Leap seconds have been added 27 times over the past century, each time effectively making time go backwards for a second. The adjustments were necessary to reconcile the ultra-precise atomic clocks that underpin Coordinated Universal Time (UTC) with the Earth’s slightly variable rotation.
Earth’s spin is not perfectly uniform. While the planet’s rotation is generally slowing down due to the Moon’s gravitational pull, shorter-term factors can cause it to speed up. Since around 2020, Earth has been spinning faster. University of Tasmania geodesist Lucia McCallum said several things could affect the spin, including “atmosphere, wind, big ocean currents, glaciers melting, big volcanoes, redistribution of mass in the inner Earth.”
The traditional definition of a second — one 86,400th of a day — is no longer precise enough for technologies such as GPS, which rely on atomic time. In the late 1960s, the BIPM adopted a definition of the second based on caesium atoms, which is now the basis for UTC. Australia’s National Measurement Institute’s leader of time standards, Michael Wouters, noted that the atomic second was as close to the traditional second as possible, but “there was no exact value to begin with, because it was changing all the time.”
If the vote succeeds, the leap second will be scrapped, and future timekeepers may be asked to implement a leap hour instead — a larger adjustment that would not need to be made for generations.