The rotation period of the Earth is a sidereal day, and its current accurate value is 23 hours, 56 minutes, and 4 seconds. However, in daily life, we use the mean solar day, which is 24 hours. Correspondingly, it currently takes about 365 mean solar days for the Earth to orbit the Sun once, so our current calendar defines a year as 365 days, with an additional day added during leap years. However, the Earth's rotation speed is not uniform, and there is a long-term slowing trend. In the 1920s, the long-term slowing of the Earth's rotation and seasonal changes were successively discovered.
The tidal effects of the moon have the greatest impact on the Earth's rotation speed. The tidal effects of the sun and moon on the Earth are the main factors that slow down the Earth's rotation, which causes the length of a mean solar day to increase by 1.6 milliseconds over a century. Over time, this will gradually reduce the number of days in a year. However, in ancient times, the situation was just the opposite. Geological research shows that 570 million years ago in the Cambrian period, the Earth had only 20.47 hours in a day and 428 days in a year; and 370 million years ago in the middle of the Devonian period, there were 22 hours in a day and 398 days in a year. The period of the Earth's orbit around the Sun is relatively stable, so the decrease in the number of days in a year is clear evidence of the long-term slowing of the Earth's rotation.
The seasonal changes in the Earth's rotation are mainly caused by the monsoon, which generally shows a slowdown in spring and an acceleration in autumn, with a change range of 20-25 milliseconds. In addition, the internal movement of the Earth's materials, the movement of its surface plates, the melting and displacement of glaciers, and even the impact of meteorites can also cause complex irregular changes in the Earth's rotation speed. Due to the uneven rotation of the Earth, the length of the "1 second" defined by it will also change irregularly, which brings trouble to work requiring extremely accurate time. Therefore, in 1967, the 13th International Conference on Weights and Measures decided to define the length of a second as 919,236,170 times the electromagnetic radiation oscillation period of a specific cesium atom. Due to the much greater stability of atomic oscillations than that of the Earth's rotation, the defined second length is extremely accurate. The time measurement system based on this is called the International Atomic Time.
The definition of the atomic second is designed to be as consistent as possible with the second defined by the Earth's rotation, which is familiar to us in daily life. However, these two time systems ultimately rely on completely different physical processes, so they cannot be absolutely consistent. According to the current standard definition, even if the Earth's rotation strictly maintains its speed unchanged, the International Atomic Time will be nearly a second faster than the mean solar time over the course of a year. This means that if we completely adopt the International Atomic Time in daily life, after tens of thousands of years of accumulated errors, we may face the embarrassing situation of welcoming the sunrise at noon. To solve this problem, the International Astronomical Union and the International Radio Consultative Committee proposed a "Coordinated Universal Time" system, which uses the atomic second as the reference and allows it to run uniformly according to the atomic time system. However, in terms of time measurement, it is required to decide whether to add a second on June 30 or December 31 according to actual needs, that is, to insert a 60-second period after 23:59:59 in Coordinated Universal Time, and then it will be 0:00:00 the next day, to compensate for the gap between Coordinated Universal Time and atomic time, called the leap second. Due to the irregular changes in the Earth's rotation speed, the specific time when the leap second is needed must be determined based on the actual observation results of the Earth's rotation.