Second No Longer Ephemeral: Cesium Transition Instead

Last fall the 13th General Conference on Weights and Measures defined the second as “the duration of 9 192 631 770 periods of the radiation corresponding to the transition between the two hyperfine levels of the fundamental state of the atom of cesium 133.” The atomic definition was deliberately chosen to make it indistinguishable from the “ephemeris second” (adopted in 1956), which was a fraction of the tropical year at 1200 hours, ephemeris time, 0 Jan. 1900 and was based on the apparent motion of the sun across the celestial sphere.