Molecular clocks, along with precise astronomical measurements, have revealed that the length of a day is suddenly getting longer, and scientists don’t know why.
This has serious implications not only for our timekeeping, but also for things like GPS and other technologies that govern our modern lives.
Over the past few decades, the Earth’s rotation on its axis – which determines the length of a day – has been accelerating. This trend is shortening our days; In fact, in June 2022 We made a record For the shortest day in the last half century.
But despite this record, since 2020 the steady pace has turned curiously into a recession – the days are getting longer again, and the reason for that has so far been a mystery.
While the clocks in our phones show that there are 24 hours in a day, the actual time it takes for the Earth to complete one revolution varies slightly. These changes occur almost instantaneously over millions of years – earthquakes and storm events can also play a role.
That rarely turns out to be the magic number of 86,400 seconds a day.
A constantly changing planet
Over millions of years, the frictional effects associated with the tides have slowed the Earth’s rotation. the moon. This process adds about 2.3 milliseconds to the length of each day of each century. A few billion years ago there was only one Earth day 19 hours.
For the past 20,000 years, another process has been working in the opposite direction, speeding up Earth’s rotation. When the last ice age ended, the melting of the polar ice sheets reduced the pressure on the surface and the Earth’s mantle began to move steadily towards the poles.
Our planet’s spin rate increases as the mantle moves closer to Earth’s axis, just as a ballet dancer moves their arms toward their body—the direction they spin—in the direction they spin. And this process slows down every day by about 0.6 milliseconds every century.
Over decades and longer, the relationship between Earth’s interior and surface also comes into play. Large earthquakes can change the length of the day, although usually by a small amount.
For example, the Great Tohoku Earthquake in Japan in 2011, which had a magnitude of 8.9, is believed to be caused by the relatively small speed of the Earth’s rotation. 1.8 microseconds.
In addition to these large-scale changes, climate and weather over short periods of time also have a significant effect on the Earth’s rotation, causing a difference in both directions.
Fortnightly and monthly tidal cycles move the mass around the planet, causing day length changes of up to a millisecond in either direction. We can see the tidal difference 18.6 in the day length record over a period of years.
The movement of our atmosphere has a particularly strong influence, and ocean currents also play a role. Seasonal snow cover and rainfall, or groundwater withdrawals, change things further.
Why is the Earth suddenly slowing down?
Since the 1960s, when radio telescopes around the planet began to develop techniques. Simultaneously observe cosmic objects such as quasarsWe have a very accurate estimate of the Earth’s rotation speed.
A comparison between these estimates and the atomic clock shows that the length of the day has always decreased over the years.
But after removing the fluctuations in rotation speed that we know are due to tidal and seasonal effects, a surprising revelation emerges. Despite Earth reaching its shortest day on June 29, 2022, the long-term trajectory appears to be shifting from shorter to longer from 2020. This change is unprecedented in the last 50 years.
The reason for this change is not clear. This may be due to changes in the climate system, including back-to-back La Niña events, although these have occurred before. Although they have not deviated much from their steady rate of ice melt in recent years, the rate of melting may increase.
It may be related to a massive volcanic eruption in Tonga Injecting large amounts of water into the atmosphere? Probably not since it happened in January 2022.
Scientists have speculated This recent, mysterious change in the planet’s rotation speed is associated with a phenomenon called the “Chandler Wobble”—a small deviation in Earth’s rotation axis with a period of about 430 days.
Radio telescope observations also show that the wobble has decreased in recent years; The two may be connected.
A final possibility, which is conceivable to us, is that nothing has changed inside or around the Earth. Long-term tidal effects may act in parallel with other periodic processes to cause a temporary change in the Earth’s rotation rate.
Do we need a ‘Negative Leap Second’?
Accurately understanding the Earth’s rotation rate is important for many applications—navigation systems like GPS wouldn’t work without it. Also, every few years the timekeepers add a leap second to our official timescales to ensure that they are out of sync with our planet.
If the Earth had changed to even larger days, we might need to include “negative leap seconds” – this would be unprecedented, and Internet may be interrupted.
The need for negative leap seconds is currently considered unlikely. For now, we can welcome the news that – at least for the time being – 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.