Earth has had the shortest day since the invention of the atomic clock

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, approximately 84,600 seconds.

The previous record was documented on July 19, 2020, when the day was 1.47 milliseconds slower than normal.

The atomic clock is a standardized unit of measurement used since the 1950s to tell time and measure the Earth’s rotation, said Dennis McCarthy, retired director of time at the US Naval Observatory.

Although June 29 broke the record for the shortest day in modern history, there have been much shorter days on Earth, he said.

When dinosaurs still roamed the planet 70 million years ago, a single day on Earth lasted around 23.5 hours, according to a 2020 study published in Paleooceanography and paleoclimatology.
Since 1820, scientists have documented the slowing of the Earth’s rotation, according to NASA. Over the past few years, it’s started to pick up speed, McCarthy said.

Why does the speed increase?

Researchers don’t have a definitive answer on how or why Earth is spinning slightly faster, but it may be due to glacial isostatic adjustment or land movement from melting glaciers, McCarthy said.

The Earth is slightly wider than it is tall, making it an oblate spheroid, he said. Glaciers at the poles press down on Earth’s crust at the North and South Poles, McCarthy said.

Since the poles are melting 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 circular shape helps the planet spin faster, McCarthy said.

It’s the same phenomenon that figure skaters use to increase and decrease their speed, he said.

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When skaters move their arms away from their bodies as they spin, it takes more force for them to spin, he said. When they bring their arms closer to their body, their speed increases because their body mass is closer to their center of gravity, McCarthy said.

As the Earth rounds out, its mass moves closer to its center, increasing its rotational speed, he said.

Some have suggested a correlation with the 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 halves.

This creates a slight wobble as the Earth spins, similar to the way a soccer ball wobbles when thrown, he said.

When a player kicks a soccer ball, it wobbles slightly as it spins because it often doesn’t spin around the axis of symmetry, he said.

“If you are a very good passer in football, you align the axis of rotation with the axis of symmetry in football, and he doesn’t waver,” McCarthy said.

However, McCarthy said the Chandler wobble probably doesn’t affect Earth’s rotational speed because the wobble is due to the shape of the planet. If the planet’s shape changes, it changes the frequency of the oscillation, not its rotational frequency, he said.

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Delete a leap second

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

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

In cases where milliseconds add up over time, the scientific community has added a leap second to the clock to slow our time to match Earth’s, he said. There have been 27 leap seconds added since 1972, according to EarthSky.

Because Earth is now spinning faster, a leap second should be removed to catch up with Earth’s increasing rotational speed, McCarthy said.

If the planet continues this rotational trend, the leap second removal probably wouldn’t need to happen for three to four years, he said.

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