Come to think of it, today is longer than any day in 2014. Or in 2013 for that matter. In fact, it's the longest day on record since June 30, 2012.
What I mean is that today, June 30, 2015, will be 86,401 seconds in length. All other days of this year and of almost any year contain only 86,400 seconds. It's just that every once and a while either June 30 or December 31 will have an extra second. Why? By international agreement, the final minute of the final hour of the day will be 61 seconds in length instead of 60. When the clock shows 23:59:59, it won't trip over to 0:00:00 as usual. Instead, it will go 23:59:59, 23:59:60, and only then 0:00:00. How cool is that?
Now, I'm not referring to the digital clock in your kitchen, although you may find it out of sync by a second after midnight unless you update it. Or perhaps you don't care about a few seconds here or there?
What I am referring to is Coordinated Universal Time, UTC for short. I'm not going to go into a long, drawn-out scientific explanation. This may be the longest day, but I'm not going to attempt a detailed explanation when many well-written ones are already to be found. (See, for example, the explanation from the Time Services Bureau at the U.S. Naval Observatory: http://tycho.usno.navy.mil/leapsec.html.) What I have in mind for these jottings is something social.
For those who may have heard of Greenwich Mean Time and like to refer to GMT, I have a cosmic message: GMT doesn't exist. It hasn't existed in any official sense for decades, yet it has held on in the public imagination. Rather, the closest thing we have to a world time from which all our local civil times are determined is UTC.
The general story goes like this. Our ancestors told time approximately by the rising and setting of the Sun, Moon, and stars. By the middle ages, astronomers were the timekeepers, and any major city worth its salt maintained its own time. Then came the great ocean explorers and the need for accurate navigation tools that led to the international conference of 1884 that chose Greenwich to be the prime meridian. At about the same time railroads and the need to maintain railroad schedules forced the need for time zones. Got it? If you've already read A Timely Remembrance of Chester A. Arthur, you already know all of this.
But the story did not end with Chester A. Arthur. As we got into the mid-20th century, the first atomic clocks appeared and allowed for the second to be defined based on oscillations of cesium atoms. At the same time astronomers began to realize the the Earth's rotation is itself non-uniform and in fact has been slowing since the early days of the solar system. Suffice it to say that at the time of the dinosaurs, the day was only about 23 hours in length as measured using today's time scales.
Through the 1950s and 1960s there was lots of scientific hand wringing over how to meld together our human concept of time with the accuracy of an atomic second. Different compromises were tried, but the one that stuck came in 1972. UTC would be defined based on the atomic second, but the International Earth Rotation and Reference Systems Service (IERS) would carefully monitor the Earth's rotation and recommend insertion of a leap second at midnight on June 30 or December 31 whenever UTC diverged from UT1 by more than 0.6 seconds.
"Hold on there," you say, "Where did that UT1 come from?"
Think of UT1 as time based on the Earth's rotation. OK, it's the Earth's rotation relative to a mean Sun instead of the real, physical Sun, but trust me. It's time based on the Earth's rotation and the rising and setting of the Sun much as time was defined by our ancestors millenia ago.
The compromise of 1972 decreed that UTC was to be based on the SI second -- i.e., the atomic second -- but that leap seconds would be added as needed to keep UTC and UT1 within a second of each other. How's that for a melding of scientific accuracy with our social concept of time?
To date some 35 leap seconds have been inserted. The one that will be inserted at midnight today is No. 36. If you had not adjusted that digital clock in your kitchen these past 43 years, it would now be off by 36 seconds. In another 30 years or so it might be off by a minute, perhaps just enough for you to notice.
So get this. There's a move afoot to get rid of leap seconds. Much of the push comes from businesses that are dependent on complex computer systems in which accurate time is of critical importance. Your run-of-the-mill programmer likely as not did not program these systems for the occasional insertion of a leap second when IERS dictates. If you are trading millions of shares of whatever on the stock market and the system crashes when it tries to cross the midnight hour on a day with a leap second, you might be upset. If you are on an airplane and the air traffic control system crashes at midnight on such a day, you might be more than upset. Proponents of eliminating leap seconds might add that as a result, you might never have cause to be upset about anything every again.
Sure enough, in 1972 the world did not depend on the complex and interlocking computer systems that rule our lives today. In that year, the year when I graduated from high school, the world was a much simpler place where computers were big things that ingested computer cards. Slide rules, typewriters, pencils, and paper were the norm. Leap seconds couldn't crash anything.
As it stands, much of the scientific community doesn't care if leap seconds are eliminated either, and the International Telecommunications Union (ITU) will take a vote on changing the definition of UTC at the end of the year.
"Stop right there," you exclaim again. "What does telecommunications have to do with time?"
Well, that goes back to those simpler days when the most accurate time was broadcast by radio. In the United States it was NIST, in those days still the National Bureau of Standards, that operated shortwave station WWV to broadcast the most accurate time available in the country. Other countries had their own national stations for time signals. That's why international time is handled by the ITU.
When the ITU meets, all member countries will get a vote. For the United States, it's my own Department of State that casts the vote.
"And what is the official U.S. position?" Speaking very unofficially and only for myself and not representing the Department of State in any way in this web journal, I will just say that my understanding is that we favor elimination of the leap second.
"So does anyone still want to preserve the leap second?"
Yes, there are some concerned communities. Many although not all astronomers want to maintain the status quo. Of course, astronomers have been trained to think about time since the dawn of modern man, so it's no surprise that astronomers get what the leap second is about and how to handle it in all their systems. The same can be said about many of the engineers who work on space systems. I had to keep track of UTC, TDB, TT, TAI, and the conversions between these time scales in the years I worked on Hubble Space Telescope. Don't worry, I won't go into what those other time scales were about, but suffice it to say that anyone who is worried about relativistic effects when accounting for velocity aberration or pointing a telescope to sub-arcsecond accuracy is going to need to know how to handle time.
Russia also wants to maintain the leap second. Perhaps the poor computers of the Soviet period forced Soviet programmers to be better than ours. Perhaps their systems handle leap seconds properly. Great Britain also wants to retain the leap second; some say the UK fears that elimination of the leap second is the first step towards diminishing the importance of the Greenwich Meridian.
Finally, there is one other group. Let's call this the group of historians, philosophers, and dreamers. Perhaps we can throw in people of a certain age, and I include myself here. Sure, we can live in a world without leap seconds. We already live in a world of time zones and daylight saving time where the idea of the Sun being anywhere near overhead at noon has long been done away with. It will be centuries before UTC and UT1 separate at anywhere near the level of what we already do administratively today with our own hands.
To me, a self-professed dreamer and woman of a certain age, the point of retaining leap seconds is that our concept of time is part of being human. Even time zones and daylight saving time are still tied to the Earth's rotation. If leap seconds are eliminated, we turn over time to atomic clocks. It will take many centuries, but at some point we will get to a point when we will no longer associate noon with being the middle of the day. We will have put our computer systems ahead of ourselves, maintaining time for the machines we created rather than for people. In doing so, we diminish ourselves.
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