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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 9 192 631 770 when expressed in the unit Hz, which is equal to s−1." [1]
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.
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.
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 kilometers).
Name | Length | Notes |
---|---|---|
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 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 | 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. |
kilosecond | 103 s | About 17 minutes. |
hour | 60 min | |
day | 24 h | Longest unit used on stopwatches and countdowns. |
week | 7 d | Historically sometimes also called "sennight". |
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. |
semester | 18 weeks | A division of the academic year. [5] 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 [6] | 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. [7] 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 two thirds of a year | a unit used in astrology, each of them represent a star sign |
terasecond | 1012 s | About 31,709 years. |
megaannum | 106 yr | Also called "Megayear." 1,000 millennia (plural of millennium), or 1 million years (in geology, abbreviated as Ma). |
petasecond | 1015 s | About 31,709,791 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 230,000,000 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 1,000,000,000 years. |
kalpa | 4.32×109 yr | Used in Hindu mythology. About 4,320,000,000 years. |
exasecond | 1018 s | About 31,709,791,983 years. Approximately 2.3 times the current age of the universe. |
zettasecond | 1021 s | About 31,709,791,983,764 years. |
yottasecond | 1024 s | About 31,709,791,983,764,586 years. |
ronnasecond | 1027 s | About 31,709,791,983,764,586,504 years. |
quettasecond | 1030 s | About 31,709,791,983,764,586,504,312 years. |
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.
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.
Generally speaking, 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. A year can also be measured by starting on any other named day of the calendar, and ending on the day before this named day in the following year. This may be termed a "year's time", but not a "calendar year". To reconcile the calendar year with the astronomical cycle certain years contain extra days. The Gregorian year, which is in use in most of the world, begins on January 1 and ends on December 31. It has a length of 365 days in an ordinary year, with 8760 hours, 525,600 minutes, or 31,536,000 seconds; but 366 days in a leap year, with 8784 hours, 527,040 minutes, or 31,622,400 seconds. With 97 leap years every 400 years, the year has an average length of 365.2425 days. Other formula-based calendars can have lengths which are further out of step with the solar cycle: for example, the Julian calendar has an average length of 365.25 days, and the Hebrew calendar has an average length of 365.2468 days. The Lunar Hijri calendar is a lunar calendar consisting of 12 months in a year of 354 or 355 days. The astronomer's mean tropical year, which is averaged over equinoxes and solstices, is currently 365.24219 days, slightly shorter than the average length of the year in most calendars.
The traditional Chinese calendar is a lunisolar calendar, combining the solar, lunar, and other cycles for various social and religious purposes. More recently, in China and Chinese communities the Gregorian calendar has been adopted and adapted in various ways, and is generally the basis for standard civic purposes, but incorporating traditional lunisolar holidays. However, there are many types and subtypes of the Chinese calendar, partly reflecting developments in astronomical observation and horology, with over a millennium plus history. The major modern form is the Gregorian calendar-based official version of the Mainland China, although diaspora versions are also notable in other parts of China and Chinese-influenced cultures; however, aspects of the traditional lunisolar calendar remain popular, 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.
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:
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.
A month is a unit of time, used with calendars, that is approximately as long as a natural orbital period of the Moon; the words month and Moon are cognates. The traditional concept of months arose with the cycle of Moon phases; such lunar months ("lunations") are synodic months and last approximately 29.53 days, making for roughly 12.37 such months in one Earth year. From excavated tally sticks, researchers have deduced that people counted days in relation to the Moon's phases as early as the Paleolithic age. Synodic months, based on the Moon's orbital period with respect to the Earth–Sun line, are still the basis of many calendars today and are used to divide the year.
The second is the unit of time in the International System of Units (SI), 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. "Minute" comes from the Latin pars minuta prima , meaning "first small part", and "second" comes from the pars minuta secunda , "second small part".
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.
Universal Time is a time standard based on Earth's rotation. While originally it was mean solar time at 0° longitude, precise measurements of the Sun are difficult. Therefore, UT1 is computed from a measure of the Earth's angle with respect to the International Celestial Reference Frame (ICRF), called the Earth Rotation Angle. UT1 is the same everywhere on Earth. UT1 is required to follow the relationship
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.
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.
The Indian national calendar, called the Shaka calendar or Śaka calendar, is a solar calendar that is used alongside the Gregorian calendar by The Gazette of India, in news broadcasts by All India Radio, and in calendars and official communications issued by the Government of India. Śaka Samvat is generally 78 years behind the Gregorian Calendar, except from January to March, when it is behind by 79 years.
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.
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.
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.