The speed of light was first accurately measured in the 17th century by a Danish astronomer named Ole Rømer. Rømer made his measurement based on observations of the moons of Jupiter.
Rømer noticed that the observed times at which the moons of Jupiter appeared to eclipse and reappear behind the planet did not match the predicted times. He hypothesized that the discrepancies were due to the varying distance between the Earth and Jupiter as they orbited the Sun. Rømer realized that the observed times were shorter when the Earth and Jupiter were closer together and longer when they were farther apart.
Using these observations, Rømer estimated that it took light about 22 minutes to travel a distance equal to the diameter of Earth's orbit around the Sun. From this, he calculated an approximate speed for light, which was about 225,000 kilometers per second.
However, it wasn't until the 19th century that more accurate and precise measurements of the speed of light were made. One notable experiment was conducted by the French physicist Hippolyte Fizeau in 1849. Fizeau used a rapidly rotating cogwheel and a beam of light reflected from a mirror placed several kilometers away. By adjusting the rotation speed of the wheel, Fizeau was able to determine the speed at which the light beam needed to travel in order to pass through the gaps in the rotating wheel. This experiment provided a more accurate measurement of the speed of light, approximately 313,000 kilometers per second.
Since then, numerous experiments and refinements have been made to measure the speed of light with increasing accuracy, using methods such as interferometry, timing techniques, and modern technologies like lasers and highly precise equipment. The currently accepted value for the speed of light in a vacuum is approximately 299,792 kilometers per second (or about 186,282 miles per second).