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A unit of time is any particular time interval, used as a standard way of measuring or expressing duration. 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.
Metric time is the measure of time intervals using the metric system. The modern SI system defines the second as the base unit of time, and forms multiples and submultiples with metric prefixes such as kiloseconds and milliseconds. Other units of time – minute, hour, and day – are accepted for use with SI, but are not part of it
When that multiplier is one, the unit is called a coherent derived unit. For example, the coherent derived SI unit of velocity is the metre per second, with the symbol m/s. [1]: 139 The base and coherent derived units of the SI together form a coherent system of units (the set of coherent SI units). A useful property of a coherent system is ...
10 −14 qs: The length of one Planck time (t P = / ≈ 5.39 × 10 −44 s) [3] is the briefest physically meaningful span of time. It is the unit of time in the natural units system known as Planck units. 10 −30: quectosecond: qs Quectosecond, (quecto-+ second), is one nonillionth of a second 10 −27: rontosecond: rs
For example, a length that is significantly longer or shorter than 1 metre can be represented in units that are a power of 10 or 1000 metres. This differs from many older systems in which the ratio of different units varied. For example, 12 inches is one foot, but the larger unit in the same system, the mile is not a power of 12 feet. It is ...
The SI system after 1983, but before the 2019 revision: Dependence of base unit definitions on other base units (for example, the metre is defined as the distance travelled by light in a specific fraction of a second), with the constants of nature and artefacts used to define them (such as the mass of the IPK for the kilogram).
In the International System of Units (SI), the unit of time is the second (symbol: s). It has been defined since 1967 as "the duration of 9 192 631 770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the caesium 133 atom", and is an SI base unit. [12]
The svedberg (S or Sv) is a unit of time used in chemistry equal to one hundred femtoseconds (100 fs). The shake is a unit of time used in nuclear physics equal to ten nanoseconds (10 ns). The sigma is a unit of time equal to one microsecond (1 μs). The jiffy is sometimes used to mean a unit of time of 10 ms. [dubious – discuss]