Length unit refers to measurementSpatial distanceOnBasic unit, is the basic unit formulated by human beings to regulate the length.Its SI unit is“rice”(symbol "m"), common units aremillimeter（mm）、centimeter（cm）、Decimeter（dm）、Kilometers(km), meter (m), micron (μ m), nanometer (nm), etc.The unit of length plays an important role in all fields[1]。
To understand the unit of length, we must first understand its upper concept - unit.Modern Chinese Dictionary explains "unit" as the name of the standard quantity for measuring things.In measurement, taking a certain quantity of the same kind of quantity as the reference quantity, determine how many times the known quantity is equivalent to the reference quantity. The reference quantity is called the unit.For example, centimeter is the unit for measuring length, gram is the unit for measuring mass, and second is the unit for measuring time.As an example of mathematics, we can also say that 1 is the unit of natural number, and the fraction with the numerator of 1 is the unit of fraction[2]。
When measuring the length of a line segment, you need to select a line segment as the measurement standard and specify its length as 1, which is called the length unit.There are many units of length, the primary unit is meters, and the rest are derived units.When measuring, select the appropriate length unit according to the actual needs.For example, the length of the runway is measured in meters, the distance between two cities is measured in kilometers, and the distance of the long jump is measured in centimeters[2]。
International unit
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International System of UnitsThe standard unit of length is "meter", which is represented by the symbol "m".The 11th International Metrology Conference in 1960: "The length of meter is equal to 2P of krypton 86 atomtenAnd 5doneThe radiation of transitions between energy levels is 1650763.73 times the wavelength in vacuum ".Since 1983lengthDefined as“lightstayvacuumIn 1/299792458secondDistance travelled inside ".The length unit adopted in China is consistent with the International System of Units, that is, "meter" is the legal length measurement unit in China[3]。
Kilometer, also known as kilometer, is a unit of length, abbreviated as "km", which is usually used to measure the distance between two places.The common conversion relationship is as follows: 1km=1000m=100000cm=1000000mm;1.61 km=1 mile[3]。
(2) Meters
International System of UnitsThe unit of length is "meter" (meter,Metre) originated in France.In May 1790, a special committee composed of French scientists recommended that one forty millionth of the total length of the Earth's meridian passing through Paris be taken as the unit of length - meter, which was approved by the French Parliament in 1791.In order to make a reference instrument for measuring the value of meters, under the leadership of French astronomer Jalilianbuer and Mishin, measurements were carried out from Dunkirk, France, to Barcelona, Spain, in 1792-1799.In 1799, according to the measurement results, a platinum meter bar with a 3.5 mm × 25 mm short section was made. The distance between the two ends of this bar was set as 1 meter, and it was handed over to the French Archives Office for safekeeping, so it is also called "archives meter".This is the earliest definition of rice[3]。
Due to the serious deformation of archive meter, the definition of "archive meter" was abandoned in 1872, and the original meter made of platinum alloy (90% platinum and 10% iridium) was used as the unit of length.The original meter was made according to the length of the "file meter". At that time, a total of 31 pieces were made, whose cross section was approximately in the shape of X. The length of the file meter was engraved on the groove (neutral side) of the ruler with two lines with a width of 6-8 microns.In 1889, at the first International Metrology Conference, the No. 6 meter prototype (the one closest to the file meter at 0 ℃ among the 31 meters) identified by the International Bureau of Metrology was selected asInternational rice prototypeAnd kept in the basement of the International Bureau of Metrology in Paris as a length benchmark, and the remaining rulers were distributed to the participating countries as sub rulers.It is specified that when the ambient air temperature is 0 ℃, the distance between the middle scribes at both ends of the meter prototype is 1 m.In 1927, the Seventh International Conference on Metrology made strict provisions on the definition of meter. In addition to the temperature requirements, it also proposed that the original meter should be kept at 1 standard atmospheric pressure, and made specific provisions on its placement method[3]。
However, there are also many shortcomings in using the meter prototype as the objective standard of meters. For example, the measurement accuracy of material deformation is not high (only 0.1 μ m), which is difficult to meet the needs of metrology and other precision measurements.In addition, if the original meter is damaged, the copy will have no basis, especially the copy is difficult to ensure that it is completely consistent with the original meter, which brings difficulties to the use of countries.Therefore, the idea of using natural quantity value as the unit benchmark has always been desirable.In the 1950s, with the development of isotope spectral light sources.With the discovery of krypton 86 isotope spectral line with very narrow width and the success of interference technology, people finally found a kind of natural standard that is not easy to destroy, that is, the natural standard with light wave length as the unit of length[3]。
This natural benchmark has stable performance, no deformation problem, is easy to reproduce, and has high reproduction accuracy.In 1963, China also established the Krypton-86 isotope length standard.After the definition of meter is changed,International rice prototypeIt is still kept in the International Bureau of Metrology according to the original regulations.With the progress of science and technology, since the 1970s, the measurement of time and speed of light has reached a high accuracy.Therefore, at the 17th International Metrology Conference held in Paris in October 1983, a new definition of meter was adopted: "meter is the length of light travel in vacuum within a time interval of 1/299792458 seconds".Thus, the definition of meter based on the wavelength of spectral line is replaced by the new definition of meter[3]。
(3) Decimeter
Decimeter (or dm) is one of the metric units of length. One decimeter is equivalent to one tenth of one meter.The common conversion relationship is as follows: 1 decimeter=0.0001 km=0.1 m=10 cm=100 mm[3]。
(4) Cm
Cm, unit of length;English: centimetre (s), abbreviation (symbol): cm.The unit conversion of centimeter is as follows: 1cm=10mm=0.1decimeter=0.01m=0.00001km[3]。
(5) Mm
Millimeter, also known as millimeter (or millimeter), is a unit of length and rainfall, with the symbol mm.One millimeter is equivalent to one thousandth of one meter (meaning "milli").The base relation is as follows: 1 mm=0.1 cm=0.01 decimeter=0.001 m=0.00001 km[3]。
(6) Micron
Micron is a unit of length, a symbol (micron), read as (mi ú).One micron is equivalent to one millionth of one meter (meaning "micro")[3]。
(7) Nanometer
Nanometer (the symbol is nm) is a unit of length, sometimes called nanometre (also the old name of nanometer) in atomic physics, which is 10-9Meters (one billionth of a meter).Like centimeter, decimeter and meter, it is a unit of measurement for length.It is four times the atomic size and smaller than the length of a single bacterium.Single bacteria and microorganisms can not be seen with the naked eye. The diameter measured with a microscope is about five microns, that is, five thousand nanometers.For example, suppose that the diameter of a hair is 0.05 mm. Divide it into 50000 radially on average, and the thickness of each hair is about one nanometer.In other words, one nanometer is about 0.000001mm. Nanoscience and technology, sometimes referred to as nanotechnology, is to study the properties and applications of materials with a structure size of 1 to 100 nanometers[3]。
(8) Pimi (pm)
Picometer (or pm) is a unit of length. One picometer is equivalent to one trillion parts of one meter.Sometimes it is called micromicron in atomic physics.The conversion relationship is as follows: 1 picometer=10-12M=0.001 nm=0.00001 μ m[3]。
Light year, a unit of length, is generally used to calculate the distance between stars.Light year refers to the distance that light travels in a vacuum for one Julian year (365.25 days, while the daily equivalent is 86400 seconds, and a year is 31557600 seconds). It is calculated by time and speed[3]。
The distance of light walking for one year is called "one light year". One light year is about 9.46 × 10twelvekm。More formally, it is defined as the distance traveled by a photon in free space and at an infinite distance from any gravitational field or magnetic field in a Julian year.Because the speed of light in vacuum is 299792458 meters per second (accurate value), a light year is equal to 9460730472580800 meters, or 5878897915661.709 miles, or 510879455193.632 nautical miles, or about 9.46 × 10fifteenM=9.46 beats m[3]。
Note: 1 kilometer (km)=0.6214 miles=0.540 nautical miles[3]；
1 Julian year=31557600 seconds[3]。
Light years are generally used to measure large distances, such as the distance between the solar system and another star.A light year is not a unit of time.In astronomy, the second difference is another commonly used unit, the 1second difference=3.26 light years[3]。
Parsec (pc) is a unit of length in astronomy.The second difference is the oldest and the most standard method to measure the distance between stars. It is based on the triangular parallax.The average radius (one astronomical unit,AU) is the inner angle of the triangle corresponding to the bottom edge, which is called parallax.When the size of the angle is 1s, the length of one side (the distance from the earth to the star) of the triangle (since the length difference between the two sides corresponding to the 1s angle can be completely ignored, the triangle can be imagined as a right triangle or an isosceles triangle) is called the 1s difference[3]。
The second gap is mainly used to measure the distance of extrasolar objects.The 1second difference is defined as the distance when the annual parallax of the celestial body is 1 ".The second difference is the reciprocal of the annual parallax. When the annual parallax of a celestial body is 0.1 '', its distance is 10 seconds; when the annual parallax of a celestial body is 0.01 '', its distance is 100 seconds, and so on.One second difference is equal to 3. 26164 light-years, or 206265 astronomical units, or 308568 trillion kilometers.When measuring distant galaxies, the unit of second difference is too small, and the units of kilosecond difference (kpc) and millisecond difference are commonly used[3]。
Astronomers usually use the second difference rather than the astronomical unit to describe the distance of celestial bodies.This is not only because it is easier to calculate when the number of second difference is small, but also for historical reasons.The greater the parallax of a celestial body, the closer its distance will be.On the contrary, the smaller the parallax, the farther away from us.The second difference of the nearest star (except the sun) is about 1.29 pc (4.22 light years).One second difference is equal to 30835997962819660.8 meters.One second difference=3.2616 light years=206265 astronomical units=30856.8 billion kilometers[3]。
The astronomical unit is a unit of length, approximately equal to the average distance between the earth and the sun.One of the astronomical constants.In astronomy, the basic unit for measuring distance, especially the distance between celestial bodies in the solar system. The average distance from the earth to the sun is one astronomical unit.One astronomical unit is about 149.6 million kilometers.In 1976,International Astronomical UnionAn astronomical unit is defined as the distance between a particle with negligible mass, undisturbed orbit and a revolution period of 365.2568983 days (one Gaussian year) and an object with mass equal to about one sun.The currently accepted astronomical unit is 149597870691 ± 30 meters (about 150 million kilometers or 93 million miles)[3]。
When the astronomical unit was first used, its actual size was not very clear, but the distance of the planets could be calculated in astronomical units with the help of heliocentric geometry and the laws of planetary motion.Later, the actual size of the astronomical unit was finally found through parallax, and in modern times, it was accurately found by radar.However, because of the uncertainty of the gravitational constant (only five or six significant bits), the mass of the sun cannot be very accurate.If the international units are used to calculate the position of planets, the accuracy will inevitably decrease in the process of unit conversion.So these calculations are usually based on the solar mass and astronomical units, rather than kilograms and kilometers[3]。
The distance of one astronomical unit is equivalent to the average distance from the earth to the sun, about 1.496 × 10eightkm[3]。
(4) Patting rice
The beat meter is also one of the units for measuring distance, and is sometimes used for measuring small and medium distances in the universe.The symbol is Pm[3]。
1Pm=10 to the 15th power m (m), that is
1Pm=100000000000000m (1 trillion kilometers or 1000 trillion meters)[3]。
(5) Megameter
Length measurement unit[3]。
Symbol: Mm (not to be confused with millimeter symbol)
Megameter (symbol Mm) is a rare unit of length, which is equivalent to 621.37 miles.There are two reasons why this unit is seldom used. One is that on this large scale, there are usually more suitable units, such as kilometers or 21 astronomical units, or they can be usedScientific notationTo represent;The second is that the symbols of megameter and millimeter are capital "M" and lowercase "m" respectively, and many people will confuse the two[3]。
The following are several daily usable length distances in megameters: the perimeter of the earth through the poles is 39.94Mm (according to the original definition of meters, from 1791 to 1875, the perimeter of the earth's poles was defined as 40.00Mm).fromAmsterdam, NetherlandsThe distance to Bordeaux, France is about 1Mm;The equatorial diameter of the earth is about 12.76Mm;The average distance from the earth to the moon is 384.4Mm;Saturn's equator has a diameter of 143Mm[3]。
In practice, a unit often used is Egsteran (abbreviated as "Ai", the symbol ∨), which is a unit of length.Egstrom is a unit named in memory of Swedish scientist Anders Egstrom.Egstrong is one of the founders of spectroscopy. He made a spectrum for the radiation wavelength of the solar spectrum-10Meters.It is not an SI unit, but it can be converted to SI units, that is, 1 ∨=10-10M=0.1 nm.It is generally used for atomic radius, bond length and wavelength of visible light.For example, the average diameter of an atom (calculated from an empirical radius) is between 0.5 angstrom (hydrogen) and 3.8 angstrom (uranium, the heaviest natural element).It is also widely used in structural biology[3]。
Meaningful minimum measurable length.Planck length is determined by the relative value of gravitational constant, speed of light and Planck constant, which is roughly equal to 1.6x10 ^ - 35m, that is, 1.6x10-33Cm, which is 10 times the diameter of a protontwenty-two1/[3]。
Silk rice, a kind of tiny sheet used to calculate length, capacity and weight.Ten threads are a thread, ten threads are a dime.Its abbreviation is dmm.oneSilk rice=1/10000 m[3]。
The flying meter (femtometer or fm) is a unit of length. One flying meter is equal to 10 meters to the negative 15th power.1 Fermi - The detail of a proton (or possibly a neutron) is about the size of a nucleus.The common conversion relationship is as follows: 1Feimi=0.001Pimi(pm) =0.000 001nanometer(nm)[3]。
The ammeter (or am) is the unit of length, 1AmiEquivalent to 10-18Meters.The conversion relationship is as follows: 1Ami=0.001Feimi(fm) =0.000 001Pimi(pm)=0.000 000 001 nm[3]。
One meter, the English symbol ym, is also called youmi.One unit meter is 1ym=10-24m = 1.0570x10-40Light year=0.001 square meters[3]。
Conversion relationship and conversion table
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Chinese units
The traditional Chinese units of length are Li, Zhang, Chi, Cun, Xun, Ren, Fu, Shu, Bi, Bu, Chang, Ya, Yan, Ji, Gui, Pheasant, Hao, Li, Fen, etc.The basic conversion relationship is as follows[3]：
A small number of European and American countries, mainly Britain and the United States, use English units, so their length units are different, mainly miles, yards, feet, inches[3]。
