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Unit of time
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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 halfmonths 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 kè (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 AlBiruni 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 Caesium133 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:
 minute (1 min = 60 sec)
 hour (1 hr = 60 min = 3.6 ks)
 Julian day (1 d = 24 hr = 86.4 ks)
 week (7 d = 604.8 ks)
 Julian year (1 a = 365.25 d = 31.5576 Ms)
 decade (10 years/annum)
 century (100 annum = 3.15576 Gs)
 millennium (1 ka = 31.5576 Gs)
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 (t_{P}) is a natural unit of time, the shortest possible interval that can be meaningfully considered in quantum mechanics. t_{P} equals about 5.4 × 10^{−44} s.
List
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^{−24}s  The amount of time light takes to travel one fermi (about the size of a nucleon) in a vacuum. 
zepatosecond  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  10 nanoseconds, also a casual term for a short period of time 
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  
dekaminute  600 seconds or 10 minutes  
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  usually calculated as 15 minutes nowadays 
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 
lunar month  27 Days 4 hours 48 minutes–29 days 12 hours  Various definitions of lunar month exist. 
month  28–31 days  
semester  an 18week division of the academic year^{[2]}  Literally "six months", also used in this sense. 
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^{[3]}  average 
Gregorian year  365 days 5:49:12 hours^{[4]}  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  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  
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 230 million years^{[5]}  The amount of time it takes the Solar System to orbit the center of the Milky Way Galaxy one time. 
eon or aeon  500,000,000 years or 1,000,000,000 years (in astronomy)  Also "An indefinite and very long period of time"^{[6]}^{[7]} 
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. 
All of the important units of time can be interrelated. 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
References
 ^ Milham, Willis I. (1945). Time and Timekeepers. New York: MacMillan. p. 190. ISBN 0780800087.
 ^ . Webster's Dictionary http://www.merriamwebster.com/dictionary/semester. Retrieved 3 December 2014. Missing or empty
title=
(help)  ^ McCarthy, Dennis D.; Seidelmann, P. Kenneth (2009). Time: from Earth rotation to atomic physics. WileyVCH. p. 18. ISBN 3527407804., Extract of page 18
 ^ Jones, Floyd Nolen (2005). The Chronology Of The Old Testament (15th ed.). New Leaf Publishing Group. p. 287. ISBN 089051416X., Extract of page 287
 ^ http://starchild.gsfc.nasa.gov/docs/StarChild/questions/question18.html NASA  StarChild Question of the Month for February 2000
 ^ http://www.merriamwebster.com/dictionary/aeon?show=0&t=1372548060
 ^ Martin Harweit (1991). Astrophysical Concepts (2nd ed.). SpringerVerlag. ISBN 3540966838. p. 4.
