metre

unit (US "meter") The fundamental SI unit of length.

From 1889 to 1960, the metre was defined to be the distance

between two scratches in a platinum-iridium bar kept in the

vault beside the Standard Kilogram at the International Bureau

of Weights and Measures near Paris.

This replaced an earlier definition as 10^-7 times the

distance between the North Pole and the Equator along a

meridian through Paris; unfortunately, this had been based on

an inexact value of the circumference of the Earth.

From 1960 to 1984 it was defined to be 1650763.73 wavelengths

of the orange-red line of krypton-86 propagating in a vacuum.

It is now defined as the length of the path traveled by light

in a vacuum in the time interval of 1/299,792,458 of a second.

(1998-02-07)