Second is a fundamental unit of time, nowadays defined in terms of radiation frequency at which the element cesium changes from one state to another. Previously defined as 1/86,400 of the mean solar day(the average period of rotation of the earth on its axis relative to the sun). This definition became inadequate by the 20th century because more precision was required. In 1956, the definition of a second was redefined to 1/31,556,925.9747 of the length of the tropical year in 1900. This definition was redefined again to the current valid definition. One second is equal to 9,192,631,770 cycles of radiation associated with the transition between the two hyperfine levels of the ground state of the cesium-133 atom. To increase the accuracy even further, it is necessary to occasionally add or subtract a second to ensure the atomic timescale. This is why the definition was redefined to the number of cycles of tradition to define the length of a second.