Scientists have recorded the shortest day on Earth since the invention of the atomic clock.
A rotation is the time it takes the Earth to rotate once on its axis, which is approximately 84,600 seconds.
The previous record was set on July 19, 2020, when the day was 1.47 milliseconds shorter than normal.
The atomic clock is a standardized unit of measurement that has been used since the 1950s to tell time and measure the Earth’s rotation, said Dennis McCarthy, retired director of weather at the U.S. Naval Observatory.
Although June 29 broke the record for the shortest day in modern history, there have been many shorter days on Earth, he said.
When dinosaurs still roamed the planet 70 million years ago, a day on Earth lasted about 23 1/2 hours, according to a 2020 study published in Paleoceanography and Paleoclimatology. Since 1820, scientists have documented a slowdown in Earth’s rotation, according to NASA. In the past few years, it has started to accelerate, McCarthy said.
Why is the speed increased?
Researchers don’t have a definitive answer as to how or why the Earth rotates a little faster, but it could be due to glacial isostatic adjustment or land movement due to melting glaciers, McCarthy said.
The Earth is slightly wider than it is tall, making it a flattened spheroid, he said. The polar ice caps weigh on the Earth’s crust at the north and south poles, McCarthy said.
As the poles melt due to the climate crisis, there is less pressure on the top and bottom of the planet, which moves the crust up and makes the Earth rounder, he said. The round shape helps the planet spin faster, McCarthy said.
It’s the same phenomenon figure skaters use to increase and decrease their speed, he said.
When skaters extend their arms away from their body as they spin, they need more power to spin, he said. When they bring their arms close to their body, their speed increases because their body mass is closer to their center of gravity, McCarthy said.
As the Earth gets rounder, its mass gets closer to the center, which increases its rotation speed, he said.
Some have suggested a connection to Chandler’s hesitation, McCarthy said. The axis on which our planet rotates is not aligned with its axis of symmetry, an invisible vertical line that divides the Earth into two equal halves.
This creates a slight wobble as the Earth rotates, similar to how a soccer ball wobbles when it is thrown, he said.
When a player throws a football, it wobbles slightly as it spins because it often doesn’t spin around the axis of symmetry, he said.
“If you’re a really good passer in soccer, you align the axis of rotation with the axis of symmetry of the soccer ball and it doesn’t wobble,” McCarthy said.
However, McCarthy said Chandler’s wobble probably doesn’t affect Earth’s rotation rate because the wobble is due to the planet’s shape. If the planet’s shape changes, it changes the oscillation frequency, not the rotation frequency, he said.
Remove leap second
Ever since researchers began measuring the Earth’s rotation rate using atomic clocks, the Earth has been slowing its rotation rate, McCarthy said.
“Our daily existence doesn’t even recognize that millisecond,” McCarthy said. “But if these things come together, it could change the rate at which we insert a leap second.”
In cases where milliseconds accumulate over time, the scientific community has added a leap second to the clock to slow our time down to match Earth’s, he said. Since 1972, 27 leap seconds have been added, according to EarthSky.
Because Earth is now spinning faster, it would take a leap second for our time to catch up with Earth’s increasing spin rate, McCarthy said.
If the planet continues this rotational trend, eliminating a leap second probably won’t have to happen for another three to four years, he said.
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