The Earth Had Its Shortest Day in Recorded History

On June 29, our planet completed one rotation in 1.59 milliseconds less than 24 hours

Satellite image of the earth
Generally, the Earth is slowing its spin ever so slightly, so why it seems to be speeding up lately is a mystery. NASA Earth Observatory / Joshua Stevens; NOAA National Environmental Satellite, Data, and Information Service

The Earth completed its shortest rotation in recorded history on June 29 this year, shaving 1.59 milliseconds off of the approximately 24-hour day. Generally, the Earth is slowing its spin ever so slightly, so why it seems to be speeding up is a mystery. 

"It's certainly odd," Matt King, a professor of geodesy at the University of Tasmania, tells Genelle Weule of the Australian Broadcasting Corporation (ABC). "Clearly something has changed, and changed in a way we haven't seen since the beginning of precise radio astronomy in the 1970s."

Because the Earth’s rotational speed varies, we use a standardized Coordinated Universal Time, or UTC, to make sure our clocks are in sync across the world. Two components determine UTC: International Atomic Time (TAI) and Universal Time (UT1). 

UT1 is a time standard based on the Earth’s rotation, whereas TAI is a time scale that provides the speed at which our clocks should tick. TAI is based on the average of hundreds of precise atomic clocks in timing labs across the world. It’s incredibly accurate, but it doesn’t take into account slight variations in how the Earth spins.

"When you start looking at the real nitty gritty, you realize that Earth is not just a solid ball that is spinning," Fred Watson, Australia's astronomer-at-large, tells ABC. "It's got liquid on the inside, it's got liquid on the outside, and it's got an atmosphere and all of these things slosh around a bit.”

The Earth’s spin is generally slowing, so a leap second (an extra second every 1.5 years or so) is added to allow it to catch up to the clocks. 

But now it seems the Earth is spinning quicker. In 2020, scientists recorded 28 of the shortest days since 1960, per's Graham Jones and Konstantin Bikos. On July 26, 2022, the Earth rotated around in 1.5 milliseconds less than 24 hours, per ABC. 

If the trend continues, scientists may need to consider skipping a second, which would be the first deletion in history. But ABC reports this likely wouldn’t happen for another eight to ten years at the current rate. 

Technology companies, such as Meta, have spoken out against the leap second, saying it is bad for both digital applications and scientists, who often use TAI or UT1.

“Every leap second is a major source of pain for people who manage hardware infrastructures,” write Oleg Obleukhov and  Ahmad Byagowi in a Meta blog post. “The impact of a negative leap second has never been tested on a large scale; it could have a devastating effect on the software relying on timers or schedulers.”

Since the 1970s, the world has added 27 leap seconds, per ABC. 

“I hope that Earth’s acceleration stops and we don’t need to subtract a second, but who knows?” Leonid Zotov at the Sternberg Astronomical Institute of Lomonosov Moscow State University tells Forbes' Jamie Carter. “Predicting variations in Earth’s rotation is almost as difficult as predicting stock prices.”

Get the latest stories in your inbox every weekday.