actually travel to foreign lands when you can just DOWNLOAD an MPEG ANIMATION? But I digress. . .
In the beginning, it was simple. "... And there was evening, and there was morning, one day." [Genesis I:5] Then, astronomy happened. Suddenly, we realized that the phases of the moon changed regularly. Months were born. The cycle of seasons came and went regularly. Once the astronomers convinced the religious leaders of the world that the earth went around the sun and not vice-versa, it was determined that this happened about every 365 days. The modern-day calendar came into being, and people were feeling pretty good about themselves. A while later, those wacky astronomers dropped another bombshell: it actually takes 365.25 days for the earth to complete its revolution around the sun. "No problem!" the Powers That Be(tm) seemed to say. "We'll just add an extra day into the calendar once every four years, and that'll even everything out!" "Well, not quite!", the astronomers found, some years later. "We were a little off. It's more like 365.242 days to a year." Things were beginning to get a bit complicated. After some debate, it was settled that the century years (1700, 1800, etc.) would NOT contain leap days, even though they were divisible by four, UNLESS the particularly century year was divisible by 400. I leave it as an exercise to the reader to determine that this brought things back into sync. The problem with leap years was solved, and everyone was happy again for quite some time.
But there was still the nagging problem of keeping accurate time on a day to day basis. Days were divided into hours, minutes and seconds. At first, people only really cared about the hour. Sundials were a pretty good measure of that. Mechanical clocks filled in the gaps between the hours and gave people a whole new definition of "on time." Their accuracy was pretty darn good, but variations in the environment would cause inevitable fluctuations. Even electrical clocks were subject to mechanical stress. The digital watch made some inroads here, but slight power differentials, irregular quartz crystal shapes and the like still caused inaccuracies. What was a modern society to do? We had evolved into a culture dependent upon accurate time-keeping, and dammit, we NEEDED a clock that didn't lose a second in 10,000 years. At least, that was the thinking which must have gone into the design of the first atomic clock. According to the NIST, an atomic clock is "a clock that keeps time using natural characteristic frequencies of atoms, such as cesium, hydrogen or rubidium. Atomic clocks are extremely stable because the atom's characteristic frequencies are not affected by factors like temperature, pressure or humidity." This is their story:
In 1967, a bunch of scientists from around the world decided to REDEFINE THE SECOND. First, they built a device which looked at the radiation corresponding to the transition between the two hyperfine levels of the ground state of the cesium atom (I assume they chose cesium because its particular radiation was easy to observe. Maybe they just liked saying "cesium" - It's not important.) Then, they used this device, the "cesium clock," to count the waves that make up that particular radiation. When it counted up to 9,192,631,770, then, BY INTERNATIONAL DECREE, a second had passed. The atomic clock is extremely accurate, and, with refinements still being developed today, is accurate to within 0.0000000000001 seconds (100 femtoseconds for all you prefix buffs). Soon, the femtosecond barrier will be broken. Since molecular vibrations occur within this time frame, we will then have a way of precisely timing just about everything in our universe that's bigger than an atom. To summarize, when you ask the NIST what time it is, you're going to get a REALLY REALLY REALLY good answer.
So what does this have to do with the leap second? Well, my fellow science buffs, enter THE PUNCH LINE! It seems that after a while on ATOMIC TIME (UTC, or Coordinated Universal Time), things started to get a bit ... off. The astronomers at the International Observatory in Greenwich, England (where the now obsolete "Greenwich Mean Time" came from) noticed that the Astronomical Time Scale, based on the rotation of the earth and the standard for time keeping from creation up until 1967, was running a bit slower than the new UTC. What was wrong? Well, the rotation of the earth was not as accurate as the atomic clock! Without a fudge factor, the two time scales would fall out of agreement. Thus, the leap second became a necessity. On June 30, 1972 (where were YOU?), one second was added between midnight and 12:00:01 to allow the earth to catch up to our perfect atomic clock. Since then, 17 additional leap seconds have been added, and while it is theoretically possible for a negative leap second to occur (if the earth were to get ahead of the atomic clock), according to the NIST, it is "unlikely in the foreseeable future, based upon what we know about the earth's rotation."
I can't help but feel a bit sorry for the old earth. For five billion years, it did a great job as our ultimate timekeeper. Now, it has been relegated to a second-rate position by a few cesium atoms. So the next time you glance down at your watch, or happen to be tuning into to TIME RADIO on your short-wave dial, think about the International Standard Atomic Clock, sitting silently in Boulder, Colorado, watching cesium atom emissions and counting to 9,192,631,770 over and over and over. And remember, it wouldn't be possible without ... SCIENCE!

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