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]

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 (1/24th 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.


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).


Units of time
UnitLength, Duration and SizeNotes
Planck time unit5.39 x 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 × 10−24sThe 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
millisecond0.001 sShortest time unit used on stopwatches.
jiffy (electronics)1/60s to 1/50sUsed to measure the time between alternating power cycles. Also a casual term for a short period of time.
second1 secSI Base unit.
minute60 seconds
moment1/40th of an hour (90 seconds)Medieval unit of time used by astronomers to compute astronomical movements.[4]
ke14 minutes and 24 secondsUsually calculated as 15 minutes, similar to "quarter" as in "a quarter past six" (6:15).
kilosecond1,000 seconds16 minutes and 40 seconds.
hour60 minutes
day24 hoursLongest unit used on stopwatches and countdowns.
week7 daysAlso called "sennight".
megasecond1,000,000 seconds277.777778333333 hours or about 1 week and 4.6 days.
fortnight2 weeks14 days
lunar month3 weeks 6 days 4 hours 48 minutes–29 days 12 hoursVarious definitions of lunar month exist.
month28–31 daysOccasionally calculated as 30 days.
quarter and season3 months
semesteran 18-week division of the academic year[5]Literally "six months", also used in this sense.
year12 months or 365/366 days
common year365 days52 weeks and 1 day.
decadeperiod consisting of ten common years
scoreperiod consisting of two decades
centuryone hundred common years or ten decades or 5 score
millennium1000 * 365.25 day years
epoch20,000 * 365.25 day years
eon100,000 * 365.25 day years
aeon1,000,000 * 365.25 day years
tropical year365 days and 5:48:45.216 hours[6]Average.
Gregorian year365 days and 5:49:12 hours[7]Average.
sidereal year365 days and 6:09:09.7635456 hours
leap year366 days52 weeks and 2 days.
biennium2 years
triennium3 years
quadrennium4 years
olympiad4 year cycle48 months, 1,461 days, 35,064 hours, 2,103,840 minutes, 126,230,400 seconds.
lustrum5 years
decade10 years
indiction15 year cycle
score20 years
gigasecond1,000,000,000 seconds16,666,666.6667 minutes or About 31.7 years.
jubilee50 years
century100 years
millennium1,000 yearsAlso called "kiloannum".
terasecond1 trillion seconds16,666,666,666.6667 minutes or about 31,700 years.
Megannum1,000,000 (106) yearsAlso called "Megayear." About 1,000 millennia (plural of millennium), or 1 million years.
petasecond1015 secondsAbout 31,700,000 years
galactic yearApproximately 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 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.
aeon1,000,000,000 years or an indefinite period of timeAlso spelled "eon"
Day of Brahman
(aka Day of God)
4,320,000,000 years or 4.32 aeonLike the galactic year which measures the time it takes all the solar systems of the Milky Way Galaxy to orbit its central nexus one time, this measurement of time is the presumed length of time it takes all the Galaxies in the Universe to orbit its presumed central nexus (aka "Ground Zero of the Big Bang"), one time. In this context, the "7 Days of Creation" mentioned in the book of Genesis are seen in a much different light, since Earth is estimated to be ~4.3 billion years old, or 1 Day of God according to the Vedic system of time.
exasecond1018 secondsAbout 31,700,000,000 years
zettasecond1021 secondsAbout 31.7 trillion years
yottasecond1024 secondsAbout 31.7 quadrillion years

Units of time interrelated

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.


  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
  7. Jones, Floyd Nolen (2005). The Chronology Of The Old Testament (15th ed.). New Leaf Publishing Group. p. 287. ISBN 0-89051-416-X., Extract of page 287
This article is issued from Wikipedia. The text is licensed under Creative Commons - Attribution - Sharealike. Additional terms may apply for the media files.