Editing Caesium standard (section)

=== Length ===

In 1983, the meter was, indirectly, defined in terms of the caesium standard with the formal definition "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. This implied:
* 1 [[metre]], m, = {{sfrac|''c s''|299,792,458}} = {{sfrac|9,192,631,770|299,792,458}} ''c'' {{math|Δ''t''<sub>Cs</sub>}} = {{sfrac|9,192,631,770|299,792,458}} {{math|Δ''λ''<sub>Cs</sub>}}
* 1 [[radian]], rad, = 1 m/m = {{math|Δ''λ''<sub>Cs</sub>}}/{{math|Δ''λ''<sub>Cs</sub>}} = 1 (dimensionless unit of angle)
* 1 [[steradian]], sr, = 1 m<sup>2</sup>/m<sup>2</sup> = {{math|Δ''λ''<sub>Cs</sub>}}<sup>2</sup>/{{math|Δ''λ''<sub>Cs</sub>}}<sup>2</sup> = 1 (dimensionless unit of [[solid angle]])

Between 1960 and 1983, the metre had been defined by the wavelength of a different transition frequency associated with the [[isotopes of krypton|krypton-86 atom]]. This had a much higher frequency and shorter wavelength than the caesium standard, falling inside the [[visible spectrum]]. The first definition, used between 1889 and 1960, was by the [[international prototype meter]].<ref>{{Cite web|url=https://www.bipm.org/en/history-si/metre|title = Metre - BIPM}}</ref>