Unit of time

Visualisation of units of time from 1 second to 1 average year of the Gregorian calendar

Units of measurement for time have historically been based on the movement of the Sun (as seen from Earth). Shorter intervals were measured by physiological periods such as drawing breath, winking or the pulse.

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-months or fortnights, and quarters or weeks. Longer periods were given in lifetimes or generations (saecula, aion), subdivisions of the solar day in hours. 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 was calibrated such that 86,400 seconds corresponded 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 as follows:

• minute (1 min = 60 sec)
• hour (1 hr = 60 min = 3.6 ks)
• Julian day (1 d = 24 hr = 86.4 ks)
• week (7 d = 604.8 ks)
• Julian year (1 a = 365.25 d = 31.5576 Ms)
• decade (10 years/annum)
• century (100 annum = 3.15576 Gs)
• millennium (1 ka = 31.5576 Gs)
• robperry (a seemingly unending two minutes)

There are a number of proposals for decimal time, or decimal calendars, 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 1hr 30 min each, or 65,536 hexadecimal seconds (1 hexsec ≈ 1.32 s).

The Planck time (t_P) is a natural unit of time, the shortest possible interval that can be meaningfully considered in quantum mechanics. t_P equals about 5.4 × 10⁻⁴⁴ s.

Horizontal logarithmic scale marked with units of time according to the internationally accepted Gregorian calendar.

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
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
fourth	1/3,600 second	medieval unit of time
millisecond	0.001 s	shortest time unit used on stopwatches
centisecond	0.01 s	used on some stopwatches
third	1/60 s	medieval unit of time
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
decisecond	0.1 s	used on some stopwatches
second	1 sec	SI base unit
dekasecond	10 seconds
minute	60 seconds
dekaminute	600 seconds or 10 minutes
moment	1/40th of an hour (~90 seconds)	medieval unit of time used by astronomers to compute astronomical movements.
hectosecond	100 seconds	1 minute and 40 seconds
ke	14 minutes and 24 seconds	usually calculated as 15 minutes nowadays
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
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 + 1 day
tropical year	365 days 5:48:45.216 hours	average
Gregorian year	365 days 5:49:12 hours	average
sidereal year	365 days 6:09:09.7635456 hours
leap year	366 days	52 weeks + 2 days
biennium	2 years	A unit of time commonly used by legislatures
triennium	3 years
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
generation	17–35 years
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
age and megaannum	1,000,000 years
epoch	10,000,000 years
petasecond	1 quadrillion seconds	About 3.17 epoches
era	100,000,000 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.
eon or aeon	500,000,000 years or 1,000,000,000 years (in astronomy)	Also "An indefinite and very long period of time"
gigaannum	1,000,000,000 years
exasecond	1 quintillion seconds	roughly 31.7 billion years, more than twice the age of the universe on current estimates
zettasecond	1 sextillion seconds	About 31.7 trillion years
yottasecond	1 septillion seconds	About 31.7 quadrillion 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.

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.

See also

• Time standard
• Orders of magnitude (time)
• Hindu time units