Why is the earth suddenly spinning faster?
By Kai Stoppel
A day is divided into 24 hours. But the Earth does not meet this specification; sometimes it spins a little slower or faster. What is striking is that it has been spinning faster and faster in recent years. But why is that?
The earth spins faster and faster. And with that, the length of the day decreases. Of June 29, 2022 was the shortest day since the 1960s. At that time, atomic clocks were used to accurately determine the length of the day. Therefore, June 29, 2022 was 1.59 milliseconds shorter than the average day. This lasts for 86,400 seconds or 24 hours. Finally, on July 26, a new record was lost: the day was 1.5 milliseconds shorter.
And a trend is emerging: “Since 2016, the earth has started to speed up,” Leonid Zotov of Lomonosov Moscow State University told US broadcaster CBS. “It’s spinning faster this year than it was in 2021 and 2020.” In 2020, the average length of the day had already been exceeded 28 times. But why does the Earth’s rotation speed up?
“The cause has not been clarified,” Ulrich Köhler, a planetary geologist at the Institute for Planetary Research at the German Aerospace Center (DLR), tells ntv.de. “But it certainly has something to do with the fact that there are massive displacements in or on Earth.” Because the earth always spins faster when the mass gets closer to its center. “You know this from a figure skater doing a pirouette. If she brings her arms closer to her body, she spins faster,” explains Köhler. Therefore, we speak of the pirouette effect.
Climate change a possible reason
But what is the mass that last approached the center in the fall of the Earth? According to Köhler, the answer to the riddle could lie in the liquid outer core of the earth. The currents could have changed the proportions of the elements iron and nickel there. So nickel is slightly heavier than iron. If their ratio increases near the center of the earth, this speeds up the rotation. However, this cannot be measured from the outside, only modeled, according to Köhler.
But many other effects can also influence the speed of rotation. The earthquake that triggered the devastating tsunami in the Indian Ocean in 2004 caused rocks to collapse and thus speed up the Earth’s rotation. As a result, the length of the day decreased by 2.7 microseconds. “The melting of glaciers on the earth’s surface as a result of climate change could also be a cause,” says Köhler. However, he believes it is “very bold” to blame the accelerated rotation of the Earth solely on climate change.
The influences on the Earth’s rotation are so numerous that it’s hard to find the exact cause, Köhler suspects. “A change in the jet stream also affects the length of the day. And strong winds, hitting the Rocky Mountains or the Andes as obstacles, for example, can slow Earth’s rotation a bit.” Volcanic eruptions move mass from the center of the earth outwards. “This, in turn, has the opposite effect and slows down the rotation of the earth a bit.”
In the long run, the rotation slows down.
But the recent acceleration is likely to remain a temporary phenomenon. Because seen in the long term, the speed of rotation of the earth decreases continuously. In the course of earth’s history to date, the days have gotten longer and longer, at an average of 1.78 milliseconds per century. In the time of the dinosaurs, a day was only 23 hours and 30 minutes instead of 24 hours. 1.4 billion years ago it was even less than 19 hours.
One reason is the moon. With its gravitational attraction, it causes the sea tides on earth. These in turn slow down the rotation of the earth. This is one of the reasons why leap seconds were officially introduced in 1972. They will be added in late July or late December and should give earth a chance to catch up again. This has already happened 27 times since the 1970s, but the last time was in 2016. There will also be no leap second this year. Due to the recent acceleration of the Earth, a negative leap second may soon be necessary; would be the first.
By the way: The long-term reduced angular momentum of the Earth is transmitted back to the Moon by the tidal braking effect. This makes it faster and it moves away from the ground over time. Each year it is about four centimeters on average.
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