Definition:Metre/Historical Note

Historical Note on Metre
The metre was initially defined as the length of a pendulum whose period is $1$ second.

It was soon established that as Acceleration Due to Gravity varies considerably according to location, this was not a sustainable definition to maintain a standard.

Hence it was changed so as to be defined as $10^{-7}$ the distance from the Earth's equator, through Paris to the North Pole (at sea level).

This definition was changed again in $1983$ to be defined as the distance travelled by light in vacuum in $\dfrac 1 {299 \ 792 \ 458}$ of a second.