Space and Physics
PUBLISHED
It is time to say goodbye to the leap second. The corrective measure is very likely to be voted out tomorrow by the International Bureau of Weights and Measures, the group that assesses units of measurement.
The leap second was first introduced in 1972, and since then it has been used 27 times to align the 24 hours of our standard day to the actual rotation of the Earth. The timing system that we use (UTC) and astronomical time (UT1) are very close to each other, but the rotation of the Earth changes with time, which can lead to them being out of step.
To keep everything synchronized, namely atomic clocks and planetary rotation, an occasional extra second has been added. This is a quirky little event that involves June 30 or December 31 ending up with the peculiar time of 23:59:60.
An upcoming threat
As the world became more complicated and interconnected, with needs for synchronicity everywhere, the reliable implementation of leap seconds became a concern. Two connected systems with two different times could lead to a series of failures.
Fortunately, the leap second has not been needed for a decade, though the reason for this creates a new level of concern.
The Earth’s rotation has been slowing down for billions of years due to the effect of the Moon. In the last decade, though, we have seen the rotation actually speeding up. If this were to continue, you’d need a negative leap second.
“Unusually, the Earth's rotation has accelerated in recent years, creating the additional possibility that a negative leap second may be required for the first time. Current estimates suggest around a 30 percent probability of this occurring within the next decade,” Setnam Shemar, Principal Scientist at the National Physical Laboratory, said in a statement shared with IFLScience.
Just like in any recipe, adding is a lot easier than removing. The chance of a possible negative leap second and the other risks made experts start reconsidering the whole enterprise.
The leap second is dead. Long live the leap hour?
“Due to the potential problems of applying leap seconds, leap hours have been proposed because it would do away with the need to apply any kind of “leap interval” to UTC (the world’s international time system) for many years,” Shemar continued.
“This would remove the risks entirely for a very long time. The resolution is likely to pass since it will be based on the outcome of a majority vote of the national delegates from many different countries, and most of them have been keen to end leap seconds for several years due to the risks posed to national infrastructure.”
The relationship between UTC and UT1 is important for some applications, such as astronomy and celestial navigation. The decision would not sever that link.
Setnam Shemar
To keep UTC and UT1 synchronized over time, the leading proposal does suggest a move to leap hours. But we would not be getting one any time soon.
A leap hour will be needed when 3,600 seconds have been accrued in the difference between the two timings. Given that we got 27 seconds added in just 50 years, the leap hour will be needed in many centuries or even a few millennia.
“Previously, leap seconds were added to UTC to keep it within 0.9 seconds of UT1. From 20 May 2027 a difference of up to 1 hour will be allowed before intervention is needed, ensuring the continuity of UTC for several centuries or more,” Shemar explained.
“The relationship between UTC and UT1 is important for some applications, such as astronomy and celestial navigation. The decision would not sever that link: UT1 will continue to be monitored and its difference from UTC will continue to be disseminated through established international services.”
This is not the only time preoccupation at the International Bureau of Weights and Measures. Thanks to the breakthrough in optical atomic clocks, the precision at which we measure time is now beyond regular atomic clocks, and there is a push for the redefinition of the second.
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