The SI unit of time, the second, is currently defined by
A) the rotation period of the Earth.
B) the motion of the Earth in orbit.
C) a standard clock at the International Bureau of Weights and Measures in France.
D) a duration of an atomic clock.
E) the time required for light to travel 1/299,792,458 of a meter.

Question

Quizplus · Accepted Answer

The second is defined by the duration of 9,192,631,770 periods of the radiation corresponding to the transition between two hyperfine levels of the ground state of the cesium-133 atom, which is a characteristic of an atomic clock.

The SI Unit of Time, the Second, Is Currently Defined