The 28 fastest days on record (since 1960) all occurred in 2020, with Earth completing its revolutions around its axis milliseconds quicker than average. That’s not particularly alarming — the planet’s rotation varies slightly all the time, driven by variations in atmospheric pressure, winds, ocean currents and the movement of the core. But it is inconvenient for international timekeepers, who use ultra-accurate atomic clocks to meter out the Coordinated Universal Time (UTC) by which everyone sets their clocks. When astronomical time, set by the time it takes the Earth to make one full rotation, deviates from UTC by more than 0.4 seconds, UTC gets an adjustment.
Until now, these adjustments have consisted of adding a “leap second” to the year at the end of June or December, bringing astronomical time and atomic time back in line. These leap seconds were tacked on because the overall trend of Earth’s rotation has been slowing since accurate satellite measurement began in the late 1960s and early 1970s. Since 1972, scientists have added leap seconds about every year-and-a-half, on average, according to the National Institute of Standards and Technology (NIST). The last addition came in 2016, when on New Year’s Eve at 23 hours, 59 minutes and 59 seconds, an extra “leap second” was added.