Atomic clocks, along with accurate astronomical measurements, have revealed that the length of the day is suddenly increasing, and scientists don’t know why.
This has crucial 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 around its axis – which determines the length of the day – has been accelerating. This trend has made our days shorter; In fact, in June 2022 We set a record! For the shortest day in half a century or so.
But despite this record, since 2020, this intriguingly steady acceleration has turned into a slowdown – the days are getting longer again, and the reason is so far a mystery.
While the clocks in our phones indicate that there are exactly 24 hours in a day, the actual time it takes the Earth to complete one cycle is slightly different than ever before. These changes occur over periods of millions of years to near instantaneous – even earthquakes and storm events can play a role.
It turns out that today is rarely the magic number of 86,400 seconds.
The ever-changing planet
Over millions of years, the Earth’s rotation has been slowing down due to the frictional effects associated with the tides that move it the moon. This process adds about 2.3 milliseconds to the length of each day every century. A few billion years ago, it was almost Earth Day 19 hours.
Over the past twenty thousand years, another process has been working in the opposite direction, accelerating the Earth’s rotation. When the last ice age ended, the melting of the polar ice sheets lowered surface pressure, and the Earth’s mantle began to move steadily toward the poles.
Just as ballerinas spin faster when they point their arms toward their body—the axis they rotate around—the rate of rotation of our planet increases as this mantle mass moves closer to the Earth’s axis. This process shortens each day by about 0.6 milliseconds per century.
For decades and longer, the relationship between the Earth’s interior and its surface also plays a role. Large earthquakes can change the length of the day, although usually in 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 amount. 1.8 microseconds.
Aside from these large-scale changes, over shorter periods, weather and climate also have important effects on the Earth’s rotation, causing differences in both directions.
Bimonthly and monthly tidal cycles move mass around the planet, causing variations in the length of a day of up to milliseconds in either direction. We can see the tidal variations Records the length of the day over periods of up to 18.6 years.
The movement of our atmosphere has a particularly strong influence, and ocean currents also play a role. Seasonal snow cover, rain or groundwater extraction changes things even more.
Why is the Earth suddenly slowing down?
Since the 1960s, when operators of radio telescopes around the planet began to invent technologies Detecting cosmic objects such as quasars at the same timewe have very accurate estimates of the rate of Earth’s rotation.
A comparison of these estimates with the atomic clock has revealed that the length of the day has apparently shortened over the past few years.
But there is a surprising discovery once we remove the rotational speed fluctuations that we know are caused by tides and seasonal influences. Although Earth reached its shortest day on June 29, 2022, the long-term path appears to have shifted from shortening to lengthening since 2020. This change is unprecedented over the past 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 they have occurred before. It could lead to increased melting of ice sheets, although it has not deviated significantly from the steady rate of melt in recent years.
Could it have something to do with the eruption of a huge volcano in Tonga Pumping huge amounts of water into the atmosphere? Probably not, given what happened in January 2022.
Scientists speculate This last mysterious change in the planet’s rotation speed is associated with a phenomenon called “Chandler’s wobble” – a small deviation in the Earth’s rotation axis for about 430 days.
Observations from radio telescopes also show that the oscillation has diminished in recent years; The two may be linked.
The last possibility, which we think is reasonable, is that nothing specific has changed in or around the Earth. It could be that long-term tidal effects work in parallel with other cyclical processes to bring about a temporary change in the rate of Earth’s rotation.
Do we need a “negative leap second”?
An accurate understanding of the Earth’s rotation rate is critical for a range of applications – navigation systems like GPS won’t work without it. Also, every few years, timers introduce leap seconds into our official timelines to make sure they don’t drift out of sync with our planet.
If the Earth turns longer days, we may need to incorporate a “negative leap second” – this would be unprecedented, and may break the internet.
The need for negative leap seconds is currently considered unlikely. For now, we can welcome the news – at least for a while – we all have a few extra milliseconds every day.
Matt Kingdirector of ARC’s Australian Center of Excellence in Antarctic Science, University of Tasmania And the Christopher Watson– Senior Lecturer at the Faculty of Geography, Planning and Spatial Sciences, University of Tasmania.
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