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 Length: Unit of length (metre)

The SI unit of length is the metre (m):

The metre is the length of the path travelled by light in vacuum during a time interval of 1/299 792 458 of a second.

 Unit of length (metre) SI Brochure, Section 2.1.1.1

The 1889 definition of the metre, based on the international prototype of platinum-iridium, was replaced by the 11th CGPM (1960) using a definition based on the wavelength of krypton 86 radiation. This change was adopted in order to improve the accuracy with which the definition of the metre could be realized, the realization being achieved using an interferometer with a travelling microscope to measure the optical path difference as the fringes were counted. In turn, this was replaced in 1983 by the 17th CGPM (1983, Resolution 1) that specified the current definition, as follows:

The metre is the length of the path travelled by light in vacuum during a time interval of 1/299 792 458 of a second.

It follows that the speed of light in vacuum is exactly 299 792 458 metres per second, c0 = 299 792 458 m/s.

The original international prototype of the metre, which was sanctioned by the 1st CGPM in 1889, is still kept at the BIPM under conditions specified in 1889.

 The symbol, c0 (or sometimes simply c), is the conventional symbol for the speed of light in vacuum.

 Decisions relating to the base units of the SI SI Brochure, from Appendix 1

Length:

 1st CGPM, 1889 sanction of the prototype metre 7th CGPM, 1927 definition and use of the prototype metre 11th CGPM, 1960 redefinition of the metre in terms of krypton 86 radiation 15th CGPM, 1975 recommends value for the speed of light 17th CGPM, 1983 redefinition of the metre using the speed of light, realization of the definition of the metre CIPM, 2002 specifies the rules for the practical realization of the definition of the metre CIPM, 2003 revision of the list of recommended radiations CIPM, 2005 revision of the list of recommended radiations CIPM, 2007 revision of the list of recommended radiations 23rd CGPM, 2007 revision of the mise en pratique of the definition of the metre and development of new optical frequency standards CIPM, 2009 updates to the list of standard frequencies 24th CGPM, 2011 on the possible future revision of the International System of Units, the SI 24th CGPM, 2011 revision of the mise en pratique of the definition of the metre and development of new optical frequency standards CIPM, 2013 updates to the list of standard frequencies
 Length Overview Unit of length (metre) Committee structure Recommended values of standard frequencies Strategic plan
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