Space & Science

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

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

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

This has critical impacts not only on our timing, 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. As this determines the length of a day, this trend has shortened our days. Indeed, in June 2022 we set a record for the shortest day in about half a century.

However, despite this record, since 2020 this constant acceleration has curiously turned into a slowdown. Now the days are getting longer again and the reason remains a mystery so far.

While our phone clocks say there are exactly 24 hours in a day, the actual time it takes 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 very rarely exactly the magic number of 86,400 seconds.

The ever-changing planet

The Earth’s rotation has been slowing for millions of years 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 every 100 years. 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 by bringing their arms towards their body – the axis around which they spin – our planet’s rotational speed increases as this mantle mass gets closer to Earth’s axis. This process has shortened each day by about 0.6 milliseconds each 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 have had very accurate estimates of the Earth’s rotation rate.


Using radio telescopes to measure the Earth’s rotation involves observations of radio sources like quasars. Credit: Nasa Godard

A comparison between these measurements 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 inject 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. Maybe the two are related.

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.

Written by:

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

This article was first published in The conversation.The conversation

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