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."[1]

Table showing quantitative relationships between common units of time

Historically 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. 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. 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 (1/24 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 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.

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.

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

Scientific time units

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 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.[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 (9 454 254 955 488 kilometers).

List

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.^[3] 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.[3]
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 proteins).
picosecond	10⁻¹² s
nanosecond	10⁻⁹ s	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	10⁻³ s	Shortest time unit used on stopwatches.
jiffy (electronics)	1/60 s or 1/50 s	Used to measure the time between alternating power cycles. Also a casual term for a short period of time.
second	1 s	SI Base unit.
decasecond	10 s
minute	60 s
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.[4]
hectosecond	100 s	1 minute and 40 seconds
decaminute	10 min
ke	14 min 24 s	Usually calculated as 15 minutes, similar to "quarter" as in "a quarter past six" (6:15).
kilosecond	1000 s	16 minutes and 40 seconds
hour	60 min
hectominute	100 min	1 hour and 40 minutes
kilominute	1000 min	16 hours and 40 minutes
day	24 h	Longest unit used on stopwatches and countdowns.
week	7 d	Also called "sennight".
megasecond	10⁶ s	277.777778333333 hours or about 1 week and 4.6 days.
fortnight	2 weeks	14 days
lunar month	27 d 4 h 48 min – 29 d 12 h	Various definitions of lunar month exist.
month	28–31 d	Occasionally calculated as 30 days.
quarter and season	3 mo
semester	18 weeks	A division of the academic year.[5] Literally "six months", also used in this sense.
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[6]	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
biennium	2 yr
triennium	3 yr
quadrennium	4 yr
olympiad	4 yr
lustrum	5 yr
decade	10 yr
indiction	15 yr
gigasecond	10⁹ s	16,666,666.6667 minutes or About 31.7 years.
jubilee	50 yr
century	100 yr
millennium	1000 yr	Also called "kiloannum".
terasecond	10¹² s	about 31,700 years.
Megannum	10⁶ yr	Also called "Megayear." 1,000 millennia (plural of millennium), or 1 million years.
petasecond	10¹⁵ s	About 31,700,000 years or 380,399,583.12 months
galactic year	2.3×10⁸ yr[2]	The amount of time it takes the Solar System to orbit the center of the Milky Way Galaxy one time. Around 230000000 years.
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	10⁹ yr	Also spelled "eon". Also refers to an indefinite period of time, otherwise is 1000000000 years.
exasecond	10¹⁸ s	About 31,700,000,000 years or 380,399,583,123.74 months
zettasecond	10²¹ s	About 31.7 trillion years or 3,803,995,983,123,744.56 months
yottasecond	10²⁴ s	About 31.7 quadrillion years or 380,399,583,123,744,510 months

Units of time interrelated

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 three.

References

"Definitions of the SI base units". The NIST reference on Constants, Units, and Uncertainty. National Institute of Standards and Technology. Retrieved 4 March 2016.
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.
"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 3-527-40780-4., Extract of page 18

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[NIST_definition-1] "Definitions of the SI base units". The NIST reference on Constants, Units, and Uncertainty. National Institute of Standards and Technology. Retrieved 4 March 2016.

[StarChild-2] ttp://starchild.gsfc.nasa.gov/docs/StarChild/questions/question18.html NASA - StarChild Question of the Month for February 2000

[3] "Meet the zeptosecond, the shortest unit of time ever measured". Retrieved 2020-10-17.

[4] Milham, Willis I. (1945). Time and Timekeepers. New York: MacMillan. p. 190. ISBN 0-7808-0008-7.

[5] "Semester". Webster's Dictionary. Retrieved 3 December 2014.

[6] 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

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 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 zone 180th meridian
Obsolete standards	Ephemeris time Greenwich Mean Time Prime meridian
Time in physics	Absolute space and time Spacetime Chronon Continuous signal Coordinate time Cosmological decade Discrete time and continuous time Planck time Proper time Theory of relativity Time dilation Gravitational time dilation Time domain Time translation symmetry T-symmetry
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
Calendar	Astronomical Dominical letter Epact Equinox Gregorian Hebrew Hindu Holocene 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 Decimal time Metric time System time Time metrology Time value of money Timekeeper