The Earth is rotating faster than it did half a century ago, and if this acceleration continues, scientists say they may need to remove a second from the atomic clock. The speed of our planet's rotation has varied throughout history. In fact, the Earth used to spin 420 times a year millions of years ago, but now it does so 365 times.
However, sometimes the speed of rotation varies slightly, impacting the global timekeeper—the atomic clock—which requires the addition of leap seconds when the world spins a bit faster. Peter Whibberley, a scientist at the UK’s National Physical Laboratory, has warned that if the rotation rate increases further, a negative leap second may be needed.
Each day on Earth contains 86,400 seconds, but rotation is not uniform, meaning that throughout the year, each day has slightly more or less than a full second. This occurs due to the movement of Earth’s core, its oceans, and atmosphere, as well as the pull of the moon. The atomic clock is incredibly precise, measuring time by the movement of electrons in atoms cooled to absolute zero. To keep the atomic clock synchronized with the number of seconds in Earth’s rotation, leap seconds have been added approximately every 18 months since 1972.
There has never been a negative leap second—removing a second from the atomic clock—and the system designed to perform this task has not been tested. The idea emerged last year when rotation began to accelerate, though it slowed down again since then, with the average day in 2021 being 0.39 milliseconds shorter than in 2020.
Judd Levin from the National Institute of Standards and Technology told Discover magazine: "Over time, there is a gradual discrepancy between atomic clock time and time measured by astronomy. To prevent this difference from becoming too large, a decision was made in 1972 to add leap seconds periodically to atomic clocks."
The International Earth Rotation and Reference Systems Service is responsible for tracking Earth's rotation speed by sending laser beams to satellites and using them to measure their movement. When that does not align with atomic clocks, scientists coordinate to stop their clocks for one second to realign them.
The rate of Earth's rotation is a complex matter. Levin explained it involves the exchange of angular momentum between the Earth, atmosphere, ocean influences and the moon's effects. No leap second has been added to the atomic clock since 2016, and while the Earth began to accelerate again, it has started to slow down once more in 2021. Levin stated, "This lack of need for leap seconds was unexpected," adding it was supposed that the Earth would continue to slow down, making this effect quite surprising.
The duration of the speed fluctuations may require scientists to take further actions, but it remains unclear what those actions might be. Whibberley noted, "There is currently concern that if Earth's rotation rate increases more, we might need what's called a negative leap second." In other words, instead of adding an extra second to allow Earth to catch up, we would have to take a second from atomic time to realign it with Earth.
While they know how to potentially implement this, scientists are unclear whether their systems would actually work or what impact it would have. The internet relies on a continuous flow of time, measured by atomic clocks, and different web companies use various methods to determine leap seconds. For example, Google uses a system that distributes the extra time over the year, adding it to every other second throughout the year.
Levin explained, "The foundational backbone of the internet is that time is continuous." He added that when there is no fixed time, the continuous flow of information collapses. Levin notes that leap seconds—added or removed—may not be worth the trouble, as they would ultimately add up to only about one minute over a century.
