What does physical phenomenon mean

The way in which the sciences interpret time naturally also determines the result of their measurements:

In physics, the speed is defined as the distance per unit of time (v = s / t):
"The units of length and time result from definitions that can be very laborious (but also exact!): The era of the validity of the Paris original meter is long gone: the meter has been defined since 1983 as the distance that the Light traverses in vacuum for a period of 1/299 792 458 second. "
(Weiss, W., 1999)

In the scientific field, too, high measuring accuracies are required for time:
"The old definition of the second as 86,400th part of the earth's rotation time was no longer sufficient for this. Therefore, in 1956, the second was redefined and linked to the earth's orbit around the sun, i.e. to the tropical year. This unit of time is called the ephemeris second ... Ephemeris are pre-calculated locations of celestial bodies. Since ... the length of the tropical year also changes slightly, a specific year had to be used as a basis. The ephemeris second is 31 556 925.9747. Part of the tropical year 1900. " (Kleffe, H., 1985)

But even this definition is no longer sufficient today. "The second is no longer the 3,600th part of an hour, this is the 24th part of a mean solar day, but the duration of 9,192,631,770 vibrations of the radiation emitted by a cesium atom 133."(Weiss, W., 1999)

In Germany, the Physikalisch-Technische Bundesanstalt, which operates two cesium clocks in Braunschweig, is legally responsible for setting the time and distributing the time signals.