The leap second, a time adjustment added intermittently to synchronize atomic clocks with Earth’s irregular rotation, is facing likely abolition by 2035. Since its introduction in 1972, the leap second has been inserted 27 times, most recently in 2016, to align the highly precise Coordinated Universal Time (UTC) with the slightly variable astronomical time governed by Earth’s spin.
Earth’s rotation slows over long periods due to gravitational interactions with the Moon, creating a divergence between atomic time maintained by ultra-precise clocks and the planet’s natural day-night cycle. When this difference approaches 0.9 seconds, a leap second is added to UTC to restore alignment. However, recent measurements indicate that changes in Earth’s core are causing the rotation to speed up, raising the possibility that a negative leap second—an unusual subtraction of one second—may be required by 2035.
UTC underpins critical infrastructure worldwide, including telecommunications, power grids, and digital systems, leading to challenges in implementing leap seconds. Operators vary in their approach; some insert the leap second abruptly, while others use a “smearing” technique that distributes the adjustment over several hours. Concerns over the risks the leap second poses to these systems have prompted most of the 69 member states of the international Bureau for Weights and Measures, the agency overseeing global time standards from its headquarters near Paris, to favor ending the leap second practice.
The bureau is expected to vote on a proposal in mid-October that would replace the leap second with a much less frequent “leap hour.” This larger adjustment would occur only once every few centuries, effectively preventing short-term disruptions caused by leap seconds. Supporters argue this would enhance stability for critical national infrastructure by removing the need for abrupt time changes.
However, some experts caution against the leap hour approach. Markus Kuhn, a computer scientist at the University of Cambridge, noted that while a negative leap second—potentially required soon—should be less disruptive than positive leap seconds because it involves skipping a second rather than adding one, the leap hour concept has not been widely tested. Kuhn has expressed opposition to the leap hour, contending that the infrequency of such adjustments would make them highly disruptive when they do occur.
Former astronomer Shemar, involved in timekeeping discussions, acknowledges that a leap hour might never be necessary within human experience due to the long timescales involved but emphasizes the immediate need to protect essential infrastructure, even if it means decoupling civil time from Earth's astronomical rhythms.
Looking ahead, atomic clock technology continues to advance, with newer models based on ytterbium atoms demonstrating extraordinary precision. Should they have been operational since the universe’s origin nearly 14 billion years ago, these clocks would have lost less than one second, underscoring the growing reliance on atomic timekeeping rather than natural cycles.
Despite its practical challenges, the leap second has served as a subtle reminder of humanity’s connection to Earth’s rotation for more than five decades, a link that may soon be severed in favor of greater technological stability.