One of the earliest attempts to measure the speed that we know of was made by Galileo in the 1600's. He used the simple method of trying to time how long a beam of light would take to travel a known distance.
He equipped himself and an assistant with lanterns that had shutters that would open, letting the light out. His assistant stood on a hill several miles away. Galileo opened the shutter, letting a beam of light travel toward his assistant. That person opened his lantern the instant he saw the light, sending a beam back to Galileo. The time lag between Galileo first opening the shutter, and eventually seeing the return beam, was supposed to enable him to calculate the speed of the light.
Unfortunately, after repeating the experiment a number of times at different distances, he discovered that the time lag did not change. It did not increase with larger distances. In fact, the time lag was just due to the length of time it took to open the shutters! Galileo concluded that light was travelling much too fast to measure this way.
Later in the 1600's, another scientist, an astronomer by the name of Roemer was observing eclipses of Jupiter's moons. By watching the moons circling Jupiter through a telescope, he was able to observe the exact time that the moon moved behind Jupiter. After many years of observations, however, he noticed that the time intervals between eclipses were not always the same. He immediately figured out the reason. Here is what he realized:
The light from the eclipsing moon of Jupiter sometimes took a relatively short time to reach earth (position 1).
At another time during the year, however, (position 2) when earth was in a farther part of its orbit, the light from Jupiter's moon would take longer to reach earth.
This caused the interval between eclipses to lengthen.
Roemer was able to use the time difference and the diameter of Earth's orbit to calculate the speed of light. He determined it to be about 185,000 miles per second, or 296,000 kilometres per second, very close to the actual value!
A more modern measurement was made by the French physicist Fizeau in the late 1800's, using a pair of toothed wheels driven by a very fast motor. As the toothed wheels turned, a beam of light shone through the gaps between the teeth of one wheel would only pass through the gaps in the other wheel if they lined up. Knowing the speed that the wheels were turning allowed him to calculate the speed of light with an accuracy similar to Roemer's.
A more easily understood experiment was performed in 1879 by a scientist named Michaelson. His apparatus consisted of a rapidly rotating hexagonal mirror, and a second mirror, as shown below:
Michelson's method was to shine a light on the rotating hexagonal mirror, which then reflected to the other mirror 35 kilometres away. The light reflecting back from this second mirror struck another face of the octagonal mirror, and could then be seen by an observer. The mirror needed to make one-eighth of a rotation in the time it took the light to make the return trip. This meant that the octagonal mirror had to be turning at about 32 000 rpm.
From the round trip distance the light travelled, and the period of rotation of the octagonal mirror, the speed of light was determined quite accurately.
Later more accurate measurements have refined the value of the speed of light, so that we now know that light will cover 299,792.458 kilometres in one second, in a vacuum. (It travels slightly slower in air and water). This is very fast. Light from the sun takes only about 8 minutes to cover that 150 million kilometre distance. A beam of light from a flashlight reaches the moon in about a second and a half.
We also know that the speed of light is the fastest speed that anything can travel.