The length of Earth’s days has increased, and scientists do not know why

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

But despite this record, since 2020 the trend has changed and it seems that the rotation of the Earth has slowed down: the days have become longer again, and the reason is still a mystery.

While the clocks on our phones show that there are exactly 24 hours in a day, a day rarely matches the magic number of 86,400 seconds exactly. The actual time it takes the Earth to complete one revolution varies slightly. These changes occur over periods ranging from millions of years to almost instantaneous; even earthquakes and storms can play their part.

An ever-changing planet

Over millions of years, the Earth’s rotation has slowed down due to frictional effects associated with tides caused by the Moon. This process adds about 2.3 milliseconds to the length of each day over a century. A few billion years ago, an Earth day lasted only about 19 hours.

For the past 20,000 years, another process has been working in the opposite direction, accelerating the Earth’s rotation. What we mean is that when the last ice age ended, the melting of the polar ice caps reduced the pressure on the surface, and the Earth’s mantle began to move steadily towards the poles.

Just as a ballerina spins faster when she brings her arms closer to her body—the axis she is spinning around—so our planet’s rotational speed increases as this mass of mantle approaches the Earth’s axis. And this process is shortening every day by about 0.6 milliseconds every century.

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

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

Biweekly and monthly tidal cycles move mass around the planet, causing changes in the length of the day to the millisecond in either direction. We can see tidal variations in day length records for periods up to 18.6 years.

The movement of our atmosphere has a particularly strong influence, with ocean currents also playing a key role. Snow cover and seasonal rainfall or groundwater extraction further aggravate the situation.

Why is the Earth suddenly slowing down?

Since the 1960s, when radio telescope operators around the world began developing methods for simultaneously observing space objects such as quasars, we have very accurate estimates of the Earth’s rotation rate.

Comparison of these estimates with atomic clocks showed that in recent years the length of the day has become increasingly shorter.

But once we remove the fluctuations in rotational speed that we know are due to tidal and seasonal effects, we have a surprising discovery. Even though the Earth reached its shortest day on June 29, 2022, as of 2020, the long-term trend appears to have changed from short to long. This change is unprecedented in the last 50 years.

The reason is not clear. This may be due to changes in weather systems, with successive La Niña events, although they have happened before. This could be further melting of the ice sheets, although they have not deviated much from their constant rate of melt in recent years. Could this be due to the powerful explosion of the Tonga volcano, which threw a huge amount of water into the atmosphere? Probably not, considering it happened in January 2022.

Scientists have speculated that this mysterious recent change in the planet’s rotational speed is due to a phenomenon called the Chandler wobble, a slight tilt of the Earth’s axis of rotation with a period of about 430 days. Observations from radio telescopes also show that the wobble has decreased in recent years. Both can be related.

The last possibility that seems plausible to us is that nothing much has changed either inside or around the Earth. These may simply be long-term tidal effects, working in parallel with other periodic processes and causing a temporary change in the speed of the Earth’s rotation.

Do we need a “negative leap second”?

Knowing exactly how fast the Earth is rotating is critical for a number of applications: navigation systems like GPS won’t work without it. In addition, every few years timekeepers introduce leap seconds into our official time scale so that they do not get out of sync with our planet.

If days were even longer on Earth, a “negative leap second” would have to be included, which would be unprecedented and could break the internet.

The need for negative leap seconds is currently considered unlikely. For now, we can be satisfied with the news that – at least for a while – we all have a few more milliseconds every day.

Matt King, Director, Australian Center of Excellence for Antarctic Sciences, University of Tasmania, and Christopher Watson, Senior Lecturer, School of Geography, Planning and Spatial Sciences, University of Tasmania.

This article was originally published on The Conversation. Read the original.

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