Unit of time

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 around 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, Δν_Cs, the unperturbed ground-state hyper-fine transition frequency of the cesium 133 atom, to be when expressed in the unit Hz, which is equal to s⁻¹."

Historically, many units of time were defined by the movements of astronomical objects.

• Sun-based: the year is based on the Earth's orbital period around the sun. Historical 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 is based on the Moon's orbital period around the Earth.
• Earth-based: the day is based on the time it takes for the Earth to rotate on its own axis, relative to the Sun. Units originally derived from this base include the week (seven days), and the fortnight (14 days). Subdivisions of the day include the hour (1/24 of a day), which is further subdivided into minutes and seconds. The second is the international standard unit (SI unit) 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 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 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.

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.

Scientific

• The Planck time is the time that light takes to travel one Planck length. (5.391 247 × 10⁻⁴⁴ sec., Full Form: .000 000 000 000 000 000 000 000 000 000 000 000 000 000 053 912 47 sec.)
• The Jiffy is the amount of time light takes to travel one femtometre (about the diameter of a nucleon).
• The atomic time relates to the orbital period of a ground state electron around a hydrogen atom and is about 24.2 attoseconds.
• The svedberg is a time unit used for sedimentation rates (usually of proteins). It is defined as 10⁻¹³ seconds (100 fs).
• The TU (for time unit) is a unit of time defined as 1024 μs for use in engineering.
• The galactic year, based on the rotation of the galaxy and usually measured in million years.
• The geological time scale relates stratigraphy to time. The deep time of Earth's past is divided into units according to events that 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. A table with approximate time lengths can be seen at Divisions of geologic time

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

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

Name	Length	Notes
Planck time	~	The amount of time light takes to travel one Planck length.
quectosecond		One nonillionth of a second.
rontosecond		One octillionth of a second.
yoctosecond		One septillionth of a second.
jiffy (physics)		The amount of time light takes to travel one fermi (about the size of a nucleon) in a vacuum.
zeptosecond		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.
attosecond		One quintillionth of a second.
atomic time	~	Derived from atomic theory of hydrogen.
femtosecond		One quadrillionth of a second.
svedberg		100 femtoseconds, time unit used for sedimentation rates (usually of proteins).
picosecond		One trillionth of a second.
nanosecond		One billionth of a second. Time for molecules to fluoresce.
shake		10 nanoseconds, also a casual term for a short period of time.
microsecond		One millionth of a second. Symbol is μs
millisecond		One thousandth of a second. Shortest time unit used on stopwatches.
centisecond		One hundredth of a second.
jiffy (electronics)	~	Used to measure the time between alternating power cycles.
decisecond		One tenth of a second.
second	1 s	SI base unit for time
decasecond
minute
hectosecond		1 minute and 40 seconds
milliday	()	1.44 minutes, or 86.4 seconds. Also marketed as a ".beat" by the Swatch corporation.
moment	( on average)	Medieval unit of time used by astronomers to compute astronomical movements, length varies with the season. Also colloquially refers to a brief period of time.
centiday	(1 % of a day)	14.4 minutes, or 864 seconds. One-hundredth of a day is 1 cd (centiday), also called "kè" in traditional Chinese timekeeping. The unit was also proposed by Lagrange and endorsed by Rey-Pailhade in the 19th century, named "centijours" (from French centi- 'hundred' and jour 'day').
kilosecond		$16.\bar{6}$ minutes.
hour		60 minutes
deciday	(10 % of a day)	2.4 hours, or 144 minutes. One-tenth of a day is 1 dd (deciday), also called "gēng" in traditional Chinese timekeeping.
day		The SI day is exactly 86 400 seconds.
week		Historically sometimes also called "sennight".
decaday	()	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		$11.5\overline{740}$ days.
fortnight		14 days
lunar month	–	There are several definitions of lunar month
month		Occasionally calculated as 30 days.
hectoday	()	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		A division of the academic year. Literally "six months", also used in this sense.
lunar year		12 Lunar months instead of normal months
common year		and
tropical year		The time it takes for the Sun to return to the same position in the sky relative to the earth or other planetary bodies
Gregorian year	365 or 366 days
Julian year		The Julian year, as used in astronomy and other sciences, is a time unit now defined as exactly 365.25 days of 86400 SI seconds each.
sidereal year		The time it takes for the Earth to complete one full revolution around the Sun relative to the background stars
leap year		and that happens every 4 years where there is an extra day in February to make up the day lost in the Gregorian calendar
olympiad		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. 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.
lustrum		In early Roman times, the interval between censuses.
decade
indiction		Interval for taxation assessments (Roman Empire).
gigasecond		About 31.71 years.
century
millennium		Also called "kiloannum".
Age	$2148.\bar{6}$ years	A superstitious unit of time used in astrology, each of them representing a star sign.
Great Year		Gradual shift in the orientation of Earth's axis of rotation in a cycle of approximately 26,000 years. At present, the rate of precession corresponds to a period of 25,772 years, so a tropical year is shorter than a sidereal year by 1,224.5 seconds (20 min 24.5 sec ≈ (365.24219 × 86400) / 25772).
terasecond		About 31 710 years.
megaannum		Also called "megayear". 1000 millennia (plural of millennium), or 1 million years (in geology, abbreviated as Ma).
petasecond		About 31 709 792 years.
Galactic year		The amount of time it takes the Solar System to orbit the center of the Milky Way Galaxy (approx years).
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		Also refers to an indefinite period of time, otherwise is years.
kalpa		Used in Hindu mythology. About years.
exasecond		About 31 709 791 984 years. Approximately 2.3 times the current age of the universe.

Interrelation

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 three.

See also

• Unit of frequency
• Orders of magnitude (time)