The speed of light was first calculated by a Danish astronomer, Olaus Roemer. He figured the rate of speed by watching the eclipses of the moons of Jupiter and estimating how fast the light moved. Although Roemer was off in his calculations, mathematics was not fully evolved enough at the time to successfully capture the correct rate of speed.
Galileo also ran into the same problem when he attempted to estimate the speed of light. Today, these calculations are studied in controlled settings, using lasers and a vacuum to capture the exact moments and get the most constant speed. The speed of light is 186,000 miles per second.
Now, there are mediums which will cause this speed to change, either slower or faster, and those mediums have been noted, and further studies are taking place. The measurements are made when a point of light goes from one location to another, and then the mathematical equations are taken into account.