Unit of time
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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 caesium133 atom.^{[1]}
Historically units of time were defined by the movements of astronomical objects.
 Sunbased: the year was the time for the earth to revolve around the sun. Yearbased units include the olympiad (four years), the lustrum (five years), the indiction (15 years), the decade, the century, and the millennium.
 Moonbased: the month was based on the moon's orbital period around the earth.
 Earthbased: 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 spherebased: 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 28day 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 [ edit ]
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.
Scientific time units [ edit ]
 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 lightyear is not a unit of time, but a unit of length of about 9.5 petametres (9 454 254 955 488 kilometres).
List [ edit ]
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. 
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 18week 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 (10^{6}) years  Also called "Megayear." About 1,000 millennia (plural of millennium), or 1 million years. 
petasecond  10^{15} 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  10^{18} seconds  About 31,700,000,000 years or 380,399,583,123.74 months 
zettasecond  10^{21} seconds  About 31.7 trillion years or 3,803,995,983,123,744.56 months 
yottasecond  10^{24} seconds  About 31.7 quadrillion years or 380,399,583,123,744,510 months 
[ edit ]
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 [ edit ]
 ^ "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 20170420.
 ^ Milham, Willis I. (1945). Time and Timekeepers. New York: MacMillan. p. 190. ISBN 0780800087.
 ^ "Semester". Webster's Dictionary. Retrieved 3 December 2014.
 ^ McCarthy, Dennis D.; Seidelmann, P. Kenneth (2009). Time: from Earth rotation to atomic physics. WileyVCH. p. 18. ISBN 3527407804., Extract of page 18