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We lost 1.59 milliseconds June 29 when the Earth spun a little faster

ALINA SELYUKH, HOST:

Does it ever feel like time is just slipping away? Well, this year, it kind of did by at least a whopping 1.6 milliseconds. On June 29, 2022, the Earth spun just a little bit faster than normal, causing some outlets to report that it was the shortest day in history. But Duncan Agnew says not so fast. He is a geophysicist at the University of California, San Diego.

DUNCAN AGNEW: It's been the shortest day since we started making measurements with atomic clocks. So that's 1960.

SELYUKH: It was merrily the shortest day in my lifetime but not everyone's.

AGNEW: There were shorter days in 1930. There were much shorter days in 1870. In the times of the Roman Empire, it was 30 milliseconds shorter.

SELYUKH: Thirty milliseconds. Just for some context, there are 1,000 milliseconds in a second. So this is not why Rome fell. On average, Agnew says, the Earth is actually slowing down. 1.4 billion years ago, for example, a day was less than 19 hours long. Isn't that mind-blowing? So why does the Earth speed up and slow down?

AGNEW: It's a physics problem.

SELYUKH: As Agnew explains, the Earth has gas and liquids that move around. For example...

AGNEW: Inside the Earth is this giant ball of molten iron.

SELYUKH: And there are irregularities in how it flows around.

AGNEW: And as those currents change and shift around, that'll cause the solid part of the earth to rotate a little faster or a little slower.

SELYUKH: Another force that could be at play, melting ice caps. That adds to the ocean and changes the Earth's shape. When you change your shape and your spin, you also change your speed, says Agnew.

AGNEW: It's the effect you see in an ice skater. When she pulls her arms in, she starts spinning around really fast.

SELYUKH: The fact is, says geophysicist Duncan Agnew, this is all really normal. You probably didn't even notice that you lost those 1.6 milliseconds, but they do matter to some folks. The International Earth Rotation and Reference System Service has occasionally added leap seconds to keep clocks in line with the Earth's slowing rotation. But there's never been a negative leap second - a leap back second? The next opportunity for the service to adjust time is when the year ends. So I guess we'll have to wait until then to see if 2022 will put us out of our misery a hair sooner than usual. Five, four, three, one. Happy New Year.

(SOUNDBITE OF MUSIC) Transcript provided by NPR, Copyright NPR.

Alina Selyukh is a business correspondent at NPR, where she follows the path of the retail and tech industries, tracking how America's biggest companies are influencing the way we spend our time, money, and energy.