The first attempt to define a metre was made in France in 1793 and was defined as the ten-millionth part of that segment of the Earth’s circumference that runs from the equator to the North Pole, via Paris of course. Oh and by the way they were 0.22 of a millimetre short due to the flattening of the Earth by its rotation (as explained in a previous factoid).
Over the years since its been tweaked a couple of timesand in 1960 the metre we have today was agreed upon as 1,650,763.74 wavelengths of the orange-red line of the spectrum of krypton-86 as measured in a vacuum.
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