The Earth inexplicably slows its rotation, soon leading to days longer than 24 hours

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A day on Earth has 24 hours, the time it takes for our planet to complete one revolution on its axis of rotation. In addition to the day/night alternation, the length of which varies by season, since 2020 the total length of a day has unexpectedly lengthened compared to atomic clocks, according to Australian researchers. Certainly, variations in the Earth’s rotational speed are known to be irregular, influenced by internal terrestrial motions, oceanic and atmospheric masses. Nevertheless, this sudden slowdown is still unexplained by science and may affect timing, the Internet, GPS and other technologies that govern modern life.

The Earth makes one complete revolution in 24 hours, which is why the Sun appears to rise and set every day. In general, over long periods of time, the Earth’s rotation slows down, mainly 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 rotation rate fluctuates, and for 20,000 years the Earth has been accelerating. From day to day, the time it takes for the Earth to rotate increases or decreases. A day therefore 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.

However, despite this record, since 2020, this ” constant acceleration transformed strangely into deceleration according to scientists from the University of Tasmania writing an article in The conversation. The days are therefore getting longer again, and the reason is currently unknown.

A “constant irregular” rotation speed

As mentioned earlier, in 20,000 years, a process other than the friction associated with the tides no longer slows the Earth down. 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. Land masses previously covered by ice were thus exposed and rebounded, a phenomenon called post-glacial rebound or isostatic adjustment. This resulted in a reduction in the Earth’s deposition 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 day. century.

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

A sudden slowdown with several hypotheses

Beginning in the 1960s, radio telescopes began to obtain very accurate estimates of the Earth’s rotation rate. By comparing these estimates and atomic clocks, the authors concluded that day lengths appear to have gotten shorter and shorter in recent years, with the Earth spinning faster and faster until 2020.

However, when rotation rate fluctuations due to tides and seasonal effects are eliminated, the long-term trajectory appears to have shifted from shortening to lengthening since 2020. This shift is unprecedented in the last 50 years. Some believe that this phenomenon would be related to Chandler’s swing. 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 fluctuation seems to have reduced significantly, in line with the earth’s deceleration.

Finally, the Australian researchers put forward another hypothesis: ” [Étant donné que] nothing specific has changed in or around Earth, it could simply be long-term tidal effects working in parallel with other periodic processes to produce a temporary change in the Earth’s rotational speed “.

It seems then that the explanation lies in an accumulation of different causes, the acceleration of the melting of the ice caps, pollution that modifies the atmospheric currents, as much as the great volcanic eruptions; but also in changes in weather systems, with successive 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 rate, such as GPS, will be undermined, forcing a correction of official time.

Towards a negative leap second and an internet collapse?

When the speed of the earth varies too much relative to the atomic clock, a leap second is requested by the International Earth Rotation and Reference Systems Service. But the impact on Internet networks is not negligible, as Arstechnica reports. Meta and many large technology companies attempt to time a global network of servers against leap seconds, which each year add between 0.1 and 0.9 seconds to Coordinated Universal Time (UTC). Specifically, at midnight on the specified day, the clocks are set 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) pushed the decision back to 2023. The next major timer conference will take place at the end of 2023 in Dubai, when a contract, that delegates UTC timing to the ITU, expires.

As a result of the change in Earth’s rotational speed, the next leap second added could be negative to keep civil time – based on atomic clocks – in step with solar time – based on the sun’s movement across the sky. In other words, a negative leap second would mean 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 said at the last annual meeting of the Asia Oceania Geosciences Society: I think there’s a 70% chance we’re at the minimum [du ralentissement de la Terre], we don’t need a negative leap second “, at least in the near future. This will please Meta and other tech companies that have been calling for a move away from leap seconds for the past few years.

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