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Unit of time

Measurement unit for time From Wikipedia, the free encyclopedia

Unit of time
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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. 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]

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Table showing quantitative relationships between common units of time

Historically, many 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 twelve 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 for scientific purposes as multiples of seconds.

Units of time based on orders of magnitude of the second follow the system of metric prefixes.

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Historical

The natural units for timekeeping used by most historical societies are the day, the solar year and the lunation. Such calendars include the Sumerian, Egyptian, Chinese, Babylonian, ancient Athenian, Buddhist, Hindu, Islamic, Icelandic, Mayan, and French Republican calendars.

The modern calendar has its origins in the Roman calendar, which evolved into the Julian calendar, and then the Gregorian calendar.

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Horizontal logarithmic scale marked with units of time in the Gregorian calendar
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Scientific

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  • The Planck time is the time that light takes to travel one Planck length.
  • The Jiffy is the amount of time light takes to travel one femtometre (about the diameter of a nucleon).
  • The atomic time relates to the orbital period of a ground state electron around a hydrogen atom and is about 24.2 attoseconds.
  • The svedberg is a time unit used for sedimentation rates (usually of proteins). It is defined as 10−13 seconds (100 fs).
  • The TU (for time unit) is a unit of time defined as 1024 μs for use in engineering.
  • The galactic year, based on the rotation of the galaxy and usually measured in million years.[2]
  • The geological time scale relates stratigraphy to time. The deep time of Earth's past is divided into units according to events that took place in each period. For example, the boundary between the Cretaceous period and the Paleogene period is defined by the Cretaceous–Paleogene extinction event. The largest unit is the supereon, composed of eons. Eons are divided into eras, which are in turn divided into periods, epochs and ages. It is not a true mathematical unit, as all ages, epochs, periods, eras, or eons don't have the same length; instead, their length is determined by the geological and historical events that define them individually.

Note: The light-year is not a unit of time, but a unit of length of about 9.5 petametres (9454254955488 km).

Note: The parsec is not a unit of time, but a unit of length of about 30.9 trillion kilometres, despite movie references otherwise.

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Interrelation

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Flowchart illustrating selected units of time. The graphic also shows the three celestial objects that are related to the units of time.

All of the formal units of time are scaled multiples of each other. The most common units are the second, defined in terms of an atomic process; the day, an integral multiple of seconds; and the year, usually 365 days. The other units used are multiples or divisions of these 3.

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See also

References

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