LIn ancient times, we humans defined a day as the time it takes for the Earth to make one rotation around its axis and for one sunrise and one sunset to occur. Our ancestors divided the day into 24 hours. But if the Earth spins a little slower, it will take a little longer than a day to complete the rotation. It’s been going on for years. Atomic clocks, which we use to regulate everything from internet communications to GPS apps to automated stock trading, never slow down, so the world’s timekeepers leap regularly to synchronize their clocks with the Earth. I’ve added seconds. Since 1972, we have made this pesky addition 27 times.
But since around 1990, the Earth’s rotation has accelerated, counteracting the slowdown and making days shorter, so we may need to subtract leap seconds for the first time. There are two explanations as to why, which we’ll get to later.
This reversal has many people wondering why they should even bother with leap seconds. Each time an adjustment is needed, a mind-boggling number of computer and communications operations must be changed. On a typical day, the National Institute of Standards and Technology, which manages atomic time in the United States and synchronizes most computers around the world, receives more than 100 billion time adjustment requests from up to 1 billion computers. Additionally, leap second adjustments can cause problems. The incident, which was added in 2012, was blamed for a sudden blackout on Reddit and a failure of Qantas’ flight systems, causing long flight delays across Australia.
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What happens if we ignore the fact that the Earth’s rotation and atomic clocks are off by one second, or even one minute? It is estimated that if nothing is done until then, this will happen a century from now. In a highly digitalized world, does the exact length of the rotation day even matter?
The Earth rotates because the solar system condensed from rotating clouds of gas and dust. In space, the drag force is virtually zero, so the planets, including Earth, continue to rotate. As the Earth rotates, the gravitational pull between the Earth, the Moon, and to a lesser extent the Sun causes ocean tides. As the tides cross the ocean floor, they create friction, which gradually slows down the planet’s rotation. In the time of the dinosaurs, a day was approximately 23.5 hours long. It has since been extended by tidal friction.
Seismic studies have shown that Earth has a solid inner core and a liquid outer core, surrounded by a solid mantle and crust. Ocean currents in the outer core cause the mantle to rotate faster or slower each year, but over centuries these changes tend to cancel out, with a general trend toward slower tidal forces.
Although the tidal force has been decreasing consistently, the increase in the Earth’s rotational speed is counteracting this trend, and the interval between additional leap seconds has increased from about 1 year in the 1970s to 3 to 4 years in the 2010s. It’s getting longer.
Jen Christiansen (timeline);Source: U.S. Naval Observatory Time Office (timeline data)
Calculations show that by 2026, ongoing speed increases will outpace slowdowns and leap seconds will have to be subtracted.
But now, global warming complicates that prediction. At the end of the most recent ice age, when the giant ice sheets between the North and South poles melted, the weight of the ice decreased and the compressed crust beneath began to rebound, a phenomenon that continues today. As a result, the Earth became more spherical. (The planet is not a perfect sphere; it is slightly wider near the equator.) This change in shape means that the Earth’s overall mass is distributed a little closer to the axis of rotation, making it easier to ice skate. Just as the athlete spins faster when pulling, the Earth moves faster. In your outstretched arms.
But as ice sheets warm, meltwater spreads across the world’s oceans, with most of the oceans at lower latitudes and farther from the axis of rotation than the ice sheets. This slows down the spin (the skater extends his arms outward). For now, this effect will become even stronger, delaying the time when the increase in rotational speed will outpace the slowdown in tidal forces. According to recent research, this reactionary force means we won’t need to subtract leap seconds until 2029.
Given so many capricious phenomena, it’s natural to ask whether we need to add or subtract leap seconds at all. And since the decline in tidal power is always a long-term trend, we may never have to subtract a second again. So why go through the trouble just once? Few computer programs are written to tolerate negative leap seconds.
Respecting the rotation date may be the only reason to keep atomic time in sync with it. Even if the two timestamps are different, “for most people, it has no practical impact,” says Scripps Institution of Oceanography geophysicist Duncan Carr Agnew, who authored the 2024 paper. nature Agnew supports the idea of waiting 100 years and making one big adjustment, rather than advocating frequent and random adjustments to the second. This is because you can prepare well in advance.
This idea has been popular for some time. In 2022, the Parties to the International Assembly of Weights and Measures resolved to end leap second adjustment by 2035. Timekeepers may then agree to revise it every 20 years, or perhaps every 100 years. Whatever choice we make, “we want consistency.” said physicist Elizabeth Donley, director of NIST’s time and frequency division. “Time is the most important unit in the International System of Units. Many other standards also depend on it.”
Some major internet providers already follow their own protocols. Rather than wait for a breakthrough, Google “smudges” the clock by one thousandth of a second, once a day. Such independent efforts don’t seem to be causing a global discontinuity, but if more and more big companies start working on them, “it becomes anarchy,” Donley said.
Waiting decades for carefully planned adjustments means astronomical (rotational) time, known as UT1, will deviate even further from Coordinated Universal Time (UTC), which is based on atomic clocks. . But Donley doesn’t think there will be any problems. “Computer networks don’t care where the sun is in the sky,” she says.
