You’ve heard of these “leap” years, especially if an acquaintance was born on the unusual date of February 29. What you might not have heard of, however, is the leap year’s sporadic sibling: the leap second. This lesser-known timekeeping fix was center stage at this week’s General Conference on Weights and Measures in France, where timekeepers decided today to return it to obscurity by getting rid of it altogether.
Traditionally, one Earth orbit around the sun determines the length of a year, and one Earth rotation on its axis determines the length of a day. Because it takes Earth around 365.25 days to go around the sun, timekeepers add that extra leap day once every four years to make sure our seasonal calendar stays aligned with the orbit. Earth’s daily rotation, however, isn’t as regular, because the speed of the planet’s spin can change.
In general, Earth’s rotational speed has been slowing down throughout history. But starting around 2020, it has increased. A number of factors can play a role in Earth’s spin, including the atmosphere, ocean currents, wind, volcanoes and melting polar ice.
This makes Earth’s rotation a “lousy” clock, Jeff Sherman, a physicist at the United States’ National Institute of Standards and Technology (NIST), tells CNN’s Katie Hunt. Earth’s rotation does not exactly match the length of a day on our clocks. Right now, the average day on Earth is slightly longer than 24 hours.
October 14, 2026, was 24 hours and 0.0006471 seconds. Even as Earth’s spin wavers, our atomic clocks—crucial to keeping ultra-precise time in this age of computers, phones, GPS and satellites—tick at a stable rate. That’s because in 1967, the General Conference on Weights and Measures chose a reliable way to define a second with the help of the isotope cesium-133.
When a cesium-133 atom undergoes a change between specific energy states, it emits radiation that oscillates in an exceptionally regular manner, Vahe Peroomian, a physicist at the University of Southern California, writes for the Conversation. As such, scientists reclassified a second as equivalent to 9,192,631,770 cesium-133 atom oscillations. Today, Coordinated Universal Time (UTC) follows this principal.
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