It is hard to remember that a lot of high tech research went on well before the arrival of electronic computers, lasers, and all the other things that used to be amazing but are now commonplace. That's why we enjoyed [Michel van Biezen's] two part post on how Michelson computed the speed of light in 1927. You can see the videos below.
Michelson wasn't the first, of course. Galileo tried. He sent an assistant to the top of a hill with a lantern. When the assistant saw Galileo's lantern, he was to uncover his lantern. They practiced near each other to account for reaction time. But when the assistant was 3 km away, it didn't take any more time. The implication was that light traveled instantaneously, but, of course, it is actually just really fast.
By 1927, Michelson tried what was in effect the same technique but with better technology, and this time they put a reflector about 35 km away meaning the light had to go to the reflector and back for a total of about 70 km.
To get the best answer requires knowing the exact distance between the emitter and the reflector, and [Michel] explains how they did that using surveying tools. They computed the maximum error at about 8 cm which is not bad for measuring 35 km.
The other important measurement is time, as the sloppier the distance and time measurements are, the worse the estimate of the speed of light will be. Michelson used an 8-sided mirror device that rotates while bouncing light to the reflector and also collecting return light. The speed of the device's rotation gave a way to compute the time, and he covers how that works.
The experiment was successful and the result was within .001% of the correct figure. Today we'd use lasers or any number of other techniques, of course. If you want to get some experience on something slower, try a bullet.
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