In 2020, the Earth broke the previous record for shortest astronomical day, set in 2005. In effect the Earth broke the record 28 times in 2020, and it's still spinning faster. While Earth is, on average, pretty reliable and takes 86,400 seconds to rotate around its axis, this is not perfect.
”When highly accurate atomic clocks were developed in the 1960s, they showed that the length of a mean solar day can vary by milliseconds (1 millisecond equals 0.001 seconds),” write Graham Jones and Konstantin Bikos on TimeandDate.com.
How fast or slow the Earth spins can be impacted by various factors.
Changes in the atmosphere, specifically atmospheric pressure around the world, and the motions of the winds that may be related to such climate signals as El Niño are strong enough that their effect is observed in the Earth’s rotation signal, David A. Salstein, an atmospheric scientist from Atmospheric and Environmental Research, pointed out
LiveScience reports the Earth has actually been slowing down for the last several decades. When the time it takes earth to make a full rotation deviates from the Coordinated Universal Time (UTC) by more than .4 seconds, the UTC is adjusted.
Scientists have been adding a “leap second” every year-and-a-half on average. The last one was added on December 31, 2016.
Since the Earth has sped up, scientists believe each astronomical day in 2021 will be 0.05 milliseconds shorter, and over the course of the year, it adds up to a 19 millisecond difference.
It's quite possible that a negative leap second will be needed if the Earth's rotation rate increases further, but it's too early to say if this is likely to happen, physicist Peter Whibberley of the National Physics Laboratory in the U.K. said.
There are also international discussions taking place about the future of leap seconds, and it's also possible that the need for a negative leap second might push the decision towards ending leap seconds for good.
The average person may not notice a leap second being taken away or the ones that have been added, but they will impact things like navigation, spaceflight, computer networks and astronomers. The last time every day of a calendar year was shorter than 86,400 seconds was in 1937, according to TimeandDate.com