![The speed of light, visualized [GIF - 48KB]](https://pic8.co/sh/Ujwhbd.gif){kind=link}
Distance was not defined relative to the speed of light, until c was precisely known. Before that, it was defined relative to a bar in France.
C is not precisely known. It has been measured in some specific conditions with imperfect methods, and the currently accepted speed is a prediction of how it would perform in a total vacuum based upon these imperfect measurements.
It is incredibly precisely known, and has been measured by an enormous variety of techniques, and many different conditions.
No, it is not precisely known. It has been measured in some very limited conditions with imperfect methods. How fast is it when completely removed from the influence of Gravity? How fast is it in a true vacuum? How fast is it where there is a lot of Dark Matter? How fast is it if there is no Dark Matter present? What if Dark Matter theory is completely wrong? The fact is that we have a very limited knowledge of this. You are simply repeating what you have been told without giving the matter any actual thought.
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