The duration of each day on Earth has been fixed at the millisecond level since the atomic clock’s invention – they measure 24 hours very accurately. Thanks to this accuracy, it turned out that the earth’s time does not always correspond to the solar time, since the Earth’s rotation slows down. Simply put, a day on Earth lasts a fraction of a second longer than the clock shows.
To coordinate UTC with the mean solar time UT1, 1 second is added “manually” to the duration of the Earth’s year approximately once every few years. Starting from the 1970s, 27 seconds have already been added to the “earthly” time.
But astronomical observations have shown that the Earth’s rotation has accelerated slightly since July 2020. The day is now about 0.5 milliseconds shorter than usual.
Now physicists from all over the world are discussing whether, for the first time in history, we should introduce the so-called negative extra second because the average solar day will be shorter than calendar days.