If it feels like there’s never enough time in the day, there may be a reason.
Earth experienced its shortest day since records began last month, with 1.59 milliseconds shaved off its normal 24-hour spin on June 29 – raising the prospect that a negative leap second may soon be needed to keep clocks in sync with the heavens .
Typically, the Earth’s average rotation rate slows slightly over time, and timekeepers have been forced to add 27 leap seconds to atomic time since the 1970s as the planet slows.
But since 2020, the phenomenon has reversed with speed records that were often broken in the last two years.
The previous fastest day was -1.47 milliseconds under 24 hours on July 19, 2020, and it was nearly broken again on July 26, when the day was -1.50 milliseconds shorter.
Although the effect is too small to be felt by humans, it can accumulate over time, potentially affecting modern satellite communication and navigation systems that rely on time matching the conventional positions of the sun, moon and stars.
That means it may soon be necessary to remove time, add a negative leap second, and speed up global clocks for the first time ever.
Scientists have been left scratching their heads about the cause, although experts have suggested that a phenomenon known as the “Chandler Wobble” may have an impact.
The speed of the Earth’s rotation varies constantly due to the complex motion of its molten core, oceans and atmosphere, as well as the effects of celestial bodies such as the Moon.
The friction of the tides and the change in the distance between the Earth and the Moon provide daily variations in the speed at which the planet rotates on its axis.
The “Chandler Wobble” is the change in the Earth’s spin on its axis, and it normally causes the Earth’s rotation to increase, meaning it takes longer to complete one turn. But in recent years, the spin has become less wobbly.
Dr Leonid Zotov, of the Sternberg Astronomical Institute at Lomonosov Moscow State University, believes this lack of wobble could be behind the faster days and will present the theory next week at the Asia Oceania Geosciences Society’s annual meeting.
“The normal amplitude of the Chandler Wobble is about three to four meters on the Earth’s surface, but from 2017 to 2020 it disappeared,” Dr Leonid Zotov told the Timeanddate website.
In the early 2000s, the amplitude of the “Chandler Wobble” began to decrease and in 2017-2020 reached an all-time low just as the length of the day began to shorten.
Global warming small contributing factor
Other factors that may affect the annual variation include snow building up on northern hemisphere mountains in the winter and then melting in the summer.
Global warming is also expected to have an effect by melting ice and snow at higher altitudes, causing the Earth to spin faster, but that is considered to be a relatively small contribution.
Changes in the length of a standard day were only discovered after highly accurate atomic clocks were developed in the 1960s and compared to fixed stars in the sky.
The last leap second was added on New Year’s Eve in 2016, when clocks around the world stopped for a second to allow the Earth’s rotation to catch up.
Then BT’s speech clock paused for a second before the third beep, while Radio 4 added an extra beep to its 1am bulletin.
The International Earth Rotation Service, based in Paris, monitors the planet’s rotation and informs countries when leap seconds need to be added or subtracted six months in advance.
However, the leap second may be abolished completely next year, when the World Radiocommunication Conference will decide whether to rely entirely on atomic time.
Britain opposes the move because it would break the connection with solar time forever.
Some experts believe that the need for a negative leap second could increase the pressure of moving to atomic time.