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 ephemeris second ( defined as 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 .