Leap seconds as a timekeeper: the role of atomic clocks and GNSS satellites in international radiocommunications and management

Earth’s rotation is an imperfect timekeeper. The time taken to make one turn varies from day to day, but the precision of atomic clock makes it clear that this is not a big deal to humans.

For millennia, people measured time using Earth’s rotation, and the second became defined as a fraction of the time it takes for the planet to turn once on its axis. But since 1967, atomic clocks — which tick using the frequency of light emitted by atoms — have served as more precise timekeepers. Today, a suite of around 450 atomic clocks defines official time on Earth, known as Coordinated Universal Time (utc), and leap seconds are used every few years to keep utc in line with the planet’s natural day.

In 1967, the internationally accepted definition of the second changed. The time measurement standard was determined by a quantum change between the 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. Earth’s movement was assumed to be regular but it was not as frequent as first thought.

Atomic clocks have enabled the development of great technologies, such as satellite navigation and, in an age of the global navigation satellite system (GNSS), celestial navigation is much less relevant than it was in 1972. GNSS satellites themselves have onboard atomic clocks that regulate their timekeeping, and the insertion of a leap second generates risk of failures. The addition of leap seconds can have a drastic effect on the infrastructure of computers in the modern world.

The maximum difference between the two times will be increased or before 2035, and the details of the new maximum and how it is to be implemented will be decided in the next meeting of the Committee on Government Relations and Management. Most delegates want the rules to be implemented quickly, but some want more time to adapt 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.

According to his analysis, global warming will push back the need for another leap second from 2026 to 2029. Leap seconds cause so much havoc for computing that scientists have voted to get rid of them, but not until 2035. Researchers are especially dreading the next leap second, because, for the first time, it is likely to be a negative, skipped second, rather than an extra one added in.

The increase in Earth’s rotation period is the result of the individual contribution of each process. The effect of this slowing may seem small but it is responsible for the 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 cope with one second missing. 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 hopes climate change will encourage some people to take action. He says he can worry about climate change without this, but it is still another way for people to see how important it is.