On June 29, 2022, the Earth set a new rotational speed record: 24 hours minus 1.59 milliseconds. This surprising acceleration, which goes back several years, is however not alarming. Explanations with Christian Bizouard, astronomer of the National Earth Rotation Service, at the Paris Observatory.
On June 29, 2022, the Earth completed one rotation in 24 hours minus 1.59 milliseconds, “the latest in a series of speed records since 2020”, explains the English site Time and date in an article of July 28, 2022. A surprising situation, which is however not alarming. Christian Bizouard, astronomer of the National Earth Rotation Service at the Paris Observatory, summarizes the situation as follows: “Since 2016, we have noticed that the speed of rotation of the Earth is accelerating, and therefore the length of the day is decreasing! »
For the French astronomer, who is studying this phenomenon in the Sirte research laboratory (time-space reference systems), this observation is, to say the least, unexpected in view of past observations. “Since 1830, we have observed a drop in the speed of rotation of the Earth and therefore an increase in the length of the day. This trend has been reversed for seven years without being able to explain it. » 2020 thus recorded the fastest 28 days since 1930.
A rotation that has always been irregular
However, this variation does not worry the French scientist who closely follows the changes in rotations and deformations of the Earth. Because the revolution of the planet on its axis is irregular. We know that its duration increases by approximately 2 milliseconds each century and, on the scale of a year, it fluctuates slightly by a few milliseconds compared to the international reference time (known as atomic) established at 86,400 seconds.
“It’s a completely normal and periodic phenomenon that we can explain and model. Over one or two years, it has been established in particular that the speed varies seasonally and several phenomena come into play, explains the astronomer. 80% of the variations in the Earth’s speed are linked to mass transport in the atmosphere, in other words the winds. »
Another cause: the tides which “have the effect of deforming the oceans but also the Earth”, he specifies. “This phenomenon is strong enough to produce 10 to 20% of the variations in rotation duration. »
No certainty about global warming
Over several years, astronomers have also observed fluctuations of one to a few milliseconds that they have trouble explaining. “It is thought to be due to internal movements in the Earth, the molten magma under the mantle. But we do not have any observations of the hard core, like those made in the atmosphere, which would validate these hypotheses. »
Just like the climate change advanced by some researchers. “Today, we put everything on the back of climate change. Does the Earth spin faster under its effect? We do not know anything. To say that is pure speculation.” believes Christian Bizouard.
Offset from atomic time
So should we be worried about this acceleration? No, according to the astronomer. This oscillation of the order of a millisecond over the duration of a day will not change the course of our existence.
For the international timekeepers, who allow us to set our clocks, it is another matter. These use ultra-precise atomic clocks to measure Coordinated Universal Time (UTC). “Because of these irregularities, the Earth’s rotation time becomes out of sync with atomic time. »
For several decades, we have been making adjustments. “Since 1972, the International Earth Rotation and Reference Systems Service has regularly added a ‘leap second’ to the year, at the end of June or the end of December. There have been 28 so far. The last dates back to December 31, 2016.
A debating second
The recent acceleration of the Earth’s rotation could therefore call into question this process. “For the first time, if the Earth continues to accelerate, we could have to remove a second. It never happened. »
This unprecedented situation also raises the question of the interest of maintaining this technique of balancing our astronomical observations and our clocks, which had been created to compensate for a slowing down of the Earth. The subject has been debated for several years.
risk of bugs
And that’s not just of interest to astronomers. Because these modifications do not facilitate the task of satellites, telecommunications networks or financial markets. The addition of leap seconds in recent years has, for example, caused a synchronization problem on the web, in particular for servers or merchant sites.
Read also: But why does the Earth rotate?