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Why is the Earth’s rotation also “in a hurry”?

  In the long run, the Earth’s rotation tends to slow down. Scientists predict that in another 100 million years, a day can increase by half an hour; on a slightly shorter time scale, such as tens of thousands or hundreds of thousands of years in the future, with the tide of the moon If the force weakens, etc., the rotation of the earth will slow down.
  On August 25, the 2nd China Space Science Conference was held in Taiyuan, Shanxi. At the meeting, the atomic clock was mentioned as an important topic.
  As the most accurate timekeeping tool, atomic clocks have made a lot of contributions in the history of human timekeeping. Not long ago, some scientists used high-precision atomic clocks to record the shortest Earth day so far.
  Generally speaking, an Earth day is about 24 hours. However, on June 29 this year, the time of the Earth’s rotation was 1.59 milliseconds less than 24 hours, making it the shortest day since the atomic clock.
  The shortening of the time it takes for the Earth to rotate once means that the Earth’s rotation is accelerating. According to the “Asia-Oceania Geoscience Society Annual Meeting Report”, scientists believe that the acceleration of the Earth’s rotation is caused by the Chandler wobble.
  So, what is the principle of atomic clock timing? Is Earth’s rotation really accelerating? How did the Chandler swing come about? Recently, the reporter interviewed relevant experts on these issues.
The most accurate timekeeping tool in the world

  ”There are three common time systems, namely Universal Time (UT) based on the Earth’s rotation period, Almanac Time (ET) based on the Earth’s revolution period around the Sun, and Atom based on the electromagnetic oscillation frequency inside the cesium atom. time (AT).” Zhai Hao, a researcher at the 510 Institute of the Fifth Research Institute of China Aerospace Science and Technology Corporation, introduced to reporters.
  The development of the time system has gone through the process of developing from astronomical time to atomic time. Astronomical time refers to the time obtained by observing astronomical phenomena, that is, the periodic motion of celestial bodies such as the sun, moon and stars, including the universal time and almanac time mentioned above. Atomic time refers to the time obtained by using atomic clocks based on the electromagnetic waves emitted by atoms when they absorb or release energy.
  Atomic time germinated in the late 1940s and was born in the early 1950s. In 1967, the 13th International Congress of Weights and Measures decided to change the definition of second from astronomical second to atomic second, that is, the duration of 9192631770 cycles of the hyperfine transition of the zero-field ground state of the cesium atom was set as 1 second, which is called atom. seconds, and took January 1, 1958 0:00:00 as the starting point of atomic time, thus ushering in a new era with microscopic quantum jumps as the timing standard.
  Zhai Hao said that the elements commonly used in atomic clocks are cesium, rubidium, hydrogen and alkaline earth metals. Because these atoms have very high-precision energy level transitions, the electromagnetic waves they output are very stable. A series of sophisticated instruments control these electromagnetic waves, making atomic clocks extremely accurate. The accuracy of a typical cesium atomic beam frequency standard is on the order of 10-14, which is orders of magnitude higher than the astronomical time accuracy of macroscopic timekeeping. Such accurate atomic time provides a strong guarantee for the development of astronomy, navigation, aerospace and other fields.
  ”From the perspective of working principle, atomic clocks are devices based on quantum mechanics and atomic physics and other physical mechanisms, using the principle of atomic transition to generate stable and accurate time-frequency signals.” Zhai Hao said that atomic clocks are divided into two categories: microwave atomic clocks and optical clocks. The cesium atomic fountain clock used as an international time and frequency reference belongs to the microwave atomic clock, and its accuracy has reached a high level. The frequency uncertainty of the cesium atomic fountain reference clock NIM6 of the Chinese Academy of Metrology is better than 5.8×10-16, which is equivalent to 54 million years or less than 1 second. The optical clocks of the National Time Service Center of the Chinese Academy of Sciences and other units have also reached the international advanced level, and the system uncertainty has reached 5 × 10-17, which is equivalent to 600 million years with no difference of 1 second.
  The world time is closely related to the rotation of the earth. If the rotation of the earth increases, the world time will speed up. If the rotation of the earth slows down, the world time will slow down. Therefore, as time goes on, the gap between atomic time and universal time will become larger and larger.
  The current international standard time is called Coordinated Universal Time (UTC). Whenever the difference between atomic time and universal time accumulates to 0.9 seconds every year, the coordinated universal time compensates for the error by plus or minus 1 leap second, while maintaining the uniformity of the time scale.

  The average period of the Earth’s rotation is 23 hours 56 minutes 04 seconds. Some scholars believe that when the earth was just born, the rotation speed was very fast, only 8 hours a day; in the age of dinosaurs, the earth’s day has been 23.5 hours; and in the more than 100 million years from the age of dinosaurs to the present, the average period of the earth’s rotation The total length is about 30 minutes, that is, the average length is about 16.4 microseconds per year.
Earth’s rotation accelerates in the short term but slows down in the long term

  Currently, the average period of the Earth’s rotation is 23 hours, 56 minutes and 04 seconds. Some scholars believe that when the earth was just born, the rotation speed was very fast, only 8 hours a day; in the age of dinosaurs, the earth’s day has been 23.5 hours; and in the more than 100 million years from the age of dinosaurs to the present, the average period of the earth’s rotation The total length is about 30 minutes, that is, the average length is about 16.4 microseconds per year.
  Ping Jinsong, a researcher at the National Astronomical Observatory of the Chinese Academy of Sciences, said that with the dynamic evolution of the earth, the earth’s rotation has been slowing down since billions of years ago. Research suggests that the most recent slowing rate is around 2 milliseconds per century. The solid tidal drag of the Earth’s Moon’s gravity slows the Earth’s rotation.
  According to precise measurements from atomic clocks, the Earth is spinning faster this year, making the day shorter. Ping Jinsong emphasized that, in fact, in the past half century, the earth’s rotation has fluctuated in a short period under the long-term slowdown trend, that is, the earth’s rotation has accelerated.
  For example, 2020 saw the 28 shortest Earth days. Among them, on July 19, 2020, the earth rotated once with a difference of 1.47 milliseconds in 24 hours, setting the record for the shortest Earth day of the year; on June 29, 2022, this record was broken.
  Ping Jinsong said that a reasonable explanation is that the intensification of the greenhouse effect causes global warming, the melting of glaciers at the poles and high altitudes of the earth, the global sea level rises by about 3 mm every year, and the material on the earth is redistributed, leading to the overall rotation around the axis of rotation. The moment of inertia decreases, and the Earth’s rotation accelerates.
  On the senses, it is difficult for humans to feel the changes in the rotation rate of the earth. However, such millisecond-level changes are significant for the various high-precision instruments and devices used by humans. For example, changes in the Earth’s rotation rate and axis of rotation are crucial to establishing an accurate dynamic Earth coordinate reference system, and also determine the accuracy of satellite navigation, positioning, and orbit determination.
  In the long run, the Earth’s rotation tends to slow down. Scientists predict that in another 100 million years, a day can increase by half an hour; on a slightly shorter time scale, such as tens of thousands or hundreds of thousands of years in the future, with the tide of the moon If the force weakens, etc., the rotation of the earth will slow down.
The mechanism of Chandler’s swing is a century-old puzzle

  Chandler wobble is the wobble of the earth’s rotation axis. It was discovered by American astronomer Seth Carlo Chandler in 1891 through astronomical observations.

  ”Chandler’s swing does not directly lead to the acceleration of the Earth’s rotation.” Yang Yi, a special-term associate researcher at Peking University, said that the two have a certain correlation, but the causality between the two is not yet clear. In fact, the migration of matter and the exchange of angular momentum within the Earth system can lead to changes in the position of the Earth’s rotation axis and the rate of its rotation.
  The fact that the Earth is an uneven spheroid is seen as one of the reasons for Chandler’s wobble. But in fact, the excitation mechanism of Chandler’s wobble, as part of the earth’s wobble, is a century-old problem in the earth science community. Scientists have proposed mechanisms such as tidal action, momentum exchange with the liquid core, and large earthquakes that also cause Chandler to wobble. In addition, scientists have found that the amplitude of Chandler’s swing is also related to the atmosphere and ocean.
  Researchers at NASA’s Jet Propulsion Laboratory have proposed that the Chandler wobble is driven by the coupling of atmospheric and oceanic motions. Their computer simulations showed that pressure fluctuations on the seafloor accounted for two-thirds of the total drive. But at least so far, there has been no movement that drives Chandler’s swing.
  Yang Yi introduced that scientists have established the International Earth Rotation Service Association. One of its tasks is to regularly measure the orientation of the Earth’s rotation axis and monitor Chandler’s wobble. Since the Chandler wobble is the wobble of the earth’s axis of rotation on the surface, it will cause changes in the earth’s latitude. Internationally, the International Latitude Observatory was established as early as 1899 to monitor latitude changes for a long time to obtain Chandler wobble information. my country’s Tianjin Latitude Station, Shanghai Sheshan Observatory and other institutions have participated in the international plan.