In 1676 a Danish astronomer named Ole Rømer was studying the orbits of the moons of Jupiter and making tables to predict when eclipses of the moons would occur. It is understandable that Galileo was unable to measure this with his pulse! We now know the speed of light very precisely, and if Galileo and his assistant were on hilltops one mile apart, light would take 0.0000054 seconds to travel from one person to the other. Galileo concluded that the speed of light was too fast to be measured by this method, and he was correct. He and his assistant tried this with different distances between them, but no matter how far apart they were, he could measure no difference in the amount of time it took the light to travel. Using the distance between the hilltops and his pulse as a timer, Galileo planned to measure the speed of light. Galileo and an assistant each stood on a different hilltop with a known distance between them, the plan was for Galileo to open the shutter of a lamp and then for his assistant to open the shutter of a lamp as soon as he saw the light from Galileo's. Galileo Galilei was the first person to attempt to measure the speed of light, in the early 1600s.

Today anyone can use Google to search for the speed of light in a vaccuum and get an accurate result in seconds: 299 792 458 m/s. But who discovered the speed of light, and how did they do it?

Gmd speed time found