What's a leap second—and why is it going away for good?
Every couple of years, an additional second is inserted into earth’s official time—but scientists have decided it’s too much of a headache to keep.
What time is it? A glimpse at your phone or computer will likely reveal a pretty accurate answer. But though time seems constant, we’ve actually been adjusting it for decades—in the form of a leap second that’s inserted every few years.
This technological solution is designed to make atomic clocks agree with the planet’s rotation and has long kept international timekeeping ticking. So why have scientists decided to abandon the leap second—and what’s at stake?
Making time
The unit of time known as a second was created using simple division that carved the 24-hour astronomical day—a single spin of planet Earth—into 86,400 parts. There was just one problem: Earth doesn’t actually spin at the same speed every day. Instead, it varies slightly, and environmental and physical disturbances such as gravity, friction, and vibrations on Earth’s surface rendered even the fanciest and most reliable clocks slightly inaccurate.
But increasing understanding of physics and the atom led to the redefinition of time itself. Unlike devices like pendulums or substances like crystals that powered earlier clocks, atoms emit and absorb radiation at a remarkably consistent rate. This reliable oscillation became the basis of the first atomic clocks developed in the 1940s and 1950s, and in 1967 the international organization that oversees standards for weights and measures proposed a new definition of the second: the duration of 9,192,631,770 oscillations of a cesium 133 atom.
A leap forward
The adoption of the atomic second was a remarkably accurate, unchanging way to define time. But there was an uncomfortable consequence: the atomic second didn’t quite match the astronomical second as defined by Earth’s rotation. To make the two agree, scientists met in the late 1960s and decided to occasionally add seconds to Coordinated Universal Time (UTC), the time scale determined by atomic clocks. These “leap seconds” allow UTC and the astronomical second to get back in sync, preventing ongoing clock creep and keeping GPS systems, power grids, and the internet, among other things, humming.
The leap second nudged astronomical and atomic time into sync. But the introduction of the leap second, which was eventually implemented in 1972, meant it was necessary to monitor and adjust UTC over time, and over the years astronomical time crept ahead of atomic time by a total of 37 seconds. Introduced roughly every four years, these seconds now represent a major technological timekeeping headache, especially given the rising importance of time coordination in a variety of industries.
Goodbye, leap second
The leap second was an imperfect solution to a ticking time bomb of a problem, and it’s fueled a longstanding debate ever since.
“There was no obvious best solution for how to how to address the problem,” says Judah Levine, a physicist at the National Institute of Standards and Technology’s Time and Frequency Division in Boulder, Colorado. Known as the “nation’s timekeeper,” Levine designed and implemented the UTC time scale for the United States and frequently works with the atomic clocks that govern time worldwide. If there had been a clear, simple solution, he said, timekeepers “would have done it years ago.”
Instead, it took decades for the international timekeeping community to settle the debate and decide to ditch the leap second entirely. In 2022, the world’s timekeepers finally resolved to get rid of leap seconds by the year 2035.
Embracing atomic time as the world’s standard as opposed to astronomical time is an ambitious goal—and one that may be tough to implement. The options range from allowing atomic and astronomical time to stray even farther each year to implementing a one-time leap minute that will cover time-keeping humans for the next few centuries. Ideally, says Levine, the solution will be seamless—one that only affects a small handful of scientists and programmers and goes virtually undetected by the general public.
Why it matters
As scientists tussle about how best to embrace atomic time once and for all, Levine says, the world becomes ever more dependent on the devices that rely on precise worldwide timekeeping.
“The applications that depend on a common, smooth time scale have become more and more important,” says Levine, citing telecommunications, power generation, and financial services as just a few of the tasks at stake. “Without atomic clocks, these applications just wouldn't be possible.”
Leap seconds and modern conveniences aren’t the only speed bumps in sight. Timekeepers face many other challenges, including a recent uptick in Earth’s rotation speed that could nudge the disagreement between atomic and astronomical time in the other direction. And the future of cesium-133 clocks themselves remains murky, especially as researchers develop and perfect even more reliable timekeepers like NIST’s experimental Ytterbium clock, a device that’s one of the most stable in the world.
Humans may disagree about how best to divvy up their days. But as the scientific community irons out these wrinkles in time, one thing remains consistent: the oscillation of the atomic clocks that make our society tick.
