 Galileo himself tried to measure the speed of light. His method was quite simple. He and an assistant each had lamps which could be covered and uncovered at will. They climbed to the tops of hills around 1.5 kilometers apart. Galileo would uncover his lamp and as soon as his assistant saw the light he would uncover his. By measuring the elapsed time until Galileo saw his assistant's light, factoring in reaction times calculated earlier, and knowing how far apart the lamps were, Galileo reasoned he should be able to determine the speed of light. Given how fast light is, we know that the time interval Galileo was trying to measure was around 10 microseconds. The clocks available to him at that time could not measure that tiny a time interval. His conclusion was that light was very fast, if not instantaneous.