annum
   HOME

TheInfoList



A year is the
orbital period The orbital period is the time a given astronomical object takes to complete one orbit around another object, and applies in astronomy Astronomy (from el, ἀστρονομία, literally meaning the science that studies the laws of th ...
of a planetary body, for example, the
Earth Earth is the third planet from the Sun and the only astronomical object known to harbour and support life. 29.2% of Earth's surface is land consisting of continents and islands. The remaining 70.8% is Water distribution on Earth, covered wit ...

Earth
, moving in its orbit around the
Sun The Sun is the star A star is an astronomical object consisting of a luminous spheroid of plasma (physics), plasma held together by its own gravity. The List of nearest stars and brown dwarfs, nearest star to Earth is the Sun. Many othe ...

Sun
. Due to the Earth's
axial tilt In , axial tilt, also known as obliquity, is the angle between an object's and its al axis, or, equivalently, the angle between its ial plane and . It differs from . At an obliquity of 0 degrees, the two axes point in the same direction; i.e., ...
, the course of a year sees the passing of the
season A season is a division of the year based on changes in weather Weather is the state of the atmosphere, describing for example the degree to which it is hot or cold, wet or dry, calm or stormy, clear or cloud cover, cloudy. On Earth, most ...

season
s, marked by change in
weather Weather is the state of the atmosphere, describing for example the degree to which it is hot or cold, wet or dry, calm or stormy, clear or cloud cover, cloudy. On Earth, most weather phenomena occur in the lowest layer of the planet's atmosp ...

weather
, the hours of
daylight Daylight is the combination of all direct and indirect sunlight Sunlight is a portion of the electromagnetic radiation given off by the Sun, in particular infrared, visible spectrum, visible, and ultraviolet light. On Earth, sunlight is lig ...

daylight
, and, consequently,
vegetation Vegetation is an assemblage of plant Plants are predominantly photosynthetic eukaryotes of the Kingdom (biology), kingdom Plantae. Historically, the plant kingdom encompassed all living things that were not animals, and included algae and f ...

vegetation
and
soil fertility Soil fertility refers to the ability of soil File:Stagnogley.JPG, Surface-water-Gley soil, gley developed in glacial till, Northern Ireland. Soil is a mixture of organic matter, minerals, gases, liquids, and organisms that together support li ...
. In
temperate In geography Geography (from Greek: , ''geographia'', literally "earth description") is a field of science devoted to the study of the lands, features, inhabitants, and phenomena of the Earth and planets. The first person to use ...
and subpolar regions around the planet, four seasons are generally recognized:
spring Spring(s) may refer to: Common uses * Spring (season), a season of the year * Spring (device), a mechanical device that stores energy * Spring (hydrology), a natural source of water * Spring (mathematics), a geometric surface in the shape of a heli ...
,
summer Summer is the hottest of the four temperate In geography, the temperate climates of Earth Earth is the third planet from the Sun and the only astronomical object known to harbor life. About 29% of Earth's surface is land ...

summer
,
autumn Autumn, also known as fall in American English American English (AmE, AE, AmEng, USEng, en-US), sometimes called United States English or U.S. English, is the set of varieties of the English language native to the United States. Curr ...

autumn
and
winter Winter is the est of the year in and . It occurs between and . The tilt of Earth's axis causes seasons; winter occurs when a is oriented away from the . Different cultures define different dates as the start of winter, and some use a de ...

winter
. In
tropical The tropics are the region of Earth surrounding the Equator. They are delimited in latitude by the Tropic of Cancer in the Northern Hemisphere at N and the Tropic of Capricorn in the Southern Hemisphere at S; these latitudes correspond to ...

tropical
and
subtropical The subtropical zones or subtropics are geographical zone, geographical and Köppen climate classification, climate zones located to the north and south of the tropics, Torrid Zone. Geographically part of the Geographical zone#Temperate zones, ...

subtropical
regions, several geographical sectors do not present defined seasons; but in the seasonal tropics, the annual wet and
dry season The dry season is a yearly period of low rainfall, especially in the tropics The tropics are the region of Earth Earth is the third planet from the Sun and the only astronomical object known to harbor life. About 29% of Earth's su ...
s are recognized and tracked. A
calendar year A calendar is a system of organizing days. This is done by giving names to periods of time Time is the indefinite continued sequence, progress of existence and event (philosophy), events that occur in an apparently irreversible process, ...
is an approximation of the number of days of the Earth's orbital period, as counted in a given
calendar A calendar is a system of organizing days. This is done by giving names to periods of time, typically days, weeks, months and years. A calendar date, date is the designation of a single, specific day within such a system. A calendar is al ...

calendar
. The
Gregorian calendar The Gregorian calendar is the used in most of the world. It was introduced in October 1582 by as a modification of the , reducing the average year from 365.25 days to 365.2425 days, and adjusting for the drift in the that the inaccuracy ha ...
, or modern calendar, presents its calendar year to be either a
common year A common year is a calendar year A calendar is a system of organizing days. This is done by giving names to periods of time Time is the indefinite continued sequence, progress of existence and event (philosophy), events that occur in ...
of 365 days or a
leap year A leap year (also known as an intercalary year or wikt:bissextile, bissextile year) is a calendar year that contains an additional day (or, in the case of a lunisolar calendar, a month) added to keep the calendar year synchronized with the astrono ...

leap year
of 366 days, as do the
Julian calendar The Julian calendar, proposed by Julius Caesar Gaius Julius Caesar (; 12 July 100 BC – 15 March 44 BC) was a Roman people, Roman general and statesman who played a critical role in Crisis of the Roman Republic, the events that led to the d ...
s . 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. In English, the abbreviations "y" and "yr" are commonly used ("a" is also used) for the unit of time, though its exact duration may be inconsistent. In astronomy, the Julian year is a
unit of time A unit of time is any particular time Time is the indefinite continued sequence, progress of existence and event (philosophy), events that occur in an apparently irreversible process, irreversible succession from the past, through the presen ...
; it is defined as 365.25
day A day is approximately the period during which the Earth completes one rotation around its axis, which takes around 24 hours. A solar day is the length of time which elapses between the Sun reaching its highest point in the sky two consecutive t ...

day
s of exactly 86,400
second The second (symbol: s, also abbreviated: sec) is the of in the (SI) (french: Système International d’unités), commonly understood and historically defined as of a – this factor derived from the division of the day first into 24 s, th ...
s (
SI base unit The SI base units are the standard units of measurement defined by the International System of Units (SI) for the seven base quantities of what is now known as the International System of Quantities: they are notably a basic set from which all ot ...

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 A fiscal year (or financial year, or sometimes budget year) is used in government accounting, which varies between countries, and for budget purposes. It is also used for financial report Financial statements (or financial reports) are formal ...
, the
academic yearAn academic year or school year is a period of time which schools, colleges and university, universities use to measure a quantity of study. School holiday School holidays (also referred to as vacations, breaks, and recess) are the periods during ...
, etc. Similarly, ''year'' can mean the orbital period of any
planet A planet is an astronomical body orbiting a star or Stellar evolution#Stellar remnants, stellar remnant that is massive enough to be Hydrostatic equilibrium, rounded by its own gravity, is not massive enough to cause thermonuclear fusion, and ...

planet
; for example, a Martian year and a
Venusian
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 The term Great Year has two major meanings. It is defined by scientific astronomy as "The period of one complete cycle of the equinoxes An equinox is the instant of time when the plane of Earth Earth is the third planet from the Sun ...
.


Etymology

English ''year'' (via West Saxon ''ġēar'' (),
Anglian Anglian may refer to: * Anglian, meaning "of the Angles", a Germanic people who settled in Britain in the post-Roman period * Anglian, a group of dialects of Old English * Anglian automobile, an English tricar manufactured from 1905 to 1907 * Angl ...
''ġēr'') continues
Proto-Germanic Proto-Germanic (abbreviated PGmc; also called Common Germanic) is the reconstructed Reconstruction may refer to: Politics, history, and sociology *Reconstruction (law), the transfer of a company's (or several companies') business to a new ...
''*jǣran'' (''*j ē₁ran''). Cognates are German language, German ''Jahr'', Old High German ''jār'', Old Norse ''ár'' and Gothic language, Gothic '':wikt:𐌾𐌴𐍂, jer'', from the Proto-Indo-European noun ' "year, season". Cognates also descended from the same Proto-Indo-European noun (with variation in suffix ablaut) are Avestan ''yārǝ'' "year", Ancient Greek, Greek () "year, season, period of time" (whence "hour"), Old Church Slavonic ''jarŭ'', and Latin ''hornus'' "of this year". Latin (a 2nd declension masculine noun; is the accusative case, accusative singular; is genitive singular and nominative plural; the dative and ablative singular) is from a Proto-Indo-European language, PIE noun ', which also yielded Gothic ''aþn'' "year" (only the dative plural ''aþnam'' is attested). Although most languages treat the word as thematic ''*yeh₁r-o-'', there is evidence for an original derivation with an ''*-r/n'' suffix, ''*yeh₁-ro-''. Both Indo-European words for year, ''*yeh₁-ro-'' and ''*h₂et-no-'', would then be derived from verbal roots meaning "to go, move", ''*h₁ey-'' and ''*h₂et-'', respectively (compare Vedic Sanskrit ''éti'' "goes", ''atasi'' "thou goest, wanderest"). A number of English words are derived from Latin , such as wikt:annual, annual, :wikt:annuity, annuity, :wikt:anniversary, anniversary, etc.; '':wikt:per annum, per annum'' means "each year", means "in the year of the Lord". The Greek word for "year", , is cognate with Latin ''vetus'' "old", from the PIE word ''*wetos-'' "year", also preserved in this meaning in Sanskrit ' "year" and ' "yearling (calf)", the latter also reflected in Latin '':wikt:vitulus, vitulus'' "bull calf", English ''wether'' "ram" (Old English ''weðer'', Gothic ''wiþrus'' "lamb"). In some languages, it is common to count years by referencing to one season, as in "summers", or "winters", or "harvests". Examples include Chinese :wikt:年, 年 "year", originally :wikt:秂, 秂, an ideographic compound of a person carrying a bundle of wheat denoting "harvest". Slavic besides '':wikt:Reconstruction:Proto-Slavic/godъ, godŭ'' "time period; year" uses '':wikt:Reconstruction:Proto-Slavic/lěto, lěto'' "summer; year".


Intercalation

Astronomical years do not have an integer number of days or lunar months. Any calendar that follows an astronomical year must have a system of Intercalation (timekeeping), intercalation such as leap years.


Julian calendar

In the Julian calendar, the average (mean) length of a year is 365.25 days. In a non-leap year, there are 365 days, in a leap year there are 366 days. A leap year occurs every fourth year, or leap year, during which a leap day is intercalation (timekeeping)#Solar calendars, intercalated into the month of February. The name "February 29, Leap Day" is applied to the added day. The Revised Julian calendar, proposed in 1923 and used in some Eastern Orthodox Churches, has 218 leap years every 900 years, for the average (mean) year length of days, close to the length of the mean tropical year, days (relative error of 9·10−8). In the year 2800 CE, the Gregorian and Revised Julian calendars will begin to differ by one calendar day.


Gregorian calendar

The Gregorian calendar attempts to cause the northward equinox to fall on or shortly before March 21 and hence it follows the tropical year, northward equinox year, or tropical year. Because 97 out of 400 years are leap years, the mean length of the Gregorian calendar year is days; with a relative error below one Parts per million, ppm (8·10−7) relative to the current length of the mean tropical year ( days) and even closer to the current ''March equinox year'' of days that it aims to match. It is estimated that by the year 4000 CE, the northward equinox will fall back by one day in the Gregorian calendar, not because of this difference, but due to the slowing of the Earth's rotation and the associated lengthening of the day.


Other calendars

Historically, lunisolar calendars intercalated entire leap months on an observational basis. Lunisolar calendars have mostly fallen out of use except for liturgical reasons (Hebrew calendar, various Hindu calendars). A modern adaptation of the historical Jalali calendar, known as the Solar Hijri calendar (1925), is a purely solar calendar with an irregular pattern of leap days based on observation (or astronomical computation), aiming to place new year (Nowruz) on the day of March equinox, vernal equinox (for the time zone of Tehran), as opposed to using an algorithmic system of leap years.


Year numbering

A calendar era assigns a cardinal number (linguistics), cardinal number to each sequential year, using a reference event in the past (called the epoch) as the beginning of the era. The Gregorian calendar era is the world's most widely used civil calendar. Its epoch is a Dionysius Exiguus#Anno Domini, 6th century estimate of the date birth of Jesus of Nazareth. Two notations are used to indicate year numbering in the Gregorian calendar: the Christian "Anno Domini" (meaning "in the year of the Lord"), abbreviated AD; and "Common Era", abbreviated CE, preferred by many of other faiths and none. Year numbers are based on inclusive counting, so that there is no "year zero". Years before the epoch are abbreviated BC for Before Christ or BCE for Before the Common Era. In Astronomical year numbering, positive numbers indicate years AD/CE, the number 0 (year), 0 designates 1 BC/BCE, −1 designates 2 BC/BCE, and so on. Other eras include that of Ancient Rome, ("from the foundation of Rome, the city), abbreviated AUC; ("year of the world"), used for the Hebrew calendar and abbreviated AM; and the Japanese emperor eras described above. (The Islamic (year of the Hegira), abbreviated AH, is a lunar calendar of twelve lunar months and thus is shorter than a solar year.)


Pragmatic divisions

Financial and scientific calculations often use a 365-day calendar to simplify daily rates.


Fiscal year

A
fiscal year A fiscal year (or financial year, or sometimes budget year) is used in government accounting, which varies between countries, and for budget purposes. It is also used for financial report Financial statements (or financial reports) are formal ...
or financial year is a 12-month period used for calculating annual financial statements in businesses and other organizations. In many jurisdictions, regulations regarding accounting require such reports once per twelve months, but do not require that the twelve months constitute a calendar year. For example, in Canada and India the fiscal year runs from April 1; in the United Kingdom it runs from April 1 for purposes of corporation tax and government financial statements, but from April 6 for purposes of personal taxation and payment of state benefits; in Australia it runs from July 1; while in the United States the fiscal year of the Federal government of the United States, federal government runs from October 1.


Academic year

An academic year is the annual period during which a student attends an educational institution. The academic year may be divided into academic terms, such as semesters or quarters. The school year in many countries starts in August or September and ends in May, June or July. In Israel the academic year begins around October or November, aligned with the second month of the Hebrew calendar. Some schools in the UK, Canada and the United States divide the academic year into ''three'' roughly equal-length terms (called ''trimesters'' or ''quarters'' in the United States), roughly coinciding with autumn, winter, and spring. At some, a shortened summer session, sometimes considered part of the regular academic year, is attended by students on a voluntary or elective basis. Other schools break the year into ''two'' main semesters, a first (typically August through December) and a second semester (January through May). Each of these main semesters may be split in half by mid-term exams, and each of the halves is referred to as a ''quarter'' (or ''term'' in some countries). There may also be a voluntary summer session and/or a short January session. Some other schools, including some in the United States, have ''four'' marking periods. Some schools in the United States, notably Boston Latin School, may divide the year into ''five or more'' marking periods. Some state in defense of this that there is perhaps a positive correlation between report frequency and academic achievement. There are typically 180 days of teaching each year in schools in the US, excluding weekends and breaks, while there are 190 days for pupils in state schools in Canada, New Zealand and the United Kingdom, and 200 for pupils in Australia. In India the academic year normally starts from June 1 and ends on May 31. Though schools start closing from mid-March, the actual academic closure is on May 31 and in Nepal it starts from July 15. Schools and universities in Australia typically have academic years that roughly align with the calendar year (i.e., starting in February or March and ending in October to December), as the southern hemisphere experiences summer from December to February.


Astronomical years


Julian year

The Julian year, as used in astronomy and other sciences, is a time unit defined as exactly 365.25 days. This is the normal meaning of the unit "year" used in various scientific contexts. The Julian century of days and the Julian millennium of days are used in astronomical calculations. Fundamentally, expressing a time interval in Julian years is a way to precisely specify how many days (not how many "real" years), for long time intervals where stating the number of days would be unwieldy and unintuitive. By convention, the Julian year is used in the computation of the distance covered by a light-year. In the Unified Code for Units of Measure, the symbol ''a'' (without subscript) always refers to the Julian year, ''aj'', of exactly
second The second (symbol: s, also abbreviated: sec) is the of in the (SI) (french: Système International d’unités), commonly understood and historically defined as of a – this factor derived from the division of the day first into 24 s, th ...
s. :365.25 d × s = 1 a = 1 aj = Second#SI multiples, Ms The #SI prefix multipliers, SI multiplier prefixes may be applied to it to form "ka" (kiloannus), "Ma" (megaannus), etc.


Sidereal, tropical, and anomalistic years

Each of these three years can be loosely called an ''astronomical year''. The sidereal year is the time taken for the Earth to complete one revolution of its orbit, as measured against a fixed frame of reference (such as the fixed stars, Latin , singular ). Its average duration is days (365 d 6 h 9 min 9.76 s) (at the epoch J2000.0 = January 1, 2000, 12:00:00 Terrestrial Time, TT). Today the mean tropical year is defined as the period of time for the mean ecliptic longitude of the Sun to increase by 360 degrees. Since the Sun's ecliptic longitude is measured with respect to the equinox, the tropical year comprises a complete cycle of the seasons and is the basis of solar calendars such as the internationally used
Gregorian calendar The Gregorian calendar is the used in most of the world. It was introduced in October 1582 by as a modification of the , reducing the average year from 365.25 days to 365.2425 days, and adjusting for the drift in the that the inaccuracy ha ...
. The modern definition of mean tropical year differs from the actual time between passages of, e.g., the northward equinox for several reasons explained below. Because of the Earth's axial precession (astronomy), axial precession, this year is about 20 minutes shorter than the sidereal year. The mean tropical year is approximately 365 days, 5 hours, 48 minutes, 45 seconds, using the modern definition ( = 365.24219 d × 86 400 s). The anomalistic year is the time taken for the Earth to complete one revolution with respect to its Apsis, apsides. The orbit of the Earth is elliptical; the extreme points, called apsides, are the perihelion, where the Earth is closest to the Sun (January 5, 07:48 UT in 2020), and the aphelion, where the Earth is farthest from the Sun (July 4, 11:35 UT in 2020). The anomalistic year is usually defined as the time between perihelion passages. Its average duration is 365.259636 days (365 d 6 h 13 min 52.6 s) (at the epoch J2011.0).


Draconic year

The draconic year, draconitic year, eclipse year, or ecliptic year is the time taken for the Sun (as seen from the Earth) to complete one revolution with respect to the same lunar node (a point where the Moon's orbit intersects the ecliptic). The year is associated with eclipses: these occur only when both the Sun and the Moon are near these nodes; so eclipses occur within about a month of every half eclipse year. Hence there are two eclipse seasons every eclipse year. The average duration of the eclipse year is : days (346 d 14 h 52 min 54 s) (at the epoch J2000.0). This term is sometimes erroneously used for the draconic or nodal period of lunar precession, that is the period of a complete revolution of the Moon's ascending node around the ecliptic: Julian years ( days; at the epoch J2000.0).


Full moon cycle

The full moon cycle is the time for the Sun (as seen from the Earth) to complete one revolution with respect to the perigee of the Moon's orbit. This period is associated with the apparent size of the full moon, and also with the varying duration of the month, synodic month. The duration of one full moon cycle is: : days (411 days 18 hours 49 minutes 35 seconds) (at the epoch J2000.0).


Lunar year

The lunar year comprises twelve full cycles of the phases of the Moon, as seen from Earth. It has a duration of approximately 354.37 days. Muslims use this for celebrating their Muslim holidays, Eids and for marking the start of the fasting month of Ramadan. A Muslim calendar year is based on the lunar cycle. The Hebrew calendar, Jewish calendar is also essentially lunar, except that an intercalary lunar month is added once every two or three years, in order to keep the calendar synchronized with the solar cycle as well. Thus, a lunar year on the Jewish (Hebrew) calendar consists of either twelve or thirteen lunar months.


Vague year

The vague year, from or wandering year, is an integral approximation to the year equaling 365 days, which wanders in relation to more exact years. Typically the vague year is divided into 12 wikt:schematic, schematic months of 30 days each plus 5 Intercalation (timekeeping), epagomenal days. The vague year was used in the calendars of Ethiopian calendar, Ethiopia, Egyptian calendar, Ancient Egypt, Iranian calendar, Iran, Armenian calendar, Armenia and in Mesoamerican calendars, Mesoamerica among the Aztec calendar, Aztecs and Haab', Maya. It is still used by many Zoroastrian communities.


Heliacal year

A heliacal year is the interval between the heliacal risings of a star. It differs from the sidereal year for stars away from the ecliptic due mainly to the precession of the equinoxes.


Sothic year

The Sothic cycle, Sothic year is the interval between heliacal risings of the star Sirius. It is currently less than the #Sidereal, tropical, and anomalistic years, sidereal year and its duration is very close to the Julian year of 365.25 days.


Gaussian year

The Gaussian year is the sidereal year for a planet of negligible mass (relative to the Sun) and unperturbed by other planets that is governed by the Gaussian gravitational constant. Such a planet would be slightly closer to the Sun than Earth's mean distance. Its length is: : days (365 d 6 h 9 min 56 s).


Besselian year

The Besselian epoch#Besselian years, Besselian year is a tropical year that starts when the (fictitious) mean Sun reaches an ecliptic longitude of 280°. This is currently on or close to January 1. It is named after the 19th-century German astronomer and mathematician Friedrich Bessel. The following equation can be used to compute the current Besselian epoch (in years): : B = 1900.0 + (Julian dateTT − ) / The TT subscript indicates that for this formula, the Julian date should use the Terrestrial Time scale, or its predecessor, ephemeris time.


Variation in the length of the year and the day

The exact length of an astronomical year changes over time. * The positions of the equinox and solstice points with respect to the apsides of Earth's orbit change: the equinoxes and solstices move westward relative to the stars because of precession, and the apsides move in the other direction because of the long-term effects of gravitational pull by the other planets. Since the speed of the Earth varies according to its position in its orbit as measured from its perihelion, Earth's speed when in a solstice or equinox point changes over time: if such a point moves toward perihelion, the interval between two passages decreases a little from year to year; if the point moves towards aphelion, that period increases a little from year to year. So a "tropical year" measured from one passage of the northward ("vernal") equinox to the next, differs from the one measured between passages of the southward ("autumnal") equinox. The average over the full orbit does not change because of this, so the length of the average tropical year does not change because of this second-order effect. * Each planet's movement is perturbed by the gravity of every other planet. This leads to short-term fluctuations in its speed, and therefore its period from year to year. Moreover, it causes long-term changes in its orbit, and therefore also long-term changes in these periods. * Tidal drag between the Earth and the Moon and Sun increases the length of the day and of the month (by transferring angular momentum from the rotation of the Earth to the revolution of the Moon); since the apparent mean solar day is the unit with which we measure the length of the year in civil life, the length of the year appears to decrease. The rotation rate of the Earth is also changed by factors such as post-glacial rebound and sea level rise. ; Numerical value of year variation Mean year lengths in this section are calculated for 2000, and differences in year lengths, compared to 2000, are given for past and future years. In the tables a day is 86,400 SI seconds long.


Summary

An average Gregorian year is 365.2425
day A day is approximately the period during which the Earth completes one rotation around its axis, which takes around 24 hours. A solar day is the length of time which elapses between the Sun reaching its highest point in the sky two consecutive t ...

day
s (52.1775 weeks, hours, minutes or
second The second (symbol: s, also abbreviated: sec) is the of in the (SI) (french: Système International d’unités), commonly understood and historically defined as of a – this factor derived from the division of the day first into 24 s, th ...
s). For this calendar, a common year is 365 days ( hours, minutes or seconds), and a leap year is 366 days ( hours, minutes or seconds). The 400-year cycle of the Gregorian calendar has days and hence exactly weeks.


Greater astronomical years


Equinoctial cycle

The Great Year, or equinoctial cycle, corresponds to a complete revolution of the equinoxes around the ecliptic. Its length is about 25,700 years.


Galactic year

The Galactic year is the time it takes Earth's Solar System to revolve once around the galactic center. It comprises roughly 230 million Earth years.


Seasonal year

A seasonal year is the time between successive recurrences of a seasonal event such as the flooding of a river, the migration of a species of bird, the flowering of a species of plant, the first frost, or the first scheduled game of a certain sport. All of these events can have wide variations of more than a month from year to year.


Symbols

A common symbol for the year as a
unit of time A unit of time is any particular time Time is the indefinite continued sequence, progress of existence and event (philosophy), events that occur in an apparently irreversible process, irreversible succession from the past, through the presen ...
is "a", taken from the Latin word . In English, the abbreviations "y" or "yr" are more commonly used in non-scientific literature but also specifically in geology and paleontology, where "kyr, myr, byr" (thousands, millions, and billions of years, respectively) and similar abbreviations are used to denote intervals of time remote from the present.


Symbol

NIST SP811 supports the symbol "a" as the unit of time for a year. In English, the abbreviations "y" and "yr" are also used. The Unified Code for Units of Measure disambiguates the varying symbologies of ISO 1000, ISO 2955 and ANSI X3.50 by using: :at = days for the mean tropical year; :aj = 365.25 days for the mean Julian year; :ag = days for the mean Gregorian year; where: :a, without a qualifier = 1 aj; :and ar, for ''Hectare#Are, are'', is a unit of area. The International Union of Pure and Applied Chemistry (IUPAC) and the International Union of Geological Sciences have jointly recommended defining the "annus", with symbol "a", as the length of the tropical year in the year 2000: :a = seconds (approximately ephemeris days) This differs from the above definition of 365.25 days by about 20 parts per million. The joint document says that definitions such as the Julian year "bear an inherent, pre-programmed obsolescence because of the variability of Earth's orbital movement", but then proposes using the length of the tropical year as of 2000 AD (specified down to the millisecond), which suffers from the same problem. (The tropical year oscillates with time by more than a minute.) The notation has proved controversial as it conflicts with an earlier convention among geoscientists to use "a" specifically for ''years ago'', and "y" or "yr" for a one-year time period.


SI prefix multipliers

For the following, there are alternative forms that elide the consecutive vowels, such as ''kilannus'', ''megannus'', etc. The exponents and exponential notations are typically used for calculating and in displaying calculations, and for conserving space, as in tables of data. * ka (for ''kiloannum'') – a
unit of time A unit of time is any particular time Time is the indefinite continued sequence, progress of existence and event (philosophy), events that occur in an apparently irreversible process, irreversible succession from the past, through the presen ...
equal to one thousand, or 103, years, or 1 E3 yr, also known as a millennium in anthropology and calendar uses. The prefix multiplier "ka" is typically used in geology, paleontology, and archaeology for the Holocene and Pleistocene period (geology), periods, where a non−radiocarbon dating technique: e.g. ice core dating, dendrochronology, uranium-thorium dating, or varve analysis; is used as the primary dating method for age determination. If age is determined primarily by radiocarbon dating, then the age should be expressed in either radiocarbon or calendar (calibrated) years Before Present. * Ma (for ''megaannum'') – a
unit of time A unit of time is any particular time Time is the indefinite continued sequence, progress of existence and event (philosophy), events that occur in an apparently irreversible process, irreversible succession from the past, through the presen ...
equal to one million, or 106, years, or 1 E6 yr. The suffix "Ma" is commonly used in scientific disciplines such as geology, paleontology, and celestial mechanics to signify very long time periods into the past or future. For example, the dinosaur species ''Tyrannosaurus rex'' was abundant approximately 66 Ma (66 million years) ago. The duration term "ago" may not always be indicated: if the quantity of a duration is specified while not explicitly mentioning a duration term, one can assume that "ago" is implied; the alternative unit "mya" does include "ago" explicitly. It is also written as "million years" (ago) in works for general public use. In astronomical applications, the year used is the Julian year of precisely 365.25 days. In geology and paleontology, the year is not so precise and varies depending on the author. * Ga (for ''gigaannum'') – a unit of time equal to 109 years, or one 1000000000 (number), billion years. "Ga" is commonly used in scientific disciplines such as physical cosmology, cosmology and geology to signify extremely long time periods in the past. For example, history of the earth, the formation of the Earth occurred approximately 4.54 Ga (4.54 billion years) ago and the age of the universe is approximately 13.8 Ga. * Ta (for ''teraannum'') – a unit of time equal to 1012 years, or one 1000000000000 (number), trillion years. "Ta" is an extremely long unit of time, about 70 times as long as the age of the universe. It is the same order of magnitude as the expected life span of a small red dwarf. * Pa (for ''petaannum'') – a unit of time equal to 1015 years, or one 10^15, quadrillion years. The half-life of the nuclide cadmium, cadmium-113 is about 8 Pa. This symbol coincides with that for the pascal (unit), pascal without a multiplier prefix, though both are infrequently used and context will normally be sufficient to distinguish time from pressure values. * Ea (for ''exaannum'') – a unit of time equal to 1018 years, or one quintillion years. The half-life of tungsten, tungsten-180 is 1.8 Ea.


Abbreviations yr and ya

In astronomy, geology, and paleontology, the abbreviation "yr" for ''years'' and "ya" for ''years ago'' are sometimes used, combined with prefixes for thousand, million, or billion. They are not SI units, using "y" to abbreviate the English ":wikt:year, year", but following ambiguous international recommendations, use either the standard English first letters as prefixes (t, m, and b) or metric prefixes (k, M, and G) or variations on metric prefixes (k, m, g). In archaeology, dealing with more recent periods, normally expressed dates, e.g. "22,000 years ago" may be used as a more accessible equivalent of a Before Present ("BP") date. These abbreviations include: Use of "mya" and "bya" is deprecated in modern geophysics, the recommended usage being "Ma" and "Ga" for dates Before Present, but "m.y." for the duration of epochs. This ''ad hoc'' distinction between "absolute" time and time intervals is somewhat controversial amongst members of the Geological Society of America. Note that on graphs, using "ya" units on the horizontal axis time flows from right to left, which may seem counter-intuitive. If the "ya" units are on the vertical axis, time flows from top to bottom which is probably easier to understand than conventional notation.


See also

* : current year * Astronomical year numbering * Century * Decade * Epoch, Epoch (reference date) * ISO 8601: standard for representation of dates and times * List of calendars * List of years * Millennium * Orders of magnitude (time) * Unit of time


References


Notes


Further reading

* *


External links

* :v:sl:letnice, Images of years {{Authority control Types of year, *