The Earth's rotation is getting faster and faster. Is the Earth also "in a hurry"?

Recently, the earth has been "acting" like a child, just like us when we were children, looking forward to "growing up".

On June 29, the Earth's rotation was 1.59 milliseconds shorter than 24 hours, the shortest day since humans began using atomic clocks. July 26 was also short, 1.5 milliseconds shorter than 24 hours. "How come a day goes by so quickly?" The acceleration of the Earth's rotation makes this feeling no longer seem like an illusion.

It has long been believed that due to the existence of the "tidal friction effect", the Earth's rotation rate has been "braking" slightly and becoming slower and slower. Duan Pengshuo, a researcher at the Shanghai Astronomical Observatory of the Chinese Academy of Sciences, confirmed this view to the reporter of Popular Science Times: "Although the Earth's rotation has accelerated in recent years, the long-term slowdown is the general trend."

The Earth's True Character: Sometimes Fast, Sometimes Slow

Duan Pengshuo introduced that from a shorter time scale, the changes in the Earth's rotation rate are very complex and uneven, showing the characteristics of "sometimes fast, sometimes slow" and quasi-periodic oscillations.

"On the interannual scale, the Earth's rotation rate will accelerate for a few years and then slow down in the next few years; in terms of seasonal changes, the Earth's rotation speed will increase in some months and slow down in other months." Duan Pengshuo analyzed that these changes in day length on different time scales actually reflect different material movement processes in the Earth system.

For example, changes in the Earth's rotation rate on an interannual scale are closely related to the movement of the Earth's core; while seasonal changes in the length of the day are more affected by factors on the Earth's surface, such as the atmosphere and the ocean.

"In other words, judging from the long-term pattern of changes in the length of the day, the accelerated rotation of the Earth in the past two years is not surprising but a normal phenomenon." Duan Pengshuo reminded that this does not mean that "the Earth has become more dynamic."

The principle of rotation is similar to the body rotation in ballet

We know that the earth itself is an independent system. When the influence of external factors is not taken into account, its own angular momentum (that is, the product of the moment of inertia and the angular velocity of rotation) is conserved and it obeys the law of conservation of angular momentum.

Duan Pengshuo explained that the faster the Earth's rotation speed is, the less its moment of inertia is, which manifests as the "contraction" of the Earth's overall matter; on the contrary, the slower the Earth's rotation speed is, the more its moment of inertia is, which manifests as the "expansion" of the Earth's overall matter.

"This process is very similar to when a ballet dancer performs a rotation in place, using the principle of conservation of angular momentum to change her own moment of inertia by contracting (stretching) her arms or legs, thereby achieving the effect of speeding up (slowing down) her own rotation speed," said Duan Pengshuo.

However, the physical factors that can cause changes in the Earth's rotation speed are not limited to the movement of the Earth's core (liquid outer core, solid inner core) and changes in the magnetic field, but also include some phenomena from the Earth's surface, such as: atmospheric movement, ocean movement, land water, glacier movement and climate change.

Internationally, there is a view that the phenomenon of the accelerated rotation of the Earth may be related to the melting of glaciers in the Arctic and Antarctic caused by recent climate change. "In this case, the pressure on the Earth's poles decreases, causing the crust to move upward and the Earth to become more round. The round shape helps the Earth rotate faster," Duan Pengshuo analyzed.

Relevant research shows that factors from the Earth's surface tend to stimulate higher-frequency changes in the Earth's rotation speed within a 5-year cycle, while changes in the Earth's rotation speed caused by factors inside the Earth usually manifest as lower-frequency changes above a 5-year cycle.

In this regard, Duan Pengshuo believes that, in theory, the actual observed changes in the Earth's rotation speed are caused by multiple factors, and are the result of the combined action of the movement of matter inside and outside the Earth, that is, the rapid changes in the length of the day caused by factors outside the Earth (such as climate change, glacier melting, etc.) are superimposed on the changes in the trend of the length of the day caused by factors inside the Earth (such as the movement of the Earth's core). Ultimately, on a certain day (such as June 29), the Earth's rotation speed accelerates to a "peak", which is manifested as the shortest day.

Rotation speed change

No necessary connection to the Chandler Swing

Previously, it was believed that the faster rotation of the Earth and the shortening of the day may be related to the "Chandler Wobble". The "Chandler Wobble" is a phenomenon in which the Earth's poles oscillate slightly relative to the Earth's surface as it rotates, because the Earth itself is not a strict sphere (nor a strict rigid body). The period of this oscillation is approximately 433 days, which is an inherent frequency of the Earth's rotation axis shaking relative to the ground, and is a natural property of the Earth.

"Earth rotation includes the Earth's rotation speed and polar motion. The Earth's rotation speed is called the 'day length', which is the length of a day, and is not strictly 24 hours; polar motion is the movement of the Earth's poles, which is manifested as the movement of the Earth's rotation axis relative to the Earth's surface." Duan Pengshuo said that there is no necessary connection between the acceleration of the Earth's rotation speed and the shortening of a day and the "Chandler Wobble". Because the "Chandler Wobble" is a change in the Earth's polar motion, which is a change in the rotation axis, not a change in the Earth's rotation speed.

No need to adjust the phone time

Many people are curious, as the earth rotates faster and faster, do we need to adjust the time on our mobile phones?

To determine the time, there are two time measurement systems in the world: "world time" based on the rotation of the earth and "atomic time" based on the oscillation period of atoms.

"Universal time" is Greenwich Mean Solar Time, which refers to the standard time at Greenwich. It is a time measurement system based on the rotation of the earth and a form of expressing the rotation rate of the earth. Before the 1970s, it was widely used as a basic time measurement system. "Due to the non-uniform rotation speed of the earth, it is no longer used as a uniform time reference," Duan Pengshuo introduced.

The 13th International Conference on Weights and Measures held in 1967 decided to use the time derived from the cesium atomic clock as the time measurement standard. The atomic clock is currently the most accurate time measurement and frequency standard known in the world.

"The advantages of 'atomic time' are good stability and very small error. It has been running continuously for 300,000 years with an error of less than 1 second. In other words, the error in the speed of time each day does not exceed one ten-millionth of a second," said Duan Pengshuo.

In addition, in addition to "world time" and "atomic time", the commonly used time system in the world now also has "Coordinated Universal Time". Duan Pengshuo introduced that the time scale of the difference between "atomic time" and "world time" within 0.9 seconds is "Coordinated Universal Time". If it is greater than 0.9 seconds, a leap second will be added.

"Here, 'Coordinated Universal Time' uses frequency modulation first and then leap seconds to achieve constraints with 'World Time'. In this way, 'Coordinated Universal Time' can not only maintain the uniformity of atomic time, but also take into account the performance of 'World Time' and the earth's rotation." Duan Pengshuo said that our mobile phones and other mobile devices use the "Coordinated Universal Time". The length of a day is reduced by 1.5 milliseconds, which is far from 0.9 seconds, so the mobile phone time does not need to be adjusted.

Duan Pengshuo said that the loss of 1.59 milliseconds is not that special, because, within a certain period of time, it is inevitable that the observed data on the change in day length will have a local minimum point, and moments smaller than -1.59ms are likely to appear again in the future.

Source: Popular Science Times

Author: Epic

Editor: Wu Tong

Review: Wang Fei

Final judge: Chen Lei