We know exactly which is the speed of light: c= 299,792,458 m/s, which always refers to the speed of light in the vacuum.
And this number is 100% exact, having been included in October 1983 as a constant in the International System of Unities, which converted the meter into a derived unit of this constant, as we will explain in more detail below.
As the speed of light is a constant (it is always the same), it is an irrefutable reference from which we can define other measurement units.
Nowadays, the definition of the metre is: “the distance light travels in a vacuum in exactly 1/299.792.458 seconds”.
In the past, the meter had been defined as the 1/10,000,000 part of the distance between the equator and the North pole, in 1799, and the length of a platinum rod held in a vault in Paris, in 1889. However, all these definitions of the metre can contain measurement errors, hence the dependence on the speed of light.
Scientists chose 299.792.458 m/s as the speed of light (and not a more convenient number such as 300,000,000 m/s), since in this way the definition of a metre was only changed by a fraction of a fraction of a percentage. In other words, to try to conserve the previous measure of the meter.
One consequence of using this definition is that any attempt to measure the speed of light is cyclic. The metre has to be used to measure it at some point, which in turn relies to the speed of light.
In conclusion, the constant of the speed of light is used as a reference with which it is possible to measure other units of distance.
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