How was the length of a meter redefined in the year 1983?

Answer 1

On October 20, 1983, the meter was officially redefined as the distance traveled by light in a vacuum during a time interval of 1/299,792,458 of a second.

Light travels at 299792.458 km/sec, so the period was chosen as 0.00000000335641 second, or 3.35641 x 10-9 second.

The original definitions (1799 and 1899) depended on a measured standard bar. In 1960, it was redefined as a relationship to the wavelength of light from ions of the element krypton. In 1980, the standard was based on the unusually cohesive wavelength of an iodine-stabilized helium-neon laser.

Answer 2

On October 20, the meter was redefined again. The definition states that the meter is the length of the path traveled by light in vacuum during a time interval of 1/299,792,458 of a second. (http://www.mel.nist.gov/div821/museum/timeline.htm)

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Answer 3

Since 1983 the meter has been defined as the distance light travels in a vacuum in exactly 1/229,792,458th of a second