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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.
This category identifies units of time, either general (in Chronology) or for specific scientific and other uses; and some closely related notions. The main article for this category is Unit of time .
Some units are deeply embedded in history and culture, and their use has not been entirely replaced by their SI alternatives. The CIPM recognised and acknowledged such traditions by compiling a list of non-SI units accepted for use with SI, [5] including the hour, minute, degree of angle, litre, and decibel.
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
An order of magnitude of time is usually a decimal prefix or decimal order-of-magnitude quantity together with a base unit of time, like a microsecond or a million years. In some cases, the order of magnitude may be implied (usually 1), like a "second" or "year". In other cases, the quantity name implies the base unit, like "century". In most ...
Unit system Domain Derivation Unit name Unit symbol Dimension symbol Quantity name Definition In SI base units In other SI units SI: Physics: Basic: second [n 1] s: T: time: 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. s: SI ...
Planck time (~ 5.4 × 10 −44 seconds) is the unit of time in the system of natural units known as Planck units. Current established physical theories are believed to fail at this time scale, and many physicists expect that the Planck time might be the smallest unit of time that could ever be measured, even in principle.
Some common examples of such units are the customary units of time, namely the minute (conversion factor of 60 s/min, since 1 min = 60 s), the hour (3600 s), and the day (86 400 s); the degree (for measuring plane angles, 1° = π ⁄ 180 rad); and the electronvolt (a unit of energy, 1 eV = 1.602 176 634 × 10 −19 J).