The meter is the basic unit of length in the International System (SI). It is defined since 1983 as the distance light travels in vacuum in 1/299792458 second.
- before 1791 : the lengths were measured in reference to humans (the thumb, foot, fathom) as every human being is different, it often took as reference the King, was a strong symbol of monarchy.
- 1668 : the meter is equal to 38 inches of Prussia (John Wilkins)
- 1675 : the meter is equal to the length of a pendulum that swings with a half-period of one second
- 1791 : Academy of Sciences defines
meter meridianwhich was based on the circumference of the earth, and worth 1 /20 000 000 a meridian.
- 1799 : Creating a yardstick
platinumfrom the meter M and became the reference.
- 1889 : the International Bureau of Weights and Measures redefined the meter as the distance between two points on a bar of an alloy of platinum and iridium. This bar is always kept at the Pavillon de Breteuil in Sèvres (like the mass, the calculation of the second or UTC)
- 1960 : The 11th General Conference on Weights and Measures (CGPM) redefined the meter as 1 650 763.73 wavelengths of orange radiation emitted by the isotope 86 of krypton4.
- 1983 : Following the discovery of relativity, the speed of light is fixed at 299,792,458 m /s and the meter is redefined as the distance light travels in a vacuum in 1 /299792458 second. This is the most accurate definition for the second unit is the International System (SI) measured with the lowest uncertainty.