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, which is the period of revolution of the Earth around its axis.
Physicists, however, don't find this good enough. In their opinion, the day 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.”
Following the decision of the 26th General Conference on Weights and Measures, the text of the definition is now:
“The second, symbol s, is the SI unit of time. It is defined by taking the fixed numerical value of the caesium frequency ∆νCs, the unperturbed ground-state hyperfine transition frequency of the caesium 133 atom, to be 9 192 631 770 when expressed in the unit Hz, which is equal to s−1”,
quoted from the SI Brochure, ed. 9, published by BIPM. The new text as well as changes to other units became effective on 20th May 2019, the date of the annual World Metrology Day.
This second, too, is approximately the 86400th part of the mean solar day. However, by means of atomic clocks, seconds can be realized equally long – much better than ever before.