A second was once defined as 1/86,400 of a mean solar day. By 1956, this definition was changed by the International Bureau of Weights and Measures to 1/31,556,925.9747 of the length of the tropical year 1900. But like most measurements, the second definition changed again in 1964, when it was assigned to be the equivalent of 9,192,631,770 cycles of radiation associated with a particular change in state of a cesium-133 atom (also seen as caesium atom; at its “ground state” at a temperature of zero degrees Kelvin).

Interestingly enough, by 1983, the second became the “definer” of the meter: Scientists determined a meter as 1/299,792,458 the distance light travels in one second. This was done because the distance light travels in one second was more accurate than the former definition of the standard meter.

One of the most famous clocks on Earth is Big Ben in London, England. Although today’s digital and atomic clocks are much more accurate, the charm of an old-fashioned analog clock still has its appeal.