Speed of light less uncertain than the previously accepted value. The remaining uncertainty was mainly related to the definition of the metre. As similar experiments found comparable results for "c", the 15th General Conference on Weights and Measures in 1975 recommended using the value for the speed of light. In 1983 the 17th CGPM found that wavelengths from frequency measurements and a given value for the speed of light are more reproducible than the previous standard. They kept the 1967 definition of second, so the caesium hyperfine frequency would now determine both the second and the metre. To do this, they redefined the
Speed of light precise measurements, in 1975 the speed of light was known to be with a measurement uncertainty of 4 parts per billion. In 1983, the metre was redefined in the International System of Units (SI) as the distance travelled by light in vacuum in 1/ of a second. The speed of light in vacuum is usually denoted by a lowercase , for "constant" or the Latin (meaning "swiftness, celerity"). In 1856, Wilhelm Eduard Weber and Rudolf Kohlrausch had used for a different constant later shown to equal times the speed of light in vacuum. Historically, the symbol "V" was used as