The End of the Leap Second?
The world may be moving toward getting rid of leap seconds—for at least 100 years.
A Request to End Leap Seconds
On November 18, the General Conference on Weights and Measures (CGPM)—an international body that decides on global standards for how things are measured—passed a resolution asking the International Telecommunication Union (ITU) to consider changing our definition of standard time.
The effect of the change would be to leave clocks running without adding or subtracting any leap seconds for at least 100 years.
If that were to happen, civil timekeeping would follow only the beat of atomic clocks, and loosen its connection with small variations in the apparent movement of the Sun across the sky.
How We Measure Time
Around the world, civil timekeeping is based on Coordinated Universal Time (UTC). Until now, UTC has been a mix of two separate timescales:
- International Atomic Time (TAI), a super-precise timescale that is defined in terms of certain properties of the cesium atom;
- Universal Time (UT1), which depends on how quickly the Earth rotates and, as a result, how quickly the Sun moves across the sky. From one day to the next, this varies by a fraction of a millisecond (1 millisecond = 0.001 seconds).
Leap Seconds Keep UTC and UT1 In Step
Since the 1970s, leap seconds have been used to ensure UTC remains close to UT1. When the difference between UT1 and UTC has approached one second, a leap second has been added to UTC to bring the two timescales back into line.
However, leap seconds have long been controversial because of the problems they create for IT systems. In the words of the CGPM resolution, leap seconds create “discontinuities that risk causing serious malfunctions in critical digital infrastructure including the Global Navigation Satellite Systems, telecommunications, and energy transmission systems”.
Additionally, in recent years the Earth’s rotation has unexpectedly speeded up. This has led to the possibility of a negative leap second, where a second is taken away from UTC, rather than added to it. According to the CGPM resolution, this “insertion has never been foreseen or tested”.
The New Proposal
The solution proposed by the CGPM is to allow the difference between civil time and Universal Time (based on Earth’s rotation and the movement of the Sun across the sky) to grow—changing the exisiting rule that keeps UTC in close step with the Sun.
Specifically, the resolution calls for consultation with the ITU and other organizations to “propose a new maximum value for the difference (UT1-UTC) that will ensure the continuity of UTC for at least a century”.
In other words, the difference between UT1 and UTC that is required to trigger the addition of leap seconds will be changed to a new value (greater than the current value of one second).
The proposed new value will be made high enough so that it will not be reached for at least 100 years.
Who Defines UTC?
Ultimately, the International Telecommunication Union (ITU) is in charge of defining UTC.
“The ITU remains the keeper of the definition of time,” says Dennis McCarthy, a former Director of Time at the United States Naval Observatory (USNO). This involvement goes back to the early 20th century, when time signals began to be broadcast by radio.
However, the ITU may decide to hand control of UTC over to the CGPM. “The ITU has lots of other things to focus on besides leap seconds,” Dr McCarthy tells timeanddate. “UTC more rightly belongs in the area of measurements and standards, not telecommunications.”