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Atomic clocks, combined with precise astronomical measurements, have revealed that the length of the day is suddenly lengthening, and scientists don’t know why. This has a major impact not only on our timekeeping, but also on 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 how long a day is – has accelerated. This trend is shortening our days; in fact, in June 2022 we set the record for the shortest day in about half a century. But despite this record, since 2020 this steady acceleration has strangely turned into 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 for the Earth to complete one rotation is slightly different. These changes occur over 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.
A constantly changing planet
Over millions of years, the Earth’s rotation has slowed due to the frictional effects associated with the tides driven by the Moon. This process adds about 2.3 milliseconds to the length of each day of each century. A few billion years ago, Earth’s day lasted only about 19 hours.
For 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 polar ice sheets reduced the surface pressure and the Earth’s mantle began to move steadily towards the poles.
Just as ballet dancers spin faster when they move their arms closer to the body – the axis around which they spin – so our planet’s rotation speed increases as this mantle mass moves closer to the Earth’s axis. And this process shortens every day by about 0.6 milliseconds every century.
Over decades and longer, the connection between the Earth’s interior and the surface also comes into play. Large earthquakes can change the length of the day, although usually by a small amount. For example, the 2011 Great Tohoku earthquake in Japan with a magnitude of 8.9 accelerated the Earth’s rotation by a relatively tiny 1.8 microseconds.
In addition to these large-scale changes, weather and climate over shorter periods also have a significant impact on the Earth’s rotation, causing changes in both directions.
Fortnightly and monthly tidal cycles move matter around the planet, causing changes in day length of up to a millisecond in either direction. We can see tidal variations in day length records over periods of up to 18.6 years. The movement of our atmosphere has a particularly strong effect, and sea currents also play a role. Seasonal snowpack and rain or groundwater extraction change things further.
Why is the Earth suddenly slowing down?
Since the 1960s, when radio telescope operators around the planet began devising techniques to simultaneously observe cosmic objects such as quasars, we have had very accurate estimates of Earth’s rotation rate.
A comparison between these estimates and atomic clocks revealed an apparently ever-shortening day length over the past few years.
But once we remove the fluctuations in rotation rate that we know occur due to tides and seasonal effects, a startling revelation emerges. Despite Earth reaching its shortest day on June 29, 2022, the long-term trajectory appears to have shifted from shortening to lengthening since 2020. This change is unprecedented in the last 50 years.
The reason for this change is not clear. This could be due to changes in weather systems, with La Nina events, although they have occurred before. It could be the increased melting of the ice sheets, even though they have not deviated significantly from their steady rate of melting in recent years. Could it be related to the huge eruption of the volcano in Tonga that injected huge amounts of water into the atmosphere? Probably not, given that it happened in January 2022.
Scientists have speculated that this recent mysterious change in the planet’s rotation rate is related to a phenomenon called the “Chandler wobble,” a small deviation in Earth’s rotation axis with a period of about 430 days. Observations from radio telescopes also show that the fluctuation has diminished in recent years; both can be combined.
The last possibility that we think is plausible is that nothing concrete has changed inside or around the Earth. It could just be long-term tidal effects that work in parallel 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 essential for a number of applications, navigation systems such as GPS would not function without it. Timekeepers also insert leap seconds into our official timelines every few years to make sure they are out of sync with our planet.
If the Earth moved to even longer days, we might have to incorporate a “negative leap second,” which would be unprecedented and could break the Internet.
The need for negative leap seconds is now considered unlikely. For now, we can welcome the news that, at least for a while, we all have a few extra milliseconds each day.