The shortest day in recorded history has been confirmed to have occurred on June 29th, 2022.
The speed of Earth's rotation, and thus its day length, has been measured by atomic clocks daily since the 1960s. On June 29th, Earth completed one full rotation in 1.59 milliseconds (ms) less than 24 hours, and continues a trend found in recent years. The previous shortest day was recorded on July 19th, 2020, at 1.47 ms fewer than 24 hours, and the one before that was on July 16th, 2019, and was shorter than 24 hours by 0.95 ms.
Earth's rotational speed has decreased over time, taking a handful of milliseconds longer to complete a rotation every century. However, fluctuations are still observed on a smaller time scale, with some days being shorter or longer than the standard 24 hours. Some scientists suggest the change in day length could be the result of various factors, including the 'Chandler wobble,' which refers to the small movement of Earth's North and South Poles.
"The normal amplitude of the Chandler wobble is about three to four meters at Earth's surface, but from 2017 to 2020 it disappeared," Dr. Leonid Zotov told timeanddate.
If Earth continues to record shorter days in upcoming years, it could require the first ever negative leap second, meaning a second will have to be subtracted from current clocks. However, Dr. Zotov believes "there's a 70 percent chance" Earth has hit a minimum for the shortest day, and a negative leap second won't be necessary.