So I was perusing Paris travel guides since I’ll be headed back there in April, and I came across this very interesting definition of a meter. Perfect example of why the metric system is mysterious and perplexing to many! The below section comes from the guide Secret Paris, penned by Jacques Garance and Maud Ratton.
How is a metre defined?
It is all too often forgotten that the metre is a French invention, being defined by the Paris Académie des Sciences in 1791 as one-ten-millionth of a quarter of a meridian of the earth. By this definition, the circumference (that is, meridian) of the Earth was 40,000km. After the establishment of the first standard metre, it was 1875 before seventeen other nations signed the “Convention du Mètre”. In 1899, the Bureau of Weights and Measures has a standard metre cast in platimun-iridium alloy, which was held to be subject to only infinitesimal variations; that original bar can still be seen at the Pavillon de Breteuil in Sèvres (Hauts-des-Seines). With the advent of laser technology, the Conférence Générale des Poids et Mesures (CGPM) in 1960 gave a definition of the metre that is rather less comprehensible to the layman: 1,650,763.73 wavelengths of orange-coloured radiation emitted by the krypton 86 atom.
In 1983 came an even more esoteric definition: the metre is the length of the path travelled by light in a vacuum during a time interval of 1/299,792,458 of a second. According to the theory of relativity, the speed of light in vacuum is the same at all points, so this definition is considered to be more accurate.