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Units of length
Which units are used to measure length ?
Do you know any history of measurent ?
Some old units of length
Cubit was used most early to measure length last 3000 years.
Cubit : the length of the arm, from the elbow to the tip of the middle finger.

cubit

Foot :
To Greek, foot : the width of 16 fingers.
To Romas, 1 foot = 12 inches
Measuring feet
Yard : the distance from the tip of the nose to the thumb of King Henry I
Yard : the arcumference of a person’s waist.
Henry I
The yardstick
(Henry I, thumb to nose). 
Some different units:
fermi (fm),
anxtrom,
micron,
RE ( radius of Earth)
AU ( astronomical unit)
LY ( light year)

Arrange from smallest to great :
inches
feet
yards
miles
Definition of metre
1799, The meter standard, a platinum The bar, is officially presented in Pari
1889, the length of that platinum bar with no longer reference to the Earth meridian.
Length measurement

1960, equal 1650763.73 wavelengths of the red-orange radiation of Kripton atom.
1983, metre is the distance travelled in vacuum by electromagnetic wave in 1 / 299792458 of a second.
1960, equal 1650763.73 wavelengths of the red-orange radiation of Kripton atom
1989, metre is the distance travelled in vacuum by electromagnetic wave in 1/299792458 of a second.
And then, metre was used as mainly unit of length.

Metric system
Metre, inclue centimetre, kilometre
Non – SI unit :
fermi ( fm) = 1 femtometre
angxtrom = 100 picometres
micron = 1 micrometre
Swedish mile = 1000 metres
Imperical unit
inch = 25.4 mm
foot = 12 inches = 0.3048 m
yard = 3 feet = 0.9144m
mile = 5282 feet = 1609.344m
Astronomical units :
Earth radius ( RE ) : ~6370 kilometres
astronomical unit (AU) : ~ 149 gigametres
light year : ~ 9.46 petametres
…..