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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 wi ...

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 An atmosphere (from the greek words ἀτμός ''(atmos)'', meaning 'vapour', and σφαῖρα ''(sphaira)'', meaning 'ball' or 'sphere') is a layer or a set of layers of gases surrounding a p ...

weather
, the hours of
daylight Daylight is the combination of all direct and indirect sunlight during the daytime. This includes direct sunlight, diffuse sky radiation, and (often) both of these reflected by Earth and terrestrial objects, like landforms and buildings. Sunligh ...

daylight
, and, consequently,
vegetation Vegetation is an assemblage of plant Plants are predominantly photosynthetic Photosynthesis is a process used by plants and other organisms to Energy transformation, convert light energy into chemical energy that, through cellular re ...

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 weather in the tropics is dominated by the tropical rain belt, which moves from the northern to the southern tropics and back over the course of the year. Rain ...
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 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, speci ...
, 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 astron ...

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 Roman or Romans usually refers to: *Rome, the capital city of Italy *Ancient Rome, Roman civilization from 8th century B ...
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 The word day has a number of meanings, depending on the context it is used such as of astronomy, physics, and various calendar systems. As a term in physics and astronomy it is approximately the period during which the Earth completes one ro ...

day
s of exactly 86,400
second The second (symbol: s, also abbreviated: sec) is the base unit of time Time is the continued sequence of existence and event (philosophy), events that occurs in an apparently irreversible process, irreversible succession from the past, th ...
s (
SI base unit The SI base units are the standard defined by the (SI) for the seven of what is now known as the : they are notably a basic set from which all other can be . The units and their physical quantities are the for , the for , the for , the f ...

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 German(s) may refer to: Common uses * of or related to Germany * Germans, Germanic ethnic group, citizens of Germany or people of German ancestry * For citizens of Germany, see also German nationality law * German language The German la ...

German
''Jahr'',
Old High German Old High German (OHG, german: Althochdeutsch, German abbr. ) is the earliest stage of the German language German ( Standard High German: , ) is a West Germanic language mainly spoken in Central Europe Central Europe is an area of Euro ...
''jār'',
Old Norse Old Norse, Old Nordic, or Old Scandinavian is a stage of development of North Germanic dialects before their final divergence into separate Nordic languages. Old Norse was spoken by inhabitants of Scandinavia Scandinavia; : ''Skades ...
''ár'' and
Gothic Gothic or Gothics may refer to: People and languages *Goths or Gothic people, the ethnonym of a group of East Germanic tribes **Gothic language, an extinct East Germanic language spoken by the Goths **Crimean Gothic, the Gothic language spoken by ...
'' jer'', from the
Proto-Indo-European Proto-Indo-European (PIE) is the theorized common ancestor of the Indo-European language family The Indo-European languages are a language family A language is a structured system of communication used by humans, including speech ( ...
noun ' "year, season". Cognates also descended from the same Proto-Indo-European noun (with variation in suffix
ablaut In linguistics, the Indo-European ablaut (, from Standard High German, German '':wikt:Ablaut#German, Ablaut'' ) is a system of apophony in the Proto-Indo-European language (PIE). An example of ablaut in English is the Germanic strong verb, stron ...
) are
Avestan Avestan , also known historically as Zend, comprises two languages: Old Avestan (spoken in the 2nd millennium BCE) and Younger Avestan (spoken in the 1st millennium BCE). The languages are known only from their use as the language of Zoroastrian ...
''yārǝ'' "year",
Greek#REDIRECT Greek Greek may refer to: Greece Anything of, from, or related to Greece Greece ( el, Ελλάδα, , ), officially the Hellenic Republic, is a country located in Southeast Europe. Its population is approximately 10.7 million as of ...
() "year, season, period of time" (whence "
hour An hour (symbol A symbol is a mark, sign, or that indicates, signifies, or is understood as representing an , , or . Symbols allow people to go beyond what is n or seen by creating linkages between otherwise very different s and s. All ...
"),
Old Church Slavonic Old Church Slavonic or Old Slavonic () was the first Slavic literary language A literary language is the form of a language A language is a structured system of communication used by humans, including speech (spoken language), gestures (S ...
''jarŭ'', and
Latin Latin (, or , ) is a classical language belonging to the Italic languages, Italic branch of the Indo-European languages. Latin was originally spoken in the area around Rome, known as Latium. Through the power of the Roman Republic, it became ...

Latin
''hornus'' "of this year". Latin (a
2nd declension Latin declension is the set of patterns according to which Latin Latin (, or , ) is a classical language belonging to the Italic languages, Italic branch of the Indo-European languages. Latin was originally spoken in the area around Rome, known ...
masculine noun; is the accusative singular; is genitive singular and nominative plural; the dative and ablative singular) is from a
PIE A pie is a baked Baking is a method of preparing food that uses dry heat, typically in an oven, but can also be done in hot ashes, or on hot stones. The most common baked item is bread but many other types of foods are baked. Heat is grad ...
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 Vedic Sanskrit was an ancient language of the Indo-AryanIndo-Aryan refers to: * Indo-Aryan languages ** Indo-Aryan superstrate in Mitanni or Mitanni-Aryan * Indo-Aryan peoples, the various peoples speaking these languages See also *Aryan invasi ...
''éti'' "goes", ''atasi'' "thou goest, wanderest"). A number of English words are derived from Latin , such as
annual Annual may refer to: *Annual publication, periodical publications appearing regularly once per year **Yearbook **Literary annual *Annual plant *Annual report *Annual giving *Annual, Morocco, a settlement in northeastern Morocco *Annuals (band), a ...
,
annuity An annuity is a series of payments made at equal intervals.Kellison, Stephen G. (1970). ''The Theory of Interest''. Homewood, Illinois: Richard D. Irwin, Inc. p. 45 Examples of annuities are regular deposits to a savings account A savings acco ...
,
anniversary An anniversary is the date on which an event took place or an institution was founded in a previous year, and may also refer to the commemoration or celebration of that event. For example, the first event is the initial occurrence or, if plan ...
, etc.; '' 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 Sanskrit (; attributively , ; nominalization, nominally , , ) is a classical language of South Asia that belongs to the Indo-Aryan languages, Indo-Aryan branch of the Indo-European languages. It arose in South Asia after its predecessor langua ...

Sanskrit
' "year" and ' "yearling (calf)", the latter also reflected in Latin '' 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 "year", originally , an ideographic compound of a person carrying a bundle of wheat denoting "harvest". Slavic besides '' godŭ'' "time period; year" uses '' lěto'' "summer; year".


Intercalation

Astronomical years do not have an
integer An integer (from the Latin Latin (, or , ) is a classical language A classical language is a language A language is a structured system of communication Communication (from Latin ''communicare'', meaning "to share" or "to ...
number of days or lunar months. Any calendar that follows an astronomical year must have a system of 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 intercalated into the month of February. The name "
Leap Day February 29, also known as leap day or leap year day, is a date added to 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 ...
" is applied to the added day. The
Revised Julian calendar The Revised Julian calendar, also known as the Milanković calendar, or less formally new calendar, is a calendar A calendar is a system of organizing days. This is done by giving names to periods of time, typically days, weeks, months an ...
, proposed in 1923 and used in some
Eastern Orthodox Church The Eastern Orthodox Church, also called the Orthodox Church, is the second-largest Christian church, with approximately 220 million baptised members. It operates as a communion Communion may refer to: Religion * The Eucharist (also cal ...
es, 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 The March equinox or northward equinox is the equinox An equinox is traditionally defined as the time when the plane In mathematics, a plane is a flatness (mathematics), flat, two-dimensional surface (mathematics), surface that extends ...
to fall on or shortly before March 21 and hence it follows the northward equinox year, or
tropical year A tropical year (also known as a solar year or tropical period) is the time Time is the continued sequence of existence and event (philosophy), events that occurs in an apparently irreversible process, irreversible succession from the past ...
. 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 ppm (8·10−7) relative to the current length of the mean
tropical year A tropical year (also known as a solar year or tropical period) is the time Time is the continued sequence of existence and event (philosophy), events that occurs in an apparently irreversible process, irreversible succession from the past ...
( 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 The Hebrew calendar ( Hebrew: , ), also called Jewish calendar, is a lunisolar calendar used today for Jewish religious observance, and as an official calendar of the state of Israel Israel (; he, יִשְׂרָאֵל; ar, إِس ...
, various
Hindu calendar The Hindu calendar, Panchanga (Sanskrit Sanskrit (, attributively , ''saṃskṛta-'', nominalization, nominally , ''saṃskṛtam'') is a classical language of South Asia belonging to the Indo-Aryan languages, Indo-Aryan branch of the Indo- ...

Hindu calendar
s). A modern adaptation of the historical
Jalali calendar The Jalali calendar is a solar calendar, was compiled during the reign of Jalaluddin Malik-Shah I of Seljuk by the order of Nizam al-Mulk and the place of observation were the cities of Isfahan (the capital of the Seljuks), Rey, and Neishabour. Va ...
, known as the
Solar Hijri calendar The Solar Hijri calendar ( fa, گاه‌شماری هجری خورشیدی, gāhshomāri-ye hejri-ye khorshidi; ps, لمريز لېږدیز کلیز, lamrez legdez kalhandara) is a solar calendar A solar calendar is a calendar A cale ...
(1925), is a purely
solar calendar A solar calendar is a 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 ...
with an irregular pattern of leap days based on observation (or astronomical computation), aiming to place new year (
Nowruz Nowruz ( fa, نوروز, ; ) is the Iranian New Year, also known as the Persian New Year, which begins on the spring equinox, marking the first day of Farvardin Farvardin ( fa, فروردین, ) is the Iranian Persian name for the first mont ...

Nowruz
) on the day of vernal equinox (for the time zone of
Tehran Tehran (; fa, تهران ) is the Capital city, capital of Iran and Tehran Province. With a population of around 8.7 million in the city and 15 million in the larger metropolitan area of Greater Tehran, Tehran is the List of largest cities o ...

Tehran
), as opposed to using an algorithmic system of leap years.


Year numbering

A
calendar era #REDIRECT Calendar era #REDIRECT Calendar era A calendar era is the period of time elapsed since one ''epoch'' of a calendar and, if it exists, before the next one. For example, it is the year as per the Gregorian calendar, which numbers its year ...
assigns a
cardinal number 150px, Aleph null, the smallest infinite cardinal In mathematics Mathematics (from Ancient Greek, Greek: ) includes the study of such topics as quantity (number theory), mathematical structure, structure (algebra), space (geometry), and ca ...
to each sequential year, using a reference event in the past (called the
epoch In chronology 222px, Joseph Scaliger's ''De emendatione temporum'' (1583) began the modern science of chronology Chronology (from Latin Latin (, or , ) is a classical language belonging to the Italic languages, Italic branch of the Indo-E ...
) as the beginning of the era. The Gregorian calendar era is the world's most widely used
civil calendar The civil calendar is the 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 wi ...
. Its epoch is a 6th century estimate of the date birth of
Jesus of Nazareth Jesus, likely from he, יֵשׁוּעַ, translit=Yēšūaʿ, label=Hebrew Hebrew (, , or ) is a Northwest Semitic languages, Northwest Semitic language of the Afroasiatic languages, Afroasiatic language family. Historically, it ...
. Two notations are used to indicate year numbering in the Gregorian calendar: the Christian "
Anno Domini The terms (AD) and before Christ (BC) are used to label or number years in the Julian and 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 , r ...
" (meaning "in the year of the Lord"), abbreviated AD; and "
Common Era Common Era (CE) is one of the year notations used for the Gregorian calendar (and its predecessor, the Julian calendar), the world's most widely used calendar era. Before the Common Era (BCE) is the era before CE. BCE and CE are alternatives ...
", abbreviated CE, preferred by many of other faiths and none. Year numbers are based on
inclusive counting Counting is the process of determining the number A number is a mathematical object used to counting, count, measurement, measure, and nominal number, label. The original examples are the natural numbers 1, 2, 3, 4, and so forth. Numbers can be ...
, so that there is no "year zero". Years before the epoch are abbreviated BC for
Before Christ The terms (AD) and before Christ (BC) are used to label or number years in the Julian and 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 , r ...
or BCE for
Before the Common Era Common Era (CE) is one of the year notations used for the Gregorian calendar The Gregorian calendar is the calendar used in most of the world. It was introduced in October 1582 by Pope Gregory XIII as a minor modification of the Julian c ...
. In
Astronomical year numbering Astronomical year numbering is based on AD/ CE year numbering, but follows normal decimal The decimal numeral system A numeral system (or system of numeration) is a writing system A writing system is a method of visually representing ...
, positive numbers indicate years AD/CE, the number 0 designates 1 BC/BCE, −1 designates 2 BC/BCE, and so on. Other eras include that of
Ancient Rome In historiography Historiography is the study of the methods of historian ( 484– 425 BC) was a Greek historian who lived in the 5th century BC and one of the earliest historians whose work survives. A historian is a person who stud ...
, ("from the foundation of
the city
the city
), abbreviated AUC; ("year of the world"), used for the
Hebrew calendar The Hebrew calendar ( Hebrew: , ), also called Jewish calendar, is a lunisolar calendar used today for Jewish religious observance, and as an official calendar of the state of Israel Israel (; he, יִשְׂרָאֵל; ar, إِس ...
and abbreviated AM; and the Japanese emperor eras described above. (The Islamic (year of the
Hegira Hegira () is a medieval Latin transliteration of the Arabic word meaning "departure" or "migration", among other definitions. Alternative transliterations of the word include Hijra or Hijrah. The word is commonly used to refer to the journey ...
), abbreviated AH, is a
lunar calendar A lunar calendar is a 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 with ...
of twelve
lunar month In lunar calendar A lunar calendar is a calendar based on the monthly cycles of the Moon's lunar phase, phases (Lunar month#Synodic month, synodic months, lunations), in contrast to solar calendars, whose annual cycles are based only directly o ...
s 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 Canada is a country in the northern part of North America North America is a continent A continent is any of several large landmasses. Generally identified by convention (norm), convention rather than any strict criteria, ...

Canada
and
India India, officially the Republic of India (Hindi Hindi (Devanagari: , हिंदी, ISO 15919, ISO: ), or more precisely Modern Standard Hindi (Devanagari: , ISO 15919, ISO: ), is an Indo-Aryan language spoken chiefly in Hindi Belt, ...

India
the fiscal year runs from April 1; in the
United Kingdom The United Kingdom of Great Britain and Northern Ireland, commonly known as the United Kingdom (UK) or Britain,Usage is mixed. The Guardian' and Telegraph' use Britain as a synonym for the United Kingdom. Some prefer to use Britain as shorth ...

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 Australia, officially the Commonwealth of Australia, is a Sovereign state, sovereign country comprising the mainland of the Australia (continent), Australian continent, the island of Tasmania, and numerous List of islands of Australia, sma ...

Australia
it runs from July 1; while in the
United States The United States of America (U.S.A. or USA), commonly known as the United States (U.S. or US) or America, is a country Continental United States, primarily located in North America. It consists of 50 U.S. state, states, a Washington, D.C., ...

United States
the fiscal year of the
federal government A federation (also known as a federal state) is a political entity A polity is an identifiable political Politics (from , ) is the set of activities that are associated with Decision-making, making decisions in Social group, groups, ...
runs from October 1.


Academic year

An academic year is the annual period during which a student attends an
educational institution An educational institution is a place where people of different ages gain an education Education is the process of facilitating learning, or the acquisition of knowledge, skills, value (ethics), values, morals, beliefs, habits, and persona ...
. The academic year may be divided into
academic term An academic term (or simply term) is a portion of an academic year, the time during which an educational institution holds classes. The schedule A train schedule informs travelers of the trains going to various locations, and indicates the ti ...
s, 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 The Boston Latin School is a public exam school in Boston Boston (, ), officially the City of Boston, is the capital city, capital and List of municipalities in Massachusetts, most populous city of the Commonwealth (U.S. state), Commonwe ...

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 A light-year, alternatively spelt lightyear, is a unit of length A unit of length refers to any arbitrarily chosen and accepted reference standard for measurement of length. The most common units in modern use are the metric system, metric un ...
. In the
Unified Code for Units of MeasureThe Unified Code for Units of Measure (UCUM) is a system of codes for unambiguously representing measurement units. Its primary purpose is machine-to-machine communication rather than communication between humans. The code set includes all units def ...
, the symbol ''a'' (without subscript) always refers to the Julian year, ''aj'', of exactly
second The second (symbol: s, also abbreviated: sec) is the base unit of time Time is the continued sequence of existence and event (philosophy), events that occurs in an apparently irreversible process, irreversible succession from the past, th ...
s. :365.25 d × s = 1 a = 1 aj = Ms The 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 A sidereal year (, ; from Latin "asterism, star") is the time taken by the Earth to orbit the Sun once with respect to the fixed stars. Hence, it is also the time taken for the Sun to return to the same position with respect to the fixed stars af ...
is the time taken for the Earth to complete one revolution of its
orbit In celestial mechanics, an orbit is the curved trajectory of an physical body, object such as the trajectory of a planet around a star, or of a natural satellite around a planet, or of an satellite, artificial satellite around an object or po ...

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 In astronomy Astronomy (from el, ἀστρονομία, literally meaning the science that studies the laws of the stars) is a natural science that studies astronomical object, celestial objects and celestial event, phenomena. It uses ma ...
= January 1, 2000, 12:00:00 TT). Today the mean
tropical year A tropical year (also known as a solar year or tropical period) is the time Time is the continued sequence of existence and event (philosophy), events that occurs in an apparently irreversible process, irreversible succession from the past ...
is defined as the period of time for the mean
ecliptic longitude The ecliptic coordinate system is a celestial coordinate system commonly used for representing the apparent place, apparent positions and orbits of Solar System objects. Because most planets (except Mercury (planet), Mercury) and many Small Solar S ...
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 calendar A solar calendar is a 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 ...
s such as the internationally used
Gregorian calendar The Gregorian calendar is the 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, speci ...
. 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 In astronomy, axial precession is a gravity-induced, slow, and continuous change in the orientation of an astronomical body's rotational axis. In particular, it can refer to the gradual shift in the orientation of Earth's axis of rotation in ...
, 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
apsides
apsides
. The orbit of the Earth is elliptical; the extreme points, called apsides, are the
perihelion upright=1.15, The two-body system of interacting primary body (yellow); both are in elliptic orbits around their center of mass">common center of mass (or barycenter), (red +). ∗Periapsis and apoapsis as distances: The smallest and largest ...

perihelion
, where the Earth is closest to the Sun (January 5, 07:48 UT in 2020), and the
aphelion upright=1.15, The two-body system of interacting primary body A primary (also called a gravitational primary, primary body, or central body) is the main physical body of a gravity, gravitationally bound, multi-object system. This object consti ...

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 lunar node is either of the two orbital nodes of the Moon, that is, the two points at which the orbit of the Moon intersection (Euclidean geometry), intersects the ecliptic. The ''ascending'' (or ''north'') node is where the Moon moves into the ...
(a point where the Moon's orbit intersects the ecliptic). The year is associated with
eclipse ECLiPSe is a software system for the development and deployment of Constraint Programming Constraint programming (CP) is a paradigm for solving combinatorial problems that draws on a wide range of techniques from artificial intelligence ...

eclipse
s: 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 seasonAn eclipse season is one of only two periods during each year when eclipse An eclipse is an astronomical event that occurs when an astronomical object or spacecraft is temporarily obscured, by passing into the shadow of another body or by havi ...
s 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 Lunar precession is a term used for three different precession Precession is a change in the orientation of the rotational axis of a rotating body. In an appropriate reference frame it can be defined as a change in the first Euler angle, wher ...
, 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 A supermoon is a full moon The full Moon of 22 October 2010, as seen through a ecliptic latitude (or northern lunar standstill">lunistice), so the southern lunar craters, craters are especially prominent and cast shadows visible from Earth ...
is the time for the Sun (as seen from the Earth) to complete one revolution with respect to the
perigee Apsis ( el, ἀψίς; plural apsides , Greek: ἀψῖδες; "orbit") denotes either of the two extreme points (i.e., the farthest or nearest point) in the orbit In celestial mechanics, an orbit is the curved trajectory of an phy ...
of the Moon's orbit. This period is associated with the apparent size of the
full moon The full moon is the lunar phase s in 2022 as viewed from the Southern Hemisphere The Southern Hemisphere is the half (hemisphere Hemisphere may refer to: * A half of a sphere As half of the Earth * A hemispheres of Earth, hemisphere o ...

full moon
, and also with the varying duration of the
synodic month In lunar calendar A lunar calendar is a 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 sing ...
. 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 Lunar most commonly means "of or relating to the Moon The Moon is Earth's only proper natural satellite. At one-quarter the diameter of Earth (comparable to the width of Australia Australia, officially the Commonwealth of Austral ...
comprises twelve full cycles of the phases of the Moon, as seen from Earth. It has a duration of approximately 354.37 days.
Muslims Muslims () are people who follow or practice Islam Islam (; ar, اَلْإِسْلَامُ, al-’Islām, "submission o God Oh God may refer to: * An exclamation; similar to "oh no", "oh yes", "oh my", "aw goodness", "ah gosh", ...
use this for celebrating their Eids and for marking the start of the fasting month of
Ramadan , type = islam , longtype = Islam, Religious , image = Ramadan montage.jpg , caption=From top, left to right: A crescent moon over Sarıçam, Turkey, marking the beginning of the Islamic month of Ramadan. Ramadan Quran reading in Bandar Torkaman, I ...

Ramadan
. A Muslim calendar year is based on the lunar cycle. The
Jewish calendar The Hebrew calendar (Hebrew: , ), also called Jewish calendar, is a lunisolar calendar used today for Jewish religious observance, and as an official calendar of the state of Israel. It determines the dates for Jewish holidays and the appropri ...
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
schematic A schematic, or schematic diagram, is a representation of the elements of a system A system is a group of Interaction, interacting or interrelated elements that act according to a set of rules to form a unified whole. A system, surrounded ...
months of 30 days each plus 5
epagomenal The intercalary month or epagomenal days. of the ancient Egyptian calendar, Egyptian, Coptic calendar, Coptic, and Ethiopian calendars are a period of five days in common years and six days in leap years in addition to those calendars' 12 standard ...
days. The vague year was used in the calendars of
Ethiopia Ethiopia, officially the Federal Democratic Republic of Ethiopia, is a landlocked country A landlocked country is a country A country is a distinct territory, territorial body or political entity. It is often referred to as the ...
,
Ancient Egypt Ancient Egypt was a civilization  A civilization (or civilisation) is a that is characterized by , , a form of government, and systems of communication (such as ). Civilizations are intimately associated with additional char ...
,
Iran Iran ( fa, ایران ), also called Persia, and officially the Islamic Republic of Iran, is a country in Western Asia Western Asia, West Asia, or Southwest Asia, is the westernmost subregion A subregion is a part of a larger regio ...
,
Armenia Armenia (; hy, Հայաստան, translit=Hayastan, ), officially the Republic of Armenia,, is a landlocked country A landlocked country is a country A country is a distinct territory, territorial body or political entity. It is ...
and in
Mesoamerica Mesoamerica is a historical and important region In geography, regions are areas that are broadly divided by physical characteristics (physical geography), human impact characteristics (human geography), and the interaction of humanity and the ...
among the
Aztecs The Aztecs () were a Mesoamerican culture that flourished in central Mexico in the post-classic period from 1300 to 1521. The Aztec peoples included different Indigenous peoples of Mexico, ethnic groups of central Mexico, particularly those ...

Aztecs
and
Maya Maya may refer to: Civilizations * Maya peoples The Maya peoples () are an ethnolinguistic group of indigenous peoples Indigenous peoples, also referred to as First people, Aboriginal people, Native people, or autochthonous people, are cu ...
. It is still used by many Zoroastrian communities.


Heliacal year

A heliacal year is the interval between the
heliacal rising The heliacal rising ( ) or star rise of a star occurs annually, or the similar phenomenon of a planet, when it first becomes visible above the eastern horizon at dawn just before sunrise (thus becoming "the Morning Star (disambiguation)#Astronomy, ...
s of a star. It differs from the
sidereal year A sidereal year (, ; from Latin "asterism, star") is the time taken by the Earth to orbit the Sun once with respect to the fixed stars. Hence, it is also the time taken for the Sun to return to the same position with respect to the fixed stars af ...
for stars away from the
ecliptic The ecliptic is the plane (geometry), plane of Earth's orbit around the Sun. From the perspective of an observer on Earth, the Sun's movement around the celestial sphere over the course of a year traces out a path along the ecliptic against the ...

ecliptic
due mainly to the
precession of the equinoxes In astronomy Astronomy (from el, ἀστρονομία, literally meaning the science that studies the laws of the stars) is a natural science that studies astronomical object, celestial objects and celestial event, phenomena. It uses ...
.


Sothic year

The
Sothic year The Sothic cycle or Canicular period is a period of 1,461  Egyptian civil years of 365 days each or 1,460  Julian years averaging 365¼ days each. During a Sothic cycle, the 365 day year loses enough time that the start of ...
is the interval between heliacal risings of the star
Sirius Sirius () is the list of brightest stars, brightest star in the night sky. Its name is derived from the Ancient Greek language, Greek word (, 'glowing' or 'scorching'). The star is designated α Canis Majoris, Latinisation of name ...

Sirius
. It is currently less than the
sidereal year A sidereal year (, ; from Latin "asterism, star") is the time taken by the Earth to orbit the Sun once with respect to the fixed stars. Hence, it is also the time taken for the Sun to return to the same position with respect to the fixed stars af ...
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 The Gaussian gravitational constant (symbol ) is a parameter used in the orbital mechanics of the solar system The Solar SystemCapitalization of the name varies. The International Astronomical Union, the authoritative body regarding astrono ...
. 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 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 Friedrich Wilhelm Bessel (; 22 July 1784 – 17 March 1846) was a German German(s) may refer to: Common uses * of or related to Germany * Germans, Germanic ethnic group, citizens of Germany or people of German ancestry * For citizens of Germa ...
. 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 Terrestrial Time (TT) is a modern astronomical time standard defined by the International Astronomical Union The International Astronomical Union (IAU; french: link=yes, Union astronomique internationale, UAI) exists to promote and safeguard t ...
scale, or its predecessor,
ephemeris time The term ephemeris time (often abbreviated ET) can in principle refer to time in association with any ephemeris In and , an ephemeris (plural: ephemerides) is a book with tables that gives the of naturally occurring as well as in the , i.e. ...
.


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 Precession is a change in the orientation Orientation may refer to: Positioning in physical space * Map orientation, the relationship between directions on a map and compass directions * Orientation (housing), the position of a building with re ...

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 Post-glacial rebound (also called isostatic rebound or crustal rebound) is the rise of land masses after the removal of the huge weight of ice sheet In , an ice sheet, also known as a continental glacier, is a mass of that covers surroundi ...
and
sea level rise Tide gauge measurements show that the current global sea level rise began at the start of the 20th century. Between 1900 and 2017, the globally averaged sea level Mean sea level (MSL) (often shortened to sea level) is an average In colloqu ...

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 The word day has a number of meanings, depending on the context it is used such as of astronomy, physics, and various calendar systems. As a term in physics and astronomy it is approximately the period during which the Earth completes one ro ...

day
s (52.1775
week A week is a time unit equal to seven days. It is the standard time period used for cycles of rest days in most parts of the world, mostly alongside—although not strictly part of—the Gregorian calendar. In many languages, the days of the we ...

week
s,
hour An hour (symbol A symbol is a mark, sign, or that indicates, signifies, or is understood as representing an , , or . Symbols allow people to go beyond what is n or seen by creating linkages between otherwise very different s and s. All ...
s,
minute The minute is a unit Unit may refer to: Arts and entertainment * UNIT Unit may refer to: Arts and entertainment * UNIT, a fictional military organization in the science fiction television series ''Doctor Who'' * Unit of action, a discrete piec ...
s or
second The second (symbol: s, also abbreviated: sec) is the base unit of time Time is the continued sequence of existence and event (philosophy), events that occurs in an apparently irreversible process, irreversible succession from the past, 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 The galactic year, also known as a cosmic year, is the duration of time required for 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 ...
is the time it takes Earth's
Solar System The Solar SystemCapitalization of the name varies. The International Astronomical Union, the authoritative body regarding astronomical nomenclature, specifies capitalizing the names of all individual astronomical objects but uses mixed "Sola ...

Solar System
to revolve once around the
galactic center The Galactic Center (or Galactic Centre) is the rotational center, the barycenter, of the Milky Way galaxy. Its central massive object is a supermassive black hole of about 4 million solar masses, which powers the compact radio source Sagitt ...
. 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 A month is a unit of time, used with calendars, that is approximately as long as a natural orbital period of the Moon; the words ''month'' and ''Moon'' are cognates. The traditional concept arose with the cycle of Moon phases; such lunar months ("l ...
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 Geology (from the Ancient Greek Ancient Greek includes the forms of the Greek language Greek ( el, label=Modern Greek Modern Greek (, , or , ''Kiní Neoellinikí Glóssa''), generally referred to by speakers simply as Greek ...

geology
and
paleontology Paleontology (), also spelled palaeontology or palæontology, is the scientific study of life that existed prior to, and sometimes including, the start of the Holocene epoch (geology), epoch (roughly 11,700 years before present). It includes th ...
, where "
kyr The abbreviation kyr means "thousand years". Kyr was formerly common in some English language English is a West Germanic language of the Indo-European language family The Indo-European languages are a language family A langua ...
,
myr The abbreviation myr, "million years", is a unit Unit may refer to: Arts and entertainment * UNIT Unit may refer to: Arts and entertainment * UNIT, a fictional military organization in the science fiction television series ''Doctor Who'' * Uni ...

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 MeasureThe Unified Code for Units of Measure (UCUM) is a system of codes for unambiguously representing measurement units. Its primary purpose is machine-to-machine communication rather than communication between humans. The code set includes all units def ...
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 Gregorian may refer to: *The thought or ideology of Pope Gregory I Pope Gregory I ( la, Gregorius I; – 12 March 604), commonly known as Saint Gregory the Great, was the bishop of Rome A bishop is an ordained, consecrated, or appointed mem ...
; where: :a, without a qualifier = 1 aj; :and ar, for '''', is a unit of area. The
International Union of Pure and Applied Chemistry The International Union of Pure and Applied Chemistry (IUPAC ) is an international federation of National Adhering Organizations that represents chemists in individual countries. It is a member of the International Science Council (ISC). IUPAC ...
(IUPAC) and the
International Union of Geological Sciences The International Union of Geological Sciences (IUGS) is an international non-governmental organization A non-governmental organization, or simply an NGO, is an that is, generally, formed independent from . They are typically s, and many o ...
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 In science Science (from the Latin word ''scientia'', meaning "knowledge") is a systematic enterprise that Scientific method, builds and Taxonomy (general), organizes knowledge in the form of Testability, testable explanations and predictio ...
. 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 1000 or one thousand is the natural number In mathematics, the natural numbers are those used for counting (as in "there are ''six'' coins on the table") and total order, ordering (as in "this is the ''third'' largest city in the country"). ...
, or 103, years, or 1 E3 yr, also known as a
millennium A millennium (plural millennia or millenniums) is a period of one thousand year A year is the orbital period of a planetary body, for example, the Earth, moving in Earth's orbit, its orbit around the Sun. Due to the Earth's axial tilt, th ...

millennium
in
anthropology Anthropology is the scientific study of human Humans (''Homo sapiens'') are the most abundant and widespread species In biology Biology is the natural science that studies life and living organisms, including their anatomy, ...
and calendar uses. The prefix multiplier "ka" is typically used in geology, paleontology, and
archaeology Archaeology or archeology is the study of human activity through the recovery and analysis Analysis is the process of breaking a complexity, complex topic or Substance theory, substance into smaller parts in order to gain a better underst ...
for the
Holocene The Holocene ( ) is the current geological epoch In geochronology, an epoch is a subdivision of the geologic timescale that is longer than an age (geology), age but shorter than a period (geology), period. The current epoch is the Holocene E ...
and
Pleistocene The Pleistocene ( , often referred to as the ''Ice Age'') is the geological Epoch (geology), epoch that lasted from about 2,580,000 to 11,700 years ago, spanning the earth’s most recent period of repeated glaciations. Before a change finally ...
periods, where a non−
radiocarbon dating Radiocarbon dating (also referred to as carbon dating or carbon-14 dating) is a method for determining the age of an object containing organic material Organic matter, organic material, or natural organic matter refers to the large source of ...
technique: e.g.
ice core An ice core is a core sample . A pied butcherbird The pied butcherbird (''Cracticus nigrogularis'') is a songbird native to Australia. Described by John Gould in 1837, it is a black and white bird long with a long hooked bill. Its head an ...
dating,
dendrochronology Dendrochronology (or tree-ring dating) is the scientific method The scientific method is an empirical Empirical evidence for a proposition is evidence, i.e. what supports or counters this proposition, that is constituted by or accessib ...
, uranium-thorium dating, or
varve A varve is an annual layer of sediment Sediment is a naturally occurring material that is broken down by processes of weathering Weathering is the deterioration of rocks A rock is any naturally occurring solid mass or aggregate of ...
analysis; is used as the primary dating method for age determination. If age is determined primarily by
radiocarbon dating Radiocarbon dating (also referred to as carbon dating or carbon-14 dating) is a method for determining the age of an object containing organic material Organic matter, organic material, or natural organic matter refers to the large source of ...
, then the age should be expressed in either radiocarbon or calendar (calibrated) years
Before Present Before Present (BP) years is a time scale used mainly in archaeology Archaeology or archeology is the study of human activity through the recovery and analysis of material culture. Archaeology is often considered a branch of socio-cultural ...
. * 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 One million (1,000,000), or one thousand thousand, is the natural number In mathematics, the natural numbers are those used for counting (as in "there are ''six'' coins on the table") and total order, ordering (as in "this is the ''third'' l ...

million
, or 106, years, or 1 E6 yr. The suffix "Ma" is commonly used in scientific disciplines such as geology, paleontology, and
celestial mechanics Celestial mechanics is the branch of astronomy that deals with the motion (physics), motions of celestial object, objects in outer space. Historically, celestial mechanics applies principles of physics (classical mechanics) to astronomical obje ...
to signify very long time periods into the past or future. For example, the
dinosaur Dinosaurs are a diverse group of reptiles of the clade Dinosauria. They first appeared during the Triassic Geological period, period, between 243 and 233.23 annum, million years ago, although the exact origin and timing of the evolution ...

dinosaur
species ''
Tyrannosaurus rex ''Tyrannosaurus'' is a genus Genus /ˈdʒiː.nəs/ (plural genera /ˈdʒen.ər.ə/) is a taxonomic rank In biological classification In biology, taxonomy () is the scientific study of naming, defining (Circumscription (taxonomy) ...

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
billion A billion is a number with two distinct definitions: *1,000,000,000 1,000,000,000 (one billion A billion is a number with two distinct definitions: *1,000,000,000 1,000,000,000 (one billion, short scale; one thousand million or milliar ...
years. "Ga" is commonly used in scientific disciplines such as
cosmology Cosmology (from Greek#REDIRECT Greek Greek may refer to: Greece Anything of, from, or related to Greece Greece ( el, Ελλάδα, , ), officially the Hellenic Republic, is a country located in Southeast Europe. Its population is appro ...
and geology to signify extremely long time periods in the past. For example, the formation of the Earth occurred approximately 4.54 Ga (4.54 billion years) ago and the
age of the universe In physical cosmology Physical cosmology is a branch of cosmology Cosmology (from Ancient Greek, Greek κόσμος, ''kosmos'' "world" and -λογία, ''-logia'' "study of") is a branch of astronomy concerned with the study of the chro ...
is approximately 13.8 Ga. * Ta (for ''teraannum'') – a unit of time equal to 1012 years, or one
trillion A trillion is a number with two distinct definitions: *1,000,000,000,000 This list contains selected positive numbers in increasing order, including counts of things, dimensionless quantity and probability, probabilities. Each number is given a n ...
years. "Ta" is an extremely long unit of time, about 70 times as long as the
age of the universe In physical cosmology Physical cosmology is a branch of cosmology Cosmology (from Ancient Greek, Greek κόσμος, ''kosmos'' "world" and -λογία, ''-logia'' "study of") is a branch of astronomy concerned with the study of the chro ...
. It is the same order of magnitude as the expected life span of a small
red dwarf A red dwarf is the smallest and coolest kind of star on the main sequence. Red dwarfs are by far the most common type of star in the Milky Way, at least in the neighborhood of the Sun, but because of their low luminosity, individual red dwarfs c ...
. * Pa (for ''petaannum'') – a unit of time equal to 1015 years, or one
quadrillion This article lists and discusses the usage and derivation of names of large numbers, together with their possible extensions. The following table lists those names of large number Numbers that are significantly larger than those typically used ...
years. The half-life of the nuclide is about 8 Pa. This symbol coincides with that for the
pascal Pascal, Pascal's or PASCAL may refer to: People and fictional characters * Pascal (given name), including a list of people with the name * Pascal (surname), including a list of people and fictional characters with the name ** Blaise Pascal, French ...
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 This article lists and discusses the usage and derivation of names of large numbers, together with their possible extensions. The following table lists those names of large numbers that are found in many English dictionaries and thus have a clai ...
years. The half-life of is 1.8 Ea.


Abbreviations yr and ya

In
astronomy Astronomy (from el, ἀστρονομία, literally meaning the science that studies the laws of the stars) is a natural science that studies astronomical object, celestial objects and celestial event, phenomena. It uses mathematics, phys ...
, 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 "
year A year is the orbital period of a planetary body, for example, the Earth, moving in Earth's orbit, 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 ...
", but following ambiguous international recommendations, use either the standard English first letters as prefixes (t, m, and b) or
metric prefix A metric prefix is a unit prefix that precedes a basic unit of measure to indicate a multiple (mathematics), multiple or submultiple of the unit. All metric prefixes used today are decimal, decadic. Each prefix has a unique symbol that is prepende ...
es (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 Before Present (BP) years is a time scale used mainly in archaeology Archaeology or archeology is the study of human activity through the recovery and analysis of material culture. Archaeology is often considered a branch of socio-cultural ...
("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 Before Present (BP) years is a time scale used mainly in archaeology Archaeology or archeology is the study of human activity through the recovery and analysis of material culture. Archaeology is often considered a branch of socio-cultural ...
, 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 Astronomical year numbering is based on AD/ CE year numbering, but follows normal decimal The decimal numeral system A numeral system (or system of numeration) is a writing system A writing system is a method of visually representing ...
*
Century A century is a period of 100 years. Centuries are numbered names of numbers in English#Ordinal numbers, ordinally in English and many other languages. The word ''century'' comes from the Latin ''centum'', meaning ''one hundred''. ''Century'' is s ...

Century
*
Decade A decade is a period of 10 years. The word is derived (via French and Latin) from the grc, δεκάς, dekas, which means ''a group of ten''. Decades may describe any ten-year period, such as those of a person's life, or refer to specific groupin ...
*
Epoch (reference date) In chronology 222px, Joseph Scaliger's ''De emendatione temporum'' (1583) began the modern science of chronology Chronology (from Latin Latin (, or , ) is a classical language belonging to the Italic languages, Italic branch of the Indo-E ...
*
ISO 8601 ISO 8601 is an international standard An international standard is a technical standard A technical standard is an established norm (social), norm or requirement for a repeatable technical task which is applied to a common and repeated use of rule ...
: standard for representation of dates and times *
List of calendars This is a list of calendars. Included are historical calendars as well as proposed ones. Historical calendars are often grouped into larger categories by cultural sphere or historical period; thus O'Neil (1976) distinguishes the groupings Egypti ...
*
List of years This page indexes the individual year A year is the orbital period of a planetary body, for example, the Earth, moving in Earth's orbit, its orbit around the Sun. Due to the Earth's axial tilt, the course of a year sees the passing of the se ...
*
Millennium A millennium (plural millennia or millenniums) is a period of one thousand year A year is the orbital period of a planetary body, for example, the Earth, moving in Earth's orbit, its orbit around the Sun. Due to the Earth's axial tilt, th ...

Millennium
*
Orders of magnitude (time) An order of magnitude An order of magnitude is an approximation of the logarithm In mathematics, the logarithm is the inverse function to exponentiation. That means the logarithm of a given number  is the exponent to which another f ...
*
Unit of time A unit of time is any particular time interval, used as a standard way of measuring or expressing duration. The SI base unit, base unit of time in the International System of Units (SI) and by extension most of the Western world, is the second, ...


References


Notes


Further reading

* *


External links

* Images of years {{Authority control *