Thought for the day / by Humperty Dumperty

The fundamental unit of time is the second, defined officially 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”. In other words, you measure the frequency of the microwave radiation emitted when electrons move between specified orbits or energy levels in caesium – and multiply it by 9,192,631,770 to get a second. It's really quite simple to the average Oxbridge graduate.