the speed of light was figured out a long time ago... but the speed of dark? Until now, was an unknown. I present to you, the means of measuring the speed of dark using a train's passenger car.

 https://ampervadasz.tumblr.com/post/669812556026822656

Figure out the length of the rail car from the far end to the camera, as you can see on the left are doors, and on the right are windows, each of a specific width, then measure the time it takes for the dark to move from that end to this end, and bingo. Distance divided by time. ft per second, or miles per hour, or whatever