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. Therefore, it is said "a million years" instead of "a megayear". Clock time and calendar time have duodecimal or sexagesimal orders of magnitude rather than decimal, e.g., 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.[1]
The largest realized amount of time, based on 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.[2] Those amounts of time together span 60 decimal orders of magnitude. Metric prefixes are defined spanning to, 60 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 the author. For everyday use and most other scientific contexts, the common units of minutes, hours (3,600 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 lengths depend on the choice of calendar and are 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 which version is being used.
Because of this, the table below does not include weeks, months, and years. Instead, the table uses the annum or astronomical Julian year (365.25 days of 86,400 seconds), denoted with the symbol a. Its definition is based on the average length of a year according to the Julian calendar, which has one leap year every four years. According to the geological science convention, this is used 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 | Symbol | Definition | Comparative examples & common units < | -- This is any multiple, within reason and which is commonly used, to express not only by comparative example the length of time but also, in other terms (common units) the interval of time. For example: 60 Seconds = 1 minute.--> | |
|---|---|---|---|---|---|
| Planck time | t | Presumed to be the shortest theoretically measurable time interval (but not necessarily the shortest increment of time—see quantum gravity) | The length of one Planck time (t = \sqrt{\hbarG/c5} | ||
| quectosecond | qs | Quectosecond, (quecto- + second), is one nonillionth of a second | |||
| rontosecond | rs | Rontosecond, (ronto- + second), is one octillionth of a second | 300 rs: The mean lifetime of W and Z bosons | ||
| yoctosecond | ys[4] | Yoctosecond, (yocto- + second), is one septillionth of a second | 23 ys: The lower estimated bound on the half-life of isotope 7 of hydrogen (Hydrogen-7) 143 ys: The half-life of the Nitrogen-10 isotope of Nitrogen 156 ys: The mean lifetime of a Higgs Boson | ||
| zeptosecond | zs | Zeptosecond, (zepto- + second), is one sextillionth of one second | 1.3 zs: Smallest experimentally controlled time delay in a photon field.[5] 2 zs: The representative cycle time of gamma ray radiation released in the decay of a radioactive atomic nucleus (here as 2 MeV per emitted photon) 4 zs: The cycle time of the zitterbewegung of an electron ( \omega=2mec2/\hbar 247 zs: The experimentally-measured travel time of a photon across a hydrogen molecule, "for the average bond length of molecular hydrogen"[6] | ||
| attosecond | as | One quintillionth of one second | 12 as: The best timing control of laser pulses.[7] 43 as: The shortest X-ray laser pulse[8] 53 as: The shortest electron laser pulse[9] [10] | ||
| femtosecond | fs | One quadrillionth of one second | 1 fs: The cycle time for ultraviolet light with a wavelength of 300 nanometres; The time it takes light to travel a distance of 0.3 micrometres (μm). 140 fs: The time needed for electrons to have localized onto individual bromine atoms 6 Ångstrom apart after laser dissociation of Br.[11] 290 fs: The lifetime of a tauon | ||
| picosecond | ps | One trillionth of one second | 1 ps: The mean lifetime of a bottom quark; the time needed for light to travel 0.3 millimetres (mm) 1 ps: The typical lifetime of a transition state one machine cycle by an IBM silicon-germanium transistor 109 ps: The period of the photon corresponding to the hyperfine transition of the ground state of cesium-133, and one 9,192,631,770th of one second by definition 114.6 ps: The time for the fastest overclocked processor to execute one machine cycle.[12] 696 ps: How much more a second lasts far away from Earth's gravity due to the effects of General Relativity | ||
| nanosecond | ns | One billionth of one second | 1 ns: The time needed to execute one machine cycle by a 1 GHz microprocessor 1 ns: The time light takes to travel 30 cm (11.811 in) | ||
| microsecond | μs | One millionth of one second | 1 μs: The time needed to execute one machine cycle by an Intel 80186 microprocessor 2.2 μs: The lifetime of a muon 4–16 μs: The time needed to execute one machine cycle by a 1960s minicomputer | ||
| millisecond | ms | One thousandth of one second | 1 ms: The time for a neuron in the human brain to fire one impulse and return to rest[13] 4–8 ms: The typical seek time for a computer hard disk | ||
| centisecond | cs | One hundredth of one second | 1.6667 cs: The period of a frame at a frame rate of 60 Hz. 2 cs: The cycle time for European 50 Hz AC electricity10–20 cs (=0.1–0.2 s): The human reflex response to visual stimuli | ||
| decisecond | ds | One tenth of a second | 1–4 ds (=0.1–0.4 s): The length of a single blink of an eye[14] |
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.
| Multiple of a second | Unit | Symbol | Common units | Comparative examples and common units < | -- This is any multiple, within reason and which is commonly used, to express not only by comparative example the length of time but also, in other terms (common units) the interval of time. For example: 60 Seconds = 1 minute.--> | ||||||||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| 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 | ||||||||||||||||||
| hectosecond | hs | minutes (1 hs = 1 min 40 s = 100 s) | 2 hs (3 min 20 s): The average length of the most popular YouTube videos as of January 2017[15] 5.55 hs (9 min 12 s): The longest videos in the above study7.1 hs (11 m 50 s): The time for a human walking at average speed of 1.4 m/s to walk 1 kilometre | ||||||||||||||||||
| kilosecond | ks | minutes, hours, days (1 ks = 16 min 40 s = 1,000 s) | 1 ks: The record confinement time for antimatter, specifically antihydrogen, in electrically neutral state as of 2011;[16] 1.477 ks: The longest period in which a person has not taken a breath.1.8 ks: The time slot for the typical situation comedy on television with advertisements included 2.28 ks: The duration of the Anglo-Zanzibar War, the shortest war in recorded history. 3.6 ks: The length of one hour (h), the time for the minute hand of a clock to cycle once around the face, approximately 1/24 of one mean solar day 7.2 ks (2 h): The typical length of feature films 35.73 ks: the rotational period of planet Jupiter, fastest planet to rotate 38.0196 ks: rotational period of Saturn, second shortest rotational period 57.996 ks: one day on planet Neptune. 62.064 ks: one day on Uranus. | ||||||||||||||||||
| megasecond | Ms | weeks to years (1 Ms = 11 d 13 h 46 min 40 s = 1,000,000 s) | 1.6416 Ms (19 d): The length of a "month" of the Baha'i calendar2.36 Ms (27.32 d): The length of the true month, the orbital period of the Moon 2.4192 Ms (28 d): The length of February, the shortest month of the Gregorian calendar, in common years 2.5056 Ms (29 d): The length of February in leap years 2.592 Ms (30 d): The length of April, June, September, and November in the Gregorian calendar; common interval used in legal agreements and contracts as a proxy for a month 2.6784 Ms (31 d): The length of the longest months of the Gregorian calendar 23 Ms (270 d): The approximate length of typical human gestational period 31.5576 Ms (365.25 d): The length of the Julian year, also called the annum, symbol a. 5.06703168 Ms: The rotational period of Mercury. 7.600544064 Ms: One year on Mercury. 19.41414912 Ms: One year on Venus. 20.9967552 Ms: The rotational period of Venus. | ||||||||||||||||||
| 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): The typical human life expectancy in the developed world 3.16 Gs: (100 a): One century 31.6 Gs: (1000 a, 1 ka): One millennium, also called a kilo-annum (ka) 63.8 Gs: The approximate time since the beginning of the Anno Domini era as of 2019 – 2,019 years, and traditionally the time since the birth of Jesus Christ 194.67 Gs: The approximate lifespan of time capsule Crypt of Civilization, 28 May 1940 – 28 May 8113 363 Gs: (11.5 ka): The time since the beginning of the Holocene epoch 814 Gs: (25.8 ka): The approximate time for the cycle of precession of the Earth's axis | ||||||||||||||||||
| 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 epoch31.6 Ts (1000 ka, 1 Ma): One mega-annum (Ma), or one million years 79 Ts (2.5 Ma): The approximate time since earliest hominids of genus Australopithecus 130 Ts (4 Ma): The typical lifetime of a biological species on Earth 137 Ts (4.32 Ma): The length of the mythic unit of mahayuga, the Great Age, in Hindu mythology. | ||||||||||||||||||
| petasecond | Ps | geological eras, history of Earth and the Universe | 2 Ps: The 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): The 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. 16 Ps (510 Ma): The approximate time since the Cambrian explosion, a massive evolutionary diversification of life which led to the appearance of most existing multicellular organisms and the replacement of the previous Ediacaran biota. 22 Ps (704 Ma): The approximate half-life of the uranium isotope U. 31.6 Ps (1000 Ma, 1 Ga): One giga-annum (Ga), one billion years, the largest fixed time unit used in the standard geological time scale, approximately the order of magnitude of an eon, the largest division of geological time. +1 Ga: The estimated remaining habitable lifetime of Earth, according to some models. At this point in time the stellar evolution of the Sun will have increased its luminosity to the point that enough energy will be reaching the Earth to cause the evaporation of the oceans and their loss into space (due to the UV flux from the Sun at the top of the atmosphere dissociating the molecules), making it impossible for any life to continue. 136 Ps (4.32 Ga): The length of the legendary unit Kalpa in Hindu mythology, or one day (but not including the following night) of the life of Brahma. 143 Ps (4.5 Ga): The age of the Earth by our best estimates. Also the approximate half-life of the uranium isotope U. 315 Ps (10 Ga): The approximate lifetime of a main-sequence star similar to the Sun. 434.8 Ps (13.787 Ga): The approximate age of the Universe | ||||||||||||||||||
| 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 300 – 600 Es (10 – 20 Ta): The estimated lifetime of low-mass stars (red dwarfs) | ||||||||||||||||||
| zettasecond | Zs | 3 Zs (+100 Ta): 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 Ta): The entire lifetime of Brahma in Hindu mythology. | |||||||||||||||||||
| yottasecond | Ys | 600 Ys : The radioactive half-life of bismuth-209 by alpha decay, one of the slowest-observed radioactive decay processes. | |||||||||||||||||||
| ronnasecond | Rs | 3.16 Rs : The estimated time until all stars are ejected from their galaxies or consumed by black holes.32 Rs : Highest estimate of the time until all stars are ejected from galaxies or consumed by black holes. | |||||||||||||||||||
| and onward | quettasecond and beyond | Qs and on | 69 Qs : The radioactive half-life of tellurium-128, the longest known half-life of any elemental isotope.1,340,009 Qs : 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 stele at the Coba Maya site, believed by archaeologist Linda Schele to be the absolute value for the length of one cycle of the universe[17] [18] : The smallest possible value for proton half-life consistent with experiment[19] : 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[20]
{10}{10{10{10{101.1 {10}{10{103,883,775,501,690 |
| 6×101 seconds | 1 minute | min | |
|---|---|---|---|
| 6×101 minutes | 1 hour | h (hr) | |
| 2.4×101 hours | 1 day | d |
| 10123 | |
| {10 |
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