Earth’s days have been mysteriously increasing—scientists don’t know why

Planet Earth Sunrise

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

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

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

The rotation of the Earth around its axis has accelerated in recent decades. Since this determines how long a day is, 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.

However, despite this record, since 2020 this steady acceleration has curiously turned into 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 the Earth to complete a single rotation can vary 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 constantly changing planet

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, a day on Earth was only about 19 hours long.

For 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 poleward.

Just as a ballet dancer spins faster as they bring their arms toward their body, the axis around which they spin, our planet’s rotation speed increases as this mantle mass approaches the axis of the Earth. This process has been shortening every day by about 0.6 milliseconds every century.

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

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

Fortnightly and monthly tidal cycles move mass around the planet, causing changes in day length of up to a millisecond in either direction. We can see tidal variations in day length records over 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 rainfall, or groundwater extraction, alters things further.

Why is the Earth suddenly slowing down?

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

Using radio telescopes to measure Earth’s rotation involves observations of radio sources such as quasars. Credit:

NASA
Founded in 1958, the National Aeronautics and Space Administration (NASA) is an independent agency of the United States federal government that succeeded the National Advisory Committee for Aeronautics (NACA). It is responsible for the civilian space program as well as aeronautical and aerospace research. His vision is “Discover and expand knowledge for the benefit of humanity.” Its core values ​​are “safety, integrity, teamwork, excellence and inclusion.”

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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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