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
Visual Gregorian calendar

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
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.

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 petametres (9 454 254 955 488 kilometres).

List

Units of time
Unit Length, Duration and Size Notes
Planck time unit5.39×10−44 sThe 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.
yoctosecond10−24 s
jiffy (physics)3×10−24 sThe amount of time light takes to travel one fermi (about the size of a nucleon) in a vacuum.
zeptosecond10−21 sTime measurement scale of the NIST strontium atomic clock. Smallest fragment of time currently measurable is 850 zeptoseconds.[3]
attosecond10−18 s
femtosecond10−15 sPulse time on fastest lasers.
Svedberg10−13 sTime unit used for sedimentation rates (usually of proteins).
picosecond10−12 s
nanosecond10−9 sTime for molecules to fluoresce.
shake10−8 s10 nanoseconds, also a casual term for a short period of time.
microsecond10−6 sSymbol is µs
millisecond10−3 sShortest time unit used on stopwatches.
jiffy (electronics)1/60 s or 1/50 sUsed to measure the time between alternating power cycles. Also a casual term for a short period of time.
second1 sSI Base unit.
decasecond10 s
minute60 s
moment1/40 solar hour (90 s on average)Medieval unit of time used by astronomers to compute astronomical movements, length varies with the season.[4]
hectosecond100 s1 minute and 40 seconds
decaminute10 min
ke14 min 24 sUsually calculated as 15 minutes, similar to "quarter" as in "a quarter past six" (6:15).
kilosecond1000 s16 minutes and 40 seconds
hour60 min
hectominute100 min1 hour and 40 minutes
kilominute1000 min16 hours and 40 minutes
day24 hLongest unit used on stopwatches and countdowns.
week7 dAlso called "sennight".
megasecond106 s277.777778333333 hours or about 1 week and 4.6 days.
fortnight2 weeks14 days
lunar month27 d 4 h 48 min – 29 d 12 hVarious definitions of lunar month exist.
month28–31 dOccasionally calculated as 30 days.
quarter and season3 mo
semester18 weeksA division of the academic year.[5] Literally "six months", also used in this sense.
year12 mo365 or 366 d
common year365 d52 weeks and 1 day.
tropical year365 d 5 h 48 min 45.216 s[6]Average.
Gregorian year365 d 5 h 49 min 12 sAverage.
sidereal year365 d 6 h 9 min 9.7635456 s
leap year366 d52 weeks and 2 d
biennium2 yr
triennium3 yr
quadrennium4 yr
olympiad4 yr
lustrum5 yr
decade10 yr
indiction15 yr
gigasecond109 s16,666,666.6667 minutes or About 31.7 years.
jubilee50 yr
century100 yr
millennium1000 yrAlso called "kiloannum".
terasecond1012 sabout 31,700 years.
Megannum106 yrAlso called "Megayear." About 1,000 millennia (plural of millennium), or 1 million years.
petasecond1015 sAbout 31,700,000 years or 380,399,583.12 months
galactic year2.3×108 yr[2]The amount of time it takes the Solar System to orbit the center of the Milky Way Galaxy one time.
cosmological decadevaries10 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.
aeon109 yrAlso spelled "eon". Also refers to an indefinite period of time.
exasecond1018 sAbout 31,700,000,000 years or 380,399,583,123.74 months
zettasecond1021 sAbout 31.7 trillion years or 3,803,995,983,123,744.56 months
yottasecond1024 sAbout 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.

gollark: Someone distracted me and now I forgot what change I was making, æ.
gollark: Yes, I said that.
gollark: pytz and also all time zones are so unintuitive.
gollark: This is wrong, but differently, at least.
gollark: --remind 1m "queued at 17:54:whatever"

References
  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.
  2. http://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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