How long is one second?

Usain Bolt is a Jamaican sprinter who holds the world record for the 100100 meters of 9.589.58 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 6060 being the minute. What has been the definition of the second since 19671967 by the International System of Units?

a) 186,400\frac{1}{86,400} of a mean solar day

b) The duration of 9,192,631,7709,192,631,770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the caesium 133133 atom

c) The fraction 131,556,925.9747\frac{1}{31,556,925.9747} of the tropical year for 19001900 January 00 at 1212 hours ephemeris time

d) Half of the period of a pendulum with a length of 39.139.1 inches (0.9940.994 m)

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