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Unit of time
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{{refimprove|date=March 2016}}(File:Gregorian year visualisation.svg|thumb|300px|Visual Gregorian calendar)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 {{val|9192631770}} periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the caesium-133 atom.WEB, Definitions of the SI base units,weblink The NIST reference on Constants, Units, and Uncertainty, National Institute of Standards and Technology, 4 March 2016,
Historically units of time were defined by the movements of astronomical objects.
  • Sun based: the year was the time for the earth to rotate 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.

Historical

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.{{wide image|Logarithmic time scale - milliseconds to years.svg|1200px|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 Âµs 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 yearsweblink NASA - StarChild Question of the Month for February 2000
  • 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}}">

List{| class"wikitable mw-collapsible mw-collapsed"Units of time}}

! Unit !! Length, Duration and Size !! Notes
Planck time>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 >|
Jiffy (time)>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 >weblinkHTTPS://WWW.RT.COM/VIRAL/366939-ZEPTOSECOND-SCIENTISTS-MEASURE-TIME/WORK=RT INTERNATIONALLANGUAGE=EN-EN,
attosecond >|
femtosecond >| Pulse time on fastest lasers.
Svedberg >| Time unit used for sedimentation rates (usually of proteins).
picosecond >|
nanosecond >| Time for molecules to fluoresce.
Shake (unit)>shake 10−8 s 10 nanoseconds, also a casual term for a short period of time.
microsecond >|Symbol is µs
millisecond >| Shortest time unit used on stopwatches.
Jiffy (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.
second >| SI Base unit.
minute >|
moment (time)>moment 1/40th of an hour (90 seconds) Medieval unit of time used by astronomers to compute astronomical movements.MILHAM >FIRST=WILLIS I. YEAR=1945 LOCATION=NEW YORK ISBN=0-7808-0008-7,
Traditional Chinese timekeeping>ke 14 minutes and 24 seconds Usually calculated as 15 minutes, similar to "quarter" as in "a quarter past six" (6:15).
kilosecond >| 16 minutes and 40 seconds.
hour >|
day >| Longest unit used on stopwatches and countdowns.
week >| Also called "sennight".
megasecond >| About 1 week and 4.6 days.
fortnight >| 14 days
lunar month >| Various definitions of lunar month exist.
month >| Occasionally calculated as 30 days.
Fiscal year>quarter and season 3 months
semester >PUBLISHER=WEBSTER'S DICTIONARY| Literally "six months", also used in this sense.
year >|
common year >| 52 weeks and 1 day.
decade >|
20 (number)>score period consisting of two decades
century >|
millennium >|
epoch >|
|
aeon >|
tropical year >| 365 days and 5:48:45.216 hoursBOOK, Time: from Earth rotation to atomic physics, Dennis D., McCarthy, P. Kenneth, Seidelmann, Wiley-VCH, 2009, 3-527-40780-4, 18,weblink , Extract of page 18
||Average.
Gregorian calendar>Gregorian year 365 days and 5:49:12 hoursBOOK, The Chronology Of The Old Testament, 15th, Floyd Nolen, Jones, New Leaf Publishing Group, 2005, 0-89051-416-X, 287,weblink , Extract of page 287
||Average.
sidereal year >|
leap year >| 52 weeks and 2 days.
biennium) 2 years
triennium >|
quadrennium >|
olympiad >| 48 months, 1,461 days, 35,064 hours, 2,103,840 minutes, 126,230,400 seconds.
lustrum >|
decade >|
indiction >|
20 (number)>score 20 years
gigasecond >| About 31.7 years.
Jubilee (biblical)>jubilee 50 years
century >|
millennium >| Also called "kiloannum".
Terasecond and longer>terasecond 1 trillion seconds About 31,700 years.
Megannum) 1,000,000 (106) years Also called "Megayear." About 1,000 millennia (plural of millennium), or 1 million years.
Terasecond and longer>petasecond 1015 seconds About 31,700,000 years
galactic year >|The amount of time it takes the Solar System to orbit the center of the Milky Way Galaxy one time.
cosmological decade >|10 times the length of the previouscosmological decade, with CÐ 1 beginningeither 10 seconds or 10 years after theBig Bang, depending on the definition.
aeon >| Also spelled "eon"
Hindu_units_of_time>Day of Brahman (aka Day of God) 4,320,000,000 years or 4.32 aeon Like 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.
Terasecond and longer>exasecond 1018 seconds About 31,700,000,000 years
Terasecond and longer>zettasecond 1021 seconds About 31.7 trillion years
Terasecond and longer>yottasecond 1024 seconds About 31.7 quadrillion years

Units of time interrelated

(File:Time units.png|thumb|Flowchart illustrating the major units of time)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 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.

References

{{reflist}}{{Time topics}}{{Time measurement and standards}}

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