The ephemeris second (1/86400 of a mean solar day) was made one of the original base units of the modern metric system, or International System of Units (SI), at the 10th General Conference on Weights and Measures (CGPM) in 1954. The SI second was later made even more exact. It was made into the time of 9,192,631,770 periods of the radiation from the transition between the two hyperfine levels of the ground state of the caesium-133 atom.