La Terre ralentit sa rotation de façon inexpliquée, menant bientôt à des journées de plus de 24 heures

ralentissement soudain rotation terre inexplique jours allonges couv

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A day on Earth has 24 hours, the time it takes for our planet to make a complete revolution on its axis of rotation. In addition to the day/night alternation, the length of which varies by season, the total length of a day has, since 2020, unexpectedly lengthened compared to atomic clocks, according to Australian researchers. It is true that variations in the rotational speed of the Earth are known to be irregular, influenced by internal terrestrial movements, those of the oceans and atmospheric masses. However, this sudden slowdown remains inexplicable to science and could affect the weather, the Internet, GPS, and other technologies that govern life today.

The Earth completes one complete revolution in 24 hours, so the Sun appears to rise and set every day. In general, over long periods, the Earth’s rotation slows down, in particular due to the frictional effects associated with the tides driven by the Moon. Every century, the Earth requires a few milliseconds or more to complete one rotation. Also, a few billion years ago, an Earth day lasted only about 19 hours.

In this general pattern, however, the Earth’s rotational speed fluctuates, and for 20,000 years the Earth has been accelerating. Day by day, the time it takes for the Earth to rotate increases or decreases. Therefore, a day does not last exactly 86,400 seconds. Thus, on June 29, 2022, the Earth recorded its shortest day since scientists began using atomic clocks to measure its rotation speed, completing a full revolution 1.59 milliseconds less than the classic 24 hours.

But despite this record, since 2020, this “ constant acceleration curiously became deceleration according to scientists at the University of Tasmania, writing an article in The conversation. Therefore, the days are longer again and the reason is currently unknown.

A “constantly irregular” rotation speed

As mentioned above, since 20,000 years ago, a process other than the friction associated with the tides no longer slows down the Earth. In fact, the change in its shape, due to the retreat of the polar caps since the last ice age, accelerates the rotation of the planet. Thus, previously ice-covered landmasses became exposed and rebounded, a phenomenon called postglacial rebound or isostatic adjustment. This resulted in a reduction in the settlement of the Earth on its axis, and the Earth’s mantle began to ” move gradually towards the poles as the authors explain. This process shortens each day by about 0.6 milliseconds per century.

In addition, changes in sea level, ocean and atmospheric currents, as well as electromagnetic forces between the Earth’s core and its rocky mantle also affect Earth’s rotation, as do precipitation or snow cover. Large earthquakes can change the length of the day, as the authors point out. For example, the 2011 Great Tōhoku Earthquake in Japan, with a magnitude of 8.9, would have sped up the Earth’s rotation by 1.8 microseconds (which is relatively small).

A sudden slowdown with multiple hypotheses

Beginning in the 1960s, radio telescopes began to obtain very precise estimates of the Earth’s rotation rate. The authors, comparing these estimates with those from atomic clocks, concluded that the length of days seems to have gotten shorter and shorter in recent years, with the Earth spinning faster and faster until 2020.

But once rotation rate fluctuations due to tidal and seasonal effects are removed, the long-term trajectory appears to have shifted from a shortening to a lengthening since 2020. This change is unprecedented in the last 50 years. Some think that this phenomenon would be related to the Chandler oscillation. This oscillation of the Earth’s axis of rotation results in an irregular movement of the geographic poles on the surface of the globe, of about three to four meters, with a period of about 430 days. Not to mention that since 2017, this wobble appears to have slowed down considerably, in line with Earth’s slowdown.

Finally, the Australian researchers proposed another hypothesis: “ [Étant donné que] nothing specific has changed on or around 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 rotation rate “.

It seems then that the explanation lies in an accumulation of various causes, the acceleration of the melting of the polar ice caps, pollution modifying atmospheric currents, as well as the great volcanic eruptions; but also in changes in climate systems, with consecutive La Niña events.

However, if this slowdown tends to continue, our timekeeping and other technologies that rely on these precise estimates of the Earth’s rotation speed, such as GPS, will be undermined, thus imposing a correction of the official time.

Towards a negative leap second and a collapse of the Internet?

When the speed of the Earth varies too much with respect to the atomic clock, the International Service for Earth Rotation and Reference Systems requests a leap second. But the impact on Internet networks is not negligible, according to Arstechnica. Meta, and many big tech companies, try to time a global network of servers against leap seconds, which add between 0.1 and 0.9 seconds to Coordinated Universal Time (UTC) each year. Specifically, at 00:00 hours on the indicated day, the clocks go from 23:59:59 to 00:00:00.

You should know that 27 leap seconds have been added since 1972, all positive. The last time there was talk of an official decision, in 2015 at the World Radiocommunication Conference in Geneva, the International Telecommunication Union (ITU) delayed the decision until 2023. The next big conference of timekeepers will take place at the end of 2023 in Dubai , when a contract that delegates UTC time to the ITU expires.

As a result of the change in the Earth’s rotational speed, the next added leap second could be negative, in order to keep civil time, based on atomic clocks, at the same rate as solar time, based on the Sun’s movement through the Sun. darling. In other words, a negative leap second would mean that our clocks would skip a second, which could have a devastating effect on software that relies on timers or schedulers.

However, Leonid Zotov of Lomonosov Moscow State University, at the latest annual meeting of the Asia-Oceania Society of Geosciences, said: I think there is a 70% chance that we are at the minimum. [du ralentissement de la Terre], we won’t need a negative leap second “, at least in the immediate future. This will delight Meta and other tech companies, who have been calling for a few years to move away from leap seconds.

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