Atomic clocks combined with precise astronomical measurements revealed that days were getting longer without scientists understanding why. This has critical implications not only for how we measure time, but also for GPS and other technologies that drive our modern lives.

In recent years, the Earth’s rotation around its axis, which determines the length of the day, has accelerated. This trend has shortened our days. We even broke the record for the shortest day of the last half-century in June 2022.

But despite this record, since 2020 the trend has changed and Earth’s rotation seems to have slowed down: the days are longer again, and why this happens with days is a mystery until now.

While the clocks on our phones show that there are exactly 24 hours in a day, a day seldom corresponds exactly to the magic number of 86,400 seconds. The actual time it takes for Earth to complete a single rotation varies slightly. These changes occur over periods of time varying almost instantaneously over millions of years; even earthquakes and storms can play a role.

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ever-changing planet

Over millions of years, the Earth’s rotation has been slowing due to the frictional effects associated with the Moon’s driven tides. This process adds about 2.3 milliseconds to the length of each day per century. A few billion years ago, an Earth day lasted only about 19 hours.

During the last 20,000 years, another process that accelerated the earth’s rotation worked in the opposite direction. By the end of the last ice age, we mean that the melting of the polar ice caps reduced the pressure on the surface and the Earth’s mantle began to constantly move towards the poles.

Just as a ballerina spins faster when her arms are closer to her body – the axis on which it rotates – the speed of our planet’s rotation increases as this mantle mass approaches the Earth’s axis. And this process shortens each day by about 0.6 milliseconds per century.

For more than decades, the connection between the Earth’s interior and its surface also comes into play. Large earthquakes can change the length of the day, though often by small amounts. For example, the 8.9-magnitude Great Tōhoku earthquake in Japan in 2011 is believed to accelerate the Earth’s rotation by a relatively small amount, up to 1.8 microseconds.

Alongside these large-scale changes, weather and climate over shorter periods of time also have major effects on Earth’s rotation, causing changes in both directions.

Bi-weekly and monthly tidal cycles move mass around the planet, causing changes in day length in any direction by up to a millisecond. We can see tidal changes in day length records for periods up to 18.6 years.

The movement of our atmosphere has a particularly strong influence, with ocean currents also playing an important role. Snow cover and seasonal precipitation or groundwater withdrawal further complicate matters.

Why is the world suddenly slowing down?

Ever since the 1960s, radio telescope operators around the world began developing techniques for simultaneously observing cosmic objects such as quasars, we’ve had very precise estimates of the Earth’s rotation rate.

A comparison between these predictions and an atomic clock has revealed increasingly shorter day lengths in recent years.

But when we remove the spin velocity fluctuations that we know are caused by tidal and seasonal effects, a surprising finding emerges. Although the world reached its shortest day on June 29, 2022, the long-term trend appears to have shifted from shortening to lengthening since 2020. This change is unprecedented in the last 50 years.

The reason is unclear. Although it has happened before, it may be due to changes in weather systems with successive La Niña events. While this hasn’t deviated much from the steady melting rate in recent years, there could be further melting of the ice sheets. Could it be related to the massive eruption of the Tonga volcano, which injected huge amounts of water into the atmosphere? Probably not, given that it was in January 2022.

Scientists have speculated that this mysterious change in the planet’s rotation rate is related to a phenomenon called the Chandler wobble, which is a small deviation in Earth’s rotation axis of about 430 days. Observations from radio telescopes also show that wobble has decreased in recent years. Both can be related.

A final possibility that seems plausible to us is that nothing specific has changed on or around Earth. There can be long-term tidal effects that work in tandem with other periodic processes to create a temporary change in the rate of Earth’s rotation.

Do we need “negative leap seconds”?

Knowing the exact speed of Earth’s rotation is crucial for a number of applications: Navigation systems such as GPS simply won’t work without it. Also, every few years timekeepers add leap seconds to our official timescales to make sure they don’t disrupt their harmony with our planet.

If Earth’s days were even longer, it would be necessary to include an unprecedented “negative leap second” that could disrupt the internet.

The need for negative leap seconds is currently considered unlikely. For now, we can settle for the news that we all have a few extra milliseconds each day, at least for a while.

Matt King, director of the ARC Australian Center of Excellence in Antarctic Science, said: University of Tasmania and Christopher Watson, Senior Lecturer, School of Geography, Planning and Spatial Sciences, University of Tasmania

This article was originally published on The Conversation. Read the original.