Why does the Earth spin faster than before?

“Is it true that the Earth currently rotates faster than before? If so, for what reasons?”, Paul Owamba asks us on our Facebook page. This is our reader question of the week. Thank you all for your participation.

Surprising but not alarming variations

At the very beginning of the summer of 2022, on June 29 to be precise, the Earth set a new rotational speed record: 24 hours minus 1.59 milliseconds. the “last in a series of speed records since 2020”explained on July 28, 2022 the English site specializing in the subject Time and Date. Indeed, as we related in a previous article, the year 2020 has been a special year for everyone, including for the Earth, which has recorded the shortest 28 days since 1960.

But nothing to panic: these small variations are normal, the rotation of the Earth does not usually occur exactly in 86,400 seconds. Indeed, several disturbances act on our planet: the movements of the oceans, the atmosphere, the attraction of the Moon whose distance varies, or the impact of the inner core, add or remove a few milliseconds from the duration of the spin.

Variations in the length of a day in milliseconds compared to the usual 86,400 seconds, for the years 2019, 2020, 2021 and 2022. Credits: timeanddate.com

A “leap second” to adapt to the Earth’s rotation time

However, these disturbances are not counted by our 200 atomic clocks which give the coordinated universal time (the UTC which we use all over the world, see the box below) and which are very stable. Thus, a shift between UTC and the time given by the earth’s rotation can occur if these disturbances are too great.

Coordinated Universal Time (UTC) is the time we use as a reference. It is measured by atomic clocks and is thus stable.
universal time (TU) is a measurement of time based on the rotation of the earth. It is measured by the motion of the stars and can vary due to varying disturbances.

When the difference between the time measured by the atomic clocks and that of the rotation of the earth differs by more than 0.9 seconds, we add or remove what is called a “leap second” on June 30 or December 31 at midnight. The last time this happened was on December 31, 2016, a day on which we were able to benefit from a beautiful “1 hour 59 minutes and 60 seconds” displayed on our dials. Since 1972, when this measurement began, 27 seconds have been added, because the rotation of the Earth has slowed down. However, in 2020, 2021 and 2022, it accelerated! No second added then, but it is possible that a second be removed, if it becomes necessary. Only time will tell.

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