How was 24 hours a day initially determined?

Dividing a day into 24 hours has its own convenience: it is convenient for people to arrange their time. The division of time is related to astronomical observation, regardless of ancient and modern times. In astronomical observation, a week is generally divided into 12 equal parts, so it is very consistent with the sky to divide a day into 12 equal parts or 24 equal parts.

Although "the earth rotates once in 23 hours, 56 minutes and 4 seconds", it refers to the time when the same star appears twice in the same position on the ground, which is called "sidereal day". People usually use the sun, but it takes 24 hours for the sun to rise to the top of the head twice at noon! This day is "Sun Day".

The unit "hour" was originally divided from the observation day, referring to the solar day instead of sidereal day.

The difference between Sun Day and sidereal day is due to the different distance between the sun and the stars. Solar Day is in line with people's daily life habits, and sidereal day only expresses the rotation time of the earth. Ancient astronomers and physicists used sundials, sundials, water clocks and simple pendulums to time by observing the changes of the sun's shadow or water level. With the progress of science and technology, modern scientists have invented simple pendulum clock and synchronous oscillator, which use the oscillation period of simple pendulum or synchronous crystal to time. However, the above-mentioned time service method is easily influenced by environment, temperature, materials, electromagnetic field and even the observation angle of observers, and its stability is not good, so it must be corrected by the period of celestial bodies (earth rotation, revolution and moon revolution).

Before 1960, CIPM was based on the earth's rotation, and its definition was based on one-eighth of the average solar day. That is, 1 sec = 1/86400, and its stability is about 10-8.

From 1960 to 1967, CIPM was changed to be based on the revolution of the earth, and 1900 was defined as the average solar year. The definition of second is amended as: one second is 3 15525.947 of the average solar year. 1 sec = 1/3 1556925.9747 The stability of solar year in 1900 is about 10-9.

In the mid-20th century, due to the development of quantum mechanics, experiments and researches such as ultra-fine structure of spectrum, magnesium emission and laser, optical pumping, magnetic vibration of molecular beam, and separation of oscillation field were developed, which made quantum frequency standard replace celestial bodies. In 1967, CIPM defines one second as the duration of 9 192, 63 1 770 radiation periods, which corresponds to the transition between two ultra-fine energy levels in the ground state of cesium 133 atom (Cs 133). This second definition has been maintained to this day.

The definition of second can be realized by cesium atomic frequency standard, and its stability varies with the manufacturing method and maintenance environment of each standard. The stability of commercial cesium frequency standard HP-507 1 is about 10- 12. The stability of the atomic clock with absolute temperature scale of 10-6 degrees made by cesium atomic fountain in LPTF laboratory of France is about 10- 16.

The earth's rotation is not 24 hours, 24 hours is the result of the earth's rotation and revolution!

The earth turns once a day.

The time for the earth to rotate once (360 degrees) is 23 hours, 56 minutes and 04 seconds, which is a little less than what we usually say "one day and one night is 24 hours". Humans themselves can't feel the rotation of the earth, so they are used to calling sunrise and sunset a day before sunrise.

The ancient Egyptians divided the day into 24 hours. Every hour is divided into 60 minutes and every minute is divided into 60 seconds. So there are 24×60=86400 seconds in a day. These hours, minutes and seconds have become time units.