The Earth’s rotation has slowed down

After accelerating in 2020, the Earth’s rotation has gradually slowed. But scientists say we may still need a “negative leap second” in the next decade.

On average, each Earth day contains 86,400 seconds. But the Earth’s rotation is not perfect; it is constantly changing slightly, depending on the motion of the core, oceans, and atmosphere. Universal Coordinated Time (UTC), the official international method of measuring time, is based on atomic clocks, which measure time by the movement of electrons in atoms cooled to absolute zero. Atomic clocks are accurate and unchanging.

Therefore, when the rotation of the Earth and the atomic clock do not exactly coincide, something happens. When astronomical time, based on the Earth’s rotation, deviates from UTC by more than 0.4 seconds, UTC is corrected as a “leap second.” Sometimes leap seconds are added, as happened on New Year’s Eve 2016, when a second was added at 23 hours, 59 minutes and 59 seconds on December 31. According to the National Institute of Standards and Technology (NIST), scientists have added a leap second on average every 18 months since 1972.

When seconds are subtracted, it is called a negative leap second. There have never been negative leap seconds in international timekeeping, but in 2020 there was a possibility that they might be needed. The Earth’s rotation accelerated that year, breaking the previous record for shortest day, set in 2005, by 28 times. The shortest day in 2020 came on July 19, when the planet completed its rotation 1.4602 milliseconds faster than the average of 86,400 seconds.

Now, according to Time and Date, Earth’s rotation has slowed. The first half of 2021 was still fast: the average day was 0.39 milliseconds less than in 2020. But from July 1 to September 30, the days lengthened by an average of 0.05 milliseconds more than in 2020.

This means that the Earth is no longer accelerating its rotation. But it is still spinning faster than average. Based on the current rate of rotation, a negative leap second might be needed in about 10 years. The final decision rests with the International Earth Rotation and Reference Systems Service (IERS) in Paris.

Of course, the planet may have other plans. It is quite possible that the Earth’s rotation will slow down again, and perhaps the next few years will require adding a second rather than subtracting a second. Predicting this is impossible: Scientists aren’t sure exactly what causes long-term changes in Earth’s rotation.

“We’ve been trying to internally model the situation for the next two years or more,” says Nick Stamatakos, one of the U.S. Naval Observatory’s representatives on the IERS Steering Board. “But we’re having trouble forecasting more than six months or one year ahead.”

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