They measured nothing at all. That's how. This is simply their calculation. And it's absolutely possible to measure this, see: https://www.thoughtco.com/measu

They measured nothing at all. That's how. This is simply their calculation. And it's absolutely possible to measure this, see: https://www.thoughtco.com/measuring-plate-motion-1441107 https://www.thoughtco.com/measuring-plate-motion-1441107 and https://www.youtube.com/watch?v=S1m1tAGbfL4 https://www.youtube.com/watch?v=S1m1tAGbfL4