To most of us it is absolutely clear that one second is the 60th part of a minute, one minute the 60th part of an hour, and one hour the 24th part of a day.
Physicists, however, don't find this good enough. In their opinion, the day (to be more precise: the duration of one complete rotation of the earth) is not a good measure of the unit of time because the earth does not really turn as uniformly as one might think. Not only has the "mean solar day" lengthened over the centuries; but there also occur periodical (seasonal) and non-periodical variations. Therefore, in 1967, the second - as one of the base units in the International System of Units SI - was defined in terms of atomic physics:
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 caesium-133 atom.
This second, too, is approximately the 86400th part of the mean solar day. However, by means of atomic clocks, each second can be realised equally long - which is much more precise than before. The clocks at railway stations also benefit from this as they get their time directly from the atomic clocks at PTB. In such an atomic clock, the length of the seconds is deduced from the periodic vibrations of the caesium atoms and transmitted directly to all station clocks (and other radio-controlled clocks) via a long-wave transmitter. Station clocks therefore show time as precisely as the atomic clock. So next time you are waiting for a delayed train, you know that it's not the clocks which are to blame!
More information on the SI and the seven base units are found under "
PTB as the guardian of the units."
© Physikalisch-Technische Bundesanstalt, last update: 2011-08-08, Volker Großmann
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