Who first measured the speed of light?

The first person who successfully measured the speed of light was Ole Roemer, and this happened in 1676. Before this period, a lot of astronomers had attempted to measure the speed of light, but their effort didn't yield any result. Most of them ended their experiments or findings on the note that the value for the speed of light is infinite. However, Galileo Galilei also tried to measure the speed of light but he ended up saying the speed of light cannot be measured by experiment because it is too fast.

However, Ole Roemer was the first person to prove that the speed of light is not infinite as concluded by ancient scientists. Ole Roemer used what he observed during eclipse of the moon of Jupiter. Based on his observation, he was able to prove that the speed of light is 210,000 km/s, a value that's very close to what James Bradley got. James Bradley, in his experiment, found the speed of light to be 301,000 km/s

Olaus Roemer, a Dutch astronomer, was the first to calculate the speed of light, in the year 1676. Galileo, an Italian astronomer, had attempted to do so, but the mathematics that he was using at the time were not sufficient to correctly calculate the speed. Two hundred years later, Einstein developed his theory of Special Relativity which said that the speed of light was constant.

Roemer relied on the moons of Jupiter to make his calculations. He looked at the eclipses that were occurring, as well as the time that elapsed between each one. Roemer correctly reasoned that the Earth was moving away from Jupiter at times, and then at others, moving closer. He then calculated the rate of speed, giving us the mean numbers for the speed of light

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Roemer’s estimation was that the speed of light was 140,000 miles per second. With mathematics still crude, it was a fair assumption and one that was close to the mean numbers that we have today.