Unit of time

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Table showing quantitative relationships between common units of time Units of Time in tabular form.png
Table showing quantitative relationships between common units of time

A unit of time 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 SI definition is "[The second] is defined by taking the fixed numerical value of the cesium frequency, ΔνCs, the unperturbed ground-state hyperfine transition frequency of the cesium 133 atom, to be 9192631770 when expressed in the unit Hz, which is equal to s−1." [1]

Contents

Historically, many 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 twelve 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 for scientific purposes 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 calendar.

Logarithmic time scale - milliseconds to years.svg
Horizontal logarithmic scale marked with units of time in the Gregorian calendar

Scientific

Note: The light-year is not a unit of time, but a unit of length of about 9.5 petametres (9454254955488 km).

Note: The parsec is not a unit of time, but a unit of length of about 30.9 trillion kilometres, despite movie references otherwise.

List

Units of time
NameLengthNotes
Planck time 5.39×10−44  s The amount of time light takes to travel one Planck length.
quectosecond 10−30  s One nonillionth of a second.
rontosecond 10−27  s One octillionth of a second.
yoctosecond 10−24  s One septillionth of a second.
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 One sextillionth of a second. Time measurement scale of the NIST and JILA strontium atomic clock. Smallest fragment of time currently measurable is 247 zeptoseconds. [3]
attosecond 10−18  s One quintillionth of a second.
femtosecond 10−15  s One quadrillionth of a second. Pulse time on fastest lasers.
svedberg 10−13  s Time unit used for sedimentation rates (usually of proteins).
picosecond 10−12  s One trillionth of a second.
nanosecond 10−9  s One billionth of a second. 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 One millionth of a second. Symbol is μs
millisecond 10−3  s One thousandth of a second. Shortest time unit used on stopwatches.
jiffy (electronics)~10−3  s Used to measure the time between alternating power cycles. Also a casual term for a short period of time.
centisecond 10−2  s One hundredth of a second.
decisecond 10−1  s One tenth of a second.
second 1 s SI base unit for time.
decasecond 10  s Ten seconds (one sixth of a minute)
minute 60  s
hectosecond 100  s
milliday 1/1000  d (0.001  d )1.44 minutes, or 86.4 seconds. Also marketed as a ".beat" by the Swatch corporation.
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. [4] Also colloquially refers to a brief period of time.
centiday0.01  d (1 % of a day)14.4 minutes, or 864 seconds. One-hundredth of a day is 1 cd (centiday), also called "kè" in tradidional Chinese timekeeping. The unit was also proposed by Lagrange and endorsed by Rey-Pailhade [5] in the 19th century, named " centijours " (from French centi- 'hundred' and jour 'day').
kilosecond 103  s About 17 minutes.
hour 60  min
deciday0.1  d (10 % of a day)2.4 hours, or 144 minutes. One-tenth of a day is 1 dd (deciday), also called "gēng" in tradidional Chinese timekeeping.
day 24  h Longest unit used on stopwatches and countdowns. The SI day is exactly 86 400 seconds.
week 7  d Historically sometimes also called "sennight".
decaday10  d (1  Dd )10 days. A period of time analogous to the concept of "week", used by different societies around the world: the ancient Egyptian calendar, the ancient Chinese calendar, and also the French Republican calendar (in which it was called a décade).
megasecond 106  s About 11.6 days.
fortnight 2  weeks 14 days
lunar month 27  d  4  h  48  min  –29  d  12  h Various definitions of lunar month exist; sometimes also called a "lunation".
month 28–31  d Occasionally calculated as 30 days.
quarantine 40 d (approximately 5.71 weeks)To retain in obligatory isolation or separation, as a sanitary measure to prevent the spread of contagious disease. Historically it meant to be isolated for 40 days. From Middle English quarentine, from Italian quarantina ("forty days"), the period Venetians customarily kept ships from plague-ridden countries waiting off port, from quaranta ("forty"), from Latin quadrāgintā.
hectoday100  d (1  hd )100 days, roughly equivalent to 1/4 of a year (91.25 days). In Chinese tradition "bǎi rì" (百日) is the hundredth day after one's birth, also called Baby's 100 Days Celebration.
semester 18  weeks A division of the academic year. [6] Literally "six months", also used in this sense.
lunar year 354.37  d
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 [7] 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
olympiad 4  yr A quadrennium (plural: quadrennia or quadrenniums) is also a period of four years, most commonly used in reference to the four-year period between each Olympic Games. [8] It is also used in reference to the four-year interval between leap years, for example when wishing friends and family a "happy quadrennium" on February 29.[ citation needed ]
lustrum 5  yr In early Roman times, the interval between censuses.
decade 10  yr
indiction 15  yr Interval for taxation assessments (Roman Empire).
gigasecond 109  s About 31.7 years.
jubilee 50  yr
century 100  yr
millennium 1000  yr Also called "kiloannum".
Age 2 148 and 2/3 of a yearA superstitious unit of time used in astrology, each of them representing a star sign.
terasecond 1012  s About 31,709 years.
megaannum 106  yr Also called "megayear". 1000 millennia (plural of millennium), or 1 million years (in geology, abbreviated as Ma).
petasecond 1015  s About 31709791 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 (approx 230000000 years [2] ).
cosmological decade logarithmic (varies)10 times the length of the previous cosmological decade, with CD 1 beginning either 10 seconds or 10 years after the Big Bang, depending on the definition.
eon 109  yr Also refers to an indefinite period of time, otherwise is 1000000000 years.
kalpa 4.32×109  yr Used in Hindu mythology. About 4320000000 years.
exasecond 1018  s About 31709791983 years. Approximately 2.3 times the current age of the universe.
zettasecond 1021  s About 31709791983764 years.
yottasecond 1024  s About 31709791983764586 years.
ronnasecond 1027  s About 31709791983764586504 years.
quettasecond 1030  s About 31709791983764586504312 years.

Interrelation

Flowchart illustrating selected units of time. The graphic also shows the three celestial objects that are related to the units of time. Time units.svg
Flowchart illustrating selected units of time. The graphic also shows the three celestial objects that are related to the units of time.

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 3.

See also

Related Research Articles

<span class="mw-page-title-main">Calendar</span> System for organizing the days of year

A calendar is a system of organizing days. This is done by giving names to periods of time, typically days, weeks, months and years. A date is the designation of a single and specific day within such a system. A calendar is also a physical record of such a system. A calendar can also mean a list of planned events, such as a court calendar, or a partly or fully chronological list of documents, such as a calendar of wills.

A calendar year begins on the New Year's Day of the given calendar system and ends on the day before the following New Year's Day, and thus consists of a whole number of days.

<span class="mw-page-title-main">Chinese calendar</span> Lunisolar calendar from China

The traditional Chinese calendar is a lunisolar calendar dating from the Han dynasty that combines solar, lunar, and other cycles for various social and agricultural purposes. While the Gregorian calendar has been adopted and adapted in various ways, and is generally the basis for China's standard civic purposes, aspects of the traditional lunisolar calendar remain, including the association of the twelve animals of the Chinese Zodiac in relation to months and years.

A day is the time period of a full rotation of the Earth with respect to the Sun. On average, this is 24 hours. As a day passes at a given location it experiences morning, noon, afternoon, evening, and night. This daily cycle drives circadian rhythms in many organisms, which are vital to many life processes.

Δ<i>T</i> (timekeeping) Measure of variation of solar time from atomic time

In precise timekeeping, ΔT is a measure of the cumulative effect of the departure of the Earth's rotation period from the fixed-length day of International Atomic Time. Formally, ΔT is the time difference ΔT = TT − UT between Universal Time and Terrestrial Time. The value of ΔT for the start of 1902 was approximately zero; for 2002 it was about 64 seconds. So Earth's rotations over that century took about 64 seconds longer than would be required for days of atomic time. As well as this long-term drift in the length of the day there are short-term fluctuations in the length of day which are dealt with separately.

The term ephemeris time can in principle refer to time in association with any ephemeris. In practice it has been used more specifically to refer to:

  1. a former standard astronomical time scale adopted in 1952 by the IAU, and superseded during the 1970s. This time scale was proposed in 1948, to overcome the disadvantages of irregularly fluctuating mean solar time. The intent was to define a uniform time based on Newtonian theory. Ephemeris time was a first application of the concept of a dynamical time scale, in which the time and time scale are defined implicitly, inferred from the observed position of an astronomical object via the dynamical theory of its motion.
  2. a modern relativistic coordinate time scale, implemented by the JPL ephemeris time argument Teph, in a series of numerically integrated Development Ephemerides. Among them is the DE405 ephemeris in widespread current use. The time scale represented by Teph is closely related to, but distinct from, the TCB time scale currently adopted as a standard by the IAU.

Intercalation or embolism in timekeeping is the insertion of a leap day, week, or month into some calendar years to make the calendar follow the seasons or moon phases. Lunisolar calendars may require intercalations of days or months.

<span class="mw-page-title-main">Second</span> SI unit of time

The second is a unit of time, historically defined as 1⁄86400 of a day – this factor derived from the division of the day first into 24 hours, then to 60 minutes and finally to 60 seconds each.

<span class="mw-page-title-main">Year</span> Time of one planets orbit around a star

A year is the time taken for astronomical objects to complete one orbit. For example, a year on Earth is the time taken for Earth to revolve around the Sun. Generally, a year is taken to mean a calendar year, but the word is also used for periods loosely associated with the calendar or astronomical year, such as the seasonal year, the fiscal year, the academic year, etc. The term can also be used in reference to any long period or cycle, such as the Great Year.

A time standard is a specification for measuring time: either the rate at which time passes or points in time or both. In modern times, several time specifications have been officially recognized as standards, where formerly they were matters of custom and practice. An example of a kind of time standard can be a time scale, specifying a method for measuring divisions of time. A standard for civil time can specify both time intervals and time-of-day.

<span class="mw-page-title-main">Metric time</span> Measure of time intervals using the metric system

Metric time is the measure of time intervals using the metric system. The modern SI system defines the second as the base unit of time, and forms multiples and submultiples with metric prefixes such as kiloseconds and milliseconds. Other units of time – minute, hour, and day – are accepted for use with SI, but are not part of it. Metric time is a measure of time intervals, while decimal time is a means of recording time of day.

<span class="mw-page-title-main">Sidereal time</span> Timekeeping system on Earth relative to the celestial sphere

Sidereal time is a system of timekeeping used especially by astronomers. Using sidereal time and the celestial coordinate system, it is easy to locate the positions of celestial objects in the night sky. Sidereal time is a "time scale that is based on Earth's rate of rotation measured relative to the fixed stars".

<span class="mw-page-title-main">Lunar day</span> Time for Moon to complete one rotation on its axis

A lunar day is the time it takes for Earth's Moon to complete on its axis one synodic rotation, meaning with respect to the Sun. Informally, a lunar day and a lunar night is each approx. 14 Earth days. The formal lunar day is therefore the time of a full lunar day-night cycle. Due to tidal locking, this equals the time that the Moon takes to complete one synodic orbit around Earth, a synodic lunar month, returning to the same lunar phase. The synodic period is about 29+1⁄2 Earth days, which is about 2.2 days longer than its sidereal period.

An order of magnitude of time is usually a decimal prefix or decimal order-of-magnitude quantity together with a base unit of time, like a microsecond or a million years. In some cases, the order of magnitude may be implied, like a "second" or "year". In other cases, the quantity name implies the base unit, like "century". In most cases, the base unit is seconds or years.

<span class="mw-page-title-main">Date of Easter</span>

As a moveable feast, the date of Easter is determined in each year through a calculation known as computus. Easter is celebrated on the first Sunday after the Paschal full moon. Determining this date in advance requires a correlation between the lunar months and the solar year, while also accounting for the month, date, and weekday of the Julian or Gregorian calendar. The complexity of the algorithm arises because of the desire to associate the date of Easter with the date of the Jewish feast of Passover which, Christians believe, is when Jesus was crucified.

In astronomy, a Julian year is a unit of measurement of time defined as exactly 365.25 days of 86400 SI seconds each. The length of the Julian year is the average length of the year in the Julian calendar that was used in Western societies until the adoption of the Gregorian Calendar, and from which the unit is named. Nevertheless, because astronomical Julian years are measuring duration rather than designating dates, this Julian year does not correspond to years in the Julian calendar or any other calendar. Nor does it correspond to the many other ways of defining a year.

<span class="mw-page-title-main">Decimal time</span> Representation of the time of day using decimally related units

Decimal time is the representation of the time of day using units which are decimally related. This term is often used specifically to refer to the French Republican calendar time system used in France from 1794 to 1800, during the French Revolution, which divided the day into 10 decimal hours, each decimal hour into 100 decimal minutes and each decimal minute into 100 decimal seconds, as opposed to the more familiar standard time, which divides the day into 24 hours, each hour into 60 minutes and each minute into 60 seconds.

A tropical year or solar year is the time that the Sun takes to return to the same position in the sky – as viewed from the Earth or another celestial body of the Solar System – thus completing a full cycle of astronomical seasons. For example, it is the time from vernal equinox to the next vernal equinox, or from summer solstice to the next summer solstice. It is the type of year used by tropical solar calendars.

<span class="mw-page-title-main">Lunar month</span> Time between successive new moons

In lunar calendars, a lunar month is the time between two successive syzygies of the same type: new moons or full moons. The precise definition varies, especially for the beginning of the month.

<span class="mw-page-title-main">Astronomical basis of the Hindu calendar</span> Applied astronomy of ancient India

The Hindu calendar is based on a geocentric model of the Solar System. A geocentric model describes the Solar System as seen by an observer on the surface of the Earth.

References

  1. ↑ "The International System of Units – 9th edition – Complete text in English and French (2019)". BIPM. Retrieved 22 July 2022.
  2. 1 2 http://starchild.gsfc.nasa.gov/docs/StarChild/questions/question18.html NASA – StarChild Question of the Month for February 2000
  3. ↑ "Meet the zeptosecond, the shortest unit of time ever measured" . Retrieved 2020-10-17.
  4. ↑ Milham, Willis I. (1945). Time and Timekeepers. New York: MacMillan. p. 190. ISBN   0-7808-0008-7.
  5. ↑ Gamez, Christophe (2020-06-17). La décimalisation du temps au prisme du Bureau des longitudes (1875-1901). Entre patriotisme, rationalité et politique (other thesis) (in French). Université de Lorraine.
  6. ↑ "Semester". Webster's Dictionary. Retrieved 3 December 2014.
  7. ↑ McCarthy, Dennis D.; Seidelmann, P. Kenneth (2009). Time: from Earth rotation to atomic physics. Wiley-VCH. p. 18. ISBN   978-3-527-40780-4., Extract of page 18
  8. ↑ "Merriam-Webster Dictionary". Merriam-Webster Incorporated. Retrieved 29 November 2016.
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