Unit of time

Table showing quantitative relationships between common units of time

Visual Gregorian calendar

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]

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 365.24 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 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 9.5 trillion kilometres (9,454,254,955,488 kilometres).

List

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 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 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	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 a solar hour (averages about 90 seconds)	Medieval unit of time used by astronomers to compute astronomical movements, length varies with the season.^[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	277.777778333333 hours or about 1 week and 4.6 days.
fortnight	2 weeks	14 days
lunar month	3 weeks 6 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 365/366 days
common year	365 days	52 weeks and 1 day.
decade	period consisting of ten common years
century	one hundred common years or ten decades
millennium	1000 * 365.25 day years
epoch	20,000 * 365.25 day years
eon	100,000 * 365.25 day years
aeon	1,000,000 * 365.25 day years
tropical year	365 days and 5:48:45.216 hours^[6]	Average.
Gregorian year	365 days and 5:49:12 hours	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
gigasecond	1,000,000,000 seconds	16,666,666.6667 minutes or About 31.7 years.
jubilee	50 years
century	100 years
millennium	1,000 years	Also called "kiloannum".
terasecond	1 trillion seconds	16,666,666,666.6667 minutes or 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"
exasecond	10¹⁸ seconds	About 31,700,000,000 years or 380,399,583,123.74 months
zettasecond	10²¹ seconds	About 31.7 trillion years or 3,803,995,983,123,744.56 months
yottasecond	10²⁴ seconds	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.
^ ^a ^b http://starchild.gsfc.nasa.gov/docs/StarChild/questions/question18.html NASA - StarChild Question of the 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 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

[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] "It only takes a zeptosecond: Scientists measure smallest fragment of time". RT International. Retrieved 2017-04-20.

[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

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v t e 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 (Italian · Thai) 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 New Earth Time
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 value of money Timekeeper