The Earth completed its normal 24-hour rotation with a speed of 1.59 milliseconds on June 29, breaking the record for the shortest day in modern history. (NASA)
Estimated reading time: 4-5 minutes
ATLANTA — If it feels like there’s less time in the day, you’re right.
Scientists have recorded the shortest day on Earth since the invention of the atomic clock.
The rotation of our planet measured at 1.59 milliseconds slower than the normal 24-hour day on June 29, according to the International Earth Rotation and Reference Systems Servicean organization in charge of world timekeeping.
A rotation is the time it takes the Earth to rotate once on its axis, which is approximately 86,400 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.
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.
Our daily existence does not even recognize this millisecond.
–Dennis McCarthy, retired weather director at the US Naval Observatory
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’re a really good passer in football, you align the axis of rotation with the axis of symmetry of the ball, and it doesn’t wobble,” 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 shape of the planet changes, it changes the frequency of the oscillation, not its rotational frequency, he said.
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 those 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.