Les journées sur Terre deviennent plus longues, et les scientifiques ne savent pas encore pourquoi – Edition du soir Ouest-France – 19/08/2022

Les journées sur Terre deviennent plus longues, et les scientifiques ne savent pas encore pourquoi - Edition du soir Ouest-France - 19/08/2022

By Matt KING, Director, Australian Research Council (ARC) Antarctic Science Center of Excellence, and Christopher WATSON, Senior Lecturer, School of Geography, Planning and Space Sciences, University of Tasmania

Although the Earth reached its shortest day on June 29, 2022, its rotation has slowed down, giving us a few extra milliseconds each day. A mystery to scientists, who nevertheless have several hypotheses to explain this strange phenomenon.

Atomic clocks combined with precise astronomical measurements have recently revealed that the length of a day on Earth is suddenly getting longer. This phenomenon has critical implications not only for our measurement of time, but also for things like GPS and other technologies that govern our modern lives.

In recent decades, the Earth’s rotation around its axis, which determines the length of a day, has accelerated. This trend has shortened our days; in fact, in June 2022 we hit the record for the shortest day in half a century.

But despite this record, since 2020, this constant acceleration has curiously turned into a slowdown: the days are longer again, and for the moment the reason is unknown.

While the clocks on our phones show that a day is exactly 24 hours long, the actual time it takes for the Earth to complete a single rotation varies very slightly. These changes occur over periods ranging from millions of years to almost instantaneously; even earthquakes and storms can play a role. So it turns out that a day very rarely corresponds to the magic number of 86,400 seconds.

The ever changing planet

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

For the past 20,000 years, another process has worked in reverse, speeding up the Earth’s rotation. At the end of the last ice age, the melting of the polar ice caps reduced the pressure on the surface and the Earth’s mantle began to move steadily toward the poles.

From même qu’un danseur de ballet tourne plus vite lorsqu’il rapproche ses bras de son corps – l’axe autour duquel il tourne–, la vitesse de rotation de notre planète augmente lorsque cette masse de manteau se rapproche de l’axe de the earth. And this process shortens each day by about 0.6 milliseconds per century.

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

Aside from these large-scale changes, over shorter time periods, weather and climate also have a big impact on the Earth’s rotation, causing variations in both directions.

Bimonthly and monthly tidal cycles move mass around the planet, causing changes in day length of up to a millisecond in any direction. We can observe tidal variations in photoperiod 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. Snow cover and seasonal precipitation, or groundwater extraction, mess things up even more.

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 quasars, we have had very precise estimates of the Earth’s rotation rate.

A comparison between these estimates and an atomic clock revealed an apparently increasingly shorter day length in recent years.

But there is a surprising revelation once we remove the fluctuations in rotational speed that we know to occur due to tidal and seasonal effects. Although the Earth reached its shortest day on June 29, 2022, the long-term trajectory seems to have changed from shortening to lengthening since 2020. This change is unprecedented in the last fifty years.

The reason for this change is not clear. It could be due to changes in weather systems, such as back-to-back La Niña weather events, even though these have happened before. It could be a further melting of the polar ice caps, although these have not deviated much from their normal rate of melting in recent years. Could it be related to the big explosion of the Tonga volcano that injected huge amounts 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 rotation rate is related to a phenomenon called “Chandler wobble,” a small shift in 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; the two phenomena could be related.

A final possibility, which seems plausible to us, is that nothing specific has changed on or around Earth. They could simply be long-term tidal effects that work in parallel with other periodic processes to produce a temporary change in the Earth’s rotation rate.

Do we need a “negative leap second”?

Precise knowledge of the Earth’s rotation rate is crucial for a host of applications: navigation systems like GPS wouldn’t work without it. Also, every few years, timekeepers insert leap seconds into our official timescales 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,” which would be unprecedented and could break the 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.

The original version of this article was published in The conversation.

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