The problem of determining the speed of light has occupied scientists since antiquity. In his Discours concernant deux nouvelles sciences (Discourses on Two New Sciences, published in Leiden in 1638), Galileo addressed the issue through a particularly illuminating dialogue between Master Sagredo and his pupils about the nature of the scientific method (see box).
John Michell's insight
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The idea that the speed of light was finite finally gained universal acceptance at the end of the 17th century following the research of Danish astronomer Ole Christensen Rømer. Focusing on eclipses of Jupiter's moons in 1675, Rømer noticed that Kepler's equations yielded predictions that differed slightly from observations. He hypothesized that the observed discrepancies were due to the relative positions of Earth, Jupiter and the Sun, and posited that light traveled at a finite, albeit very high, speed c. His conjectures, confirmed by several subsequent observations, led him to an initial approximation of c: around 220,000 km/s. In 1728, British astronomer James Bradley (1693–1762) refined this estimate to 301,000 km/s. The speed of light is now known with great precision: c = 299,792,458 m/s in the "MKS" system.
