Earth is whipping around quicker than it has in a half-century

By Stephanie Pappas - Live Science Contributor , Jan 6, 2021

https://www.livescience.com/earth-spinning-faster-negative-leap-second.html

(Image: © Stocktrek Images via Getty Images)

It could mean a "negative" leap second.

Even time did not escape 2020 unscathed.

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.

However, according to Time and Date, the recent acceleration in Earth's spin has scientists talking for the first time about a negative leap second. Instead of adding a second, they might need to subtract one. That's because the average length of a day is 86,400 seconds, but an astronomical day in 2021 will clock in 0.05 milliseconds shorter, on average. Over the course of the year, that will add up to a 19 millisecond lag in atomic time. 

"It's quite possible that a negative leap second will be needed if the Earth's rotation rate increases further, but it's too early to say if this is likely to happen," physicist Peter Whibberley of the National Physics Laboratory in the U.K., told The Telegraph. "There are also international discussions taking place about the future of leap seconds, and it's also possible that the need for a negative leap second might push the decision towards ending leap seconds for good."

The year 2020 was already faster than usual, astronomically speaking (cue sighs of relief). According to Time and Date, Earth broke the previous record for shortest astronomical day, set in 2005, 28 times. That year's shortest day, July 5, saw Earth complete a rotation 1.0516 milliseconds faster than 86,400 seconds. The shortest day in 2020 was July 19, when the planet completed one spin 1.4602 milliseconds faster than 86,400 seconds. 

According to the NIST, leap seconds have their pros and cons. They're useful for making sure that astronomical observations are synced with clock time, but they can be a hassle for some data-logging applications and telecommunications infrastructure. Some scientists at the International Telecommunication Union have suggested letting the gap between astronomical and atomic time widen until a "leap hour" is needed, which would minimize disruption to telecommunications. (Astronomers would have to make their own adjustments in the meantime.)

The International Earth Rotation and Reference Systems Service (IERS) in Paris, France, is responsible for determining whether adding or subtracting a leap second is necessary. Currently, the IERS shows no new leap seconds scheduled to be added, according to the service's Earth Orientation Center.

Originally published on Live Science.

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