# How long is one second?

Usain Bolt is a Jamaican sprinter who holds the world record for the $$100$$ meters of $$9.58$$ seconds, which is the highest since the start of fully automatic time measurements. The second (symbol: s) is the base unit of time in the International System of Units (SI) and is also a unit of time in other systems of measurement (abbreviated s or sec); it is the second division of the hour by sixty, the first division by $$60$$ being the minute. What has been the definition of the second since $$1967$$ by the International System of Units?

a) $$\frac{1}{86,400}$$ of a mean solar day

b) 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

c) The fraction $$\frac{1}{31,556,925.9747}$$ of the tropical year for $$1900$$ January $$0$$ at $$12$$ hours ephemeris time

d) Half of the period of a pendulum with a length of $$39.1$$ inches ($$0.994$$ m)

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