meter
Lengthexact by definition
The metre is the SI base unit of length. Since 1983 it has been defined by fixing the speed of light in vacuum at exactly 299,792,458 metres per second, so the metre is the distance light travels in 1/299,792,458 of a second. That definition is exact by construction, and it makes the metre depend on the definition of the second.
| 1 m | 1 m |
The metre began in 1793 as one ten-millionth of the distance from the North Pole to the equator along the meridian through Paris. In 1889 it became the length of a platinum-iridium bar kept near Paris, in 1960 it became 1,650,763.73 wavelengths of a particular krypton-86 emission, and in 1983 it became the light-travel definition still in force.
About the meter
Every definition of the metre has been an attempt to replace something perishable with something reproducible. The 1793 version tied it to the Earth: one ten-millionth of the quadrant from pole to equator. Surveying that arc took years and the answer was slightly wrong, which is why the Earth's polar circumference is not exactly 40,000 km but about 40,008 km. In 1889 the definition moved to a physical artefact, the International Prototype Metre, a platinum-iridium bar with two scratches on it. An artefact can be dropped, and it can drift, and every country that wanted the metre had to compare its own bar against that one.
The 1960 krypton-86 definition broke that dependence by using an atomic emission any properly equipped laboratory could reproduce. Then in 1983 came the inversion that still governs: rather than measure the speed of light in metres, the CGPM fixed \(c = 299{,}792{,}458\) m/s exactly and let the metre follow from it. Since that day the speed of light has no measurement uncertainty at all, because it is a defined constant, and every improvement in clock accuracy quietly improves the metre. The chain runs from the caesium atom to the second to the metre, and no object anywhere needs to be preserved for it to work.