Letters Blogatory Opposes Abolition of the Leap Second

Readers, I am going to subject you today to an editorial on a topic that periodically gets me worked up even though it has nothing to do with private international law: the proposed abolition of the leap second. I’m sorry to say that the United States is the main proponent of the change. Here is the story.

What is a Leap Second?

Today, the world’s time standard is coordinated universal time, or UTC. Why UTC instead of CUT? Well, in French, it’s temps universel coordonné, which could be abbreviated TUC. UTC, which stands for neither the French name for the standard nor the English name, was apparently a compromise between the Francophone world and the Anglophone world.

The Royal Observatory, Greenwich
The Royal Observatory, Greenwich. Credit: Shizhao
The old way of keeping time, known as Universal Time, or UT (the English won that round!), was based on astronomical observations of the sun. We observed the length of the mean solar day, and then divided that day into hours, minutes, and seconds. So the second was in a sense defined to be 1/86,400 of the solar day and depended on the period of the earth’s rotation about its axis. To be sure, the solar day is not the only natural way to measure time: we could also measure time by the length of the year—by the period of the Earth’s revolution around the Sun rather than its rotation about its own axis. Astronomers and other scientists made use of this so-called ephemeris time because they knew that the length of the solar day was not perfectly uniform: the earth’s rotation is slowing down over time. And they had a system of converting UT to ephemeris time, or in other words, a system of tying the natural length of the day to the natural length of the year.

US Naval Observatory atomic clock
The master atomic clock ensemble at the US Naval Observatory, Washington, D.C. Credit: Coenen.
But with the development of highly regular atomic clocks in the 1950s, we began to think of the second not as a subdivision of a fundamental unit of time like a day or a year, but as the fundamental unit of time in its own right, and to define the second with reference to the number of “ticks” of the atomic clock. And since the period of the Earth’s rotation was not perfectly uniform, it became necessary to insert occasional “leap seconds” into UTC to keep the official time in line with the natural period of the day. (The more familiar leap year serves a similar function, keeping the official time in line with the natural period of the year). There have been 25 leap seconds since 1972, and thus UTC is now 25 seconds behind the time scale that relies only on the ticking of the atomic clock without reference to days and years, called International Atomic Time, or TAI (the French must have won that round).

Why Do Some People Want to Get Rid of the Leap Second?

GPS Satellite In Orbit
Artistic Impression of a GPS Satellite In Orbit. Credit: NASA
Now that computers rely on highly precise time synchronization throughout the Internet and where airplanes, ships, and cars rely on highly precise measurements of time to determine their position using GPS, there is a hue and cry each time a leap second is declared (the next leap second will be in June 2015). A group called the Civil GPS Interface Committee has declared the leap second a hazard to navigation. IT people seem to be unable to ensure that their computers can properly account for leap seconds, leading to computer glitches and crashes. And so the United States is proposing that the leap second be abolished. The effect of this proposal would be to remove the link in principle between days and years on the one hand and the inexorable ticking of the atomic clock on the other. To be sure, because leap seconds are so rare, in practice time as measured by atomic clocks will be more or less the same as time measured by the sun or the stars for many, many years to come.

Why Should You Care?

Days and years are meaningful for human beings. We live our lives in days and years. In my view we should take the meaningful units of measurement as the fundamental units of measurement, and then subdivide as appropriate. The SI definition of the second, useful in atomic clocks, “the duration of 9192631770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the caesium 133 atom,” is meaningless for human beings. It might just as well be 9,192,631,771 or 9,192,631,772, or 10,000,000,000 periods. And even granted that we can measure the absolute interval of time between two events with such precision, it’s meaningless, on a human scale, to do so. I see no reason why we should move from a standard of time that is meaningful to a standard of time that is meaningless, especially because over time the result will be that the natural, meaningful definition of time we all use everyday will become progressively more “wrong.”

Well, what about high-speed computers and other technology that require a very high degree of precision and synchronization? It’s a technical problem that the people who have a vested interest in ultra-precise time should bear the cost of fixing. And while I’m no expert, it seems to me there is a pretty easy answer: let the airlines and the Internet companies use TAI. In other words, let them simply stop adjusting for for leap secondsuse a time scale that does not include leap seconds, while the rest of the world continues to use UTC as it is now defined. Let the atomic clocks TAI become progressively more wrongout of sync with official civil time.

It seems to me we have our priorities backwards when we worry about how to keep our everyday natural time in sync with our atomic clocks instead of vice versa!

Further Reading

I had thought about quoting at length Hannah Arendt’s prologue to The Human Condition, a great book, which at the dawn of the space age expressed similar thoughts about cutting the necessary link between humankind and the earth (she didn’t have the measurement of time in mind). But instead of quoting it, I will just say: go read it! It’s a great book.

This Post Has 17 Comments

  1. Rob Seaman

    Interested readers can delve further into this subject through the presentations and preprints of two international workshops that were organized on this topic in 2011 and 2013: http://futureofutc.org

  2. Gerry Ashton

    I think it will be quite difficult to keep TAI in a walled garden. Technical, legal, and personal matters will be intersecting more, not less, in the future. But I also feel it just isn’t right to divorce time from the Sun.

      1. Steve Allen

        I have seen that the folks making the proposal understand the consequences of abandoning leap seconds, but many of them are constrained not to speak anything contrary to the policies of the agencies and national governments under whose auspices they work. Furthermore, the culture of the ITU-R is one of diplomats and bureaucrats operating in camera, so ironically the folks who make things happen at the International Telecommunications Union are not good at communicating plainly and openly.

  3. Nero Imhard

    Perhaps even more importantly, there’s an element of betrayal in fundamentaly changing an existing definition. It breaks your ability to meaningfully choose the right time scale for your purpose.

      1. Rob Seaman

        For example, astronomers and astronautical engineers have made the natural assumption throughout their software and systems that Coordinated Universal Time is indeed a flavor of Universal Time, that is, an approximation to Greenwich Mean Time. If UTC is redefined to no longer reflect Earth orientation then a lot of our software will break. The software is not at fault, the naive standards process is.

        Also see: http://www.cacr.caltech.edu/futureofutc/aas223/presentations/2-1-ISOterminologyAAS.pdf

          1. Rob Seaman

            Hi Ted. Yes, that would be my understanding. It might also be worth pointing out that redefining UTC would not improve access to unsegmented timescales (that is, without leaps) since projects and systems can already use TAI, GPS and related timescales. Rather, all it would accomplish is to eliminate straightforward access to mean solar time. As a result confusion would increase, not decrease.

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