First time speed of light was measured

The astronomer Jean Picard (France, 1620-82) of the Observatoire de Paris -- who was the first to measure the earth's diameter precisely -- had a young Dane, Olaus Romer, as his assistant. The latter carried out observations of the four large satellites of Jupiter discovered by Galileo some 70 years earlier. He measured the intervals of time between the successive eclipses of the satellites by Jupiter and noted that these were regularly shorter or longer depending on whether Jupiter and the earth were closer together or further apart in their respective orbits around the sun.

Romer understood that this phenomenon was due to delays in the light from Jupiter reaching earth. He then calculated that, to explain all these observations, light traveled at a speed of 186,400 miles per second. At last, after 2,000 years of controversy, it was established that light did not travel instantaneously, but has a finite and measurable speed. (‘Inventions and Discoveries’)