An order of magnitude of time is (usually) a decimal prefix or decimal order-of-magnitude quantity together with a base unit of time, like a microsecond or a million years. In some cases, the order of magnitude may be implied (usually 1), like a "second" or "year". In other cases, the quantity name implies the base unit, like "century". In most cases, the base unit is seconds or years. Prefixes are not usually used with a base unit of years, so we say "a million years", not "a megayear". Clock time and calendar time have duodecimal or sexagesimal orders of magnitude rather than decimal, i.e. a year is 12 months, and a minute is 60 seconds.
The smallest meaningful increment of time is the Planck time, the time light takes to traverse the Planck distance, many decimal orders of magnitude smaller than a second. The largest realized amount of time, given known scientific data, is the age of the universe, about 13.8 billion years - the time since the Big Bang as measured in the cosmic microwave background rest frame. Those amounts of time together span 60 decimal orders of magnitude. Metric prefixes are defined spanning 10^{−24} to 10^{24}, 48 decimal orders of magnitude which may be used in conjunction with the metric base unit of second. Metric units of time larger than the second are most commonly seen only in a few scientific contexts such as observational astronomy and materials science although this depends on author; for everyday usage and most other scientific contexts the common units of minutes (60 s), hours (3600 s or 3.6 ks), days (86 400 s), weeks, months, and years (of which there are a number of variations) are commonly used. Weeks, months and years are significantly variable units whose length crucially depends on the choice of calendar and is often not regular even with a calendar, e.g. leap years versus regular years in the Gregorian calendar. This makes them problematic for use against a linear and regular time scale such as that defined by the SI since it is not clear as to which version of these units we are to be using. Because of this, in the table below we will not use weeks and months and the year we will use is the Julian year of astronomy, or 365.25 days of 86 400 s exactly, also called an annum and denoted with the symbol a, whose definition is based on the average length of a year of the Julian calendar which had one leap year every and always every 4 years against common years of 365 days each. This unit is used, following the convention of geological science, to form larger units of time by the application of SI prefixes to it at least up to giga-annum, or Ga, equal to 1 000 000 000 a (short scale: one billion years, long scale: one milliard years).
Unit (s) | Multiple | Symbol | Definition | Comparative examples & common units |
---|---|---|---|---|
10^{−44} | 1 Planck time | t_{P} | Presumed to be the shortest theoretically measurable time interval (but not necessarily the shortest increment of time - see quantum gravity) | 10^{−20} ys: One Planck time t_{P} = ≈ 5.39×10^{−44} s^{[1]} is the briefest physically meaningful span of time. It is the unit of time in the natural units system known as Planck units. |
10^{−24} | 1 yoctosecond | ys^{[2]} | Yoctosecond, (yocto- + second), is one septillionth of a second | 0.3 ys: mean lifetime of W and Z bosons 156 ys: mean lifetime of a Higgs Boson |
10^{−21} | 1 zeptosecond | zs | Zeptosecond, (zepto- + second), is one sextillionth of one second | 2 zs: representative cycle time of gamma ray radiation released in the decay of a radioactive atomic nucleus (here as 2 MeV per emitted photon) |
10^{−18} | 1 attosecond | as | One quintillionth of one second | 12 attoseconds: best timing control of laser pulses.^{[3]} |
10^{−15} | 1 femtosecond | fs | One quadrillionth of one second | 1 fs: Cycle time for 300 nanometre light; ultraviolet light; light travels 0.3 micrometres (µm). 140 fs: Electrons have localized onto individual bromine atoms 6Å apart after laser dissociation of Br_{2}.^{[4]} |
10^{−12} | 1 picosecond | ps | One trillionth of one second | 1 ps: mean lifetime of a bottom quark; light travels 0.3 millimeters (mm) 1 ps: typical lifetime of a transition state 4 ps: Time to execute one machine cycle by an IBM Silicon-Germanium transistor |
10^{−9} | 1 nanosecond | ns | One billionth of one second | 1 ns: Time to execute one machine cycle by a 1 GHz microprocessor 1 ns: Light travels 30 centimetres (12 in) |
10^{−6} | 1 microsecond | µs | One millionth of one second | 1 µs: Time to execute one machine cycle by an Intel 80186 microprocessor 2.2 µs: Lifetime of a muon 4–16 µs: Time to execute one machine cycle by a 1960s minicomputer |
10^{−3} | 1 millisecond | ms | One thousandth of one second | 1 ms: time for a neuron in human brain to fire one impulse and return to rest^{[5]} 4–8 ms: typical seek time for a computer hard disk |
10^{−2} | 1
centisecond |
cs | One hundredth of one second | 18–30 cs (=0.2–0.3 s): Human reflex response to visual stimuli
1.6667 cs period of a frame at a frame rate of 60 Hz. |
10^{−1} | 1
decisecond |
ds | One tenth of a second | 1–4 ds (=0.1–0.4 s): Blink of an eye^{[6]} |
In this table, large intervals of time surpassing one second are catalogued in order of the SI multiples of the second as well as their equivalent in common time units of minutes, hours, days, and Julian years.
Unit (s) | Multiple | Symbol | Common units | Comparative examples & common units |
---|---|---|---|---|
10^{1} | 1 decasecond | das | single seconds
(1 das = 10 s) |
6 das: one minute (min), the time it takes a second hand to cycle around a clock face |
10^{2} | 1 hectosecond | hs | minutes (1 hs = 1 min 40 s = 100 s) |
2.6 hs (4 min 20 s): average length of the most popular YouTube videos as of January 2017^{[7]} 5.55 hs (9 min 12 s): longest videos in above study 7.1 hs(11 m 50 s): time for a human walking at average speed of 1.4 m/s to walk 1 kilometre |
10^{3} | 1 kilosecond | ks | minutes, hours, days (1 ks = 16 min 40 s = 1,000 s) |
1 ks: record confinement time for antimatter, specifically antihydrogen, in electrically neutral state as of 2011^{[8]} 1.8 ks: time slot for the typical situation comedy on television with advertisements included |
10^{6} | 1 megasecond | Ms | weeks to years (1 Ms = 11 d 13 h 46 min 40 s = 1,000,000 s) |
1.641 6 Ms (19 d): length of a "month" of the Baha'i calendar 2.36 Ms (27.32 d): length of the true month, the orbital period of the Moon |
10^{9} | 1 gigasecond | Gs | decades, centuries, millennia (1 Gs = over 31 years and 287 days = 1,000,000,000 s) |
1.5 Gs: UNIX time as of Jul 14 02:40:00 UTC 2017. UNIX time being the number of seconds since 1970-01-01T00:00:00Z ignoring leap seconds. 2.5 Gs: (79 a): typical human life expectancy in the developed world |
10^{12} | 1 terasecond | Ts | millennia to geological epochs (1 Ts = over 31,600 years = 1,000,000,000,000 s) |
3.1 Ts (100 ka): approximate length of a glacial period of the current Quaternary glaciation epoch 31.6 Ts (1000 ka, 1 Ma): one mega-annum (Ma), or one million years |
10^{15} | 1 petasecond | Ps | geological eras, history of Earth and the Universe | 2 Ps: approximate time since the Cretaceous-Paleogene extinction event, believed to be caused by the impact of a large asteroid into Chicxulub in modern-day Mexico. This extinction was one of the largest in Earth's history and marked the demise of most dinosaurs, with the only known exception being the ancestors of today's birds. 7.9 Ps (250 Ma): approximate time since the Permian-Triassic extinction event, the actually largest known mass extinction in Earth history which wiped out 95% of all extant species and believed to have been caused by the consequences of massive long-term volcanic eruptions in the area of the Siberian Traps. Also, the approximate time to the supercontinent of Pangaea. Also, the length of one galactic year or cosmic year, the time required for the Sun to complete one orbit around the Milky Way Galaxy. |
10^{18} | 1 exasecond | Es | future cosmological time | All times of this length and beyond are currently theoretical as they surpass the elapsed lifetime of the known universe. 1.08 Es (+34 Ga): time to the Big Rip according to some models, but this is not favored by existing data. This is one possible scenario for the ultimate fate of the Universe. Under this scenario, dark energy increases in strength and power in a feedback loop that eventually results in the tearing apart of all matter down to subatomic scale due to the rapidly increasing negative pressure thereupon |
10^{21} | 1 zettasecond | Zs | 3 Zs (+100 000 Ga): The remaining time until the end of Stelliferous Era of the universe under the heat death scenario for the ultimate fate of the Universe which is the most commonly-accepted model in the current scientific community. This is marked by the cooling-off of the last low-mass dwarf star to a black dwarf. After this time has elapsed, the Degenerate Era begins. 9.85 Zs (311 000 Ga): The entire lifetime of Brahma in Hindu mythology. | |
10^{24} and onward | 1 yottasecond and beyond | Ys and on | 600 Ys (9 × 10^{18} a): The radioactive half-life of bismuth-209 by alpha decay, one of the slowest-observed radioactive decay processes. 1.310 019 × 10^{12} Ys (4.134 105 × 10^{28} years) – The time period equivalent to the value of 13.13.13.13.13.13.13.13.13.13.13.13.13.13.13.13.13.13.13.13.0.0.0.0 in the Mesoamerican Long Count, a date discovered on a stela at the Coba Maya site, believed by archaeologist Linda Schele to be the absolute value for the length of one cycle of the universe^{[9]}^{[10]} 10^{29} Ys (3.2×10^{45} years) – the largest possible value for the proton half-life, assuming that the Big Bang was inflationary and that the same process that made baryons predominate over antibaryons in the early Universe also makes protons decay^{[12]} |
An attosecond is 1×10−18 of a second (one quintillionth of a second). For context, an attosecond is to a second what a second is to about 31.71 billion years.The word "attosecond" is formed by the prefix atto and the unit second. Atto- was derived from the Danish word for eighteen (atten). Its symbol is as.
An attosecond is equal to 1000 zeptoseconds, or 1⁄1000 of a femtosecond. Because the next higher SI unit for time is the femtosecond (10−15 seconds), durations of 10−17 s and 10−16 s will typically be expressed as tens or hundreds of attoseconds:
Times which can be expressed in attoseconds:
1 attosecond: the time it takes for light to travel the length of two hydrogen atoms
24 attoseconds: the atomic unit of time
43 attoseconds: the shortest pulses of laser light yet created
53 attoseconds: the second-shortest pulses of laser light created
100 attoseconds: fastest-ever view of molecular motion
200 attoseconds (approximately): half-life of beryllium-8, maximum time available for the triple-alpha process for the synthesis of carbon and heavier elements in stars
320 attoseconds: estimated time it takes electrons to transfer between atoms
Billion yearsA billion years (109 years) is a unit of time on the petasecond scale, more precisely equal to 3.16×1016 seconds.
It is sometimes abbreviated Gy, Ga ("giga-annum"), Byr and variants. The abbreviations Gya or bya are for "billion years ago", i.e. billion years before present.
The terms are used in geology, paleontology, geophysics, astronomy and physical cosmology.
The prefix giga- is preferred over billion- to avoid confusion in the long and short scales over the meaning of billion; the postfix annum may be further qualified for precision as a sidereal year or Julian year:
1 Gaj=3.15576×1016 s,
1 Gas=3.15581×1016 s (epoch J2000.0).
1 Gas=1×109 yByr was formerly used in English-language geology and astronomy as a unit of one billion years. Subsequently, the term gigaannum (Ga) has increased in usage, with Gy or Gyr still sometimes used in English-language works (at the risk of confusion with Gy as abbreviation for the gray, a unit of radiation exposure). Astronomers use Gyr or Gy as an abbreviation for gigayear.
Byabya or b.y.a. is an abbreviation for "billion years ago". It is commonly used as a unit of time to denote length of time before the present in 109 years. This initialism is often used in the sciences of astronomy, geology, and paleontology.
The "billion" in bya is the 109 "billion" of the short scale of the U.S., not the long-scale 1012 "billion" of some European usage. Billion by this convention (109) is often called a "thousand million" in the UK and a "milliard" in some other countries. For this reason, there is potential for some confusion, and some scientists prefer the unit Gya, while others prefer Ga (Giga-annum), however bya remains in more widespread use. In 1974, the UK switched from the long scale to the short scale.Related units are mya ("million years ago"), and byr ("billion years"). These are traditionally written in lowercase. Ga or Gya has a capitalized first letter instead.
DayA day is approximately the period of time during which the Earth completes one rotation around its axis. A solar day is the length of time which elapses between the Sun reaching its highest point in the sky two consecutive times.In 1960, the second was redefined in terms of the orbital motion of the Earth in year 1900, and was designated the SI base unit of time. The unit of measurement "day", was redefined as 86,400 SI seconds and symbolized d. In 1967, the second and so the day were redefined by atomic electron transition. A civil day is usually 86,400 seconds, plus or minus a possible leap second in Coordinated Universal Time (UTC), and occasionally plus or minus an hour in those locations that change from or to daylight saving time.Day can be defined as each of the twenty-four-hour periods, reckoned from one midnight to the next, into which a week, month, or year is divided, and corresponding to a rotation of the earth on its axis. However its use depends on its context, for example when people say 'day and night', 'day' will have a different meaning. It will mean the interval of light between two successive nights; the time between sunrise and sunset, in this instance 'day' will mean time of light between one night and the next. However, in order to be clear when using 'day' in that sense, "daytime" should be used to distinguish it from "day" referring to a 24-hour period; this is since daytime usually always means 'the time of the day between sunrise and sunset. The word day may also refer to a day of the week or to a calendar date, as in answer to the question, "On which day?" The life patterns (circadian rhythms) of humans and many other species are related to Earth's solar day and the day-night cycle.
FemtosecondA femtosecond is the SI unit of time equal to 10^{−15} or ^{1}/_{1,000,000,000,000,000} of a second; that is, one quadrillionth, or one millionth of one billionth, of a second. For context, a femtosecond is to a second as a second is to about 31.71 million years; a ray of light travels approximately 0.3 µm (micrometers) in 1 femtosecond, a distance comparable to the diameter of a virus.
The word femtosecond is formed by the SI prefix femto and the SI unit second. Its symbol is fs.
A femtosecond is equal to 1000 attoseconds, or 1/1000 picosecond. Because the next higher SI unit is 1000 times larger, times of 10^{−14} and 10^{−13} seconds are typically expressed as tens or hundreds of femtoseconds.
HourAn hour (symbol: h; also abbreviated hr.) is a unit of time conventionally reckoned as 1⁄24 of a day and scientifically reckoned as 3,599–3,601 seconds, depending on conditions.
The hour was initially established in the ancient Near East as a variable measure of 1⁄12 of the night or daytime. Such seasonal, temporal, or unequal hours varied by season and latitude. The hour was subsequently divided into 60 minutes, each of 60 seconds. Equal or equinoctial hours were taken as 1⁄24 of the day as measured from noon to noon; the minor seasonal variations of this unit were eventually smoothed by making it 1⁄24 of the mean solar day. Since this unit was not constant due to long term variations in the Earth's rotation, the hour was finally separated from the Earth's rotation and defined in terms of the atomic or physical second.
In the modern metric system, hours are an accepted unit of time defined as 3,600 atomic seconds. However, on rare occasions an hour may incorporate a positive or negative leap second, making it last 3,599 or 3,601 seconds, in order to keep it within 0.9 seconds of UT1, which is based on measurements of the mean solar day.
Jiffy (time)Jiffy is an informal term for any unspecified short period of time, as in "I will be back in a jiffy". From this it has acquired a number of more precise applications for short, very short, extremely short, ultra short or hyper short periods of time. First attested in 1785, the word's origin is unclear, though one suggestion is that it was thieves' cant for lightning.
Logarithmic timelineA logarithmic timeline is a timeline laid out according to a logarithmic scale. This necessarily implies a zero point and an infinity point, neither of which can be displayed. The most natural zero point is the Big Bang, looking forward, but the most common is the ever-changing present, looking backward. (Also possible is a zero point in the present, looking forward to the infinite future.)
The idea of presenting history logarithmically goes back at least to 1932, when John B. Sparks copyrighted his chart "Histomap of Evolution". Around the same time it was also explored by the cyberneticist Heinz von Foerster, who used it to propose that memories naturally fade in an exponential manner. Logarithmic timelines have also been used in future studies to justify the idea of a technological singularity.
A logarithmic scale enables events throughout time to be presented accurately, but enables more events to be included closer to one end. Sparks explained this by stating:
As we travel forward in geological time the more complex is the evolution of life forms and the more are the changes to be recorded. Further, the most recent periods of evolution hold the most interest for us. We need therefore increasingly more space for our outline the nearer we approach modern times, and the logarithmic scale fulfills just this condition without any break in the continuity.Two examples of such timelines are shown below, while a more comprehensive version (similar to that of Sparks' "Histomap") can be found at Detailed logarithmic timeline.
MicrosecondA microsecond is an SI unit of time equal to one millionth (0.000001 or 10−6 or 1⁄1,000,000) of a second. Its symbol is μs, sometimes simplified to us when Unicode is not available.
A microsecond is equal to 1000 nanoseconds or 1⁄1,000 of a millisecond. Because the next SI prefix is 1000 times larger, measurements of 10−5 and 10−4 seconds are typically expressed as tens or hundreds of microseconds.
MillisecondA millisecond (from milli- and second; symbol: ms) is a thousandth (0.001 or 10−3 or 1/1000) of a second.A unit of 10 milliseconds may be called a centisecond, and one of 100 milliseconds a decisecond, but these names are rarely used.
To help compare orders of magnitude of different times, this page lists times between 10−3 seconds and 100 seconds (1 millisecond and one second). See also times of other orders of magnitude.
MinuteThe minute is a unit of time or angle. As a unit of time, the minute is most of times equal to 1⁄60 (the first sexagesimal fraction) of an hour, or 60 seconds. In the UTC time standard, a minute on rare occasions has 61 seconds, a consequence of leap seconds (there is a provision to insert a negative leap second, which would result in a 59-second minute, but this has never happened in more than 40 years under this system). As a unit of angle, the minute of arc is equal to 1⁄60 of a degree, or 60 seconds (of arc). Although not an SI unit for either time or angle, the minute is accepted for use with SI units for both. The SI symbols for minute or minutes are min for time measurement, and the prime symbol after a number, e.g. 5′, for angle measurement. The prime is also sometimes used informally to denote minutes of time.
MonthA month is a unit of time, used with calendars, which is approximately as long as a natural period related to the motion of the Moon; month and Moon are cognates. The traditional concept arose with the cycle of Moon phases; such months (lunations) are synodic months and last approximately 29.53 days. From excavated tally sticks, researchers have deduced that people counted days in relation to the Moon's phases as early as the Paleolithic age. Synodic months, based on the Moon's orbital period with respect to the Earth-Sun line, are still the basis of many calendars today, and are used to divide the year.
NanosecondA nanosecond (ns) is an SI unit of time equal to one thousand-millionth of a second (or one billionth of a second), that is, 1/1,000,000,000 of a second, or 10−9 seconds.
The term combines the prefix nano- with the basic unit for one-sixtieth of a minute.
A nanosecond is equal to 1000 picoseconds or 1⁄1000 microsecond. Time units ranging between 10−8 and 10−7 seconds are typically expressed as tens or hundreds of nanoseconds.
Time units of this granularity are commonly encountered in telecommunications, pulsed lasers, and related aspects of electronics.
PicosecondA picosecond is an SI unit of time equal to 10−12 or 1/1,000,000,000,000 (one trillionth) of a second. That is one trillionth, or one millionth of one millionth of a second, or 0.000 000 000 001 seconds. A picosecond is to one second as one second is to approximately 31,689 years. Multiple technical approaches achieve imaging within single-digit picoseconds: for example, the streak camera or intensified CCD (ICCD) cameras are able to picture the motion of light.The name is formed by the SI prefix pico and the SI unit second. It is abbreviated as ps.
One picosecond is equal to 1000 femtoseconds, or 1/1000 nanoseconds. Because the next SI unit is 1000 times larger, measurements of 10−11 and 10−10 second are typically expressed as tens or hundreds of picoseconds. Some notable measurements in this range include:
1.0 picoseconds (1.0 ps) – cycle time for electromagnetic frequency 1 terahertz (THz) (1 x 1012 hertz), an inverse unit. This corresponds to a wavelength of 0.3 mm, as can be calculated by multiplying 1 ps by the speed of light (approximately 3 x 108 m/s) to determine the distance traveled. 1 THz is in the Far infrared.
1 picosecond – time taken by light in a vacuum to travel approximately 0.30 mm
1 picosecond – half-life of a bottom quark
~1 picosecond – lifetime of a single H3O+ (hydronium) ion in water at 20 °C
picoseconds to nanoseconds – phenomena observable by dielectric spectroscopy
1.2 picoseconds – switching time of the world's fastest transistor (845 GHz, as of 2006)
1.7 picoseconds - rotational correlation time of water
3.3 picoseconds (approximately) – time taken for light to travel 1 millimeter
10 picoseconds after the Big Bang – electromagnetism separates from the other fundamental forces
10–150 picoseconds – rotational correlation times of a molecule (184 g/mol) from hot to frozen water
108.7827757 picoseconds – transition time between the two hyperfine levels of the ground state of the caesium-133 atom at absolute zero
330 picoseconds (approximately) – the time it takes a common 3.0 GHz computer CPU to complete a processing cycle
Planck time
In quantum mechanics, the Planck time (t_{P}) is the unit of time in the system of natural units known as Planck units. A Planck time unit is the time required for light to travel a distance of 1 Planck length in a vacuum, which is a time interval of approximately 5.39 × 10 ^{−44} s. The unit is named after Max Planck, who was the first to propose it.
The Planck time is defined as:
where:
Using the known values of the constants, the approximate equivalent value in terms of the SI unit, the second, is
where the two digits between parentheses denote the standard error of the approximated value.
SecondThe second (symbol: s) is the base unit of time in the International System of Units (SI), commonly understood and historically defined as 1⁄86400 of a day – this factor derived from the division of the day first into 24 hours, then to 60 minutes and finally to 60 seconds each. Analog clocks and watches often have sixty tick marks on their faces, representing seconds, and a "second hand" to mark the passage of time in seconds. Digital clocks and watches often have a two-digit seconds counter. The second is also part of several other units of measurement like meters per second for velocity, meters per second per second for acceleration, and per second for frequency.
Although the historical definition of the unit was based on this division of the Earth's rotation cycle, the formal definition in the International System of Units (SI) is a much steadier timekeeper: it is defined by taking the fixed numerical value of the caesium frequency ∆νCs, the unperturbed ground-state hyperfine transition frequency of the caesium 133 atom, to be 9 192 631 770 when expressed in the unit Hz, which is equal to s−1.
Because the Earth's rotation varies and is also slowing ever so slightly, a leap second is periodically added to clock time to keep clocks in sync with Earth's rotation.
Multiples of seconds are usually counted in hours and minutes. Fractions of a second are usually counted in tenths or hundredths. In scientific work, small fractions of a second are counted in milliseconds (thousandths), microseconds (millionths), nanoseconds (billionths), and sometimes smaller units of a second. An everyday experience with small fractions of a second is a 1-gigahertz microprocessor which has a cycle time of 1 nanosecond. Camera shutter speeds are often expressed in fractions of a second, such as 1⁄30 second or 1⁄1000 second.
Sexagesimal divisions of the day from a calendar based on astronomical observation have existed since the third millennium BC, though they were not seconds as we know them today. Small divisions of time could not be counted back then, so such divisions were figurative. The first timekeepers that could count seconds accurately were pendulum clocks invented in the 17th century. Starting in the 1950s, atomic clocks became better timekeepers than earth's rotation, and they continue to set the standard today.
Term (time)A term is a period of duration, time or occurrence, in relation to an event. To differentiate an interval or duration, common phrases are used to distinguish the observance of length are near-term or short-term, medium-term or mid-term and long-term.
It is also used as part of a calendar year, especially one of the three parts of an academic term and working year in the United Kingdom: Michaelmas term, Hilary term / Lent term or Trinity term / Easter term, the equivalent to the American semester. In America there is a midterm election held in the middle of the four-year presidential term, there are also academic midterm exams.
In economics, it is the period required for economic agents to reallocate resources, and generally reestablish equilibrium. The actual length of this period, usually numbered in years or decades, varies widely depending on circumstantial context. During the long term, all factors are variable.
In finance or financial operations of borrowing and investing, what is considered long-term is usually above 3 years, with medium-term usually between 1 and 3 years and short-term usually under 1 year. It is also used in some countries to indicate a fixed term investment such as a term deposit.
In law, the term of a contract is the duration for which it is to remain in effect (not to be confused with the meaning of "term" that denotes any provision of a contract). A fixed-term contract is one concluded for a pre-defined time, although it may also include provision for it to be extended. A contractor required to deliver against a term contract is often referred to as a "term contractor".
Time scaleTime scale may refer to:
Time standard, a specification of either the rate at which time passes, points in time, or both
A duration or quantity of time:
Orders of magnitude (time) as a power of 10 in seconds;
A specific unit of time
Geological time scale, a scale that divides up the history of Earth into scientifically meaningful periodsIn astronomy and physics:
Dynamical time scale, in stellar physics, the time in which changes in one part of a body can be communicated to the rest of that body, or in celestial mechanics, a realization of a time-like argument based on a dynamical theory
Nuclear timescale, an estimate of the lifetime of a star based solely on its rate of fuel consumption
Thermal time scale, an estimate of the lifetime of a star once the fuel reserves at its center are used upIn cosmology and particle physics:
Planck time, the time scale beneath which quantum effects are comparable in significance to gravitational effectsIn mathematics:
Time-scale calculus, the unification of the theory of difference equations with differential equationsIn music:
Rhythm, a temporal pattern of events
Time scale (music), which divides music into sections of time
YearA year is the orbital period of the Earth moving in its orbit around the Sun. Due to the Earth's axial tilt, the course of a year sees the passing of the seasons, marked by change in weather, the hours of daylight, and, consequently, vegetation and soil fertility.
In temperate and subpolar regions around the planet, four seasons are generally recognized: spring, summer, autumn, and winter. In tropical and subtropical regions, several geographical sectors do not present defined seasons; but in the seasonal tropics, the annual wet and dry seasons are recognized and tracked.
A calendar year is an approximation of the number of days of the Earth's orbital period as counted in a given calendar. The Gregorian calendar, or modern calendar, presents its calendar year to be either a common year of 365 days or a leap year of 366 days, as do the Julian calendars; see below. For the Gregorian calendar, the average length of the calendar year (the mean year) across the complete leap cycle of 400 years is 365.2425 days. The ISO standard ISO 80000-3, Annex C, supports the symbol a (for Latin annus) to represent a year of either 365 or 366 days. In English, the abbreviations y and yr are commonly used.
In astronomy, the Julian year is a unit of time; it is defined as 365.25 days of exactly 86,400 seconds (SI base unit), totalling exactly 31,557,600 seconds in the Julian astronomical year.The word year is also used for periods loosely associated with, but not identical to, the calendar or astronomical year, such as the seasonal year, the fiscal year, the academic year, etc. Similarly, year can mean the orbital period of any planet; for example, a Martian year and a Venusian year are examples of the time a planet takes to transit one complete orbit. The term can also be used in reference to any long period or cycle, such as the Great Year.
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