Earth breaks record for shortest day since atomic clock was invented

Earth completed its normal 24-hour rotation on June 29 at a rate of 1.59 milliseconds, breaking the record for the shortest day in modern history. (NASA)

Estimated reading time: 4-5 minutes

ATLANTA – If you think there are fewer hours of the day, you’re right.

Scientists have recorded the shortest day on Earth since the invention of the atomic clock.

On June 29, the Earth’s rotation was 1.59 milliseconds shorter than its normal 24-hour period, according to the International Earth Rotation and Reference System Service, the organization responsible for global timekeeping.

Rotation is the length of time it takes for the Earth to rotate once around its axis, approximately 86,400 seconds.

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

Dennis McCarthy, retired time director at the U.S. Naval Observatory, said the atomic clock is a standardized unit of measurement that has been used to time and measure the Earth’s rotation since the 1950s.

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

When dinosaurs were still roaming the Earth 70 million years ago, a day on Earth lasted about 23.5 hours, according to a 2020 study published in Paleooceanography and Paleoclimatology.

Since 1820, scientists have recorded a slowing of Earth’s rotation, according to NASA. Over the past few years, it has started to accelerate, McCarthy said.

Why is it getting faster and faster?

McCarthy said researchers don’t have a clear answer to how or why the planet is spinning slightly faster, but it could be due to an equilibrium adjustment in glaciers, or land shifts due to melting glaciers.

The Earth is slightly wider than its height, which makes it an oblate spheroid, he said. McCarthy said glaciers at the poles press on the crust of the Arctic and Antarctic.

Because 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 upwards, making the planet more round, he said. The circular shape helps planets spin faster, McCarthy said.

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

When skaters remove their arms from their bodies as they spin, they need more force to spin, he said. McCarthy said that when they brought their arms closer to their body, their speed increased because their weight was closer to the center of gravity.

As the Earth becomes more round, its mass becomes closer to its center, which increases its rotational speed, he said.


Our everyday life doesn’t even recognize that millisecond.

—Dennis McCarthy, retired time director, U.S. Naval Observatory


McCarthy said some believe it has something to do with Chandler’s swing. The axis of our planet’s rotation is not aligned with its axis of symmetry, the invisible vertical line that divides the Earth in equal halves.

As the earth spins, this creates a slight wobble, similar to the way a football is thrown, he said.

When a player throws a football, it wobbles slightly as it rotates because it doesn’t usually rotate around an axis of symmetry, he said.

“If you’re a really good football passer and you align the axis of rotation with the axis of symmetry of the football, it won’t wobble,” McCarthy said.

However, McCarthy said the Chandler wobble probably didn’t affect the 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 wobble, not the frequency of its rotation, he said.

remove leap seconds

The Earth has been slowing down since researchers started using atomic clocks to measure its rotation, McCarthy said.

“Our daily lives don’t even recognize milliseconds,” McCarthy said. “But if these things add up, then it could change the speed at which we insert leap seconds.”

With milliseconds increasing with time, the scientific community has added a leap second to the clock to slow down our synchronization with Earth, he said. According to EarthSky, 27 leap seconds have been added since 1972.

McCarthy said that because the Earth is spinning faster now, a leap second needs to be removed to keep up with the Earth’s increasing rotational speed.

If the Earth continues this rotational trend, the elimination of leap seconds could take another three to four years, he said.

Recent Science Stories

More stories you might be interested in

Leave a Comment

Your email address will not be published.