Unit of time
Encyclopedia
Units of measurement
for time
have historically been based on the movement of the Sun
(as seen from Earth
; giving the solar day and the year
) and the Moon
(giving the month
). Shorter intervals were measured by physiological periods such as drawing breath, wink
ing or the heartbeat
.
Units of time consisting of a number of years include the lustrum
(five years) and the olympiad
(four years). The month could be divided into half-month
s or fortnight
s, and quarters or week
s.
Longer periods were given in lifetimes or generations (saecula, aion
), subdivisions of the solar day in hour
s. The Sothic cycle
was a period of 1,461 years of 365 days in the Ancient Egyptian calendar. Medieval (Pauranic) Hindu cosmology
is notorious for introducing names for fabulously long time periods, such as kalpa (4.32 billion years).
In classical antiquity, the hour
divided the daylight period into 12 equal parts. The duration of an hour thus varied over the course of the year.
In classical China, the kè
(刻) was a unit of decimal time
, dividing a day into 100 equal intervals of 14.4 minutes. Alongside the ke, there were double hours (shíchen) also known as watches. Because one cannot divide 12 double hours into 100 ke evenly, each ke was subdivided into 60 fēn (分).
The introduction of the minute
(minuta; ′) as the 60th part of an hour, the second
(seccunda; ′′) as the 60th part of a minute, and the third (tertia; ′′′) as the 60th part of the second dates to the medieval period, used by Al-Biruni
around AD 1000, and by Roger Bacon
in the 13th century. Bacon further subdivided the tertia into a quarta or fourth (′′′′).
Hindu chronology
divides the civil day (daylight hours) into vipalas, palas and ghatikas. A tithi
is the 30th part of the synodic month.
The introduction of the division of the solar day into 24 hours of equal length, as it were the length of a classical hour at equinox
used regardless of daylight hours, dates to the 14th century, due to the development of the first mechanical clocks.
Today, the fundamental unit of time suggested by the International System of Units
is the second
, since 1967 defined as the second of International Atomic Time
, based on the radiation emitted by a Caesium-133 atom in the ground state. Its definition is still so calibrated that 86,400 seconds correspond to a solar day. 31,557,600 (86,400 × 365.25) seconds are a Julian year
, exceeding the true length of a solar year by about 21 ppm
.
Based on the second as the base unit, the following time units are in use:
There are a number of proposals for decimal time
, or decimal calendar
s, notably in the French Republican Calendar
of 1793. Such systems have either ten days per week, a multiple of ten days in a month, or ten months per year.
A suggestion for hexadecimal time
divides the Julian day into 16 hexadecimal hours of 1h 30 min each, or 65,536 hexadecimal seconds (1 hexsec ≈ 1.32 s).
The Planck time
(tP) is a natural unit of time, the shortest possible interval that can be meaningfully considered in quantum mechanics
. tP equals about 5.4 × 10−44 s.
Units of measurement
A unit of measurement is a definite magnitude of a physical quantity, defined and adopted by convention and/or by law, that is used as a standard for measurement of the same physical quantity. Any other value of the physical quantity can be expressed as a simple multiple of the unit of...
for time
Time
Time is a part of the measuring system used to sequence events, to compare the durations of events and the intervals between them, and to quantify rates of change such as the motions of objects....
have historically been based on the movement of the Sun
Sun
The Sun is the star at the center of the Solar System. It is almost perfectly spherical and consists of hot plasma interwoven with magnetic fields...
(as seen from Earth
Geocentric model
In astronomy, the geocentric model , is the superseded theory that the Earth is the center of the universe, and that all other objects orbit around it. This geocentric model served as the predominant cosmological system in many ancient civilizations such as ancient Greece...
; giving the solar day and the year
Year
A year is the orbital period of the Earth moving around the Sun. For an observer on Earth, this corresponds to the period it takes the Sun to complete one course throughout the zodiac along the ecliptic....
) and the Moon
Moon
The Moon is Earth's only known natural satellite,There are a number of near-Earth asteroids including 3753 Cruithne that are co-orbital with Earth: their orbits bring them close to Earth for periods of time but then alter in the long term . These are quasi-satellites and not true moons. For more...
(giving the month
Month
A month is a unit of time, used with calendars, which was first used and invented in Mesopotamia, as a natural period related to the motion of the Moon; month and Moon are cognates. The traditional concept arose with the cycle of moon phases; such months are synodic months and last approximately...
). Shorter intervals were measured by physiological periods such as drawing breath, wink
Wink
A wink is a facial expression made by briefly closing one eye. A wink is an informal mode of non-verbal communication usually signalling shared hidden knowledge or intent, which may also include, in some contexts, sexual attraction....
ing or the heartbeat
Heartbeat
Heartbeat may refer to:* Cardiac cycle of the heart* Heart sounds* Pulse* The interval for keeping alive a push e-mail connection* Heartbeat , a British police drama* HeartBeat Heartbeat may refer to:* Cardiac cycle of the heart* Heart sounds* Pulse* The interval for keeping alive a push e-mail...
.
Units of time consisting of a number of years include the lustrum
Lustrum
A lustrum was a term for a five-year period in Ancient Rome.The lustration was originally a sacrifice for expiation and purification offered by one of the censors in the name of the Roman people at the close of the taking of the census...
(five years) and the olympiad
Olympiad
An Olympiad is a period of four years, associated with the Olympic Games of Classical Greece. In the Hellenistic period, beginning with Ephorus, Olympiads were used as calendar epoch....
(four years). The month could be divided into half-month
Half-month
A half-month is a unit of time typically used in astronomy. Each month is separated into two parts:*days 1-15*day 16 through the end of the month....
s or fortnight
Fortnight
The fortnight is a unit of time equal to fourteen days, or two weeks. The word derives from the Old English fēowertyne niht, meaning "fourteen nights"....
s, and quarters or week
Week
A week is a time unit equal to seven days.The English word week continues an Old English wice, ultimately from a Common Germanic , from a root "turn, move, change"...
s.
Longer periods were given in lifetimes or generations (saecula, aion
Aion
Aion may refer to:In music:*Aion , a Japanese metal band**Aion , their 1992 album*Aion , a Polish gothic metal band*Aion , a 2003 album...
), subdivisions of the solar day in hour
Hour
The hour is a unit of measurement of time. In modern usage, an hour comprises 60 minutes, or 3,600 seconds...
s. The Sothic cycle
Sothic cycle
The Sothic cycle or Canicular period is a period of 1,461 ancient Egyptian years or 1,460 Julian years...
was a period of 1,461 years of 365 days in the Ancient Egyptian calendar. Medieval (Pauranic) Hindu cosmology
Hindu cosmology
In Hindu cosmology the universe is, according to Hindu mythology and Vedic cosmology, cyclically created and destroyed.-Description:The Hindu cosmology and timeline is the closest to modern scientific timelines and even more which might indicate that the Big Bang is not the beginning of everything...
is notorious for introducing names for fabulously long time periods, such as kalpa (4.32 billion years).
In classical antiquity, the hour
Hour
The hour is a unit of measurement of time. In modern usage, an hour comprises 60 minutes, or 3,600 seconds...
divided the daylight period into 12 equal parts. The duration of an hour thus varied over the course of the year.
In classical China, the kè
Ke (unit)
The ke is a traditional Chinese unit of decimal time lasting approximately a quarter of a western hour. Traditionally the ke divides a day into 100 equal intervals of 14.4 minutes . The ke is equivalent to the centiday , a non-SI prefixed unit...
(刻) was a unit of decimal time
Decimal time
Decimal time is the representation of the time of day using units which are decimally related. This term is often used to refer specifically to French Revolutionary Time, which divides the day into 10 decimal hours, each decimal hour into 100 decimal minutes and each decimal minute into 100 decimal...
, dividing a day into 100 equal intervals of 14.4 minutes. Alongside the ke, there were double hours (shíchen) also known as watches. Because one cannot divide 12 double hours into 100 ke evenly, each ke was subdivided into 60 fēn (分).
The introduction of the minute
Minute
A minute is a unit of measurement of time or of angle. The minute is a unit of time equal to 1/60th of an hour or 60 seconds. In the UTC time scale, a minute on rare occasions has 59 or 61 seconds; see leap second. The minute is not an SI unit; however, it is accepted for use with SI units...
(minuta; ′) as the 60th part of an hour, the second
Second
The second is a unit of measurement of time, and is the International System of Units base unit of time. It may be measured using a clock....
(seccunda; ′′) as the 60th part of a minute, and the third (tertia; ′′′) as the 60th part of the second dates to the medieval period, used by Al-Biruni
Al-Biruni
Abū al-Rayḥān Muḥammad ibn Aḥmad al-BīrūnīArabic spelling. . The intermediate form Abū Rayḥān al-Bīrūnī is often used in academic literature...
around AD 1000, and by Roger Bacon
Roger Bacon
Roger Bacon, O.F.M. , also known as Doctor Mirabilis , was an English philosopher and Franciscan friar who placed considerable emphasis on the study of nature through empirical methods...
in the 13th century. Bacon further subdivided the tertia into a quarta or fourth (′′′′).
Hindu chronology
Hindu chronology
Hindu chronology may refer to:*Indian astronomy*Hindu calendar*Yuga - in Hindu philosophy, the name of an "epoch" or "era" within a cycle of four ages...
divides the civil day (daylight hours) into vipalas, palas and ghatikas. A tithi
Tithi
In vedic timekeeping, a tithi is a lunar day, or the time it takes for the longitudinal angle between the moon and the sun to increase by 12°. Tithis begin at varying times of day and vary in duration from approximately 19 to approximately 26 hours. There are 30 tithis in each lunar month, named...
is the 30th part of the synodic month.
The introduction of the division of the solar day into 24 hours of equal length, as it were the length of a classical hour at equinox
Equinox
An equinox occurs twice a year, when the tilt of the Earth's axis is inclined neither away from nor towards the Sun, the center of the Sun being in the same plane as the Earth's equator...
used regardless of daylight hours, dates to the 14th century, due to the development of the first mechanical clocks.
Today, the fundamental unit of time suggested by the International System of Units
International System of Units
The International System of Units is the modern form of the metric system and is generally a system of units of measurement devised around seven base units and the convenience of the number ten. The older metric system included several groups of units...
is the second
Second
The second is a unit of measurement of time, and is the International System of Units base unit of time. It may be measured using a clock....
, since 1967 defined as the second of International Atomic Time
International Atomic Time
International Atomic Time is a high-precision atomic coordinate time standard based on the notional passage of proper time on Earth's geoid...
, based on the radiation emitted by a Caesium-133 atom in the ground state. Its definition is still so calibrated that 86,400 seconds correspond to a solar day. 31,557,600 (86,400 × 365.25) seconds are a Julian year
Julian year
A Julian year can refer to:* The Julian year is a time interval of exactly 365.25 Earth days, used in astronomy.* The Julian year is a year in the Julian calendar which has started on different days, at different times, in different countries and is equal to either 365 or 366 days, or 365.25 days...
, exceeding the true length of a solar year by about 21 ppm
PPM
- Culture :*Peter, Paul and Mary, a 1960s folk music trio*Picture Postcard Monthly, a magazine for collectors of postcards*Please Please Me, the first album by The Beatles- Health :*Permanent pacemaker or artificial pacemaker...
.
Based on the second as the base unit, the following time units are in use:
- minuteMinuteA minute is a unit of measurement of time or of angle. The minute is a unit of time equal to 1/60th of an hour or 60 seconds. In the UTC time scale, a minute on rare occasions has 59 or 61 seconds; see leap second. The minute is not an SI unit; however, it is accepted for use with SI units...
(1 min = 60 s) - hourHourThe hour is a unit of measurement of time. In modern usage, an hour comprises 60 minutes, or 3,600 seconds...
(1 h = 60 min = 3.6 ks) - Julian dayJulian dayJulian day is used in the Julian date system of time measurement for scientific use by the astronomy community, presenting the interval of time in days and fractions of a day since January 1, 4713 BC Greenwich noon...
(1 d = 24 h = 86.4 ks) - weekWeekA week is a time unit equal to seven days.The English word week continues an Old English wice, ultimately from a Common Germanic , from a root "turn, move, change"...
(7 d = 604.8 ks) - Julian yearJulian yearA Julian year can refer to:* The Julian year is a time interval of exactly 365.25 Earth days, used in astronomy.* The Julian year is a year in the Julian calendar which has started on different days, at different times, in different countries and is equal to either 365 or 366 days, or 365.25 days...
(1 a = 365.25 d = 31.5576 MsMegasecondA megasecond is 1 million seconds, or roughly 11.6 days. There are roughly 31.5 megaseconds in a year. This page lists times between 1 and 1000 megaseconds , or 11.6 days and 31.7 years.*shorter times-1 year:...
) - centuryCenturyA century is one hundred consecutive years. Centuries are numbered ordinally in English and many other languages .-Start and end in the Gregorian Calendar:...
(100 a = 3.15576 GsGigasecondA gigasecond is 1 billion seconds, or roughly 31.7 years. This table lists times of between 1 and 1000 gigaseconds, or between 31.7 and 31,700 years...
) - millenniumMillenniumA millennium is a period of time equal to one thousand years —from the Latin phrase , thousand, and , year—often but not necessarily related numerically to a particular dating system....
(1 ka = 31.5576 Gs)
There are a number of proposals for decimal time
Decimal time
Decimal time is the representation of the time of day using units which are decimally related. This term is often used to refer specifically to French Revolutionary Time, which divides the day into 10 decimal hours, each decimal hour into 100 decimal minutes and each decimal minute into 100 decimal...
, or decimal calendar
Decimal calendar
A decimal calendar contains either ten days per week, a multiple of ten days in a month, or ten months per year. Examples that have been adopted are the calendar of Romulus, the Egyptian calendar, the Alexandrian calendar, the Coptic calendar, the Ethiopian calendar, and the French Republican...
s, notably in the French Republican Calendar
French Republican Calendar
The French Republican Calendar or French Revolutionary Calendar was a calendar created and implemented during the French Revolution, and used by the French government for about 12 years from late 1793 to 1805, and for 18 days by the Paris Commune in 1871...
of 1793. Such systems have either ten days per week, a multiple of ten days in a month, or ten months per year.
A suggestion for hexadecimal time
Hexadecimal time
Hexadecimal time is the representation of the time of day as a hexadecimal number in the interval [0,1). The day is divided in 1016 hexadecimal hours, each hour in 10016 hexadecimal minutes and each minute in 1016 hexadecimal seconds.This time format was first proposed by the...
divides the Julian day into 16 hexadecimal hours of 1h 30 min each, or 65,536 hexadecimal seconds (1 hexsec ≈ 1.32 s).
The Planck time
Planck time
In physics, the Planck time, , is the unit of time in the system of natural units known as Planck units. It is the time required for light to travel, in a vacuum, a distance of 1 Planck length...
(tP) is a natural unit of time, the shortest possible interval that can be meaningfully considered in quantum mechanics
Quantum mechanics
Quantum mechanics, also known as quantum physics or quantum theory, is a branch of physics providing a mathematical description of much of the dual particle-like and wave-like behavior and interactions of energy and matter. It departs from classical mechanics primarily at the atomic and subatomic...
. tP equals about 5.4 × 10−44 s.
List
| Unit | Size | Notes |
|---|---|---|
| yoctosecond | 10−24 s | |
| zeptosecond | 10−21 s | |
| attosecond | 10−18 s | shortest time now measurable |
| femtosecond | 10−15 s | pulse time on fastest lasers |
| picosecond Picosecond A picosecond is 10−12 of a second. That is one trillionth, or one millionth of one millionth of a second, or 0.000 000 000 001 seconds. A picosecond is to one second as one second is to 31,700 years.... |
10−12 s | |
| nanosecond Nanosecond A nanosecond is one billionth of a second . One nanosecond is to one second as one second is to 31.7 years.The word nanosecond is formed by the prefix nano and the unit second. Its symbol is ns.... |
10−9 s | time for molecules to fluoresce |
| microsecond Microsecond A microsecond is an SI unit of time equal to one millionth of a second. Its symbol is µs.A microsecond is equal to 1000 nanoseconds or 1/1000 millisecond... |
10−6 s | |
| millisecond Millisecond A millisecond is a thousandth of a second.10 milliseconds are called a centisecond.... |
0.001 s | |
| second Second The second is a unit of measurement of time, and is the International System of Units base unit of time. It may be measured using a clock.... |
1 s | SI Si Si, si, or SI may refer to :- Measurement, mathematics and science :* International System of Units , the modern international standard version of the metric system... base unit |
| minute Minute A minute is a unit of measurement of time or of angle. The minute is a unit of time equal to 1/60th of an hour or 60 seconds. In the UTC time scale, a minute on rare occasions has 59 or 61 seconds; see leap second. The minute is not an SI unit; however, it is accepted for use with SI units... |
60 seconds | |
| ke Ke (unit) The ke is a traditional Chinese unit of decimal time lasting approximately a quarter of a western hour. Traditionally the ke divides a day into 100 equal intervals of 14.4 minutes . The ke is equivalent to the centiday , a non-SI prefixed unit... |
~15 minutes | |
| hour Hour The hour is a unit of measurement of time. In modern usage, an hour comprises 60 minutes, or 3,600 seconds... |
60 minutes | |
| day Day A day is a unit of time, commonly defined as an interval equal to 24 hours. It also can mean that portion of the full day during which a location is illuminated by the light of the sun... |
24 hours | |
| week Week A week is a time unit equal to seven days.The English word week continues an Old English wice, ultimately from a Common Germanic , from a root "turn, move, change"... |
7 days | Also called sennight |
| fortnight Fortnight The fortnight is a unit of time equal to fourteen days, or two weeks. The word derives from the Old English fēowertyne niht, meaning "fourteen nights".... |
14 days | 2 weeks |
| lunar month Lunar month In lunar calendars, a lunar month is the time between two identical syzygies . There are many variations. In Middle-Eastern and European traditions, the month starts when the young crescent moon becomes first visible at evening after conjunction with the Sun one or two days before that evening... |
27.2–29.5 days | Various definitions of lunar month exist. |
| month Month A month is a unit of time, used with calendars, which was first used and invented in Mesopotamia, as a natural period related to the motion of the Moon; month and Moon are cognates. The traditional concept arose with the cycle of moon phases; such months are synodic months and last approximately... |
28–31 days | |
| quarter | 3 months | |
| year Year A year is the orbital period of the Earth moving around the Sun. For an observer on Earth, this corresponds to the period it takes the Sun to complete one course throughout the zodiac along the ecliptic.... |
12 months | |
| common year Year A year is the orbital period of the Earth moving around the Sun. For an observer on Earth, this corresponds to the period it takes the Sun to complete one course throughout the zodiac along the ecliptic.... |
365 days | 52 weeks + 1 day |
| leap year Leap year A leap year is a year containing one extra day in order to keep the calendar year synchronized with the astronomical or seasonal year... |
366 days | 52 weeks + 2 days |
| tropical year Tropical year A tropical year , for general purposes, is the length of time that the Sun takes to return to the same position in the cycle of seasons, as seen from Earth; for example, the time from vernal equinox to vernal equinox, or from summer solstice to summer solstice... |
365.24219 days | average |
| Gregorian year Gregorian calendar The Gregorian calendar, also known as the Western calendar, or Christian calendar, is the internationally accepted civil calendar. It was introduced by Pope Gregory XIII, after whom the calendar was named, by a decree signed on 24 February 1582, a papal bull known by its opening words Inter... |
365.2425 days | average |
| Olympiad Olympiad An Olympiad is a period of four years, associated with the Olympic Games of Classical Greece. In the Hellenistic period, beginning with Ephorus, Olympiads were used as calendar epoch.... |
4 year cycle | |
| lustrum Lustrum A lustrum was a term for a five-year period in Ancient Rome.The lustration was originally a sacrifice for expiation and purification offered by one of the censors in the name of the Roman people at the close of the taking of the census... |
5 years | Also called pentad |
| decade Decade A decade is a period of 10 years. The word is derived from the Ancient Greek dekas which means ten. This etymology is sometime confused with the Latin decas and dies , which is not correct.... |
10 years | |
| Indiction Indiction An indiction is any of the years in a 15-year cycle used to date medieval documents throughout Europe, both East and West. Each year of a cycle was numbered: first indiction, second indiction, etc... |
15 year cycle | |
| generation Generation Generation , also known as procreation in biological sciences, is the act of producing offspring.... |
17–35 years | approximate |
| jubilee (Biblical) Jubilee (Biblical) The Jubilee year is the year at the end of seven cycles of Sabbatical years , and according to Biblical regulations had a special impact on the ownership and management of land in the territory of the kingdoms of Israel and of Judah; there is some debate whether it was the 49th year The Jubilee... |
50 years | |
| century Century A century is one hundred consecutive years. Centuries are numbered ordinally in English and many other languages .-Start and end in the Gregorian Calendar:... |
100 years | |
| millennium Millennium A millennium is a period of time equal to one thousand years —from the Latin phrase , thousand, and , year—often but not necessarily related numerically to a particular dating system.... |
1,000 years | |
| exasecond | 1018 s | roughly 32 billion years, more than twice the age of the universe on current estimates |
| cosmological decade Cosmological decade A cosmological decade is a division of the lifetime of the cosmos. The divisions are logarithmic in size, on base 10. Each successive cosmological decade represents a ten-fold increase in the total age of the universe.... |
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. |