The historical definition of seconds originates from astronomy and is closely related to the time scale of the earth's motion cycle. Before atomic seconds appear, the time scale is always defined first, and then the second is defined; But the definition of atomic second itself has nothing to do with astronomical time scale, so in order to coordinate with universal time, atomic second is defined first, and then atomic time is defined.
Recognized second
Under the current international system of units, the second is defined as 9 19263 1770 times the electromagnetic wave period radiated by cesium-133 atom when it transits between two ultra-fine energy levels.
The cesium atom mentioned in this definition must be stationary at absolute zero, and its environment is zero magnetic field. The second defined in this case is equivalent to the second defined in the astronomical almanac.
The international standard symbol of seconds can be referred to.