Midnight Comes a Fraction Sooner as Earth Spins Faster

If time feels tighter than ever of late, blame it on the revolution. On 29 June this year, Earth racked up an unusual record: its shortest day since the 1960s, when scientists began measuring the planet’s rotation with high-precision atomic clocks. From a report: 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.

So is the world speeding up? Over the longer term — the geological timescales that compress the rise and fall of the dinosaurs into the blink of an eye — the Earth is actually spinning more slowly than it is used to. Wind the clock back 1.4bn years and a day would pass in less than 19 hours. On average, then, Earth days are getting longer rather than shorter, by about one 74,000th of a second each year. The moon is mostly to blame for the effect: the gravitational tug slightly distorts the planet, producing tidal friction that steadily slows the Earth’s rotation. To keep clocks in line with the planet’s spin, the International Telecommunication Union, a United Nations body, has taken to adding occasional leaps 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.

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