Limited by the precision of his measurements, Fizeau calculated the speed of light to be 315,000 km/s. Using this data, Fizeau was then able to calculate the speed of light. The rotational speed of the wheel was then increased until the returning light was blocked by the tooth on the wheel just adjacent to the space through which it had passed. The light passing between teeth of that wheel was then projected to a mirror at a distance of about 8 km, where it was collected and then reflected back to the point of origin. A light source was focused through a beamsplitter onto an image plane where a spinning toothed wheel was located. Fizeau's experiment is illustrated in Fig. The first terrestrial measurement of the speed of light was recorded by the French scientist Armand Fizeau (1819-1896) in 1849. The amount of telescope tilt required allowed Bradley to calculate the speed of light, which he found to be 301,000 km/s. It was found that in order to compensate for the speed of the incident light, the telescope's axis would have to be tilted through a small angle in the direction in which the earth is traveling. This experiment involved the observation of a star using a telescope with its axis set perpendicular to the plane of the earth's rotation. Contemporaries of Roemer would modify his findings, including more accurate data on the earth's orbital radius, and arrive at a value close to 300,000 km/s.Īpproximately 50 years later, in 1728, the noted British astronomer James Bradley (1693-1762) made an entirely different type of astronomical observation from which he was able to calculate the speed of light. While there is no record that Roemer actually did the final calculation, his data would lead to the conclusion that light travels at a speed of 200,000 km/s. Essentially, when the earth was nearest to Jupiter the eclipses would occur several minutes ahead of the predicted time and when the earth was farthest from Jupiter, that eclipse would occur several minutes later than was predicted. Roemer noted a significant difference in the timing of these eclipses, depending on the relative positions of the Sun, Earth, and Jupiter when the observations were made. In 1675 the Danish astronomer Olaf Roemer (1644-1710) made the first scientific determination of the speed of light based on observations of the eclipses of the innermost moon of Jupiter. As early as the eleventh century it was thought that light did travel at a finite speed, but much too fast to be measured using normal methods. Initially, it was thought that light traveled with infinite speed. Over the years the history of optics has been tied inexorably to the quest to determine the velocity of the propagation of light within various media.
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