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)

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 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

References