Simplify the following text for a Grade 15 native English-speaking person.
Original: Under the International System of Units ( via the International Committee for Weights and Measures , or CIPM ) , since 1967 the second has been defined as the duration of periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the caesium 133 atom .
Simplified: Under the International System of Units , the second is currently defined as the duration of 9,192,631,770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the caesium-133 atom .