Today anyone can use Google to search for the speed of light in a vaccuum and get an accurate result in seconds: 299 792 458 m/s. But who discovered the speed of light, and how did they do it?
Galileo Galilei was the first person to attempt to measure the speed of light, in the early 1600s. Galileo and an assistant each stood on a different hilltop with a known distance between them, the plan was for Galileo to open the shutter of a lamp and then for his assistant to open the shutter of a lamp as soon as he saw the light from Galileo's.
Using the distance between the hilltops and his pulse as a timer, Galileo planned to measure the speed of light. He and his assistant tried this with different distances between them, but no matter how far apart they were, he could measure no difference in the amount of time it took the light to travel.
Galileo concluded that the speed of light was too fast to be measured by this method, and he was correct. We now know the speed of light very precisely, and if Galileo and his assistant were on hilltops one mile apart, light would take 0.0000054 seconds to travel from one person to the other. It is understandable that Galileo was unable to measure this with his pulse!
In 1676 a Danish astronomer named Ole Rømer was studying the orbits of the moons of Jupiter and making tables to predict when eclipses of the moons would occur. He noticed that when Jupiter and Earth are far apart (near conjunction), the eclipses of the moons occurred several minutes later than when Jupiter and the Earth are closer (near opposition.) He reasoned that this could be because of the time light takes to travel from Jupiter to Earth.
Rømer found the maximum variation in timing of these eclipses to be 16.6 minutes. He interpreted this to be the amount of time it takes light to travel across the diameter of Earth's orbit. He didn't actually calculate the speed of light as the diameter of Earth's orbit was not well known in his day. But using his method with the knowledge of distances we have today, we get a value of approximately 301,204.8 km/s for the speed of light. This is only about 0.5% off the modern known value of the speed of light.
In the 1850s, French physicist Jean Foucault measured the speed of light in a laboratory using a light source, a rapidly rotating mirror and a stationary mirror. This method was based on a similar apparatus built by Armand-Hippolyte Fizeau. For the first time the speed of light could be measured on Earth, and the speed of light was measured to very great accuracy.
In the 1970s, interferometry was used to get the most accurate value for the speed of light that had been measured yet: 299,792.4562±0.0011 km/s. Then, in 1983, the meter was redefined in the International System of Units (SI) as the distance traveled by light in vacuum in 1/299,792,458 of a second. As a result, the numerical value of the speed of light (c) in meters per second is now fixed exactly by the definition of the meter. It is always slower in other materials such as water or glass. For most calculations the value 3.00 x 105 km/s is used.