Science

The length of Earth’s days has mysteriously increased – Scientists do not know why

Planet Earth Sunrise
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Planet Earth Sunrise

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

Precise astronomical observations, along with atomic clocks, revealed that the length of a day suddenly lengthened. Scientists don’t know why.

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

The rotation of the Earth on its axis has been accelerating in the last few decades. Because it determines how long a day is, this tendency shortens our days. In fact, in June 2022 we made a record for the shortest day of the last half century.

However, despite this record, since 2020 this steady pace has curiously slowed down. Now the days are getting longer again and the reason remains a mystery until now.

Although the clocks on our phones show exactly 24 hours in a day, the actual time it takes for the Earth to complete a single rotation can vary very little. These changes sometimes take place over millions of years, other times almost instantaneously. For example, even earthquakes and storm events can play a role.

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

A constantly changing planet

Earth’s rotation slows down over millions of years due to frictional effects associated with the moon’s tides. This process adds about 2.3 milliseconds to the length of each day every 100 years. A few billion years ago, Earth’s day was only approx 19 hours.

Over the past 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 sheets reduced the surface pressure and the Earth’s mantle began to move steadily towards the poles.

Just as a ballet dancer spins faster as she brings her arms toward her body—the axes on which they spin—our planet’s spin speeds up as this mantle mass moves closer to Earth’s axis. This process shortens by about 0.6 milliseconds every day every century.

Over decades and longer, the relationship between Earth’s interior and surface also comes into play. Large earthquakes, although usually small in magnitude, can change the length of the day. For example, the Great Tohoku earthquake that struck Japan in 2011 with a magnitude of 8.9 is believed to have caused a relatively small acceleration of the Earth’s rotation. 1.8 microseconds.

In addition to these large-scale changes, shorter weather and climate cycles also have an important effect on the Earth’s rotation, causing changes in both directions.

Biweekly and monthly tidal cycles move mass around the planet, causing the length of the day to vary by milliseconds in either direction. We can see the tide changes In records of day length for periods longer than 18.6 years. The movement of our atmosphere has a particularly strong influence, and ocean currents also play a role. Seasonal snow cover and precipitation or groundwater withdrawals change things further.

Why is the Earth suddenly slowing down?

Since the 1960s, radio telescope operators around the planet have been developing the techniques. at the same time observe cosmic objects such as quasarsWe have very accurate estimates of the Earth’s rotation speed.


Using radio telescopes to measure Earth’s rotation involves observations 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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