Excerpts from How is the speed of light measured?
Before the seventeenth century, it was generally thought that light is transmitted instantaneously. This was supported by the observation that there is no noticeable lag in the position of the Earth’s shadow on the Moon during a lunar eclipse, which would otherwise be expected if c were finite.
Instantaneously means a pace of zero. This is a basis for the Galilean transformation.
The first successful measurement of c was made by Olaus Roemer in 1676. He noticed that, depending on the Earth–Sun–Jupiter geometry, there could be a difference of up to 1000 seconds between the predicted times of the eclipses of Jupiter’s moons, and the actual times that these eclipses were observed. He correctly surmised that this is due to the varying length of time it takes for light to travel from Jupiter to Earth as the distance between these two planets varies.
This was actually the pace of light since the measured travel time was dependent on the known distance.
In 1728 James Bradley made another estimate by observing stellar aberration, being the apparent displacement of stars due to the motion of the Earth around the Sun. He observed a star in Draco and found that its apparent position changed throughout the year. All stellar positions are affected equally in this way. … Bradley measured this angle for starlight, and knowing the speed of the Earth around the Sun, he found a value for the speed of light of 301,000 km/s.
The distance that the stars appear to move is proportional to the speed that the Earth moves divided by the speed of light, which is equivalent to the pace of light divided by the Earth’s orbital pace.
The first measurement of c that didn’t make use of the heavens was by Armand Fizeau in 1849. He used a beam of light reflected from a mirror 8 km away. The beam was aimed at the teeth of a rapidly spinning wheel. The speed of the wheel was increased until its motion was such that the light’s two-way passage coincided with a movement of the wheel’s circumference by one tooth. This gave a value for c of 315,000 km/s. Leon Foucault improved on this result a year later using rotating mirrors, which gave the much more accurate value of 298,000 km/s.
Again, this was actually the pace of light since the measured travel time was dependent on the known distance.
According to the conventionality of simultaneity, the speed of light is actually the harmonic mean speed of the two-way speed of light. The harmonic mean is used because the length of the trips is the same. The pace of light is actually the arithmetic mean pace of the two-way pace of light.
The speed of light adopted as an international standard in 1983 is 299 792.458 km/s, which is equivalent to a pace of light that is 3.335 640 952 s/Gm.