Earth Is Spinning Faster — Could It Trigger The First-Ever "Negative Leap Second"?

You wouldn’t have noticed, unless your brain possesses an atomic clock, but Earth has been spinning unusually fast in recent weeks. On a handful of days, the days are over 1 millisecond shorter than the usual 24 hours. 

Earth’s days are rarely 24 hours on the dot because numerous factors can influence the speed of its rotation, including shifts in Earth’s core, atmospheric pressure changes, ocean currents, and even large-scale weather patterns like El Niño. Typically, these forces will give or take a fraction of a millisecond, but July and August have seen some freakishly short days. 

According to preliminary data from TimeandDate, July 9 was approximately 1.23 milliseconds shorter than usual, while July 10 was 1.36 milliseconds shorter. July 22 is predicted to have been 1.34 milliseconds shorter, while August 5 is set to be 1.25 milliseconds shorter than a usual day.

Among the many factors at play, the Moon is the leading suspect. Its gravitational pull is a major driver of tidal forces on Earth, and these tidal interactions can shift the distribution of water (and therefore mass) across the planet, altering its spin. At this point in time, the Moon is relatively far away from Earth’s equator, so its gravitational pull is marginally weaker. This “eases the brakes” and allows Earth to spin just a fraction faster. 

While variations of a millisecond might sound trivial, they are incredibly important to technologies that rely on ultra-precise timekeeping, such as GPS navigation, telecommunications, financial transactions, and certain scientific experiments.

If the trend of Earth's spin speeding up continues, it could raise the possibility of needing a “negative leap second” in the future to keep our atomic timekeeping in sync with astronomical time.

Since 1972, international timekeepers have agreed to add a leap second every now and again to ensure our clocks are aligned with Earth's rotation speed as accurately as possible. This is because the general trend over the past century has been Earth’s rotation slowing down, resulting in slightly longer days.

Now, however, we’re seeing  Earth’s rotation has upped its pace, resulting in slightly shorter days. As a result, a “negative leap second” might be needed to bring UTC back into alignment with astronomical time. In other words, timekeepers will subtract a single leap second to make sure the atomic clocks are in tune with the astronomical movement of our Solar System. 

Scientists have discussed this issue for several years, with the suggestion that climate change and the rate of melting ice caps may mean we require a negative leap second in 2029, but the ongoing changes we’ve seen this summer could fire up the debate again. 

However, leap seconds are not popular with everyone. Meta, Google, Amazon, Microsoft, and other tech companies have previously spoken against leap seconds, arguing they cause more problems than they solve. 

"As an industry, we bump into problems whenever a leap second is introduced. And because it’s such a rare event, it devastates the community every time it happens. With a growing demand for clock precision across all industries, the leap second is now causing more damage than good, resulting in disturbances and outages," Meta said in a 2022 blog post.

Their concerns are not without precedent. In 2012, Reddit was struck with a major outage because the Linux operating system it was using failed to take into account the leap second. 

Since the idea of negative leap seconds is equally unpopular among the tech giants, and judging by the power and influence they wield, it seems unlikely they’ll passively accept such a change.

“With the Earth’s rotation pattern changing, it’s very likely that we will get a negative leap second at some point in the future,” Meta said. “The impact of a negative leap second has never been tested on a large scale; it could have a devastating effect on the software relying on timers or schedulers.”