Spring is cold and steep, and basking in the sun at noon is a kind of enjoyment. But have you noticed? The February sun is a bit "lazy": the midday hour is always delayed. Why?
Wang Kechao, director of science popularization at the Purple Mountain Observatory of the Chinese Academy of Sciences, introduced that the so-called noon is the time when the sun comes to the south. It is generally believed that the sun will run in the south direction at 12 o'clock every day, and at this time it will arrive at noon. But this is not the case. There is often a time difference between noon and 12 o'clock, which is related to the time system we use.
"Sunrise and rest". Since ancient times, people have arranged production and life according to the Sunday movement of the sun. This time system, which is determined based on the apparent motion of the true sun, is called true solar time. However, due to the influence of the eccentricity of the Earth's orbit and the equatorial intersection of the ecliptic, the true solar time is uneven and cannot meet the modern human need for time accuracy. In order to get a more uniform daily time, people assume that the sun moves average and get flat solar hours. We know Greenwich Mean Time, also known as Greenwich Flat Solar Time.
There is a time difference between the normal solar time and the true solar time, and this difference ranges between minus 14 minutes and positive 16 minutes, and the noon hour is therefore correspondingly advanced or postponed. In fact, there are only 4 days of zero time difference in a year, which generally occurs around April 16, June 13, September 1, and December 25. The time difference in February remained near the minimum, i.e. minus 14 minutes. That is to say, the most recent period of the year when the sun has postponed "duty" for the longest time.
With the development of technology, the basis for people's timekeeping is no longer limited to astronomical observations. At present, science adopts a more precise time system: atomic time. Atomic time is based on the electromagnetic oscillation period emitted from within the atom, which is a relatively constant and uniform time system. The time unit "second" of the International System of Units is defined in atomic time and is one of the basic units of the three major physical quantities.
"The time people use on a daily basis is now about 1 second apart every 1 to 2 years compared to atomic time, adjusted by dialing 1 second in advance or backward." Wang Kechao said.
Producer: Hu Qinghai
Planner: Liu Xinhui
Reporter: Wang Jueyue
Poster Maker: Chen Jie
Editors: Lian Changyan, Tang Jianhui
It was co-produced by Xinhua News Agency and Jiangsu Branch of Xinhua News Agency
Produced by Starry Sky Studios