Simplify the following text for a Grade 8 native English-speaking person.
Original: The SI second was later redefined more precisely 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 .
Simplified: 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 .