The unit of time - the second |
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The second was once exactly 1/86,400th part of a mean solar day. In 1956 the definition was refined to be a small part of the year in 1900. This was again unsatisfactory. The second isnow the time interval equal to 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. In terms of this definition (the atomic clock) the length of the mean solar day has increased due to tidal friction by roughly 2 milliseconds since it was exactly 86,400 seconds of atomic time about 180 years ago. Over the course of one year, the difference accumulates to almost one second, which is added as a leap second a little less than once per year. For details try... |
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