Unit of time

Table showing quantitative relationships between common units of time

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 second, symbol s, is the SI unit of time. It is defined by taking the fixed numerical value of the cesium frequency Δν_Cs, 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."

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

• Sun-based: the year was the time for the earth to revolve 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 (1/24 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 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 as multiples of seconds.

Units of time based on orders of magnitude of the second include the nanosecond and the millisecond.

Historical

Main article: History of calendars

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.

Horizontal logarithmic scale marked with units of time in the Gregorian calendar

Scientific

• 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 µs 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 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.

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).

List

Units of time

Unit	Length, Duration and Size	Notes
yoctoyoctosecond	10−48 s	One quindecillionth of a second. The smallest unit of time.
zeptoyoctosecond	10−45 s	One quattuordecillionth of a second.
Planck time	5.39×10−44 s	The amount of time light takes to travel one Planck length.
attoyoctosecond	10−42 s	One tredecillionth of a second.
yoctoyoctoannum	10−48 yr	One quindecillionth of a year or 31.536 attoyoctoseconds.
femtoyoctosecond	10−39 s	One duodecillionth of a second.
zeptoyoctoannum	10−45 yr	One quattuordecillionth of a year or 31.536 femtoyoctoseconds.
picoyoctosecond	10−36 s	One undecillionth of a second.
attoyoctoannum	10−42 yr	One tredecillionth of a year or 31.536 picoyoctoseconds.
nanoyoctosecond	10−33 s	One decillionth of a second.
femtoyoctoannum	10−39 yr	One duodecillionth of a year or 31.536 nanoyoctoseconds.
microyoctosecond	10−30 s	One nonillionth of a second.
picoyoctoannum	10−36 yr	One undecillionth of a year or 31.536 microyoctoseconds.
milliyoctosecond	10−27 s	One octillionth of a second.
nanoyoctoannum	10−33 yr	One decillionth of a year or 31.536 milliyoctoseconds.
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.
microyoctoannum	10−30 yr	One nonillionth of a year or 31.536 yoctoseconds.
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.
milliyoctoannum	10−27 yr	One octillionth of a year or 31.536 zeptoseconds.
attosecond	10−18 s	One quintillionth of a second.
yoctoannum	10−24 yr	One septillionth of a year or 31.536 attoseconds.
femtosecond	10−15 s	One quadrillionth of a second. Pulse time on fastest lasers.
zeptoannum	10−21 yr	One sextillionth of a year or 31.536 femtoseconds.
svedberg	10−13 s	Time unit used for sedimentation rates (usually of proteins).
picosecond	10−12 s	One trillionth of a second.
attoannum	10−18 yr	One quintillionth of a year or 31.536 picoseconds.
nanosecond	10−9 s	One billionth of a second. Time for molecules to fluoresce.
femtoannum	10−15 yr	One quadrillionth of a year or 31.536 nanoseconds.
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
picoannum	10−12 yr	One trillionth of a year or 31.536 microseconds.
millisecond	10−3 s	One thousandth of a second. Shortest time unit used on stopwatches.
centisecond	10−2 s	One hundredth of a second.
jiffy (electronics)	1/60 s or 1/50 s	Used to measure the time between alternating power cycles. Also a casual term for a short period of time.
nanoannum	10−9 yr	One billionth of a year or 31.536 milliseconds.
decisecond	10−1 s	One tenth of a second.
semisecond / half second	0.5 s	One half of a second.
second	1 s	SI Base unit.
decasecond	10 s
microannum	10−6 yr	One millionth of a year or 31.536 seconds.
minute	60 s
milliday	1/1000 d	Also marketed as a ".beat" by the Swatch corporation; Milliday is a more accurate name.
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.
hectosecond	100 s	1 minute and 40 seconds
kilosecond	1000 s	16 minutes and 40 seconds
hour	60 min
milliannum	10−3 yr	One thousandth of a year or 31,536 seconds.
day	24 h	Longest unit used on stopwatches and countdowns.
centiannum	10−2 yr	One hundredth of a year or 315,360 seconds.
week	7 d	Also called "sennight".
megasecond	106 s	277.777778333333 hours or about 1 week and 4.6 days.
fortnight	2 weeks	14 days
lunar month	27 d 4 h 48 min – 29 d 12 h	Various definitions of lunar month exist.
month	28–31 d	Occasionally calculated as 30 days.
deciannum	10−1 yr	One tenth of a year or 3,153,600 seconds.
quarter and season	3 mo
quadrimester	4 mo
semester	18 weeks	A division of the academic year. Literally "six months", also used in this sense.
half year	6 mo
lunar year	354.37 days
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	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
biennium	2 yr
triennium	3 yr
quadrennium	4 yr
olympiad	4 yr
lustrum	5 yr
decade	10 yr
indiction	15 yr
gigasecond	109 s	16,666,666.6667 minutes or about 31.7 years.
jubilee	50 yr
century	100 yr
millennium	1000 yr	Also called "kiloannum".
terasecond	1012 s	About 31,709 years.
megaannum	106 yr	Also called "Megayear." 1,000 millennia (plural of millennium), or 1 million years.
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 one time. Around 230,000,000 years.
cosmological decade	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.
gigaannum	109 yr	Also refers to an indefinite period of time, otherwise is 1,000,000,000 years.
exasecond	1018 s	About 31,709,791,983 years.
teraannum	1012 yr	About 1,000,000,000,000 years.
zettasecond	1021 s	About 31,709,791,983,764 years.
petaannum	1015 yr	About 1,000,000,000,000,000 years.
yottasecond	1024 s	About 31,709,791,983,764,584 years.
exaannum	1018 yr	About 1,000,000,000,000,000,000 years.
kiloyottasecond	1027 s	About 3.1709791983765E+19 years.
zettaannum	1021 yr	About 1E+21 years.
megayottasecond	1030 s	About 3.1709791983765E+22 years.
yottaannum	1024 yr	About 1E+24 years.
gigayottasecond	1033 s	About 3.1709791983765E+25 years.
kiloyottaannum	1027 yr	About 1E+27 years.
terayottasecond	1036 s	About 3.1709791983765E+28 years.
megayottaannum	1030 yr	About 1E+30 years.
petayottasecond	1039 s	About 3.1709791983765E+31 years.
gigayottaannum	1033 yr	About 1E+33 years.
exayottasecond	1042 s	About 3.1709791983765E+34 years.
terayottaannum	1036 yr	About 1E+36 years.
zettayottasecond	1045 s	About 3.1709791983765E+37 years.
petayottaannum	1039 yr	About 1E+39 years.
yottayottasecond	1048 s	About 3.1709791983765E+40 years.
exayottaannum	1042 yr	About 1E+42 years.
zettayottaannum	1045 yr	About 1E+45 years.
yottayottaannum	1048 yr	About 1E+48 years. The biggest unit of time.

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.