The unit of time equal to 1/(31 556 925. 974 7) part of the tropical year for 1900 January 0 at 12 hours ephemeris time. This definition was adopted in 1960 by the general Conference on Weights and Measures, the previous definition in terms of the rotation of the Earth being abolished. It is theoretically equivalent to the system of time used by Simon Newcomb in his theory of the motions of the Solar System. The ephemeris second was replaced, in 1967, by the atomic second.