Unit of time

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 exact modern SI definition is "[The second] is defined by taking the fixed numerical value of the cesium frequency, ΔνCs, the unperturbed ground-state hyperfine transition frequency of the cesium 133 atom, to be 9192631770 when expressed in the unit Hz, which is equal to s−1.

"[1] Historically, many units of time were defined by the movements of astronomical objects.

Units of time based on orders of magnitude of the second include the nanosecond and the millisecond.

The natural units for timekeeping used by most historical societies are the day, the solar year and the lunation.

Table showing quantitative relationships between common units of time
Flowchart illustrating selected units of time. The graphic also shows the three celestial objects that are related to the units of time.