| Intro | Metre | Kilogram | kelvin | candela | mole | second | helen(joke) | Bibliography |
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The second was initially based on a day having 86,400(24*60*60) seconds in it. This proved to be unreliable as the length of a day is getting shorter due to gravitational pull, and there are natural variances in the rotation of the earth causing additional unreliability.
A better method was needed. In 1997 the BiPM(International Bureau of Weights and Measures)defined the second as "The second is 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 cesium 133 atom." furthermore this was later defined as the frequency at absolute zero.