Earth experienced its shortest day since records began last month, with 1.59 milliseconds shaved off its usual 24-hour rotation on June 29 – raising the prospect that a negative leap second may soon be needed to keep up the clocks in correspondence with the heavens. Typically, the Earth’s average rotation speed decreases slightly over time. Timekeepers have been forced to add 27 bisepta seconds to atomic time since the 1970s as the planet slows down. But as of 2020, the phenomenon has been reversed – with speed records being broken frequently in the past two years. The previous fastest day was -1.47 milliseconds under 24 hours on July 19, 2020. It was nearly broken again on July 26, when the day was -1.50 milliseconds shorter. Although the effect is too small to be perceived by humans, it can accumulate over time – potentially affecting modern satellite communication and navigation systems that rely on time according to the conventional positions of the Sun, Moon and stars . It means you may soon need to subtract time, add a negative leap second, and speed up world clocks for the first time.
The “Chandler Wobble”
Scientists have been left scratching their heads as to the cause, although experts have suggested that a phenomenon known as the ‘Chandler Velocity’ may be having 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 influence of celestial bodies such as the Moon. Tidal friction and the changing distance between Earth and the Moon cause daily fluctuations in the speed at which the planet spins on its axis. The “Chandler Oscillation” is the change in the Earth’s rotation on its axis and usually causes the Earth’s spin to increase, meaning it takes longer to complete one turn. But in recent years the rotation has become less oscillating. Dr Leonid Zotov, of the Sternberg Astronomical Institute at Lomonosov Moscow State University, believes this lack of oscillation may be behind the faster days and will present the theory next week at the annual meeting of the Asia Oceania Geosciences Society. “The Chandler Wobble’s normal width 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 width of the “Chandler Wobble” began to decrease, and in 2017-20 it reached an all-time low as day length began to shorten.
Global warming is a small contributing factor
Other factors that can have an impact on the annual variation include the accumulation of snow on mountains in the northern hemisphere 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 this is thought to be a relatively small contribution. The changes in the length of a typical 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 one second to allow the Earth’s rotation to catch up. BT’s talk clock then added a second pause before its third tone, while BBC Radio 4 introduced an extra pip to its 1am bulletin. The International Earth Rotation Service, based in Paris, monitors the planet’s rotation and tells countries when to add or subtract leap seconds six months in advance. However, the leap second could be abolished entirely next year when the World Radiocommunication Conference decides whether to rely entirely on atomic time. Britain opposes the move because it would sever the link to solar time forever. Some experts believe that the need for a negative leap second may increase pressure to move to atomic time.
