This year 2020 has been a special year for everyone, including the Earth. Indeed, its rotation has accelerated, creating days a little shorter than usual. So, we have lived through the shortest 28 days since 1960 and July 19, 2020 was our shortest day, with a rotation that lasted 1.4602 milliseconds less than the usual 86,400 seconds or 24 hours making up the rotation of Earth.
Variations in the length of a day in milliseconds compared to the usual 86,400 seconds, for the year 2020. Credits: timeanddate.com
A normal shift
But nothing to panic: these small variations are normal, the rotation of the Earth is not usually done 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 internal core, add or subtract a few milliseconds to the duration of the rotation.
However, these disturbances are not counted by our 200 atomic clocks which give Coordinated Universal Time (UTC which is used all over the world, see box below) and which are very stable. Thus, an offset between UTC and the time given by the rotation of the earth can occur if these disturbances are too important.
Coordinated Universal Time (UTC) is the time we use as a reference. It is measured by atomic clocks and thus is stable.
Universal time (TU) is a measure of time based on the rotation of the earth. It is measured by the movement of the stars and can vary due to varying disturbances.
One minute of 59 seconds to make up for lost time?
When the difference between the time measured by atomic clocks and that of the Earth’s rotation differs by more than 0.9 seconds, we add or subtract what is called a “leap second” on June 30 or December 31 at midnight. The last time this happened was January 31, 2016, a day for which we were able to benefit from a beautiful “1 hour 59 minutes and 60 seconds” displayed on our dials. Since 1972, date of the start of this measurement, 27 seconds have been added, because the rotation of the Earth has slowed down. Yet in 2020 it has accelerated, and measurements predict that 2021 should benefit from an acceleration too. No second added then, but it is possible that a second is removed, if it becomes necessary. Only time will tell.