Unit Of Time

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= Unit of time =
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From Wikipedia, the free encyclopedia
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link=https://en.wikipedia.org/wiki/File:Units_of_Time_in_tabular_form.png|thumb|Table showing quantitative relationships between common units of time
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 9 192 631 770 when expressed in the unit Hz, which is equal to s−1."
Historically, many units of time were defined by the movements of astronomical objects.
* Sun-based: the year was the time for the Earth to revolve around the Sun. Historical year-based units include the Olympiad (four years), the lustrum(five years), the indiction (15 years), the decade, the century, and the millennium.
* 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[citation needed]. Units originally derived from this base include the week (seven days), and the fortnight (14 days). Subdivisions of the day include the hour (1/24 of a day), which was further subdivided into minutes and finally seconds. The second became the international standard unit (SI units) for science.
* 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 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 include the nanosecond and the millisecond.
== Historical ==
Main article: 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, 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.
https://upload.wikimedia.org/wikipedia/commons/thumb/c/ca/Logarithmic_time_scale_-_milliseconds_to_years.svg/1200px-Logarithmic_time_scale_-_milliseconds_to_years.svg.png
Horizontal logarithmic scale marked with units of time in the Gregorian calendar
== Scientific ==
* The Jiffy is the amount of time light takes to travel one fermi (about the size of a nucleon) in a vacuum.
* The Planck time is the time light takes to travel one Planck length.
* The TU (for time unit) is a unit of time defined as 1024 µs for use in engineering.
* The 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.
* 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 (9 454 254 955 488 kilometers).
== List ==
Units of time 
Name 	Length 	Notes
Planck time 	5.39×10−44 s 	The amount of time light takes to travel one Planck length.
quectosecond 	10−30 s 	One nonillionth of a second.
rontosecond 	10−27 s 	One octillionth of a second.
yoctosecond 	10−24 s 	One septillionth of a second.
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 	One sextillionth of a second. Time measurement scale of the NIST strontium atomic clock. Smallest fragment of time currently measurable is 247 zeptoseconds.
attosecond 	10−18 s 	One quintillionth of a second.
femtosecond 	10−15 s 	One quadrillionth of a second. Pulse time on fastest lasers.
Svedberg 	10−13 s 	Time unit used for sedimentation rates (usually of proteins).
picosecond 	10−12 s 	One trillionth of a second.
nanosecond 	10−9 s 	One billionth of a second. 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 	One millionth of a second. Symbol is µs
millisecond 	10−3 s 	One thousandth of a second. Shortest time unit used on stopwatches.
jiffy(electronics) 	~10−3 s 	Used to measure the time between alternating power cycles. Also a casual term for a short period of time.
centisecond 	10−2 s 	One hundredth of a second.
decisecond 	10−1 s 	One tenth of a second.
second 	1 s 	SI base unit for time.
decasecond 	10 s 	Ten seconds (one sixth of a minute)
minute 	60 s 	
hectosecond 	100 s 	
milliday 	1/1000 d 	Also marketed as a ".beat" by the Swatch corporation.
moment 	1/40 solar hour (90 s on average)	Medieval unit of time used by astronomers to compute astronomical movements, length varies with the season. Also colloquially refers to a brief period of time.
kilosecond 	103 s 	About 17 minutes.
hour 	60 min 	
day 	24 h 	Longest unit used on stopwatches and countdowns.
week 	7 d 	Historically sometimes also called "sennight".
megasecond 	106 s 	About 11.6 days.
fortnight 	2 weeks 	14 days
lunar month 	27 d 4 h 48 min – 29 d 12 h 	Various definitions of lunar month exist; sometimes also called a "lunation."
month 	28–31 d 	Occasionally calculated as 30 days.
quarantine 	40 d (approximately 5.71 weeks) 	To retain in obligatory isolation or separation, as a sanitary measure to prevent the spread of contagious disease. Historically it meant to be isolated for 40 days. From Middle English quarentine, from Italian quarantina (“forty days”), the period Venetians customarily kept ships from plague-ridden countries waiting off port, from quaranta (“forty”), from Latin quadrāgintā.
semester 	18 weeks 	A division of the academic year. Literally "six months", also used in this sense.
lunar year 	354.37 d 	
year 	12 mo 	365 or 366 d
common year 	365 d 	52 weeks and 1 day.
tropical year 	365 d 5 h 48 min 45.216 s 	Average.
Gregorian year 	365 d 5 h 49 min 12 s 	Average.
sidereal year 	365 d 6 h 9 min 9.7635456 s 	
leap year 	366 d 	52 weeks and 2 d
olympiad 	4 yr 	A quadrennium (plural: quadrennia or quadrenniums) is also a period of four years, most commonly used in reference to the four-year period between each Olympic Games. It is also used in reference to the four-year interval between leap years, for example when wishing friends and family a "happy quadrennium" on February 29.[citation needed]
lustrum 	5 yr 	In early Roman times, the interval between censuses.
decade 	10 yr 	
indiction 	15 yr 	Interval for taxation assessments (Roman Empire).
gigasecond 	109 s 	About 31.7 years.
jubilee 	50 yr 	
century 	100 yr 	
millennium 	1000 yr 	Also called "kiloannum".
Age 	2,148 and two thirds of a year 	a unit used in astrology, each of them represent a star sign
terasecond 	1012 s 	About 31,709 years.
megaannum 	106 yr 	Also called "Megayear." 1,000 millennia (plural of millennium), or 1 million years (in geology, abbreviated as Ma).
petasecond 	1015 s 	About 31,709,791 years.
galactic year 	2.3×108 yr 	The amount of time it takes the Solar System to orbit the center of the Milky Way Galaxy (approx 230,000,000 years).
cosmological decade	logarithmic (varies) 	10 times the length of the previous cosmological decade, with CD 1 beginning either 10 seconds or 10 years after the Big Bang, depending on the definition.
eon 	109 yr 	Also refers to an indefinite period of time, otherwise is 1,000,000,000 years.
kalpa 	4.32×109 yr 	Used in Hindu mythology. About 4,320,000,000 years.
exasecond 	1018 s 	About 31,709,791,983 years. Approximately 2.3 times the current age of the universe.
zettasecond 	1021 s 	About 31,709,791,983,764 years.
yottasecond 	1024 s 	About 31,709,791,983,764,586 years.
ronnasecond 	1027 s 	About 31,709,791,983,764,586,504 years.
quettasecond 	1030 s 	About 31,709,791,983,764,586,504,312 years.
== Interrelation ==
link=https://en.wikipedia.org/wiki/File:Time_units.svg|thumb|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.
== See also ==
* Unit of frequency
* Orders of magnitude (time)
== References ==
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# http://starchild.gsfc.nasa.gov/docs/StarChild/questions/question18.html NASA – StarChild Question of the Month for February 2000
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# , Extract of page 18
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Time	 	
Key concepts 	
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Measurementand standards	
{| class="fandom-table"	Chronometry	
* UTC
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* Unit of time
* Orders of magnitude (time)
Measurementsystems 	
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Calendars 	
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Time measurement and standards	
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Time measurement and standards	
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Time measurement and standards
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* Chronometry
* Orders of magnitude
* Metrology	
* Chronometry
* Orders of magnitude
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International standards	
* Coordinated Universal Time 
** offset
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* Time zone
* 180th meridian	https://upload.wikimedia.org/wikipedia/commons/thumb/7/70/Marine_sandglass_MMM.jpg/75px-Marine_sandglass_MMM.jpghttps://upload.wikimedia.org/wikipedia/commons/thumb/6/63/Aleutian_Islands_with_180th_meridian_and_International_Date_Line_%28cropped%29.png/75px-Aleutian_Islands_with_180th_meridian_and_International_Date_Line_%28cropped%29.png
Obsolete standards 	
* Ephemeris time
* Greenwich Mean Time
* Prime meridian	https://upload.wikimedia.org/wikipedia/commons/thumb/7/70/Marine_sandglass_MMM.jpg/75px-Marine_sandglass_MMM.jpghttps://upload.wikimedia.org/wikipedia/commons/thumb/6/63/Aleutian_Islands_with_180th_meridian_and_International_Date_Line_%28cropped%29.png/75px-Aleutian_Islands_with_180th_meridian_and_International_Date_Line_%28cropped%29.png
Time in physics 	
* Absolute space and time
* Spacetime
* Chronon
* Continuous signal
* Coordinate time
* Cosmological decade
* Discrete time and continuous time
* Proper time
* Theory of relativity
* Time dilation
* Gravitational time dilation
* Time domain
* Time-translation symmetry
* T-symmetry	https://upload.wikimedia.org/wikipedia/commons/thumb/7/70/Marine_sandglass_MMM.jpg/75px-Marine_sandglass_MMM.jpghttps://upload.wikimedia.org/wikipedia/commons/thumb/6/63/Aleutian_Islands_with_180th_meridian_and_International_Date_Line_%28cropped%29.png/75px-Aleutian_Islands_with_180th_meridian_and_International_Date_Line_%28cropped%29.png
Horology 	
* Clock
* Astrarium
* Atomic clock
* Complication
* History of timekeeping devices
* Hourglass
* Marine chronometer
* Marine sandglass
* Radio clock
* Watch 
** stopwatch
* Water clock
* Sundial
* Dialing scales
* Equation of time
* History of sundials
* Sundial markup schema	https://upload.wikimedia.org/wikipedia/commons/thumb/7/70/Marine_sandglass_MMM.jpg/75px-Marine_sandglass_MMM.jpghttps://upload.wikimedia.org/wikipedia/commons/thumb/6/63/Aleutian_Islands_with_180th_meridian_and_International_Date_Line_%28cropped%29.png/75px-Aleutian_Islands_with_180th_meridian_and_International_Date_Line_%28cropped%29.png
Calendar 	
* Gregorian
* Hebrew
* Hindu
* Holocene
* Islamic (lunar Hijri)
* Julian
* Solar Hijri
* Astronomical
* Dominical letter
* Epact
* Equinox
* Intercalation
* Julian day
* Leap year
* Lunar
* Lunisolar
* Solar
* Solstice
* Tropical year
* Weekday determination
* Weekday names	https://upload.wikimedia.org/wikipedia/commons/thumb/7/70/Marine_sandglass_MMM.jpg/75px-Marine_sandglass_MMM.jpghttps://upload.wikimedia.org/wikipedia/commons/thumb/6/63/Aleutian_Islands_with_180th_meridian_and_International_Date_Line_%28cropped%29.png/75px-Aleutian_Islands_with_180th_meridian_and_International_Date_Line_%28cropped%29.png
Archaeology and geology	
* Chronological dating
* Geologic time scale
* International Commission on Stratigraphy	https://upload.wikimedia.org/wikipedia/commons/thumb/7/70/Marine_sandglass_MMM.jpg/75px-Marine_sandglass_MMM.jpghttps://upload.wikimedia.org/wikipedia/commons/thumb/6/63/Aleutian_Islands_with_180th_meridian_and_International_Date_Line_%28cropped%29.png/75px-Aleutian_Islands_with_180th_meridian_and_International_Date_Line_%28cropped%29.png
Astronomical chronology	
* Galactic year
* Nuclear timescale
* Precession
* Sidereal time	https://upload.wikimedia.org/wikipedia/commons/thumb/7/70/Marine_sandglass_MMM.jpg/75px-Marine_sandglass_MMM.jpghttps://upload.wikimedia.org/wikipedia/commons/thumb/6/63/Aleutian_Islands_with_180th_meridian_and_International_Date_Line_%28cropped%29.png/75px-Aleutian_Islands_with_180th_meridian_and_International_Date_Line_%28cropped%29.png
Other units of time 	
* Instant
* Flick
* Shake
* Jiffy
* Second
* Minute
* Moment
* Hour
* Day
* Week
* Fortnight
* Month
* Year
* Olympiad
* Lustrum
* Decade
* Century
* Saeculum
* Millennium	https://upload.wikimedia.org/wikipedia/commons/thumb/7/70/Marine_sandglass_MMM.jpg/75px-Marine_sandglass_MMM.jpghttps://upload.wikimedia.org/wikipedia/commons/thumb/6/63/Aleutian_Islands_with_180th_meridian_and_International_Date_Line_%28cropped%29.png/75px-Aleutian_Islands_with_180th_meridian_and_International_Date_Line_%28cropped%29.png
Related topics 	
* Chronology
* Duration 
** music
* Mental chronometry
* Decimal time
* Metric time
* System time
* Time metrology
* Time value of money
* Timekeeper	https://upload.wikimedia.org/wikipedia/commons/thumb/7/70/Marine_sandglass_MMM.jpg/75px-Marine_sandglass_MMM.jpghttps://upload.wikimedia.org/wikipedia/commons/thumb/6/63/Aleutian_Islands_with_180th_meridian_and_International_Date_Line_%28cropped%29.png/75px-Aleutian_Islands_with_180th_meridian_and_International_Date_Line_%28cropped%29.png
Category: 
* Units of time
* This page was last edited on 9 April 2024, at 01:49 (UTC).
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