The natural cycle of day and night, the change of seasons, etc. are the earliest concepts of time established by human beings. In the struggle against nature, people need to carry out productive labor better according to time. Humans gradually made use of the principles of sun shadow movement, fuel combustion and material flow to create early timing tools. For example, in the Xia Dynasty (2 1 century BC to16th century BC), the vertical rod shadow measurement method was established in China. According to the orientation change of the rod shadow, different times are determined. The sundial is a kind of timer developed on this basis. A sundial has a fixed arm or needle, and a disk engraved with numbers and scales. Divide the disc into many parts, and observe the position where the shadow is cast on the disc, so that different times can be distinguished. The sundial can be timed accurately to the minute (15 minutes).
The ancients also measured time by the flowing phenomenon of certain substances, such as the amount of water flowing from a leaking container or a leaking pot. In ancient China, carving leaks were carved into 100 spaces on carved arrows made of bamboo and wood, and each space was an instant, so it was called "Hundred Carvings". Since the Western Han Dynasty, 12 hour has been used to indicate the change of day and night, and every hour is today's 2 hours. Twelve hours are named after twelve earthly branches (Zi, Chou, Yin, Mao, Chen, Si, Wu, Wei, Shen, You, Xu and Hai). From 1 1 in the evening to a child's time, from 1 in the evening to 3 o'clock to an ugly time, and so on.
The ancient Egyptians expressed the change of day and night by setting the day as 10 hour and the night as 12 hour. Due to the changes of the four seasons, the length of day and night varies. Later, the change of day and night was divided into 24 hours, 60 minutes per hour and 60 seconds per minute. This timing method has been used up to now and has become a universal unit of time measurement all over the world.
Due to the development of science and technology, it is required to unify the measurement system and form a complete system. 1 1 960, adopted by the first international metrology conference, officially regards "second" as one of the seven basic units of the international system of units. The State Council 1984 issued the "Order on Unified Implementation of Legal Units of Measurement", which also takes seconds as the basic time unit, and selects non-international time units such as day (day), [hour] and minute as auxiliary units.