Physics and measurement of time
The second
Also: SI unit of time
The second is the base unit of time in the International System of Units (SI). Since 1967 it is no longer tied to the Earth's rotation but to nature: one second is the duration of 9,192,631,770 periods of the radiation from the transition between two hyperfine levels of the ground state of the caesium-133 atom.
This atomic definition makes the second reproducible anywhere in the world and forms the basis of TAI and UTC.