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Units of measurement for time have historically been based on the movement of the Sun (as seen from Earth). Shorter intervals were measured by physiological periods such as drawing breath, winking or the pulse.

Units of time consisting of a number of years include the lustrum (five years) and the olympiad (four years). The month could be divided into half-months or fortnights, and quarters or weeks. Longer periods were given in lifetimes or generations (saecula, aion), subdivisions of the solar day in hours. The Sothic cycle was a period of 1,461 years of 365 days in the Ancient Egyptian calendar. Medieval (Pauranic) Hindu cosmology is notorious for introducing names for fabulously long time periods, such as kalpa (4.32 billion years).

In classical antiquity, the hour divided the daylight period into 12 equal parts. The duration of an hour thus varied over the course of the year. In classical China, the (刻) was a unit of decimal time, dividing a day into 100 equal intervals of 14.4 minutes. Alongside the ke, there were double hours (shíchen) also known as watches. Because one cannot divide 12 double hours into 100 ke evenly, each ke was subdivided into 60 fēn (分).

The introduction of the minute (minuta; ′) as the 60th part of an hour, the second (seccunda; ′′) as the 60th part of a minute, and the third (tertia; ′′′) as the 60th part of the second dates to the medieval period, used by Al-Biruni around AD 1000, and by Roger Bacon in the 13th century. Bacon further subdivided the tertia into a quarta or fourth (′′′′). Hindu chronology divides the civil day (daylight hours) into vipalas, palas and ghatikas. A tithi is the 30th part of the synodic month.

The introduction of the division of the solar day into 24 hours of equal length, as it were the length of a classical hour at equinox used regardless of daylight hours, dates to the 14th century, due to the development of the first mechanical clocks.

Today, the fundamental unit of time suggested by the International System of Units is the second, since 1967 defined as the second of International Atomic Time, based on the radiation emitted by a Caesium-133 atom in the ground state. Its definition was calibrated such that 86,400 seconds corresponded to a solar day. 31,557,600 (86,400 × 365.25) seconds are a Julian year, exceeding the true length of a solar year by about 21 ppm.

Based on the second as the base unit, the following time units are in use as follows:

There are a number of proposals for decimal time, or decimal calendars, notably in the French Republican Calendar of 1793. Such systems have either ten days per week, a multiple of ten days in a month, or ten months per year.

A suggestion for hexadecimal time divides the Julian day into 16 hexadecimal hours of 1hr 30 min each, or 65,536 hexadecimal seconds (1 hexsec ≈ 1.32 s).

The Planck time (tP) is a natural unit of time, the shortest possible interval that can be meaningfully considered in quantum mechanics. tP equals about 5.4 × 10−44 s.

Horizontal logarithmic scale marked with units of time according to the internationally accepted Gregorian calendar.


Units of time
Unit Length, Duration and Size Notes Other
Planck time unit 5.39 x 10–44 s The amount of time light takes to travel one Planck length. This is the shortest time unit possible to understand in physics. All smaller time units have no use in physics.
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
attosecond 10−18 s shortest time now measurable
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 Also a casual term for a short period of time 10 nanoseconds
microsecond 10−6 s
fourth 1/3,600 second medieval unit of time
millisecond 0.001 s shortest time unit used on stopwatches
centisecond 0.01 s used on some stopwatches
third 1/60 s medieval unit of time
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
decisecond 0.1 s used on some stopwatches
second 1 sec SI base unit
dekasecond 10 seconds
minute 60 seconds
moment 1/40th of an hour (~90 seconds) medieval unit of time used by astronomers to compute astronomical movements.[1]
hectosecond 100 seconds 1 minute and 40 seconds
ke 14 minutes and 24 seconds
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 About 11.6 days
fortnight 2 weeks 14 days may not be common
lunar month 27 Days 4 hours 48 minutes–29 days 12 hours Various definitions of lunar month exist.
month 28–31 days
quarter and season 3 months
year 12 months or 365 days
common year 365 days 52 weeks + 1 day
tropical year 365 days 5:48:45.216 hours[2] average
Gregorian year 365 days 5:49:12 hours[3] average
sidereal year 365 days 6:09:09.7635456 hours
leap year 366 days 52 weeks + 2 days
biennium 2 years A unit of time commonly used by legislatures
triennium 3 years
Olympiad 4 year cycle
lustrum 5 years
decade 10 years
Indiction 15 year cycle
score 20 years "four score and seven years.." = 87 years
generation 17-35 years
gigasecond 1,000,000,000 seconds About 31.7 years
jubilee 50 years
century 100 years
millennium 1,000 years also called "kiloannum"
terasecond 1 trillion seconds About 31,700 years
age and megaannum 1,000,000 years
epoch 10,000,000 years
petasecond 1 quadrillion seconds About 3.17 epoches
era 100,000,000 years
galactic year Approximately 2.3 eras[4] The amount of time it takes the Solar System to orbit the center of the Milky Way Galaxy one time.
eon 500,000,000 years Also "An indefinite and very long period of time""[5]
gigaannum 1,000,000,000 years
exasecond 1 quintillion seconds roughly 31.7 billion years, more than twice
the age of the universe on current estimates
zettasecond 1 sextillion seconds About 31.7 trillion years
yottasecond 1 septillion seconds About 31.7 quadrillion 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.

Units of time interrelated[edit]

Flowchart illustrating the major units of time

All these units are not used in time. The key units are the second, defined in terms of an atomic process; the day, an integral multiple of seconds; and the year, usually 365.25 days. Most of the other units used are multiples or divisions of these three. The graphic also shows the three heavenly bodies whose orbital parameters relate to the units of time.

See also[edit]


  1. ^ Milham, Willis I. (1945). Time and Timekeepers. New York: MacMillan. p. 190. ISBN 0-7808-0008-7. 
  2. ^ 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
  3. ^ 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
  4. ^ http://starchild.gsfc.nasa.gov/docs/StarChild/questions/question18.html NASA - StarChild Question of the Month for February 2000
  5. ^ http://www.merriam-webster.com/dictionary/aeon?show=0&t=1372548060

Original courtesy of Wikipedia: http://en.wikipedia.org/wiki/Unit_of_time — Please support Wikipedia.
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