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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."
Historically, many units of time were defined by the movements of astronomical objects.
- Sun-based: the year is based on the Earth's orbital period 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 is based on the Moon's orbital period around the Earth.
- Earth-based: the day is based on the time it takes 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 is further subdivided into minutes and seconds. The second is the international standard unit (SI unit) 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
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.
Scientific
- The Jiffy is the amount of time light takes to travel one femtometre (about the diameter of a nucleon).
- The Planck time is the time that 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 (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.
List
| 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 and JILA 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 (0.001 d) | 1.44 minutes, or 86.4 seconds. 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. |
| centiday | 0.01 d (1 % of a day) | 14.4 minutes, or 864 seconds. One-hundredth of a day is 1 cd (centiday), also called "kè" in tradidional Chinese timekeeping. The unit was also proposed by Lagrange and endorsed by Rey-Pailhade in the 19th century, named "centijours" (from French centi- 'hundred' and jour 'day'). |
| kilosecond | 103 s | About 17 minutes. |
| hour | 60 min | |
| deciday | 0.1 d (10 % of a day) | 2.4 hours, or 144 minutes. One-tenth of a day is 1 dd (deciday), also called "gēng" in traditional Chinese timekeeping. |
| day | 24 h | Longest unit used on stopwatches and countdowns. The SI day is exactly 86 400 seconds. |
| week | 7 d | Historically sometimes also called "sennight". |
| decaday | 10 d (1 Dd) | 10 days. A period of time analogous to the concept of "week", used by different societies around the world: the ancient Egyptian calendar, the ancient Chinese calendar, and also the French Republican calendar (in which it was called a décade). |
| 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. |
| hectoday | 100 d (1 hd) | 100 days, roughly equivalent to 1/4 of a year (91.25 days). In Chinese tradition "bǎi rì" (百日) is the hundredth day after one's birth, also called Baby's 100 Days Celebration. |
| 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 2/3 of a year | A superstitious unit of time used in astrology, each of them representing a star sign. |
| terasecond | 1012 s | About 31,709 years. |
| megaannum | 106 yr | Also called "megayear". 1000 millennia (plural of millennium), or 1 million years (in geology, abbreviated as Ma). |
| petasecond | 1015 s | About 31709791 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 230000000 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 1000000000 years. |
| kalpa | 4.32×109 yr | Used in Hindu mythology. About 4320000000 years. |
| exasecond | 1018 s | About 31709791983 years. Approximately 2.3 times the current age of the universe. |
| zettasecond | 1021 s | About 31709791983764 years. |
| yottasecond | 1024 s | About 31709791983764586 years. |
| ronnasecond | 1027 s | About 31709791983764586504 years. |
| quettasecond | 1030 s | About 31709791983764586504312 years. |
Interrelation
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
- "The International System of Units – 9th edition – Complete text in English and French (2019)". BIPM. Retrieved 22 July 2022.
- http://starchild.gsfc.nasa.gov/docs/StarChild/questions/question18.html NASA – StarChild Question of the Month for February 2000
- "Meet the zeptosecond, the shortest unit of time ever measured". Retrieved 2020-10-17.
- Milham, Willis I. (1945). Time and Timekeepers. New York: MacMillan. p. 190. ISBN 0-7808-0008-7.
- Gamez, Christophe (2020-06-17). La décimalisation du temps au prisme du Bureau des longitudes (1875-1901). Entre patriotisme, rationalité et politique (other thesis) (in French). Université de Lorraine.
- "Semester". Webster's Dictionary. Retrieved 3 December 2014.
- McCarthy, Dennis D.; Seidelmann, P. Kenneth (2009). Time: from Earth rotation to atomic physics. Wiley-VCH. p. 18. ISBN 978-3-527-40780-4., Extract of page 18
- "Merriam-Webster Dictionary". Merriam-Webster Incorporated. Retrieved 29 November 2016.
This article needs additional citations for verification Please help improve this article by adding citations to reliable sources Unsourced material may be challenged and removed Find sources Unit of time news newspapers books scholar JSTOR March 2016 Learn how and when to remove this message 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 DnCs the unperturbed ground state hyperfine transition frequency of the cesium 133 atom to be 9192 631 770 when expressed in the unit Hz which is equal to s 1 Table showing quantitative relationships between common units of time Historically many units of time were defined by the movements of astronomical objects Sun based the year is based on the Earth s orbital period 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 is based on the Moon s orbital period around the Earth Earth based the day is based on the time it takes 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 is further subdivided into minutes and seconds The second is the international standard unit SI unit 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 HistoricalThe 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 Horizontal logarithmic scale marked with units of time in the Gregorian calendarScientificThe Jiffy is the amount of time light takes to travel one femtometre about the diameter of a nucleon The Planck time is the time that light takes to travel one Planck length The TU for time unit is a unit of time defined as 1024 ms 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 9454 254 955 488 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 ListUnits of time Name Length NotesPlanck 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 and JILA 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 msmillisecond 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 shectosecond 100 smilliday 1 1000 d 0 001 d 1 44 minutes or 86 4 seconds 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 centiday 0 01 d 1 of a day 14 4 minutes or 864 seconds One hundredth of a day is 1 cd centiday also called ke in tradidional Chinese timekeeping The unit was also proposed by Lagrange and endorsed by Rey Pailhade in the 19th century named centijours from French centi hundred and jour day kilosecond 103 s About 17 minutes hour 60 mindeciday 0 1 d 10 of a day 2 4 hours or 144 minutes One tenth of a day is 1 dd deciday also called geng in traditional Chinese timekeeping day 24 h Longest unit used on stopwatches and countdowns The SI day is exactly 86 400 seconds week 7 d Historically sometimes also called sennight decaday 10 d 1 Dd 10 days A period of time analogous to the concept of week used by different societies around the world the ancient Egyptian calendar the ancient Chinese calendar and also the French Republican calendar in which it was called a decade megasecond 106 s About 11 6 days fortnight 2 weeks 14 dayslunar 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 quadraginta hectoday 100 d 1 hd 100 days roughly equivalent to 1 4 of a year 91 25 days In Chinese tradition bǎi ri 百日 is the hundredth day after one s birth also called Baby s 100 Days Celebration semester 18 weeks A division of the academic year Literally six months also used in this sense lunar year 354 37 dyear 12 mo 365 or 366 dcommon 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 sleap year 366 d 52 weeks and 2 dolympiad 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 yrindiction 15 yr Interval for taxation assessments Roman Empire gigasecond 109 s About 31 7 years jubilee 50 yrcentury 100 yrmillennium 1000 yr Also called kiloannum Age 2 148 and 2 3 of a year A superstitious unit of time used in astrology each of them representing a star sign terasecond 1012 s About 31 709 years megaannum 106 yr Also called megayear 1000 millennia plural of millennium or 1 million years in geology abbreviated as Ma petasecond 1015 s About 31709 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 230000 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 1000 000 000 years kalpa 4 32 109 yr Used in Hindu mythology About 4320 000 000 years exasecond 1018 s About 31709 791 983 years Approximately 2 3 times the current age of the universe zettasecond 1021 s About 31709 791 983 764 years yottasecond 1024 s About 31709 791 983 764 586 years ronnasecond 1027 s About 31709 791 983 764 586 504 years quettasecond 1030 s About 31709 791 983 764 586 504 312 years InterrelationFlowchart 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 alsoUnit of frequency Orders of magnitude time References The International System of Units 9th edition Complete text in English and French 2019 BIPM Retrieved 22 July 2022 http starchild gsfc nasa gov docs StarChild questions question18 html NASA StarChild Question of the Month for February 2000 Meet the zeptosecond the shortest unit of time ever measured Retrieved 2020 10 17 Milham Willis I 1945 Time and Timekeepers New York MacMillan p 190 ISBN 0 7808 0008 7 Gamez Christophe 2020 06 17 La decimalisation du temps au prisme du Bureau des longitudes 1875 1901 Entre patriotisme rationalite et politique other thesis in French Universite de Lorraine Semester Webster s Dictionary Retrieved 3 December 2014 McCarthy Dennis D Seidelmann P Kenneth 2009 Time from Earth rotation to atomic physics Wiley VCH p 18 ISBN 978 3 527 40780 4 Extract of page 18 Merriam Webster Dictionary Merriam Webster Incorporated Retrieved 29 November 2016
