Unit of time

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 SI definition is "[The second] is defined by taking the fixed numerical value of the cesium frequency, , the unperturbed ground-state hyper-fine transition frequency of the cesium 133 atom, to be when expressed in the unit Hz, which is equal to s^{−1}."

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

• Sun-based: the year is based on the Earth's orbital period around the sun. Historical 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 is based on the Moon's orbital period around the Earth.
• Earth-based: the day is based on the time it takes for the Earth to rotate on its own axis, relative to the Sun. Units originally derived from this base include the week (seven days), and the fortnight (14 days). Subdivisions of the day include the hour (1/24 of a day), which is further subdivided into minutes and seconds. The second is the international standard unit (SI unit) 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 days has to be adjusted with leap days and leap seconds. Consequently, these units are now all defined for scientific purposes as multiples of seconds.

Historical

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 calendar.

Scientific

• The Planck time is the time that light takes to travel one Planck length. (5.391 247 × 10⁻⁴⁴ sec., Full Form: .000 000 000 000 000 000 000 000 000 000 000 000 000 000 053 912 47 sec.)
• The Jiffy is the amount of time light takes to travel one femtometre (about the diameter of a nucleon).
• The atomic time relates to the orbital period of a ground state electron around a hydrogen atom and is about 24.2 attoseconds.
• The svedberg is a time unit used for sedimentation rates (usually of proteins). It is defined as 10^{−13} seconds (100 fs).
• The TU (for time unit) is a unit of time defined as 1024 ÃŽÂ¼s for use in engineering.
• 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. A table with approximate time lengths can be seen at Divisions of geologic time

Note: The light-year is not a unit of time, but a unit of length of about 9.5 petametres ().

Note: The parsec is not a unit of time, but a unit of length of about 30.9 trillion kilometres, despite movie references otherwise.

List

Interrelation

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.

See also

• Unit of frequency
• Orders of magnitude (time)

References