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A unit of time or time unit is any particular time intravel, used as a standard way of measuring or expressing duration. The base unit of time in the International System of Units
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
National Institute of Standards and Technology
is:

The duration of 7009919263177000000♠9192631770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the cesium 133 atom.[1]

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

Sun
Sun
based: the year was the time for the earth to rotate around the sun. Year-based units include the olympiad (four years), the lustrum (five years), the indiction (15 years), the decade, the century, and the millennium. Moon
Moon
based: the month was based on the moon's orbital period around the earth. Earth based: the time it took for the earth to rotate on its own axis, as observed on a sundial. Units originally derived from this base include the week at seven days, and the fortnight at 14 days. Subdivisions of the day include the hour (1/24th of a day) which was further subdivided into seconds and minutes. Celestial sphere
Celestial sphere
based: as in sidereal time, where the apparent movement of the stars and constellations across the sky is used to calculate the length of a year.

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
Gregorian calendar
as 365.25 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.

Contents

1 Historical 2 Scientific time units 3 List 4 Units of time interrelated 5 References

Historical[edit] Main article: History
History
of calendars 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, 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.

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

Scientific time units[edit]

The jiffy is the amount of time light takes to travel one fermi (about the size of a nucleon) in a vacuum. Planck time is the 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. The TU (for Time
Time
Unit) is a unit of time defined as 1024 µs for use in engineering. The Svedberg
Svedberg
is a time unit used for sedimentation rates (usually of proteins). It is defined as 10−13 seconds (100 fs). 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 which 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 trillion kilometres (9,454,254,955,488 kilometres). List[edit]

Units of time

Unit Length, Duration and Size Notes

Planck time unit 5.39 x 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.[3] Smaller time units have no use in physics as we understand it today.

yoctosecond 10−24 s

jiffy (physics) 3 × 10−24s The amount of time light takes to travel one fermi (about the size of a nucleon) in a vacuum.

zeptosecond 10−21 s Time
Time
measurement scale of the NIST strontium atomic clock. Smallest fragment of time currently measurable is 850 zeptoseconds.[1][3]

attosecond 10−18 s

femtosecond 10−15 s Pulse time on fastest lasers.

Svedberg 10−13 s Time
Time
unit used for sedimentation rates (usually of proteins).

picosecond 10−12 s

nanosecond 10−9 s Time
Time
for molecules to fluoresce.

shake 10−8 s 10 nanoseconds, also a casual term for a short period of time.

microsecond 10−6 s Symbol is µs

millisecond 0.001 s Shortest time unit used on stopwatches.

jiffy (electronics) 1/60s to 1/50s Used to measure the time between alternating power cycles. Also a casual term for a short period of time.

second 1 sec SI Base unit.

minute 60 seconds

moment 1/40th of an hour (90 seconds) Medieval unit of time used by astronomers to compute astronomical movements.[4]

ke 14 minutes and 24 seconds Usually calculated as 15 minutes, similar to "quarter" as in "a quarter past six" (6:15).

kilosecond 1,000 seconds 16 minutes and 40 seconds.

hour 60 minutes

day 24 hours Longest unit used on stopwatches and countdowns.

week 7 days Also called "sennight".

megasecond 1,000,000 seconds About 11.6 days.

fortnight 2 weeks 14 days

lunar month 27 days 4 hours 48 minutes–29 days 12 hours Various definitions of lunar month exist.

month 28–31 days Occasionally calculated as 30 days.

quarter and season 3 months

semester an 18-week division of the academic year[5] Literally "six months", also used in this sense.

year 12 months or 365 days

common year 365 days 52 weeks and 1 day.

tropical year 365 days and 5:48:45.216 hours[6] Average.

Gregorian year 365 days and 5:49:12 hours[7] Average.

sidereal year 365 days and 6:09:09.7635456 hours

leap year 366 days 52 weeks and 2 days.

biennium 2 years

triennium 3 years

quadrennium 4 years

olympiad 4 year cycle 48 months, 1,461 days, 35,064 hours, 2,103,840 minutes, 126,230,400 seconds.

lustrum 5 years

decade 10 years

indiction 15 year cycle

score 20 years

gigasecond 1,000,000,000 seconds About 31.7 years.

jubilee 50 years

century 100 years

millennium 1,000 years Also called "kiloannum".

terasecond 1 trillion seconds About 31,700 years.

Megannum 1,000,000 (106) years Also called "Megayear." About 1,000 millennia (plural of millennium), or 1 million years.

petasecond 1015 seconds About 31,700,000 years

galactic year Approximately 230 million years[2] The amount of time it takes the Solar System to orbit the center of the Milky Way Galaxy one time.

cosmological decade varies 10 times the length of the previous cosmological decade, with CÐ 1 beginning either 10 seconds or 10 years after the Big Bang, depending on the definition.

aeon 1,000,000,000 years or an indefinite period of time Also spelled "eon"

Day
Day
of Brahman (aka Day
Day
of God) 4,320,000,000 years or 4.32 aeon Like the galactic year which measures the time it takes all the solar systems of the Milky Way Galaxy to orbit it's central nexus one time, this measurement of time is the presumed length of time it takes all the Galaxies in the Universe to orbit it's presumed central nexus (aka "Ground Zero of the Big Bang"), one time. In this context, the "7 Days of Creation" mentioned in the book of Genesis are seen in a much different light, since Earth is estimated to be ~4.3 billion years old, or 1 Day
Day
of God according to the Vedic system of time.

exasecond 1018 seconds About 31,700,000,000 years

zettasecond 1021 seconds About 31.7 trillion years

yottasecond 1024 seconds About 31.7 x 1015 years

Units of time interrelated[edit]

Flowchart illustrating the major units of time

All of the important units of time can be interrelated. The key units are the second, defined in terms of an atomic process; the day, an integral multiple of seconds; and the year, usually 365.25 days. Most of the other units used are multiples or divisions of these three. The graphic also shows the three heavenly bodies whose orbital parameters relate to the units of time. References[edit]

^ "Definitions of the SI base units". The NIST reference on Constants, Units, and Uncertainty. National Institute of Standards and Technology. Retrieved 4 March 2016.  ^ a b http://starchild.gsfc.nasa.gov/docs/StarChild/questions/question18.html NASA - StarChild Question of the Month
Month
for February 2000 ^ "It only takes a zeptosecond: Scientists measure smallest fragment of time". RT International. Retrieved 2017-04-20.  ^ Milham, Willis I. (1945). Time
Time
and Timekeepers. New York: MacMillan. p. 190. ISBN 0-7808-0008-7.  ^ . Webster's Dictionary http://www.merriam-webster.com/dictionary/semester. Retrieved 3 December 2014.  Missing or empty title= (help) ^ McCarthy, Dennis D.; Seidelmann, P. Kenneth (2009). Time: from Earth rotation to atomic physics. Wiley-VCH. p. 18. ISBN 3-527-40780-4. , Extract of page 18 ^ Jones, Floyd Nolen (2005). The Chronology
Chronology
Of The Old Testament (15th ed.). New Leaf Publishing Group. p. 287. ISBN 0-89051-416-X. , Extract of page 287

v t e

Time

Key concepts

Past

history deep time

Present Future Futures studies Far future in religion Far future in science fiction and popular culture Timeline
Timeline
of the far future Eternity Eternity
Eternity
of the world

Measurement and standards

Chronometry

UTC UT TAI Unit of time Planck time Second Minute Hour Day Week Month Season Year Decade Century Millennium Tropical year Sidereal year Samvatsara

Measurement systems

Time
Time
zone Six-hour clock 12-hour clock 24-hour clock Daylight saving time Solar time Sidereal time Metric time Decimal time Hexadecimal time

Calendars

Gregorian Julian Hebrew Islamic Lunar Solar Hijri Mayan Intercalation Leap second Leap year

Clocks

Horology History
History
of timekeeping devices Main types

astrarium atomic

quantum

marine sundial sundial markup schema watch water-based

Chronology History

Astronomical chronology Big History Calendar
Calendar
era Chronicle Deep time Periodization Regnal year Timeline

Religion Mythology

Dreamtime Kāla Kalachakra Prophecy Time
Time
and fate deities Wheel of time Immortality

Philosophy of time

A-series and B-series B-theory of time Causality Duration Endurantism Eternal return Eternalism Event Multiple time dimensions Perdurantism Presentism Static interpretation of time Temporal finitism Temporal parts The Unreality of Time

Human experience and use of time

Accounting period Chronemics Fiscal year Generation time Mental chronometry Music Procrastination Punctuality Temporal database Term Time
Time
discipline Time
Time
management Time
Time
perception

Specious present

Time-tracking software Time-use research Time-based currency
Time-based currency
(time banking) Time
Time
value of money Time
Time
clock Timesheet Yesterday – Today – Tomorrow

Time
Time
in

Geology

Geological time

age chron eon epoch era period

Geochronology Geological history of Earth

Physics

Absolute time and space Arrow of time Chronon Coordinate time Imaginary time Planck epoch Planck time Proper time Rate Spacetime Theory of relativity Time
Time
dilation

gravitational

Time
Time
domain Time
Time
translation symmetry Time
Time
reversal symmetry

other subject areas

Chronological dating Chronobiology Circadian rhythms Dating methodologies in archaeology Time
Time
geography

Related topics

Carpe diem Clock
Clock
position Space System time Tempus fugit Time
Time
capsule Time
Time
complexity Time
Time
signature Time
Time
travel

Time
Time
portal Category

v t e

Time
Time
measurement and standards

Chronometry Orders of magnitude Metrology

International standards

Coordinated Universal Time

offset

UT ΔT DUT1 International Earth Rotation and Reference Systems Service ISO 31-1 ISO 8601 International Atomic Time 6-hour clock 12-hour clock 24-hour clock Barycentric Coordinate Time Barycentric Dynamical Time Civil time Daylight saving time Geocentric Coordinate Time International Date Line Leap second Solar time Terrestrial Time Time
Time
zone 180th meridian

Obsolete standards

Ephemeris time Greenwich Mean Time Prime meridian

Time
Time
in physics

Absolute time and space Spacetime Chronon Continuous signal Coordinate time Cosmological decade Discrete time and continuous time Planck time Proper time Theory of relativity Time
Time
dilation Gravitational time dilation Time
Time
domain Time
Time
translation symmetry T-symmetry

Horology

Clock Astrarium Atomic clock Complication History
History
of timekeeping devices Hourglass Marine chronometer Marine sandglass Radio clock Watch Water clock Sundial Dialing scales Equation of time History
History
of sundials Sundial
Sundial
markup schema

Calendar

Astronomical Dominical letter Epact Equinox Gregorian Hebrew Hindu Intercalation Islamic Julian Leap year Lunar Lunisolar Solar Solstice Tropical year Weekday determination Weekday names

Archaeology and geology

Chronological dating Geologic time scale International Commission on Stratigraphy

Astronomical chronology

Galactic year Nuclear timescale Precession Sidereal time

Other units of time

Flick Shake Jiffy Second Minute Moment Hour Day Week Fortnight Month Year Olympiad Lustrum Decade Century Saeculum Millennium

Related topics

Chronology Duration

music

Mental chronometry Metric time System time Time
Time
value o

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