Scientists recorded the shortest day on Earth since the invention of the atomic clock.
A rotation is the length of time the Earth takes to spin once on its axis, which is roughly 86,400 seconds.
The previous record was documented on July 19, 2020, when the day measured 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 time at the US Naval Observatory.
Despite June 29 breaking a record for the shortest day in modern history, there have been much shorter days on Earth, he said.
Why is the speed increasing?
Researchers do not have a definitive answer on how or why Earth is turning slightly faster, but it may be due to glacial isostatic adjustment, or the movement of land due to melting glaciers, McCarthy said.
Earth is slightly wider than it is tall, which makes it an oblate spheroid, he said. The glaciers at the poles weigh down on the 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.
When skaters stretch their arms away from their body as they spin, it takes more force for them to rotate, he said. When they tuck their arms close 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 becomes closer to its center, which increases its rotational speed, he said.
Some have suggested a correlation with the Chandler wobble, McCarthy said. The axis our planet rotates on is not lined up 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 football wobbles when it is thrown, he said.
When a player tosses a football, it wobbles slightly as it rotates since it often doesn’t spin around the axis of symmetry, he said.
“If you’re a really good passer in football, you line up the axis of rotation with the axis of symmetry of the football, and it doesn’t wobble,” McCarthy said.
However, McCarthy said the Chandler wobble likely does not affect the rotational speed of Earth because the wobble is due to the planet’s shape. If the planet’s shape changes, it changes the frequency of the wobble, not its rotation frequency, he said.
Removing a leap second
Since researchers began measuring the Earth’s rotational speed using atomic clocks, Earth had been slowing down its rotational speed, McCarthy said.
“Our day-to-day existence doesn’t even recognize that millisecond,” McCarthy said. “But if these things add up, then it could change the rate at which we insert a leap second.”
Because Earth is now rotating faster, a leap second would need to be taken away to catch our timekeeping up with Earth’s increasing rotational speed, McCarthy said.
If the planet continues this rotational trend, the removal of a leap second likely wouldn’t need to happen for another three to four years, he said.
Correction: An earlier version of this story gave the incorrect number of seconds it takes the Earth to spin once on its axis.