Unit of time

Visualisation of units of time from one second to one average year of the Gregorian calendar

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 definition, from the National Institute of Standards and Technology is:

The duration of 9192631770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the cesium 133 atom.

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

• Sun based: the year was the time for the earth to rotate 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 (1/24th of a day) which was further subdivided into seconds and minutes.
• 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 12 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 365.25 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, 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 time units

• 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 which 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.

The light-year is not a unit of time, but a unit of length about 9 trillion kilometres.

List

Units of time

Unit	Length, Duration and Size	Notes
Planck time unit	5.39 x 10−44 s	The amount of time light takes to travel one Planck length. Theoretically, this is the smallest time measurement that will ever be possible.[3] Smaller time units have no use in physics as we understand it today.
yoctosecond	10−24 s
jiffy (physics)	3 × 10−24s	The amount of time light takes to travel one fermi (about the size of a nucleon) in a vacuum.
zeptosecond	10−21 s	Time measurement scale of the NIST strontium atomic clock
attosecond	10−18 s	Shortest time now measurable.
femtosecond	10−15 s	Pulse time on fastest lasers.
Svedberg	10−13 s	Time unit used for sedimentation rates (usually of proteins).
picosecond	10−12 s
nanosecond	10−9 s	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	Symbol is µs
millisecond	0.001 s	Shortest time unit used on stopwatches.
jiffy (electronics)	1/60s to 1/50s	Used to measure the time between alternating power cycles. Also a casual term for a short period of time.
second	1 sec	SI Base unit.
minute	60 seconds
moment	1/40th of an hour (~90 seconds)	Medieval unit of time used by astronomers to compute astronomical movements.
ke	14 minutes and 24 seconds	Usually calculated as 15 minutes, similar to "quarter" as in "a quarter past six" (6:15).
kilosecond	1,000 seconds	16 minutes and 40 seconds.
hour	60 minutes
day	24 hours	Longest unit used on stopwatches and countdowns.
week	7 days	Also called "sennight".
megasecond	1,000,000 seconds	About 11.6 days.
fortnight	2 weeks	14 days
lunar month	27 Days 4 hours 48 minutes–29 days 12 hours	Various definitions of lunar month exist.
month	28–31 days	Occasionally calculated as 30 days.
semester	an 18-week division of the academic year	Literally "six months", also used in this sense.
quarter and season	3 months
year	12 months or 365 days
common year	365 days	52 weeks and 1 day.
tropical year	365 days and 5:48:45.216 hours	Average.
Gregorian year	365 days and 5:49:12 hours	Average.
sidereal year	365 days and 6:09:09.7635456 hours
leap year	366 days	52 weeks and 2 days.
Olympiad	4 year cycle	48 months, 1,461 days, 35,064 hours, 2,103,840 minutes, 126,230,400 seconds.
lustrum	5 years
decade	10 years
Indiction	15 year cycle
gigasecond	1,000,000,000 seconds	About 31.7 years.
jubilee	50 years
century	100 years
millennium	1,000 years	Also called "kiloannum".
terasecond	1 trillion seconds	About 31,700 years.
galactic year	Approximately 230 million years	The amount of time it takes the Solar System to orbit the center of the Milky Way Galaxy one time.
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.
aeon	1,000,000,000 years or an indefinite period of time	Also spelled "eon"

Units of time interrelated

Flowchart illustrating the major units of time

All of the important units of time can be interrelated. The key units are the second, defined in terms of an atomic process; the day, an integral multiple of seconds; and the year, usually 365.25 days. Most of the other units used are multiples or divisions of these three. The graphic also shows the three heavenly bodies whose orbital parameters relate to the units of time.