How long is a second and how is it determined?

Meteorologists have observed and calculated the "average solar days" and divided 1 average solar days into 86,400, of which 1 is 1s (seconds). After astronomical observation and quartz clock calibration, the uncertainty is one in a billion, that is, the three-year difference is1s.

After the Han dynasty, "time" no longer represents the season, but is used to represent the unit of calculating time. At that time, people divided a day into 12 hours on average, and "one hour" was equal to two hours now. People are used to calling this "hour".

In some classical books, we often see the word "time". Many people mistakenly think that an hour is an hour now, which is actually incorrect. The ancient word "night watch" and "night watch" refers to time, and there are two hours between the first watch and the second watch and the third watch.

Later, people divided the day into 24 parts on average, and each part was still expressed by "time". At this time, "a moment" is an hour now, only half of the past (after the Han Dynasty), and the time referred to is shortened. Then, people divide an hour into 60 equal parts, each time called 1 minute, and divide 1 minute into 60 equal parts, each time called 1 second. This determines the hours, minutes and seconds.

Extended data:

In the international system of units, the basic unit of time is seconds, and the symbol is S, 20 18 1 16. The 26th International Conference on Weights and Measures defined seconds as 965433, which corresponds to the radiation of two ultra-fine energy level transitions of undisturbed cesium-133.

The cesium atom mentioned in this definition must be at rest at absolute zero, and the environment on the ground is zero magnetic field. The second defined in this case is equivalent to the second defined in the astronomical almanac. Common time units in life are: millisecond ms, minute min, hour h, day) d, month m, year y, etc.

Time is an abstract concept, which shows the continuity and sequence of material movement and change. The concept of time includes two concepts: time and time period. Time is a parameter used by human beings to describe the process of material movement or the occurrence of events. The determination of time depends on the law of material circulation that is not affected by the outside world.

For example, the cycle of the moon around the earth, the cycle of the earth around the sun, the cycle of the earth's rotation, and the cycle of atomic oscillation. Einstein said that time and space are an illusion of people's cognition.

References:

Baidu Encyclopedia-Second