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’)
