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Science and Scientists The Speed of Light The Science Ole Rømer's measurement of the speed of light was the first clear demonstration that light travels with a finite velocity. Although his value was off by about 25 percent, his method was correct in principle. A more accurate value was obtained fifty years later by James Bradley using another astronomical method. The Scientists Ole Rømer (1644-1710), Danish astronomer whose study of Jupiter's moons helped determine the speed of light Galileo Galilei (1564-1642), Italian physicist and astronomer who discovered the moons of Jupiter and made one of the first attempts to measure the speed of light Christiaan Huygens (1629-1695), Dutch physicist and astronomer who invented the pendulum clock and assisted Rømer in calculating the speed of light James Bradley (1693-1762), third Astronomer Royal in England whose measurements on the aberration of starlight led to the first accurate measurement of the speed of light Gian Domenico Cassini (1625-1712), Italian astronomer and first director of the Paris Observatory Beyond Infinity Before the seventeenth century, scientists believed that the speed of light was infinite. In about 1607, Galileo attempted to measure the speed of light with the aid of an assistant on a hilltop at some distance away with a covered lamp. When the assistant saw Galileo uncover a similar lamp, he then uncovered his lamp and Galileo tried to observe the time for the light to travel to the assistant and back again. He concluded that the speed of light was either instantaneous or extremely rapid. The first observations showing that the speed of light is finite were made by the Danish astronomer Ole Rømer in Paris in 1675. Using the new pendulum clock invented in 1657 by Christiaan Huygens, a fellow foreign member of the Royal Academy of Sciences, Rømer determined that the 42.5-hour period of Jupiter's moon Io had an orbital period that was a maximum of 13 seconds longer when the Earth was moving away from Jupiter and 13 seconds less time when it was approaching (42.5 hr. ± 13 seconds). He recognized that this phenomenon occurred because the light took longer to reach the Earth as it moved away from Jupiter and shorter as the Earth moved toward Jupiter in each 42.5-hour orbit of Io. Rømer's Calculations To determine the range of variations in the orbital period, Rømer observed consecutive eclipses of Io as it passed behind Jupiter, noting the times when it emerged from each eclipse. Since these emergences of the Moon from eclipses were not instantaneous events, there were some errors in his measurements. From these variations, he calculated that light would take about 22 minutes to cross Earth's orbit (compared with a modern value of about 16 minutes). On November 22, 1675, Rømer read a paper to the science academy, in which he announced that an eclipse of Jupiter's moon Io would occur about 10 minutes later than the time predicted from the average orbital period as measured in 1668 by Gian Domenico Cassini, director of the Paris Observatory and also a foreign member of the science academy. Working with the aid of Huygens, Rømer combined the 22-minute time for light to cross Earth's orbit with the diameter of Earth's orbit as determined by Cassini in 1671, a value that was 7 percent too small. By taking the ratio of the distance to the time, he found the speed of light to be about 230 million meters per second, or about three-fourths of the modern value of nearly 300 million meters per second. Rømer published his discovery in a short paper entitled "Demonstration touchant le mouvement de la lumière trouvé" (demonstration concerning the discovery of the movement of light) in the Journal des Savants on December 7, 1676. At the request of the Danish king, Rømer returned to Denmark in 1681 as royal mathematician and professor of astronomy at Copenhagen University. Bradley's Calculations The first accurate measurement of the speed of light was made some fifty years after Rømer's measurements by the English astronomer James Bradley in 1728, also using an astronomical method. Bradley was trying to find evidence for the Earth's motion around the Sun by measuring the annual stellar parallax, the shifting angle of the stars that should result from Earth's motion in a six-month period. Rømer also had attempted to measure this parallax, but he had failed to detect any change. Although Bradley also failed to measure any parallax, he did notice a relatively large shift in angle of one second of arc in just three days and in the wrong direction to qualify as the annual parallax. According to some accounts, Bradley's explanation of the anomalous star angles he observed occurred to him while sailing on the Thames River and noticing how a steady wind caused the wind vane on the mast to shift relative to the boat as it changed directions. He reasoned that the apparent shift in star angles resulted from the orbital motion of the Earth relative to the constant speed of light. This "aberration of star light" is similar to the apparent angle of vertically falling raindrops relative to a moving observer. The angle of stellar aberration is given approximately by the ratio of the Earth's forward orbital speed to the speed of light. Careful measurements of this angle combined with the known speed of the Earth allowed Bradley to obtain a value of 295 million meters per second for the speed of light, slightly too small (but by less than 2 percent). Bradley's precise measurements of stellar aberration not only improved the value for the finite speed of light but also provided the first direct evidence for the motion of the Earth as suggested by the Copernican theory some two hundred years earlier. Further careful measurements of star angles by Bradley revealed in 1732 the nodding motion of the Earth's axis, called nutation, resulting from variations in the direction of the gravitational pull of the Moon. For these achievements, he was named the third Astronomer Royal in England. His value for the speed of light was not corrected until terrestrial measurements were begun in mid-nineteenth century France, when the original method of Galileo was improved by using reflected light and rapid timing by rotating wheels. Impact Even though Ole Rømer's value for the speed of light was about one-quarter too small, his method was correct and revealed that light has a finite speed. By showing that light travels nearly one million times faster than sound, Rømer provided evidence that eventually showed that light cannot consist of a mechanical propagation like sound, but is actually an electromagnetic wave as demonstrated in the nineteenth century. Bradley's improved method for measuring the speed of light began a quest for precision that finally revealed the true nature of light and gave the first direct evidence for the motion of the Earth. Terrestrial measurements a century after his work gave the most accurate values for the speed of light and revealed that light travels more slowly in water than in air, confirming the wave nature of light. When electromagnetic studies showed that light is propagated by electric and magnetic fields, the speed of the resulting electromagnetic waves could be calculated from electric and magnetic constants as measured in the laboratory, and the result matched the observed speed of light. In Albert Einstein's theory of relativity, the speed of light is seen as one of the fundamental constants of the universe. See Also Heisenberg's Uncertainty Principle; Lasers; Mössbauer Effect; Optics; Pendulum; Photoelectric Effect; Radio Astronomy; Relativity; Schrödinger's Wave Equation; Wave-Particle Duality of Light. Further Reading Alioto, Anthony M. A History of Western Science. 2d ed. Upper Saddle River, N.J.: Prentice Hall, 1993. Cohen, I. Bernard. Roemer and the First Determination of the Velocity of Light. New York: Burndy Library, 1944. Crump, Thomas. A Brief History of Science as Seen Through the Development of Scientific Instruments. New York: Carroll & Graf, 2001. Huygens, Christiaan. Treatise on Light. Vol. 34 in Great Books of the Western World, edited by R. M. Hutchins. Chicago: William Benton, 1952. Joseph L. Spradley |
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