Since 2019, the metre has been defined as the length of the path travelled by light in vacuum during a time interval of 1/299792458 of a second, where the second is defined by a hyperfine transition frequency of caesium.
It’s not that bad once you accept that there is no correct answer for what should be considered “1 length” unless you want to use Planck units, which are absurdly uselessly large or small.
The metre was originally defined in 1791 by the French National Assembly as one ten-millionth of the distance from the equator to the North Pole along a great circle through Paris, setting 10000 km as that quarter of the Earth’s polar circumference.
There was the navigation rationale for setting the meter/metre at the length that it is, but the original and subsequent definitions proved to be inconsistent and difficult to measure precisely.
Wikipedia says:
Of course. It’s so obvious.
It’s not that bad once you accept that there is no correct answer for what should be considered “1 length” unless you want to use Planck units, which are absurdly uselessly large or small.
There was the navigation rationale for setting the meter/metre at the length that it is, but the original and subsequent definitions proved to be inconsistent and difficult to measure precisely.
It’s also not accurate because one of the people who measured that distance made a small mistake in their calculations.