The role of a leap second in establishing a coordinated universal time standard and its consequences for the radiocommunication sector of the International Telecommunication Union
“Enough ice has melted to move sea level enough that we can actually see the rate of theEarth’s rotation has been affected,” says Duncan Agnew, a geophysicist at the Scripps Institution of Oceanography in La Jolla, California, and author of the study.
But sailors still relied on the Sun and stars to navigate, and they wanted a time standard that remained tied in some way to Earth’s rotation. It was therefore decided that the new international reference, known as coordinated universal time (utc), would be set by atomic clocks, but kept apace with the rotational angle of Earth, which is known as universal time (ut1). Since 1972, utc has been adjusted to meet this goal by adding a leap second whenever the discrepancy between the two standards approaches one second.
In 1967, the internationally accepted definition of the second changed. The standard of time measurement was determined by the quantum transition between two states of a caesium atom. The change was motivated by accuracy: caesium atomic clocks keep time on the basis of the ultrastable frequency of the photons exchanged in the quantum transition. This seemed like a safer bet than Earth’s movements, which weren’t as regular as was first assumed.
Metroologists would approve of a delay in the leap second. According to Elizabeth Donley, the time and frequency division at the National Institute of Standards and Technology in Boulder, Colorado, Leap seconds are a big problem because they lead to system failures in a society that is increasingly based on precise timing.
The maximum difference between the two times will be increased in or before the year 2035, and the details of the new maximum will be decided at the next meeting of the CGPM. The majority of delegates want the new rules to be implemented quickly, while some want more time to modify their systems. The radiocommunication sector of the International Telecommunication Union — the organization that regulates the transmission of time signals — endorsed the CGPM decisions at the World Radiocommunication Conference in 2023.
A negative leap second has never been added or tested, so the problems it could create are without precedent. Metrologists around the world are following the unfolding discussion attentively, with the view to avoiding any unnecessary risks. In metrological practice, it is needed to determine the uncertainty associated with predictions of Earth’s rotation. This information would allow researchers to evaluate the probability that a negative leap second will be required — and assess the related risks — so that they can anticipate any such change before 2035. Unfortunately, this task remains formidable (ref. 3 and visit nature.com. Agnew suggested that the change could be delayed.
Although the individual contribution of each process is somewhat uncertain, their sum is known precisely: it has led to an increase in Earth’s rotation period of 6 millionths of a second per year4. This slowing might seem trivially small, but its effect is responsible for a phenomenon known as clock error. This error describes a discrepancy in the timing of eclipses: events recorded by ancient astronomers seem to have occurred at times that differ from those predicted by assuming that Earth’s rotation rate has remained unchanged since ancient times. Clock error increases with the age of the eclipse and reaches around 4 hours for eclipses that were observed 2,500 years ago5.
We do not know how to make up for lost time. This is why time metrologists are worried,” says Felicitas Arias, former director of the Time Department at the International Bureau of Weights and Measures in Sèvres, France.
Agnew thinks that seeing the influence of climate change on timekeeping will be enough to wake people up. He says that he worried about climate change for a long time without this and that it was yet another way of impressing on people just how big of a deal this is.