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, winking 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-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 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 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 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.
| 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 | 10−12 s | |
| nanosecond | 10−9 s | time for molecules to fluoresce |
| microsecond | 10−6 s | |
| millisecond | 0.001 s | |
| second | 1 s | SI base unit |
| minute | 60 seconds | |
| ke | ~15 minutes | |
| hour | 60 minutes | |
| day | 24 hours | |
| week | 7 days | Also called sennight |
| fortnight | 14 days | 2 weeks |
| lunar month | 27.2–29.5 days | Various definitions of lunar month exist. |
| month | 28–31 days | |
| quarter | 3 months | |
| year | 12 months | |
| common year | 365 days | 52 weeks + 1 day |
| leap year | 366 days | 52 weeks + 2 days |
| tropical year | 365.24219 days[1] | average |
| Gregorian year | 365.2425 days[2] | average |
| Olympiad | 4 year cycle | |
| lustrum | 5 years | Also called pentad |
| decade | 10 years | |
| Indiction | 15 year cycle | |
| generation | 17–35 years | approximate |
| jubilee (Biblical) | 50 years | |
| century | 100 years | |
| millennium | 1,000 years | |
| exasecond | 1018 s | roughly 32 billion years, more than twice the age of the universe on current estimates |
| 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. |
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