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, Egli ha detto.
When dinosaurs still roamed the planet
70 milioni di anni fa, a single day on Earth lasted about
23 1/2 ore, secondo a 2020 study published in Paleoceanography and Paleoclimatology
Da 1820, scientists documented Earth’s rotation slowing down
, secondo la NASA
. In the past few years
, it began speeding up
, McCarthy ha detto.
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 ha detto.
Earth is slightly wider than it is tall, which makes it an oblate spheroid, Egli ha detto. The glaciers at the poles weigh down on the Earth’s crust at the North and South poles, McCarthy ha detto.
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, Egli ha detto. The circular shape helps the planet spin faster, McCarthy ha detto.
It’s the same phenomenon that figure skaters use to increase and decrease their speed, Egli ha detto.
When skaters stretch their arms away from their body as they spin, it takes more force for them to rotate, Egli ha detto. When they tuck their arms close to their body, their speed increases because their body mass is closer to their center of gravity, McCarthy ha detto.
As Earth becomes rounder, its mass becomes closer to its center, which increases its rotational speed, Egli ha detto.
Some have suggested a correlation with the Chandler wobble, McCarthy ha detto. 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, Egli ha detto.
When a player tosses a football, it wobbles slightly as it rotates since it often doesn’t spin around the axis of symmetry, Egli ha detto.
“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 ha detto.
tuttavia, 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, Egli ha detto.
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 ha detto.
“Our day-to-day existence doesn’t even recognize that millisecond,” McCarthy ha detto. “But if these things add up, then it could change the rate at which we insert a leap second.”
In the instances when the 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
, Egli ha detto. Ci sono stati 27 leap seconds added since
1972, secondo EarthSky
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 ha detto.
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, Egli ha detto.