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The base unit of time in the International System of Units (SI), and by extension most of the Western world, is the second, defined as about 9 billion oscillations of the caesium atom.
"The second, symbol s, is the SI unit of time. It is defined by taking the fixed numerical value of the caesium frequency, ∆ ν Cs , 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 ."
The SI comprises a coherent system of units of measurement starting with seven base units, which are the second (symbol s, the unit of time), metre (m, length), kilogram (kg, mass), ampere (A, electric current), kelvin (K, thermodynamic temperature), mole (mol, amount of substance), and candela (cd, luminous intensity).
The SI unit of time, the second, symbol s, is easy enough to define, but time itself can only be defined in reference to other quantities. As we will see, time isn't even the steady thing that it seems to be, but can change how quickly or slowly is passes depending on speed and mass.
The SI unit of time is the second, which has been further accurately defined as “the time interval equal to 9192631770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the caesium-133 atom (13th CGPM, 1967).”
May 29, 2019 · Second – Unit of Time. Symbol: s. The second is defined by taking the fixed numerical value of the cesium frequency ∆ν Cs, the unperturbed ground-state hyperfine transition frequency of the cesium-133 atom, to be 9,192,631,770 when expressed in the unit Hz, which is equal to s −1. Learn More.
The International System of Units (SI units) is founded on seven base quantities. Each of the quantities, such as length and time, are assumed to be independent of each other. All seven units are explained on these pages, together with worked examples:
