Earth’s days are mysteriously getting longer, scientists say

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 only on our timing, but also on things like GPS and other technologies that govern our modern lives.

In recent decades, the rotation of the Earth around its axis – which determines the length of a day – has accelerated. This trend has shortened our days; in fact, in June 2022 we set a record for the shortest day in about half a century.

But despite this record, since 2020 this steady acceleration has curiously turned into a slowdown – the days are getting longer again, and the reason is so far a mystery.

While our phone clocks say there are exactly 24 hours in a day, the actual time it takes Earth to complete a single rotation varies very slightly. These changes occur over periods of millions of years almost instantaneously – even earthquakes and storms can play a role.

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

The ever-changing planet

For millions of years, the rotation of the Earth has slowed down due to the frictional effects associated with the tides driven by the moon. This process adds about 2.3 milliseconds to the length of each day each century. A few billion years ago, an Earth day was only about 19 hours.

For 20,000 years, another process has been acting in the opposite direction, accelerating the rotation of the Earth. At the end of the last ice age, the melting of the polar ice caps reduced the surface pressure and the Earth’s mantle began to move gradually towards the poles.

Just as a ballet dancer spins faster as he brings his arms closer to his body – the axis around which he spins – our planet’s rotational speed increases as this mass of mantle gets closer to Earth’s axis. And this process is shortening every day by about 0.6 milliseconds per century.

As decades and more go by, the connection between the Earth’s interior and surface also comes into play. Major earthquakes can alter the length of the day, although normally by small amounts.

For example, the 2011 Great Tōhoku earthquake in Japan, with a magnitude of 8.9, is said to have accelerated the Earth’s rotation by a relatively minute rate. 1.8 microseconds.

In addition to these large-scale changes, over shorter time periods, weather and climate also have large impacts on the Earth’s rotation, causing variations in both directions.

Bi-weekly and monthly tidal cycles move mass around the planet, causing changes in day length of up to a millisecond in either direction. You can see tidal variations in daylength records over periods as long as 18.6 years.

The movement of our atmosphere has a particularly strong effect, and ocean currents also play a role. Snow cover and seasonal precipitation, or groundwater extraction, further alter things.

Why is the Earth suddenly slowing down?

Since the 1960s, when radio telescope operators around the planet began devising techniques to simultaneously observe cosmic objects like quasarswe had very accurate estimates of the Earth’s rotation rate.

A comparison between these estimates and an atomic clock revealed a seemingly shorter day length over the past few years.

But there is a startling revelation once we rule out the fluctuations in rotational speed that we experience due to tides and seasonal effects. Although Earth reached its shortest day on June 29, 2022, the long-term trajectory appears to have shifted from shortening to lengthening since 2020. This change is unprecedented in the past 50 years.

The reason for this change is unclear. This could be due to changes in weather systems, with back-to-back La Niña events, although these have happened before. It could be an increased melting of the ice caps, although these have not deviated greatly from their regular rate of melting in recent years.

Could this be related to the huge volcano explosion in Tonga injecting huge amounts of water into the atmosphere? Probably not, given that it happened in January 2022.

Scientists have hypothesized This recent and mysterious change in the rotational speed of the planet is linked to a phenomenon called the “Chandler Oscillation” – a small deviation of the axis of rotation of the Earth with a period of about 430 days.

Observations from radio telescopes also show that the oscillation has diminished in recent years; the two can be linked.

A final possibility, which we think is plausible, is that nothing specific has changed in or around the Earth. They could simply be long-term tidal effects working in parallel with other periodic processes to produce a temporary change in the Earth’s rate of rotation.

Do we need a “negative leap second”?

Understanding the Earth’s rotation rate precisely is crucial for a host of applications – navigation systems such as GPS wouldn’t work without it. Additionally, every few years, timekeepers insert leap seconds into our official time scales to ensure they don’t get out of sync with our planet.

If Earth were to transition to even longer days, we may need to incorporate a “negative leap second” – this would be unprecedented, and can break internet.

The need for negative leap seconds is considered unlikely at this time. For now, we can rejoice in the news that – at least for a while – we all get a few extra milliseconds every day.The conversation

Matt Kingdirector of the ARC Australian Center for Excellence in Antarctic Science, University of Tasmania and Christopher WatsonLecturer, School of Geography, Planning and Space Sciences, University of Tasmania.

This article is republished from The conversation under Creative Commons license. Read it original article.

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