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Earth’s days have mysteriously increased in length: scientists don’t know why

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Precise measurements show that the Earth’s rotation has mysteriously slowed down since 2020, making the day longer.

Accurate astronomical observations, combined with atomic clocks, have revealed that the length of a day suddenly lengthens. Scientists don’t know why.

This has a critical impact not only on our timekeeping, but also on things like GPS and other precision technologies that govern our modern lives.

The Earth’s rotation around its axis has accelerated in recent decades. Since this determines the length of a day, this trend has made our days shorter. In fact, in June 2022 we set a record for the shortest day in the last half century or so.

Yet despite this record, since 2020 that steady acceleration has curiously changed to a slowdown. Now, the days are getting longer again, and the reason so far remains a mystery.

Although the clocks on our phones indicate that there are exactly 24 hours in a day, the actual time it takes for the Earth to complete a single rotation can vary very slightly. These changes sometimes occur over periods of millions of years and other times almost instantaneously. For example, even earthquakes and storms can play a role.

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

The ever changing planet

Earth’s rotation has been slowing for millions of years due to the frictional effects associated with tides driven by the Moon. That process adds about 2.3 milliseconds to the length of each day every 100 years. A few billion years ago, an Earth day was only about 19 hours.

For the last 20,000 years, another process has been working in the opposite direction, speeding up the Earth’s rotation. When the last ice age ended, the melting of the polar ice caps lowered the surface pressure and the Earth’s mantle began to move steadily toward the poles.

Just like a ballet dancer spins faster when she brings her arms in toward her body, the axis around which she spins, our planet’s spin rate increases as this mantle mass approaches Earth’s axis. This process has been shortening every day by about 0.6 milliseconds every century.

For decades and more, the connection between the interior and the surface of the Earth also comes into play. Large earthquakes can change the length of the day, although usually by small amounts. For example, the 2011 Great Tōhoku Earthquake in Japan, with a magnitude of 8.9, is believed to have accelerated the Earth’s rotation by a relatively small amount. 1.8 microseconds.

Apart from these large-scale changes, over shorter periods the weather and climate also have major impacts on the Earth’s rotation, causing variations in both directions.

Biweekly and monthly tidal cycles move mass around the planet, causing changes in day length of up to a millisecond in any direction. We can see tidal variations in day length records for periods of up to 18.6 years. The movement of our atmosphere has a particularly strong effect, and ocean currents also play a role. Seasonal snow cover and rains, or groundwater extraction, mess things up even more.

Why is the Earth suddenly slowing down?

Since the 1960s, when radio telescope operators around the globe began devising techniques to simultaneously observe cosmic objects such as quasarswe have had very precise estimates of the Earth’s rotation rate.


The use of radio telescopes to measure the Earth’s rotation involves the observation of radio sources such as quasars. Credit:[{” attribute=””>NASA Goddard

A comparison between these measurements and an atomic clock has revealed a seemingly ever-shortening length of day over the past few years.

But there’s a surprising reveal once we take away the rotation speed fluctuations we know happen due to the tides and seasonal effects. Despite Earth reaching its shortest day on June 29, 2022, the long-term trajectory seems to have shifted from shortening to lengthening since 2020. This change is unprecedented over the past 50 years.

The reason for this change is not clear. It could be due to changes in weather systems, with back-to-back La Niña events, although these have occurred before. It could be increased melting of the ice sheets, although those have not deviated hugely from their steady rate of melt in recent years. Could it be related to the huge volcano explosion in Tonga injecting huge amounts of water into the atmosphere? Probably not, given that occurred in January 2022.

Scientists have speculated this recent, mysterious change in the planet’s rotational speed is related to a phenomenon called the “Chandler wobble” – a small deviation in Earth’s rotation axis with a period of about 430 days. Observations from radio telescopes also show that the wobble has diminished in recent years. Perhaps the two are linked.

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

Do we need a ‘negative leap second’?

Precisely understanding Earth’s rotation rate is crucial for a host of applications – navigation systems such as GPS wouldn’t work without it. Also, every few years timekeepers insert leap seconds into our official timescales to make sure they don’t drift out of sync with our planet.

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

The need for negative leap seconds is regarded as unlikely right now. For now, we can welcome the news that – at least for a while – we all have a few extra milliseconds each day.

Written by:

  • Matt King – Director of the ARC Australian Centre for Excellence in Antarctic Science, University of Tasmania
  • Christopher Watson – Senior Lecturer, School of Geography, Planning, and Spatial Sciences, University of Tasmania

This article was first published in The Conversation.The Conversation

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