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KF5JRV > TECH 19.12.16 14:27l 9 Lines 1960 Bytes #999 (0) @ WW
BID : 7224_KF5JRV
Read: GUEST
Subj: Light Speed
Path: IW8PGT<CX2SA<N0KFQ<KF5JRV
Sent: 161219/1215Z 7224@KF5JRV.#NWAR.AR.USA.NA BPQ6.0.13
Velocity of light in vacuum (c)
To determine a velocity requires knowledge of both a distance and a time. Attempts to achieve measurements of the speed of light (c) date back to the 16th- and 17th-century Italian scientist Galileo Galilei, who supposedly tried unsuccessfully to determine c by having two men stand at a known distance from each other and alternately cover and uncover their handheld lanterns as soon as they saw the light from the other man's lamp, thus seeking to determine the elapsed time for light to travel the known distance between the two men. A 17th-century Danish astronomer, Ole Rømer, calculated a value of c from the dependence of the period of revolution of a moon of Jupiter on the Earth's orbital position about the Sun. Similarly, in 1726, an English astronomer, James Bradley, determined c from the apparent change in position of a number of stars in the sky as the Earth moved about the Sun.
The problem of overcoming the short time interval associated with light traveling a readily measured distance on the Earth's surface was first solved by a French physicist, Armand-Hippolyte-Louis Fizeau, in the mid 19th century. He did this by having the light pass through a gap between the teeth of a toothed wheel rotating at a known rate, reflect off a fixed mirror a known distance away, and return to the wheel. A related method utilizing a rotating mirror was also employed by another French physicist, Jean Foucault, in 1862.
The classic pre-World War II measurements of the constant c are associated with Albert A. Michelson, a physicist in the United States. From 1924 to 1926, Michelson measured c by reflecting light between a rotating mirror with a number of faces and a fixed mirror some 35 kilometers (22 miles) away. A second measurement using essentially the same method but in a 1.6 kilometer (one-mile) evacuated tube was carried out by Michelson and his associates over the period 1931 to 1935.
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