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This summer, the Earth rotates faster, shortening days and attracting the attention of scientists and timekeepers.
July 10 was the shortest day of the year to date, lasting less than 24 hours, according to data from the International Earth Rotation and Reference Systems Service and Timeanddate.com edited by the International Earth Rotation and Reference Systems Service and the U.S. Naval Observatory. On July 22 and August 5, more short dates will arrive on July 22, and are currently expected to be less than 24 hours in 1.34 and 1.25 milliseconds, respectively.
The length of day is the time it takes for a planet to complete a complete rotation on its axis – 24 hours or an average of 86,400 seconds. But in fact, each rotation is slightly irregular due to various factors such as the gravity of the moon, the seasonal changes of the atmosphere, and the influence of the Earth's liquid core. As a result, full rotation usually requires a slightly lower or slightly greater than 86,400 seconds, which is a millisecond difference, which has no obvious effect on daily life.
However, in the long run, these differences may affect computers, satellites and telecommunications, which is why the minimum time deviation is tracked using atomic clocks, which were introduced into 1955. Some experts believe this may lead to a similar scheme to Y2K, which threatens the threat of modern civilization to Halt.
The atomic clock calculates the oscillation of the atoms held in the vacuum cavity in the clock book, and calculates it with maximum accuracy for 24 hours. We call the final UTC (or coordinated universal time) based on approximately 450 atomic clocks and is the global standard for timing management, as well as the time set by all phones and computers.
Astronomers can also track Earth's rotation—for example, using satellites that examine Earth's position relative to a fixed star, and can detect slight differences between atomic clock times and how long it takes to actually require the Earth to complete its full rotation. Last year, July 5, 2024, Earth experienced the shortest day since the appearance of atomic clocks 65 years ago, less than 1.66 milliseconds and less than 24 hours.
“We have been on a slightly faster trend since 1972,” said Scripps Institute of Oceanography and a geophysicist at San Diego. “But there is volatility. It's like looking at the stock market. There is indeed a long-term trend, and then there are peaks and peaks.”
In 1972, after decades of relatively slow rotation, the Earth's rotation accumulated so late with respect to atomic time that the International Earth Rotation and Reference System Services required the addition of “leap second” to UTC. This is similar to the leap year, which adds an additional day every four years February to illustrate the difference between the Gregorian calendar and the time it takes the Earth to complete an orbit around the Sun.
UTC has added 27 seconds in total since 1972, but the increase is getting slower and slower due to Earth acceleration. Throughout the 1970s, nine seconds were added, and no new leap seconds were added since 2016.
In 2022, a weight and measure (CGPM) vote was held to retire in 2035, meaning we may never see others increasing in the clock. However, according to Agnew, if the earth keeps spinning faster for a few years, it may eventually take a second to be removed from UTC. “There has never been a negative leap, but the chance of having one between now and 2035 is about 40 percent.”
Agnew said the shortest change in Earth's rotation comes from the moon and tides, and when the satellite is above the equator, it rotates slowly when it is at a higher or lower altitude. This effect makes the Earth's natural rotation faster in summer – the result of the slowing of the atmosphere itself due to seasonal changes, such as jet flows northward or southward; the laws of physics indicate that the overall angle of the Earth and its atmosphere must remain constant, so the Earth itself loses the rotational speed of the atmosphere loss. Similarly, the earth's liquid core has been slowing down over the past 50 years, and the surrounding solid earth is accelerating.
By looking at the combination of these effects, scientists can predict whether the upcoming day is particularly short. “These fluctuations have short-term correlations, which means that if the earth accelerates in one day, it tends to accelerate the next day as well,” said Judah Levine, an academician of the National Institute of Standards and Technology. “But, as you get longer and longer, that correlation disappears. When you reach one year, the prediction becomes very uncertain. In fact, the predictions for the International Earth Rotation and Reference Systems Service are not going to go further than the year.”
While there is no difference between shorter days, the trend of shorter days is increasing negative leap seconds, Levin said. “When the leap second system was defined in 1972, no one really thought that a negative leap second system would happen,” he noted. “It's just something in the standard, because you have to do it to be complete. Everyone thinks that only positive leap seconds is needed, but now, the shortening of days (negative seconds) creates the danger of happening, so to speak.”
Levine explained that Levine still had a positive leap second issue after 50 years, which caused the prospect of a negative leap, which attracted attention. “There are still some places that did something wrong or at the wrong time, or (using) the wrong numbers, etc. It's a positive leap second, which is done over and over again. The second worry about the negative leap, because it's never tested, never tried, never tried.”
Because so many basic technology systems rely on clocks and functional time, such as telecommunications, financial transactions, power grids and GPS satellites, to name just a few, the second negative leap is Levine's statement, which, according to the Y2K problem, is similar to the Y2K problem – at that moment in the last century, when a person in the world might encounter a certain situation, because another attitude may appear, because it is an incredible thing. '99' to '00.
Climate change is also a factor in the second problem of LEAP, but in a surprising way. Although global warming has had a considerable negative impact on the planet, in terms of our timing, it has offset the force that is accelerating the rotation of the planet. A study published last year by Agnew details how melting ice in Antarctica and Greenland spreads across the ocean, slowing the Earth's rotation, just like a skater's arms spinning overhead, but if the arms are stuffed along the body more slowly.
“If that ice hasn't melted, if we don't have global warming, then we're already second in Leap for Leap, otherwise we'll be very close to it,” Agnew said. According to NASA, meltwater from Greenland and Antarctic ice sheets has caused a third of global sea level rise since 1993.
According to an assistant professor in the Department of Civil, Environmental and Geological Engineering at the Swiss Federal Institute of Technology in Switzerland, this large-scale transfer of melted ice will not only cause changes in the speed of the earth's rotation, but also the axis of rotation. If the heating continues, its impact may dominate. “By the end of this century, under a pessimistic situation (humans continue to emit more greenhouse gases) The impact of climate change may outweigh the impact of the moon, which has indeed driven the rotation of the earth over the past few billion years. ” Soja said.
Given the uncertainty of long-term predictions of Earth’s rotational behavior, it is currently helpful to have more time to prepare for action. “I think (faster spins) are still within reasonable boundaries, so it might be natural variability,” Soja said. “Maybe in a few years we can see different situations again, and in the long run we can see the Earth slow down again. That would be my intuition, but you will never know.”
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