When measuring a speed, the most common way to calculate it is by recording how far something went and the time it took to go that far. In the case of light, this is very difficult. One could conceivably shine a light over a vast distance and have someone else record when they see the light, but this would be difficult even at large distances. The person recording when they see it will need to have terrific reflexes to accurately measure a correct time as the time will be very short. A better method involves the use of a quickly rotating mirror and a beam of light. By aiming a beam of light o the rotating mirror, then reflecting it o a second stationary mirror back into the rotating mirror, calculations can be made on the speed of light. After first hitting the rotating mirror, the mirror will rotate very slightly in the time it takes the beam of light to return and will reflect back to a different position from where it came from. By measuring the displacement of the round trip, a measurement of the speed of light can be made.
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