Unit of time




A unit of time or midst unit 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. The exact modern definition, from the National Institute of Standards and Technology is: "The duration of 9192631770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the caesium-133 atom."

Historically units of time were defined by the movements of astronomical objects.

These units do not have a consistent relationship with each other and require intercalation. For example, the year cannot be divided into 12 28-day months since 12 times 28 is 336, well short of 365. The lunar month (as defined by the moon's rotation) is not 28 days but 28.3 days. The year, defined in the Gregorian calendar as 365.2425 days has to be adjusted with leap days and leap seconds. Consequently, these units are now all defined as multiples of seconds.

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

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This article may require cleanup to meet Wikipedia's quality standards. The specific problem is: inconsistent notes column Please help improve this article if you can. ( June 2020 ) (Learn how and when to remove this template message) Units of time Unit Length, Duration and Size Notes Planck time unit 5.39 × 10−44 s The amount of time light takes to travel one Planck length. Theoretically, this is the smallest time measurement that will ever be possible. Smaller time units have no use in physics as we understand it today. yoctosecond 10−24 s jiffy (physics) 3 × 10−24 s The amount of time light takes to travel one fermi (about the size of a nucleon) in a vacuum. zeptosecond 10−21 s Time measurement scale of the NIST strontium atomic clock. Smallest fragment of time currently measurable is 247 zeptoseconds. attosecond 10−18 s femtosecond 10−15 s Pulse time on fastest lasers. Svedberg 10−13 s Time unit used for sedimentation rates (usually of pr...
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