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 and demonstrated the value of using careful measurements of time with the pendulum clock developed by Christiaan Huygens, which provided a substantial improvement in accuracy to less than a second. A more-accurate value was obtained fifty years later by James Bradley using another astronomical method.
Before the seventeenth century, scientists believed that the speed of light was infinite. In about 1607, Galileo
Rømer had been born in �arhus, the largest city in Jutland, Denmark, on September 25, 1644, and studied astronomy in Copenhagen. He assisted in determining the exact location of Tycho Brahe’s observatory on the island of Hveen in 1671 and then went to Paris in 1672. He remained for nine years at the new Paris Observatory of the Académie Royale de Sciences (Royal Academy of Sciences), making careful observations of the moons of Jupiter. In 1675, he discovered an inequality in the motion of the moon Io, the closest and fastest of the four large moons of Jupiter discovered by Galileo in 1610.
Using the new pendulum clock
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 are 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,
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 titled “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.
The first accurate measurement of the speed of light was made some fifty years after Rømer’s measurements by the English astronomer James Bradley
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 Earth relative to the constant speed of light. This“aberration of starlight” 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 Earth’s forward orbital speed to the speed of light. Careful measurements of this angle combined with the known speed of Earth allowed Bradley to obtain a value of 295 million meters per second for the speed of light, slightly too small (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 Earth as suggested by the Copernican theory some 200 years earlier. Further careful measurements of star angles by Bradley revealed in 1732 the nodding motion of 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 improved until terrestrial measurements were begun in mid-nineteenth century France, improving on the original method of Galileo by using reflected light and rapid timing by rotating wheels.
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.
James 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 Einstein’s theory of relativity, the speed of light is seen as one of the fundamental constants of the universe.
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Alioto, Anthony M. A History of Western Science. 2d ed. Upper Saddle River, N.J.: Prentice Hall, 1993. This book includes a brief discussion of the work of Rømer and Bradley in the context of the debate between particle and wave theories of light. -
Cohen, I. Bernard. Roemer and the First Determination of the Velocity of Light. New York: Burndy Library, 1944. This fifty-page monograph of Rømer’s work on the speed of light also contains illustrations and a bibliography. -
Crump, Thomas. A Brief History of Science as Seen Through the Development of Scientific Instruments. New York: Carroll & Graf, 2001. This book includes brief discussions of the work of Rømer, Huygens, Cassini, and Bradley. -
Huygens, Christiaan. Treatise on Light. Vol. 34 in Great Books of the Western World, edited by R. M. Hutchins. Chicago: William Benton, 1952. Translation by Silvanus P. Thompson of Traite de la lumiere (wr. 1678, pb. 1690). Rømer’s measurement of the speed of light is discussed in chapter 1 of Huygens’s classic Treatise on Light.
Gian Domenico Cassini; Galileo; Francesco Maria Grimaldi; Christiaan Huygens; Johannes Kepler; Sir Isaac Newton.