How many centimeters is the traditional unit of length in China? We old people say that one bow is five feet. Do you know that one bow is five feet?

How many centimeters is the traditional unit of length in China? We old people say that one bow is five feet. Do you know that one bow is five feet?

In Du Di Lun, two feet is one cubit, four cubits is one bow, and three hundred bows are one li. Three hundred and sixty steps are one Li, even three hundred bows. In Xiyu Ji, five hundred bows are heard. In notes, five hundred bows are two and a half Li

Why do we have the concept of length and the unit of length, and why do we define 1 meter as such a length? Is this an abstract concept?

The concept of length can be said to be a human understanding of our living space, which can be digitized for description and communication
The source of length unit "meter" is as follows:
In May 1790, a special committee composed of French scientists suggested that one fourth of the total length of the meridian of the earth passing through Paris should be taken as the unit of length - meter, which was approved by the French parliament in 1791, Under the leadership of the French astronomer jeream Buhl and mishen, measurements were made from Dunkirk, France to Barcelona, Spain in 1792-1799. In 1799, according to the measurement results, a platinum metal bar with a short section of 3.5 mm × 25 mm was made. The distance between the two ends of the bar was determined to be 1 m, and it was handed over to the French archives for safekeeping, So it is also called "file meter". This is the earliest definition of meter
Due to the serious deformation of archival meters, the definition of "archival meters" was abandoned in 1872, and the original meters made of platinum alloy (90% platinum and 10% iridium) were used as the unit of length, In 1889, at the first International Conference on metrology, the No.6 meter prototype (the one closest to the length of the file meter at 0 ℃ among the 31 meters certified by the International Bureau of Metrology) was selected as the international meter prototype, As the most authoritative length standard in the world, it is kept in the basement of the International Bureau of Metrology in Paris, and the rest of the rulers are distributed to the participating countries as auxiliary rulers. It is stipulated that when the ambient air temperature is 0 ℃, the distance between the two ends of the original meter is 1 meter. In 1927, the Seventh International Conference on Metrology made strict provisions on the definition of meter, It is also proposed that the rice container should be kept at 1 standard atmospheric pressure and the specific regulations for its placement are made
However, there are many shortcomings in using the original meter as the objective standard, such as material deformation, low measurement accuracy (only up to 0.1 μ m). It is difficult to meet the needs of Metrology and other precision measurement. In addition, if the original meter is damaged, there will be no basis for reproduction, especially the reproduction is difficult to ensure that it is completely consistent with the original, which brings difficulties to the use of various countries, In the 1950s, with the development of isotope spectral light source, krypton-86 isotope spectral line with very narrow width was discovered. With the success of interference technology, people finally found a natural standard which is not easy to destroy, that is, the natural standard with light wavelength as the unit of length
In 1960, the Eleventh International Conference on Metrology made the following changes to the definition of meter: "the length of meter is equal to 1650763.73 times of the wavelength of radiation in vacuum of the transition between 2p10 and 5d1 levels of krypton-86 atom.", In 1963, the krypton-86 isotope length standard was established in China. After the definition of meter was changed, the international meter is still kept in the International Bureau of metrology
With the development of science and technology, the measurement of time and the speed of light has reached a high accuracy since the 1970s. Therefore, the 17th International metrology conference held in Paris in October 1983 adopted a new definition of meter: "meter is the length of light's journey in vacuum within a time interval of 1 / 299792458 seconds.", The definition of meter based on spectral line wavelength is replaced by the new definition of meter
In fact, the meter is defined as the distance that light travels in 0.000000003335640952 seconds measured by a platinum atomic clock (the reason for this particular number is that it corresponds to the historical definition of meter - the distance between two scales stored on a specific platinum rod in Paris), This is simply defined as the distance that light travels in one second. Now, we define the distance according to time and speed of light in relativity, so that each observer can automatically measure the same speed of light (defined as 1 meter per 0.000000003335640952 seconds). There is no need to introduce the concept of ether, as shown in the Michelson Morey experiment, However, relativity forces us to fundamentally change the concept of time and space. We must accept that time cannot be completely separated from and independent of space, but must be combined with space to form the so-called space, the object of time

What's the meaning of D in the unit of length

Diameter is the abbreviation of diameter