The metre began as a slice of the distance from equator to pole
In 1791 revolutionary France declared the metre to be a ten-millionth part of the stretch running from the equator up to the North Pole, measured along a meridian through Paris. Since then it has been a metal bar, a count of krypton light waves, and now the distance light covers in a tiny fraction of a second.
The French National Assembly's original choice made a quarter of Earth's polar circumference exactly 10,000 kilometres. Practicality soon intervened. In 1799 the metre was redefined by a prototype bar, and a replacement bar took over in 1889. In 1960 the standard left metal behind, becoming a set number of wavelengths of a particular emission line of krypton-86.
Since 1983 the metre has been tied to light: the length of the path light travels in a vacuum in 1/299,792,458 of a second. A 2002 tweak clarified that this is a proper length, and the 2019 revision of the SI rephrased the definition to include the second's own basis in a caesium atomic frequency.
The word reaches back to the Greek metreo, to measure or count, and metron, a measure, from a Proto-Indo-European root meaning to measure. The same family covers poetic metre and the idea of a measured, moderate response. English began using metre for the French unit by at least 1797. Every English-speaking country spells it metre except the United States and the Philippines, which write meter.
Older units hint at what came before. The ancient Egyptian cubit was about half a metre, the Scottish and English ells were 0.941 and 1.143 metres, and the old Parisian toise, just under two metres, was later fixed at exactly two. Sweden's mil, once 10.688 kilometres, was trimmed to 10 when the country went metric. For quick conversion, the three 3s rule says a metre is about 3 feet 3 and 3/8 inches, overshooting by an eighth of a millimetre. People tend to say 30 centimetres rather than 3 decimetres, and astronomers prefer light-years or parsecs to vast multiples of the metre.
Source: Metre
