The Velocity of Light was long thought to be instantaneous. In 1675, however, Römer, a Danish astronomer, determined the velocity of light by a study of the satellite of Jupiter. One of the moons of Jupiter, in its path around the planet, passes into the shadow of the planet once in 42 hr. 28 min. and 36 sec., on the average. Römer noticed that while the earth was passing from E to E' the observed times of the eclipses were later than the computed times, and that the differences between them kept increasing until, after six months, at E', the total retardation was 16 min. 36 sec. This means that it takes the light 16 min. 36 sec. to cross the orbit of the earth, about 196,000,000 miles. This gives a velocity of about 186,000 miles or 300,000 km. per second.
The determination has been made in other ways, and the results confirm Römer's measurement. The velocity is so great that light could travel around the earth nearly 7½ times in 1 second.
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* Römer’s calculations were not numerically precise by modern standards, but they were fundamentally correct and brilliant in concept.
