The SI unit of length defined (since 1983) as the length of the path travelled by light in vacuum during a time interval of 1/299 792 458 of a second. It was previously defined as the length equal to 1 650 763.73 wavelengths in a vacuum of the radiation associated with the transition between the levels 2p10 and 5d5 of the krypton-86 atom. The wavelength of this radiation is about 605.8 nanometres. The metre can be measured very accurately and, unlike the original international prototype metre, which was a bar of platinum, does not change with time.