Atomic clocks, combined with precise astronomical measurements, have revealed that the length of a day is suddenly getting longer, and scientists don’t know why.

This has critical impacts not just on our timekeeping, but also things like GPS and other technologies that govern our modern life.

Over the past few decades, Earth’s rotation around its axis – which determines how long a day is – has been speeding up. This trend has been making our days shorter; in fact, in June 2022 we set a record for the shortest day over the past half a century or so.

But despite this record, since 2020 that steady speedup has curiously switched to a slowdown – days are getting longer again, and the reason is so far a mystery.

While the clocks in our phones indicate there are exactly 24 hours in a day, the actual time it takes for Earth to complete a single rotation varies ever so slightly. These changes occur over periods of millions of years to almost instantly – even earthquakes and storm events can play a role.

It turns out a day is very rarely exactly the magic number of 86,400 seconds.

The ever-changing planet
Over millions of years, Earth’s rotation has been slowing down due to friction effects associated with the tides driven by the Moon. That process adds about about 2.3 milliseconds to the length of each day every century. A few billion years ago an Earth day was only about 19 hours.

For the past 20,000 years, another process has been working in the opposite direction, speeding up Earth’s rotation. When the last ice age ended, melting polar ice sheets reduced surface pressure, and Earth’s mantle started steadily moving toward the poles.

Just as a ballet dancer spins faster as they bring their arms toward their body – the axis around which they spin – so our planet’s spin rate increases when this mass of mantle moves closer to Earth’s axis. And this process shortens each day by about 0.6 milliseconds each century.

Read more: https://theconversation.com/the-length-of-earths-days-has-been-mysteriously-increasing-and-scientists-dont-know-why-188147