How do we know how fast light is?
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We measure it. Either by measuring something [very far away](https://www.amnh.org/learn-teach/curriculum-collections/cosmic-horizons-book/ole-roemer-speed-of-light#:~:text=The%20speed%20of%20light%20could,is%20186%2C000%20miles%20per%20second.) (TL:DR: he measured the eclipses of Jupiter’s moons at one time of year and at a different time of year, and used the difference between when he measured the eclipses to calculate the speed of light.)
Or by using something a bit [closer to home](https://wonders.physics.wisc.edu/measure-the-speed-of-light/#:~:text=Note%3A%20MHz%20%3D%2010%5E6,10%5E10%20cm%2Fs.)
The most precise measurements of c come from the fact that we actually defined the meter to be based on c, so c is actually exactly known.
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