ENCYCLOPEDIA BRITANNICA

 Romer, Ole (Christensen)
b. Sept. 25, 1644, rhus, Jutland
d. Sept. 23, 1710, Copenhagen

Romer also spelled ROMER, OR ROEMER, OLE also spelled OLAUS OR OLAF, astronomer who demonstrated conclusively that light travels at a finite speed. Romer went to Paris in 1672, where he spent nine years working at the Royal Observatory. While there he noticed that the time elapsed between eclipses of Jupiter's moons by Jupiter became shorter as Earth moved closer to Jupiter and became longer as Earth and Jupiter drew farther apart. He correctly deduced that this phenomenon was caused by the time needed for light to cross the increased distance between the two planets and in 1676 announced that, according to his observations, the speed of light was 140,000 miles (225,000 km) per second. (Modern measurements have established a figure of 186,282 miles, or 299,792 km, per second.) In 1679 Romer went on a scientific mission to England, where he met Sir Isaac Newton and the astronomers John Flamsteed and Edmond Halley. Upon his return to Denmark in 1681, he was appointed royal mathematician and professor of astronomy at the University of Copenhagen. At the university observatory he set up an instrument with altitude and azimuth circles and a telescope, which accurately measured the position of celestial objects. He also held several public offices, including that of mayor of Copenhagen in 1705.

ENCYCLOPEDIA BRITANNICA

Speed of light

Much effort has been devoted to measuring the speed of light, beginning with the aforementioned work of Romer in 1676. Romer noticed that the orbital period of Jupiter's first moon, Io, is apparently slowed as the Earth and Jupiter move away from each other. The eclipses of Io occur later than expected when Jupiter is at its most remote position. This effect is understandable if light requires a finite time to reach the Earth from Jupiter. From this effect, Romer calculated the time required for light to travel from the Sun to the Earth as 11 minutes. In 1728 James Bradley, an English astronomer, determined the speed of light from the apparent orbital motion of stars that is produced by the orbital motion of the Earth. He computed the time for light to reach the Earth from the Sun as eight minutes, 12 seconds. The first terrestrial measurements were made in 1849 by Fizeau and a year later by Foucault. Michelson improved on Foucault's method and obtained an accuracy of one part in 100,000. Any measurement of velocity requires, however, a definition of the measure of length and of time. Current techniques allow a determination of the velocity of electromagnetic radiation to a substantially higher degree of precision than permitted by the unit of length that scientists had applied earlier. In 1983 the value of the speed of light was fixed at exactly 299,792,458 metres per second, and this value was adopted as a new standard. As a consequence, the metre was redefined as the length of the path traveled by light in a vacuum over a time interval of 1/299,792,458 of a second. Furthermore, the second--the international unit of time--has been based on the frequency of electromagnetic radiation emitted by a cesium-133 atom.