Definition of one second

9192631770

One second is defined by exactly 9,192,631,770 periods of radiation corresponding to a transition between two hyperfine levels of the ground state of the caesium-133 atom.

Atom
Caesium-133
The reference atom used by the SI second.
Frequency
9.192631770 GHz
The microwave transition counted by atomic clocks.
Unit
1 second
A duration anchored to an invariant quantum transition.

Time, counted by nature.

The number 9192631770 is not a slogan or a serial number. It is the exact count of caesium-133 radiation periods that realizes one second in the International System of Units.

Stable
Atomic transitions provide a reproducible reference independent of a mechanical clock.
Exact
The defining frequency is fixed by convention, so the second can be realized anywhere.
Universal
Modern navigation, networks, finance, and science all depend on this shared definition.