by The changing spin of the Earth is threatening to toy with our sense of time, clocks and computerized society in a way never seen before — just for a second.
For the first time in history, the world’s timekeepers may have to consider subtracting a second from our clocks in a few years because the planet is spinning a little faster than it used to. Clocks may have to skip a second – known as a “second negative leap” – around 2029, a study in the journal Nature said on Wednesday.
“This is an unprecedented situation and it’s a big deal,” said lead study author Duncan Agnew, a geophysicist at the Scripps Institution of Oceanography at the University of California, San Diego. “It’s not going to be a huge change in the Earth’s rotation as a result of a disaster or anything, but it’s something significant. It is yet another sign that we are in a very unusual time.”
Melting ice at both Earth’s poles is counteracting the planet’s speed burst and this second global reckoning is likely delayed by about three years, Agnew said.
“We’re headed for a second negative jump,” said Dennis McCarthy, a retired director of time for the US Naval Observatory who was not part of the study.
It is a complex situation involving, physics, global power politics, climate change, technology and two types of time.
The Earth takes about 24 hours to rotate, but the key word is around.
For thousands of years, the Earth has generally been slowing down, with the rate changing from time to time, said Agnew and Judah Levine, a physicist for the time and frequency division of the National Institute of Standards and Technology.
The slowdown is mostly caused by the effect of the tides, and the pull of the moon, McCarthy said.
This was not important until atomic clocks were adopted as the official time standard more than 55 years ago. That didn’t slow them down.
That established two versions of time – astronomical and atomic – and they were not the same. Astronomical time fell behind atomic time by 2.5 milliseconds each day. That meant the atomic clock would say it was midnight and for Earth it was midnight a fraction of a second later, Agnew said.
Those daily fractions of seconds add up to whole seconds every few years. Beginning in 1972, international timekeepers decided to add a “second leap” in June or December for astronomical time to catch up with atomic time, known as Coordinated Universal Time or UTC. Instead of turning 11:59 and 59 seconds to midnight, there would be another second at 11:59 and 60 seconds. The second negative jump would run from 11:59 and 58 seconds exactly to midnight, skipping 11:59:59.
Between 1972 and 2016, 27 separate leap seconds were added as the Earth slowed down. But the rate of slowdown was decreasing.
“In 2016 or 2017 or maybe 2018, the rate of deceleration had slowed to the point where the Earth was actually accelerating,” Levine said.
Earth is accelerating because its hot liquid core — “a big ball of molten fluid” — behaves in unpredictable ways, and eddies and flows change, Agnew said.
Agnew said the core has been driving acceleration for about 50 years, but rapid melting of ice at the poles since 1990 has masked that effect. Melting ice shifts Earth’s mass from the poles to the bulging center, which slows the rotation much like an ice skater spins as they extend their arms out to their sides, he said.
Without the effect of the melting ice, the Earth would need that second negative jump in 2026 instead of 2029, Agnew calculated.
For years, astronomers have been keeping universal and astronomical time along with those handy little seconds. But computer system operators said those additions aren’t easy for all the precision technology the world now relies on. In 2012, several computer systems mishandled the second jump, causing problems for Reddit, Linux, Qantas Airlines and others, experts said.
“What is the need for this adjustment and causing so many problems?” said McCarthy.
But Russia’s satellite system relies on astronomical time, so eliminating leap seconds would cause problems, Agnew and McCarthy said. Astronomers and others wanted to maintain the system that would add a second whenever the difference between atomic and astronomical time was close to a second.
In 2022, the world’s timekeepers decided, starting in the 2030s, that they would change the standards for adding or deleting leap seconds, making it less likely.
Tech companies such as Google and Amazon unilaterally initiated their own solutions to the leap second edition by gradually adding fractions of a second over a full day, Levine said.
“The fights are so serious because the stakes are so small,” Levine said.
Then add in the “strange” effect of subtracting, not adding a second, Agnew said. It will likely be more difficult to drop a second because software programs are designed to add, rather than subtract, time, McCarthy said.
McCarthy said the trend toward a second negative jump is clear, but he thinks it has more to do with the Earth becoming more rounded from geological shifts since the end of the last ice age.
Three other outside scientists said Agnew’s study makes sense, saying its evidence is strong.
But Levine doesn’t think a negative second will really be needed. He said the overall slowing trend from tides has been around for centuries and continues, but the shorter trends in the Earth’s core come and go.
“This is not a process where the past is a good predictor of the future,” Levine said. “Anyone who makes long-term predictions about the future is on very clever ground.”
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