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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 caesium-133 atom."
Historically units of time were defined by the movements of astronomical objects.
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.
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.
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).
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|Unit||Length, Duration and Size||Notes|
|Planck time unit||5.39×10−44 s||The amount of time light takes to travel one Planck length. Theoretically, this is the smallest time measurement that will ever be possible. Smaller time units have no use in physics as we understand it today.|
|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.|
|femtosecond||10−15 s||Pulse time on fastest lasers.|
|Svedberg||10−13 s||Time unit used for sedimentation rates (usually of proteins).|
|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||10−3 s||Shortest time unit used on stopwatches.|
|jiffy (electronics)||1/60 s or 1/50 s||Used to measure the time between alternating power cycles. Also a casual term for a short period of time.|
|second||1 s||SI Base unit.|
|moment||1/40 solar hour (90 s on average)||Medieval unit of time used by astronomers to compute astronomical movements, length varies with the season.|
|ke||14 min 24 s||Usually calculated as 15 minutes, similar to "quarter" as in "a quarter past six" (6:15).|
|kilosecond||1000 s||16 minutes and 40 seconds|
|day||24 h||Longest unit used on stopwatches and countdowns.|
|week||7 d||Also called "sennight".|
|megasecond||106 s||277.777778333333 hours or about 1 week and 4.6 days.|
|fortnight||2 weeks||14 days|
|lunar month||27 d 4 h 48 min – 29 d 12 h||Various definitions of lunar month exist.|
|month||28–31 d||Occasionally calculated as 30 days.|
|quarter and season||3 mo|
|semester||18 weeks||A division of the academic year. Literally "six months", also used in this sense.|
|year||12 mo||365 or 366 d|
|common year||365 d||52 weeks and 1 day.|
|tropical year||365 d 5 h 48 min 45.216 s||Average.|
|Gregorian year||365 d 5 h 49 min 12 s||Average.|
|sidereal year||365 d 6 h 9 min 9.7635456 s|
|leap year||366 d||52 weeks and 2 d|
|gigasecond||109 s||16,666,666.6667 minutes or About 31.7 years.|
|millennium||1000 yr||Also called "kiloannum".|
|terasecond||1012 s||16,666,666,666.6667 minutes or about 31,700 years.|
|Megannum||106 yr||Also called "Megayear." About 1,000 millennia (plural of millennium), or 1 million years.|
|petasecond||1015 s||About 31,700,000 years|
|galactic year||2.3×108 yr||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||109 yr||Also spelled "eon". Also refers to an indefinite period of time.|
|exasecond||1018 s||About 31,700,000,000 years or 380,399,583,123.74 months|
|zettasecond||1021 s||About 31.7 trillion years or 3,803,995,983,123,744.56 months|
|yottasecond||1024 s||About 31.7 quadrillion years or 380,399,583,123,744,510 months|
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.