On June 29, the National Physical Laboratory in England recorded the shortest day in history; 1.59 milliseconds shy of 24 hours. The Earth completed one spin in 1.59 milliseconds less than 24 hours, making it the shortest day since the advent of atomic clocks. On July 26, Earth was 1.50 milliseconds shy of 24 hours. If this trend continues, timekeepers might develop the first-ever negative leap second to keep the clocks in sync with Earth’s rotation.
Around 27 leap seconds have been introduced in the history of atomic clocks, the last one being in 2016. But in 2020, the trend reversed as Earth clocked up 28 days that came in under 24 hours. The addition of leap seconds has caused problems with IT in the past. Deleting a second, on the other hand, makes you skip forward in time. Broadly speaking, Earth completes one full turn on its axis every 24 hours. That single spin marks out a day and drives the cycle of sunrise and sunset that has shaped patterns of life for billions of years. But the curtains fell early on 29 June, with midnight arriving 1.59 milliseconds sooner than expected.
The past few years have seen a flurry of records fall, with shorter days being notched up ever more frequently. In 2020, the Earth turned out 28 of the shortest days in the past 50 years, with the shortest of those, on 19 July, shaving 1.47 milliseconds off the 86,400 seconds that make up 24 hours. The 29 June record came close to being broken again last month when 26 July came in 1.5 milliseconds short.
To keep clocks in line with the planet’s spin, the International Telecommunication Union, a United Nations body, has taken to adding occasional leap seconds in June or December – most recently in 2016 – effectively stopping the clocks for a second so that the Earth can catch up. The first leap second was added in 1972. The next opportunity is in December 2022, although with Earth spinning so fast of late, it is unlikely to be needed.
According to NASA, stronger winds in El Niño years can slow down the planet’s spin, extending the day by a fraction of a millisecond. Earthquakes, on the other hand, can have the opposite effect. The 2004 earthquake that unleashed a tsunami in the Indian Ocean shifted enough rock to shorten the length of the day by nearly three microseconds.
While engineers clamor for the abolition of the leap second, period, scientists are still trying to figure out just why Earth’s rotational speed is changing. Of course, Earth could slow back to a more average speed and the worry subsides. Or, in a more extreme scenario, the world begins spinning even more quickly.
