The SI Units Adopt
a Constant Value for c
The Standard Second in 1967
Considering that a very precise definition of the unit of time is indispensable
for the International System, the 13th CGPM (1967) decided to replace the
definition of the second by the following (affirmed by the CIPM in 1997 that
this definition refers to a cesium atom in its ground state at a temperature of 0 K):
The second is the duration of 9 192 631 770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the cesium 133 atom.
The Standard Meter in 1983
[ ... ] to further reduce the uncertainty, in 1983 the CGPM replaced this latter definition by the following definition:
The meter is the length of the path travelled by light in vacuum during a time interval of 1/299 792 458 of a second.
Note that the effect of this definition is to fix the speed of light in vacuum at exactly 299 792 458 mĚs-1.
This transfers the remaining uncertainty in the speed of light to the definition of the meter.
This makes a lot of sense, of course, because the remaining uncertainty in the
speed of light was less than that in the length of the meter.
The decision led to an exact definition of c and an improved definition
of the meter.