GeezerVison is showing en.wikipedia.org — Unit of time - Wikipedia
Table showing quantitative relationships between common units of time[1]
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 around 9 billion oscillations of the caesium atom.[2] The exact modern SI definition is "[The second] is defined by taking the fixed numerical value of the cesium frequency, ΔνCs, the unperturbed ground-state hyper-fine transition frequency of the cesium 133 atom, to be 9192631770 when expressed in the unit Hz, which is equal to s−1."[2]
Historically, many units of time were defined by the movements of astronomical objects.
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.
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 365.2425 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.
The Planck time is the time that light takes to travel one Planck length. (5.391 247 × 10⁻⁴⁴ sec., Full Form: 0.000 000 000 000 000 000 000 000 000 000 000 000 000 000 053 912 47 sec.)[12][13]
The Jiffy is the amount of time light takes to travel one femtometre (about the diameter of a nucleon).
The svedberg is a time unit used for sedimentation rates (usually of proteins). It is defined as 10−13 seconds (100fs).[15]
The TU (for time unit) is a unit of time defined as 1024μs for use in engineering.
The galactic year, based on the rotation of the galaxy and usually measured in million years.[16]
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[17]
Note: The light-year is not a unit of time, but a unit of length of about 9.5 petametres (9454254955488km).[18]
Note: The parsec is not a unit of time, but a unit of length of about 30.9 trillion kilometres, despite movie references otherwise.[19]
One sextillionth of a second. Time measurement scale of the NIST and JILA strontium atomic clock. Smallest fragment of time currently measurable is 247 zeptoseconds.[22][23]
Medieval unit of time used by astronomers to compute astronomical movements, length varies with the season.[30] Also colloquially refers to a brief period of time.
14.4 minutes, or 864 seconds. One-hundredth of a day is 1 cd (centiday), also called "kè" in traditional Chinese timekeeping. The unit was also proposed by Lagrange and endorsed by Rey-Pailhade[31] in the 19th century, named "centijours" (from French centi- 'hundred' and jour 'day').
100 days, roughly equivalent to 1/4 of a year (91.25 days). In Chinese tradition "bǎi rì" (百日) is the hundredth day after one's birth, also called Baby's 100 Days Celebration.
A quadrennium (plural: quadrennia or quadrenniums) is also a period of four years, most commonly used in reference to the four-year period between each Olympic Games.[37] It is also used in reference to the four-year interval between leap years, for example when wishing friends and family a "happy quadrennium" on February 29.[38]
Gradual shift in the orientation of Earth's axis of rotation in a cycle of approximately 26,000 years. At present, the rate of precession corresponds to a period of 25,772 years, so a tropical year is shorter than a sidereal year by 1,224.5 seconds (20 min 24.5 sec ≈ (365.24219 × 86400) / 25772).[44][45]
10 times the length of the previous cosmological decade, with CD 1 beginning either 10 seconds or 10 years after the Big Bang, depending on the definition.
About 31 709 791 984 years. Approximately 2.3 times the current age of the universe.
Interrelation
Flowchart illustrating selected units of time. The graphic also shows the three celestial objects that are related to the units of time.
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.
You are visitor 01402503 — GeezerVison does not store the pages you look up. Best viewed with Netscape Navigator 4.7 or Internet Explorer 6 at 800×600. No cookies required.
