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Earth had the shortest days since the invention of atomic clocks

Earth had the shortest days since the invention of atomic clocks
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Scientists have recorded the shortest day on Earth since the invention of atomic clocks.

A rotation is roughly the time it takes the Earth to rotate once on its axis 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 standard unit of measurement that has been used since the 1950s to tell time and measure Earth’s rotation, said Dennis McCarthy, retired director of the US Naval Observatory.

Despite June 29 breaking the record for the shortest day in modern history, there are many shorter days on Earth, he said.

70 million years ago when dinosaurs still roamed the planet, a day on Earth lasted about 23 1/2 hours, according to a study published in 2020. Paleoceanography and Paleoclimatology.
Since the 1820s, scientists have documented that Earth’s rotation is slowing, According to NASA. In the past few years, that has started to accelerate, McCarthy said.

Why is the speed increasing?

Researchers don’t have a definitive answer as to how or why the Earth is bending slightly faster, but it could be due to isostatic adjustment of glaciers or land movement caused by melting glaciers, McCarthy said.

He said that the Earth is slightly wider than it is tall, which makes it an obtuse sphere. Polar glaciers weigh on the Earth’s crust at the North and South Poles, McCarthy said.

Because the poles are melting due to the climate crisis, there is less pressure above and below the planet, which pushes the crust up and makes the Earth rounder, he said. The circular shape helps the planet rotate faster, McCarthy said.

It’s the same phenomenon that figure skaters use to accelerate and decelerate, he said.

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When skaters move their arms away from their body while spinning, it takes more force to spin, he said. When they pull their arms closer to their body, their speed increases because their body mass is closer to their center of gravity, McCarthy said.

As Earth becomes rounder, its mass gets closer to its center, which speeds up its rotation, he said.

Some have suggested a relationship with Chandler Wobble, 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 parts.

As the Earth rotates, it creates a slight wobble, like the wobble of throwing a football, he said.

When a player tosses a football, it wobbles a bit as it spins because it often doesn’t rotate around an axis of symmetry, he said.

“If you’re a really good passer in football, you align the axis of rotation with the axis of symmetry of the football, And it can’t waver,” McCarthy said.

However, McCarthy said that the Chandler double likely does not affect Earth’s rotation speed because of the shape of the double planet. If the planet’s shape changes, it changes the frequency of oscillations, not the frequency of its rotation, he said.

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Moving a leap second

Since researchers began measuring Earth’s rotation using atomic clocks, Earth has slowed its rotation, McCarthy said.

“Our day-to-day existence doesn’t even recognize that millisecond,” McCarthy said. “But if these things add up, it could change the rate at which we insert a leap second.”

As milliseconds build up over time, the scientific community has added a leap second to the clock to slow down our time to match Earth’s, he said. 27 leap seconds have been added since 1972, According to EarthSky.

Because Earth is now rotating faster, our timekeeping needs to take a leap second to catch up with Earth’s increasing rotational speed, McCarthy said.

If the planet continues this rotation trend, a leap second removal likely won’t happen for another three to four years, he said.

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