American Scientist

In July-August 2011, David Finkleman and his colleagues authored "The future of time: UTC and the leap second." The article described efforts to keep ever more precise timekeeping methods in sync with Earth’s actual rotation, and how some of those measures were being reconsidered. In this blog, Dr. Finkleman provides an update to the debate on whether or not leap seconds should be used to maintain clock alignment with celestial motions.

The Moon is rising, the stars and planets appear, and eventually the Sun rises. But where and when? It depends on where you are on the Earth. More importantly, it depends on how rapidly the Earth rotates and on where the Earth is with respect to distant, effectively fixed, reference points in the Universe. Our perception of time must be consistent with where the Earth is and how rapidly the Earth rotates. For millennia, fundamental units of time were referenced this way. The ever-changing Earth rotation interval was divided into 86,400 seconds. But Earth rotation is not constant and is unpredictable. This meant that the duration of a second had to be changed occasionally to maintain essential synchronization with Earth rotation.

The unchanging atomic second emerged in the 1970s as a constant value of this fundamental time interval. Atomic seconds are measured using the natural resonance frequency of cesium-133 atoms. 86,400 atomic seconds no longer represent the true, variable rotation rate of the Earth, which can be monitored using interferometer measurement of very precise benchmarks in distant constellations. The difference between atomic time and Earth’s rotation, called Coordinated Universal Time (abbreviated UTC), is monitored and published to aid activities that depend on precise synchronization with the planet’s rotation. When the difference approaches a whole second, a leap second adjustment to UTC is necessary. Many important processes are designed to accommodate the adjustment. The adjustment can be scheduled for the final day of any month, but the convention is the last day of December or June. The clock must keep counting seconds—57, 58, 59, 60, 0, 1—and not roll over from 59 to zero. The last minute of the UTC day will have 61 seconds. (Negative leap seconds are possible, if the Earth starts to rotate more rapidly, but have never occurred yet.) It sounds simple, and the rules have existed for more than 40 years.

The leap second is not just a technical oddity. UTC as currently defined with leap seconds is mean solar time, a subdivision of calendar date required for many religions and cultures. In Japan every document of significance is time stamped with UTC. The breadth of application is exposed in several publications, linked below. Temps Atomique International (TAI), a time scale without leap seconds, is available worldwide. It is a running count of atomic seconds beginning at the inception of UTC. UTC is TAI minus accumulated leap seconds. Those who do not care about leap seconds can use TAI instead. GPS time is always precisely 19 seconds behind TAI by convention. There is no need to deprecate the leap second in UTC.

Nonetheless, systems that are not time critical to within one second sometimes have problems with the transition. There have not been any permanent consequences due to leap second ignorance. However, leap seconds are an inconvenience to some. That group has for two decades petitioned the International Telecommunications Union (ITU) to stop using the leap second. The last World Radiocommunication Conference (WRC 2012) mandated further study and consideration at the next (WRC 2015) to occur shortly.

As Steve Allen writes on his website about the leap second debate: The 2012 World Radiocommunication Conference produced Resolution 653 [COM6/20] (WRC-12), (which has become WRC-15 Agenda Item 1.14, and directs the ITU-R to prepare this meeting for another vote on the fate of leap seconds in UTC. The result of that vote will decide whether the definition of the word "day" will change to depend solely on cesium atoms or whether the word "day" will continue to be related to the rotation of the Earth, as specified in existing international agreements.

If there is a change, the transition will extend over about two decades. Those who need the leap second, such as astrophysicists and navigators, would suffer costly and risky burdens, as major software and hardware updates would be required. However, even those who do not want the leap second would have to accommodate the change.

Convenience should not trump necessity. We create and enforce standards for those who need them, not for those who do not need them. Save the leap second.