The meter is defined by a metal bar in Paris
An old model or guideline was retired in favor of a new one.
TaughtThe meter is defined by the International Prototype Metre, a platinum-iridium bar kept near Paris, or by a specific number of wavelengths of krypton-86 light.
NowSince 1983 the meter has been the distance light travels in a vacuum in 1/299,792,458 of a second. The definition is built from the speed of light, which is now fixed by definition rather than measured.
What actually happened
The chain of definitions is a good story. The meter began in 1793 as one ten-millionth of the distance from the North Pole to the equator, an elegant idea undermined by the fact that surveying that distance was extremely hard and the original survey was slightly off. It became a physical bar in 1889, then a krypton wavelength in 1960, and finally a light-second fraction in 1983.
The 1983 move has a subtle consequence worth appreciating. Because the meter is now defined in terms of the second and a fixed value for c, the speed of light is no longer something we measure. It is exactly 299,792,458 m/s by definition, and any improvement in measurement precision refines the meter rather than the speed. This same philosophy, defining units from fixed constants of nature, is what the kilogram finally adopted in 2019, 36 years later.
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Sources
- BIPM: SI base unit: metrebipm.org
- NIST: Definition of the meternist.gov
Who was taught this
Still standard through 1990, so anyone who finished school between 1950 and 1983 learned the earlier version.