How did Galileo attempt to measure the speed of light?

Answer 1

Galileo used two distantly-separated lanterns in his experiment.

Galileo knew that light traveled very quickly, but did not appreciate how fast it really is. Standing on a peak at night with a shuttered lantern, he sent a colleague with another lantern to a peak some miles away, where the lights would still be visible to each other when opened.

At a prearranged time, Galileo would uncover his lantern. The colleague, when he saw the light from Galileo's lantern, would uncover his own lantern, so that Galileo could then see it. The time between Galileo's uncovering and the time he saw the other lantern would be the time it took light for a round trip equal to twice the distance they were apart.

The result, as would be expected today, was that the colleague saw the light practically as soon as Galileo uncovered it, and uncovered his own, which was then visible practically instantaneously to Galileo. No matter how far apart they were, the brief time lag was identical.

Considering the reaction time by the assistant, and the time taken to open the lantern, Galileo reasoned that light traveled far too quickly to be measured.

Answer 2

Galileo spoke of how he tried to measure the speed of light by having two people with covered lanterns stand far apart, having one open the lantern, then having the other open HIS lantern when he saw the first lantern open, and finally having the first person measure the time it took between his initial opening of his lantern and his seeing of the second lantern.

Galileo could not measure any difference between the delay of the first person seeing the second lantern opening, whether the two people were very close to each other or half a mile apart. Galileo thus concluded that the speed of light was "if not infinite, then exceedingly fast."

Answer 3

Mirrors