Atomic clocks, combined with precise astronomical measurements, revealed that the length of the day was suddenly lengthening, and scientists did not know why.
This has a critical impact not only on our timekeeping, but also on things like GPS and other technologies that govern our modern lives.
In the past few decades, the rotation of the Earth on its axis – which determines the length of the day – has been accelerating. This trend is making our days shorter; in fact in June 2022 we set a record for the shortest day in the last half century or so.
But despite this record, since 2020 this steady acceleration has strangely changed to a slowdown – the days are getting longer again, and the reason is still a mystery.
While the clocks in our phones show that there are exactly 24 hours in a day, the actual time it takes the Earth to complete one rotation varies only slightly. These changes occur over periods of millions of years to almost instantaneously—even earthquakes and storms can play a role.
It turns out that a day is very rarely exactly the magic number of 86,400 seconds.
The ever-changing planet
Over millions of years, the Earth’s rotation has slowed due to the frictional effects associated with tides driven by The moon. This process adds about 2.3 milliseconds to the length of each day every century. A few billion years ago, Earth’s day was only approx 19 hours.
Over the past 20,000 years, another process has been working in the opposite direction, speeding up the Earth’s rotation. When the last ice age ended, the melting of the polar ice sheets reduced the surface pressure and the Earth’s mantle began to steadily move towards the poles.
Just as a ballet dancer spins faster as she brings her arms closer to her body—the axis around which she spins—so our planet’s spin speed increases as this mantle mass moves closer to Earth’s axis. And this process shortens every day by about 0.6 milliseconds every century.
For decades and longer, the connection between Earth’s interior and surface also comes into play. Large earthquakes can change the length of the day, although usually by small amounts.
For example, the 2011 Great Tohoku earthquake in Japan, with a magnitude of 8.9, is believed to have accelerated the Earth’s rotation by a relatively small 1.8 microseconds.
In addition to these large-scale changes, over shorter periods, weather and climate also have an important effect on the Earth’s rotation, causing variations in both directions.
Biweekly and monthly tidal cycles move mass around the planet, causing changes in day length by up to a millisecond in each direction. We can see tidal variations in day length records for periods up to 18.6 years.
The movement of our atmosphere has a particularly strong effect, and ocean currents also play a role. Seasonal snowpack and precipitation or groundwater extraction change things further.
Why is the Earth suddenly slowing down?
Since the 1960s, when radio telescope operators around the planet began creating techniques to simultaneously observe space objects such as quasarswe had very accurate estimates of the Earth’s rotation rate.
A comparison between these estimates and the atomic clock revealed a seemingly continuous shortening of day length over the past few years.
But there is a surprising revelation once we remove the fluctuations in rotation rate that we know occur due to tides and seasonal effects. Although Earth reached its shortest day on June 29, 2022, the long-term trajectory appears to have changed from shortening to lengthening since 2020. This change is unprecedented in the last 50 years.
The reason for this change is not clear. It may be due to changes in weather systems, with successive La Niña events, although such have occurred before. It may be due to increased melting of the ice sheets, although they have not deviated significantly from their constant melting rate in recent years.
Could it be related to the huge volcano explosion in Tonga injecting huge amounts of water into the atmosphere? Probably not, given that it happened in January 2022.
Scientists speculated this recent, mysterious change in the planet’s spin rate is linked to a phenomenon called the “Chandler wobble,” a small deviation in Earth’s spin axis with a period of about 430 days.
Observations from radio telescopes also show that the wobble has decreased in recent years; the two may be related.
One final possibility that we think is plausible is that nothing in particular has changed inside or around Earth. They may simply be long-term tidal effects working in tandem with other periodic processes to cause a temporary change in the Earth’s rotation rate.
Do we need a “negative leap second”?
An accurate understanding of the Earth’s rotation rate is critical to many applications – navigation systems such as GPS would not work without it. Also, every few years timekeepers insert leap seconds into our official time scales to make sure they don’t drift out of sync with our planet.
If Earth moves to even longer days, we may have to include a “negative leap second”—this would be unprecedented and it could break the internet.
The need for negative leap seconds is considered unlikely at this time. For now, we can welcome the news that – at least for a while – we all have a few extra milliseconds each day.
Matt Kingdirector of the ARC Australian Center for Excellence in Antarctic Science, University of Tasmania and Christopher WatsonSenior Lecturer, School of Geography, Planning and Spatial Sciences, University of Tasmania.