View source for Year

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{{Short description|Unit of time based on orbit of Earth}}
{{Use mdy dates|date=January 2024|cs1-dates=ll}}
{{redirect|Years}}
{{Redirects here|12 months|3=The Twelve Months (disambiguation)}}
[[File:Solar system orrery inner planets.gif|thumb|alt=see caption|upright=1.5|An animation of the [[inner Solar System]] planets' orbit around the Sun. The duration of the year is the time taken to go around the Sun.]]
A '''year''' is a [[unit of time]] based on how long it takes the [[Earth]] to orbit the [[Sun]].<ref>[[OED]], s.v. "year".</ref>  In scientific use, the [[tropical year]] (approximately 365 [[Synodic day|solar days]], 5 hours, 48 minutes, 45 seconds) and the [[sidereal year]] (about 20 minutes longer) are more exact. The modern [[calendar year]], as reckoned according to the [[Gregorian calendar]], approximates the tropical year by using a system of [[leap years]]. 

The term 'year' is also used to indicate other periods of roughly similar duration, such as the lunar year (a roughly 354-day cycle of twelve of the Moon's phases{{snd}}see [[lunar calendar]]), as well as periods loosely associated with the calendar or astronomical year, such as the [[seasonal year]], the [[fiscal year]], the [[academic year]], etc.

Due to the Earth's [[axial tilt]], the course of a year sees the passing of the [[season]]s, marked by changes in [[weather]], the hours of [[daylight]], and, consequently, [[vegetation]] and [[soil fertility]]. In [[temperate]] and [[subpolar climate|subpolar]] regions around the planet, four seasons are generally recognized: [[spring (season)|spring]], [[summer]], [[autumn]], and [[winter]]. In [[tropics|tropical]] and [[subtropics|subtropical]] regions, several geographical sectors do not present defined seasons; but in the [[tropics#Seasons and climate|seasonal tropics]], the annual [[wet season|wet]] and [[dry season]]s are recognized and tracked.

By extension, the term 'year' can also be applied to the time taken for the [[orbit]] of any [[astronomical object]] around its [[Primary body|primary]]{{snd}} for example the [[Timekeeping on Mars#Year|Martian year]] of roughly 1.88 Earth years.

The term can also be used in reference to any long period or cycle, such as the [[Great Year]].<ref>[[OED]], s.v. "year", entry 2.b.: "''transf.''  Applied to a very long period or cycle (in chronology or mythology, or vaguely in poetic use)."</ref>

== Calendar year ==
A [[calendar year]] is an approximation of the number of days of the Earth's orbital period, as counted in a given [[calendar]]. The [[Gregorian calendar]], or modern calendar, presents its calendar year to be either a [[common year]] of 365 days or a [[leap year]] of 366 days, as do the [[Julian calendar]]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 (97 out of 400 years are leap years).<ref>{{cite web |url=https://www.grc.nasa.gov/www/k-12/Numbers/Math/Mathematical_Thinking/calendar_calculations.htm |title= Calendar Calculations |author=<!--Not stated--> |date= |website=Glenn Learning Technologies Project  |publisher=National Aeronautics Space Administration |access-date=November 11, 2023 |quote=}}</ref>

== Abbreviation ==
In English, the [[unit of time]] for year is commonly abbreviated as "y" or "yr". The symbol "a" (for {{langx|la|annus}}, year) is sometimes used in scientific literature, though its exact duration may be inconsistent.{{cn|date=December 2024}}

== Etymology ==
English ''year'' (via [[West Saxon dialect (Old English)|West Saxon]] ''ġēar'' ({{IPA|/jɛar/}}), [[Anglian dialects|Anglian]] ''ġēr'') continues [[Proto-Germanic]] ''*jǣran'' (''*j[[Proto-Germanic language#ē₁ and ē₂|ē₁]]<nowiki>ran</nowiki>''). Cognates are [[German language|German]] ''Jahr'', [[Old High German]] ''jār'', [[Old Norse]] ''ár'' and [[Gothic language|Gothic]] ''[[:wikt:𐌾𐌴𐍂|jer]]'',<!--(Gothic ''e'' is always a long vowel)--> from the [[Proto-Indo-European]] noun {{lang|ine-x-proto|[[:wikt:Reconstruction:Proto-Indo-European/yeh₁-|*yeh₁r-om]]}} "year, season". Cognates also descended from the same Proto-Indo-European noun (with variation in suffix [[ablaut]]) are [[Avestan]] ''yārǝ'' "year", [[Ancient Greek|Greek]] {{lang|grc|ὥρα}} ({{Lang|grc-Latn|hṓra}}) "year, season, period of time" (whence "[[hour]]"), [[Old Church Slavonic]] ''jarŭ'', and [[Latin]] ''hornus'' "of this year".{{citation needed||date=May 2023}}

Latin {{Lang|la|[[annus]]}} (a [[2nd declension]] masculine noun; {{Lang|la|annum}} is the [[accusative case|accusative singular]]; {{Lang|la|annī}} is [[genitive]] singular and [[nominative]] plural; {{Lang|la|annō}} the [[dative]] and [[ablative]] singular) is from a [[Proto-Indo-European language|PIE]] noun {{lang|ine-x-proto|h₂et-no-}}, 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 {{Lang|la|annus}}, such as [[wikt:annual|annual]], [[:wikt:annuity|annuity]], [[:wikt:anniversary|anniversary]], etc.; ''[[:wikt:per annum|per annum]]'' means "each year", {{Lang|la|[[anno Domini|annō Dominī]]}} means "in the year of the Lord".

The Greek word for "year", {{lang|grc|ἔτος}}, is cognate with Latin ''vetus'' "old", from the PIE word ''*wetos-'' "year", also preserved in this meaning in [[Sanskrit]] ''{{IAST|vat-sa-ras}}'' "year" and ''{{IAST|vat-sa-}}'' "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 during which a leap day is [[intercalation (timekeeping)#Solar calendars|intercalated]] into the month of February. The name "Leap Day" is applied to the added day.

In astronomy, the [[Julian year (astronomy)|Julian year]] is a unit of time defined as 365.25 days, each of exactly {{val|86400}} [[second]]s ([[SI base unit]]), totaling exactly 31,557,600 seconds in the Julian astronomical year.<ref>{{cite web |publisher=International Astronomical Union IAU |url=http://www.iau.org/science/publications/proceedings_rules/units/ |title=SI units |access-date=February 18, 2010 |archive-date=July 16, 2013 |archive-url=https://web.archive.org/web/20130716232020/http://www.iau.org/science/publications/proceedings_rules/units/ }}</ref><ref>{{cite journal |first=George A. |last=Wilkins |url=http://www.iau.org/static/publications/stylemanual1989.pdf |title=The IAU Style Manual |year=1989 |journal=IAU Transactions |volume=XXB |archive-date=July 26, 2011 |access-date=February 18, 2010 |archive-url=https://web.archive.org/web/20110726170213/http://www.iau.org/static/publications/stylemanual1989.pdf }}</ref>

==== Revised Julian calendar ====
The [[Revised Julian calendar]], proposed in 1923 and used in some [[Eastern Orthodox Church]]es, has 218 leap years every 900 years, for the average (mean) year length of {{val|365.2422222}} days, close to the length of the mean tropical year, {{val|365.24219}} days (relative error of 9·10). In the year 2800 CE, the Gregorian and Revised Julian calendars will begin to differ by one calendar day.<ref>{{Cite journal |bibcode = 1924PA.....32..407S|title = The new calendar of the eastern churches|last1 = Shields |first1 = Miriam Nancy|journal = Popular Astronomy|year = 1924|volume = 32|page = 407}}</ref>

=== Gregorian calendar ===
The [[Gregorian calendar]] aims to ensure that the [[northward equinox]] falls on or shortly before March 21 and hence it follows the [[tropical year|northward equinox year]], or [[tropical year]].<ref>{{Cite book |last=Ziggelaar |first=A. |year=1983 | chapter-url=http://articles.adsabs.harvard.edu/cgi-bin/nph-iarticle_query?journal=grc..&year=1983&volume=book&letter=.&db_key=GEN&page_ind=230&plate_select=NO&data_type=GIF&type=SCREEN_GIF&classic=YES |chapter=The Papal Bull of 1582 Promulgating a Reform of the Calendar |editor1=G. V. Coyne|editor2=M. A. Hoskin |editor3=O. Pedersen  |title=Gregorian Reform of the Calendar: Proceedings of the Vatican Conference to Commemorate its 400th Anniversary |location=Vatican City |publisher=Pontifical Academy of Sciences|page=223}}</ref> Because 97 out of 400 years are leap years, the mean length of the Gregorian calendar year is {{val|365.2425}} days; with a relative error below one [[Parts per million|ppm]] (8·10) relative to the current length of the mean [[tropical year]] ({{gaps|365.242|189}} days) and even closer to the current ''March equinox year'' of {{gaps|365.242|374}} 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.{{cn|date=June 2022}} This not because of this difference, but due to [[Tidal acceleration|the slowing of the Earth's rotation]] and the associated lengthening of the day.{{cn}} [Statement suspended because uncited in over a year and unattributed. OR violation?] -->

=== Other calendars ===
{{see|Lunisolar calendar}}
Historically, lunisolar calendars intercalated entire [[leap month]]s on an observational basis. Lunisolar calendars have mostly fallen out of use except for liturgical reasons ([[Hebrew calendar]], various [[Hindu calendar]]s).

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]].<ref>{{cite book |last=Richards |first=E.G. |date=2013 |contribution=Calendars |editor1-first=S.E. |editor1-last=Urban |editor2-first=P.K. |editor2-last=Seidelmann |url=http://aa.usno.navy.mil/publications/docs/c15_usb_online.pdf |title=Explanatory Supplement to the Astronomical Almanac |edition=3rd |pages=585, 590 |location=Mill Valley, CA |publisher=University Science Books |isbn= 978-1-891389-85-6 |access-date= May 9, 2018 |archive-date= April 30, 2019 |archive-url=https://web.archive.org/web/20190430134555/http://aa.usno.navy.mil/publications/docs/c15_usb_online.pdf }}</ref> Its epoch is a [[Dionysius Exiguus#Anno Domini|6th century estimate]] of the date of 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]], {{lang|la|[[Ab Urbe Condita]]}} ("from the foundation of [[Rome|the city]]), abbreviated AUC; {{lang|la|[[Anno Mundi]]}} ("year of the world"), used for the [[Hebrew calendar]] and abbreviated AM; and the [[Japanese calendar|Japanese imperial eras]]. The Islamic [[Hijri year]], (year of the [[Hijrah]], {{lang|la|[[Anno Hegirae]]}} abbreviated AH), is a [[lunar calendar]] of twelve [[lunar month]]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 ===
{{main|Fiscal year}}
A [[fiscal year]] 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 ===
{{main|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 term]]s, such as semesters or quarters. The school year in many countries in the [[Northern Hemisphere]] starts in August or September and ends in May, June or July, providing a summer break from study between academic years. 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 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.{{cn|date=August 2025}}

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.{{citation needed|date=June 2010}}

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 ==
{{see also|Astronomical month}}

=== Julian year ===
{{Main|Julian year (astronomy)}}
The Julian year, as used in astronomy and other sciences, is a time unit now defined as exactly 365.25 days of {{val|86400}} [[Second|SI seconds]] each<ref name="ESttAA">{{cite book |author=<!-- not stated --> |date=1992 |title=Explanatory supplement to the Astronomical Almanac |url=https://archive.org/details/explanatorysuppl0003unse/page/698/mode/2up |location=Mills Valley, California |editor-last1=Seidelmann |editor-first1=P. Kenneth |publisher=University Science Books |page=698 |isbn=0-935702-68-7 |access-date=11 April 2025}}</ref> ("[[ephemeris day]]s"). This is one meaning of the unit "year" used in various scientific contexts. The Julian century of {{val|36525}} ephemeris days and the Julian millennium of {{val|365250}} ephemeris days are used in astronomical calculations. Fundamentally, expressing a time interval in Julian years is a way to precisely specify an amount of time (not how many "real" years), for long time intervals where stating the number of ephemeris 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]] (but not according to the [[International Union of Pure and Applied Physics]] or the [[International Union of Geological Sciences]], see below), the symbol 'a' (without subscript) always refers to the Julian year, 'a<sub>j</sub>', of exactly {{val|31557600}} [[second]]s.

: 365.25 d × {{val|86400|u=s}} = 1&nbsp;a = 1&nbsp;a<sub>j</sub> = {{val|31.5576}} [[Second#SI multiples|Ms]]

The [[#SI prefix multipliers|SI multiplier prefixes]] may be applied to it to form "ka", "Ma", etc.<ref name="UCUM" />

The scientific Julian year is not to be confused with a year in the Jullian calendar. The scientific Julian year is a multiple of the SI&nbsp;second; it is today "astronomical" only in the sense "used in astronomy", whilst true astronomical years are based on the movements of celestial bodies.

=== Sidereal, tropical, and anomalistic years ===
{{main|Sidereal year|Tropical year}}
{{hatnote|The relations among these are considered more fully in [[Axial precession (astronomy)]].}}

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 {{Lang|la|sidera}}, singular {{Lang|la|sidus}}). Its average duration is {{val|365.256363004}} days (365 d 6 h 9 min 9.76 s) (at the epoch [[J2000.0]] = January 1, 2000, 12:00:00 [[Terrestrial Time|TT]]).<ref>International Earth Rotation and Reference System Service. (2010).[http://hpiers.obspm.fr/eop-pc/models/constants.html IERS EOP PC Useful constants.] {{Webarchive|url=https://web.archive.org/web/20121029154942/http://hpiers.obspm.fr/eop-pc/models/constants.html |date=October 29, 2012 }}</ref>

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.<ref>Richards, E.G. (2013). Calendars. In S.E. Urban & P.K. Seidelmann (Eds.), ''Explanatory Supplement to the Astronomical Almanac'' (3rd ed.). Mill Valley, CA: University Science Books. p. 586.</ref> Since the Sun's ecliptic longitude is measured with respect to the equinox,<ref>{{cite web |url-status=live |url=https://aa.usno.navy.mil/faq/asa_glossary#longitude-ecliptic |title=longitude, ecliptic |archive-url=https://web.archive.org/web/20230908233641/https://aa.usno.navy.mil/faq/asa_glossary#longitude-ecliptic |archive-date= September 8, 2023 |postscript=,}} and {{cite web |url-status=live |url=https://aa.usno.navy.mil/faq/asa_glossary#dynamical-equinox |title=dynamical equinox |archive-url=https://web.archive.org/web/20230908233641/https://aa.usno.navy.mil/faq/asa_glossary#dynamical-equinox |archive-date= September 8, 2023 |postscript=,}} (c. 2022). In "Glossary", ''The Astronomical Almanac Online''. United States Naval Observatory.</ref> the tropical year comprises a complete cycle of the seasons and is the basis of [[solar calendar]]s such as the internationally used [[Gregorian calendar]]. The modern definition of mean tropical year differs from the actual time between passages of, e.g., the northward equinox, by a minute or two, 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<ref>
{{cite web
 | title = Glossary
 | work = Astronomical Applications Department
 | publisher = United States Naval Observatory
 | access-date = November 6, 2023
 | date = c. 2022| url = https://aa.usno.navy.mil/faq/asa_glossary
 | at=s.v. year, tropical
 | archive-date=September 8, 2023
 | archive-url = https://web.archive.org/web/20230908233641/https://aa.usno.navy.mil/faq/asa_glossary
 | url-status=live
}}</ref> (= {{val|365.24219|u=d}} × {{val|86400|u=s}}). The length of the tropical year varies a bit over thousands of years because the rate of axial precession is not constant.

{{anchor|Anomalistic}}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, and the [[aphelion]], where the Earth is farthest from the Sun. 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).<ref>
{{cite book
 |title=Astronomical Almanac for the Year 2011
 |year=2009
 |publisher = [[US Government Printing Office]] and the [[UK Hydrographic Office]]
 |location = Washington and Taunton
 |pages = A1, C2
}}</ref>

=== Draconic year {{anchor|Draconitic year}} ===
{{further|Draconic period}}
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 [[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 season]]s every eclipse year. The average duration of the eclipse year is
: {{val|346.620075883}} 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: {{val|18.612815932}} Julian years ({{val|6798.331019}} 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:
: {{val|411.78443029}} 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 religious purposes, including calculating the date of the [[Hajj]] and the fasting month of [[Ramadan]], and thus also the [[Muslim holidays|Eids]]. The [[Hebrew calendar|Jewish calendar]] is also mainly lunar, but with the addition of an intercalary lunar month once every two or three years, designed to keep the calendar broadly synchronous with the solar cycle. Thus, a lunar year on the Jewish (Hebrew) calendar consists of either twelve or thirteen lunar months.

=== Vague year ===
The vague year, from {{Lang|la|annus vagus}} 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]].<ref>[http://www.mayacalendar.com/descripcion.html Calendar Description and Coordination] {{Webarchive|url=https://web.archive.org/web/20120426010158/http://www.mayacalendar.com/descripcion.html |date= April 26, 2012 }} Maya World Studies Center</ref> It is still used by many Zoroastrian communities.

=== Heliacal year ===
A heliacal year is the interval between the [[heliacal rising]]s 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 heliacal year, 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:
: {{val|365.2568983}} 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):<ref>
{{cite book
 |title=Astronomical Almanac for the Year 2010
 |year=2008
 |publisher = U.S. Government Printing Office and the U.K. Hydrographic Office
 |location = Washington and Taunton
 |page = B3
}}</ref>
: B = 1900.0 + (Julian date<sub>TT</sub> − {{val|2415020.31352}}) / {{val|365.242198781}}

The TT subscript indicates that for this formula, the Julian date should use the [[Terrestrial Time]] scale, or its predecessor, [[ephemeris time]].<!-- Capitalization follows that of the glossary of the ''Astronomical Almanac for the Year 2010'' -->

=== Variation in the length of the year and the day ===
{{more citations needed section|date=October 2012}}
{{further|ΔT (timekeeping)}}

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<br />
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 {{val|86400}} SI seconds long.<ref>
{{cite book
 |title=Astronomical Almanac for the year 2011
 |location=Washington
 |publisher=U.S. Government Printing Office
 |author=U.S. Naval Observatory Nautical Almanac Office and Her Majesty's Nautical Almanac Office
 |year=2010
 |pages=C2, L8
}}</ref><ref>
{{cite journal
 |title = Numerical expressions for precession formulae and mean elements for the Moon and planets
 |journal=Astronomy and Astrophysics
 |volume=282
 |issue=2
 |pages=663–683
 |date=February 1994
 |last1=Simon |first1=J.L.
 |last2=Bretagnon |first2=P.
 |last3=Chapront |first3=J.
 |last4=Chapront-Touzé |first4=M.
 |last5=Francou |first5=G.
 |last6=Laskar | first6=J.
 |bibcode=1994A&A...282..663S
}}</ref><ref>
{{cite book
 |last1=Taff
 |first1=Lawrence G.
 |title=Celestial Mechanics: A Computational Guide for the Practitioner
 |year=1985
 |isbn=978-0-471-89316-5
 |place=New York
 |publisher=John Wiley & Sons
 |page=103
}} Values in tables agree closely for 2000, and depart by as much as 44 seconds for the years furthest in the past or future; the expressions are simpler than those recommended in the ''Astronomical Almanac for the Year 2011''.</ref><ref>
{{cite book
 | edition = 3
 | publisher = Univ Science Books
 | isbn = 978-1-891389-85-6
 | last = Seidelmann
 | first = P. Kenneth
 | others = Sean E. Urban (ed.)
 | title = Explanatory Supplement to the Astronomical Almanac
 | year = 2013
 | page = 587
}} Tabulates length of tropical year from −500 to 2000 at 500 year intervals using a formula by Laskar (1986); agrees closely with values in this section near 2000, departs by 6 seconds in −500.</ref>

{| border = "1" class="wikitable"
|+ Mean year lengths for 2000
! Type of year !! Days !! Hours !! Minutes !! Seconds
|-
| [[Tropical year|Tropical]] || 365 ||  5 || 48 || 45
|-
| [[Sidereal year|Sidereal]] || 365 ||  6 ||  9 || 10
|-
| Anomalistic                || 365 ||  6 || 13 || 53
|-
| Eclipse                    || 346 || 14 || 52 || 55
|}

{| border = "1" class="wikitable"
|+ Year length difference from 2000 <br />(seconds; positive when length for tabulated year is greater than length in 2000)
! Year !! Tropical !! Sidereal !! Anomalistic !! Eclipse
|-
| −4000 ||  −8 || −45 || −15 || −174
|-
| −2000 ||   4 || −19 || −11 || −116
|-
|     0 ||   7 ||  −4 ||  −5 ||  −57
|-
|  2000 ||   0 ||   0 ||   0 ||    0
|-
|  4000 || −14 ||  −3 ||   5 ||   54
|-
|  6000 || −35 || −12 ||  10 ||  104
|}

=== Summary ===
Some of the year lengths in this table are in average [[solar day]]s, which are slowly getting longer (at a rate that cannot be exactly predicted in advance) and are now around {{val|86400.002}} [[Second|SI seconds]].

{| class="wikitable"
! scope="col" | Days
! scope="col" | Year type
|-
| 346.62             || Draconic, also called eclipse
|-
| 354.37             || Lunar
|-
| 365                || Solar days: vague, and a [[common year]] in many [[solar calendar]]s
|-
| {{val|365.24219}}  || Tropical, also called solar, averaged and then rounded for epoch [[J2000.0]]
|-
| {{val|365.2425}}   || Gregorian, solar days averaged over the 400-year cycle
|-
| {{val|365.25}}     || Julian, solar days averaged over the four-year cycle
|-
| {{val|365.25636}}  || Sidereal, for epoch [[J2000.0]]
|-
| {{val|365.259636}} || Anomalistic, averaged and then rounded for epoch J2011.0
|-
| 366                || [[Leap year]] in many [[solar calendar]]s
|}

An average Gregorian year may be said to be 365.2425 [[day]]s (52.1775 [[week]]s, and if an hour is defined as one twenty-fourth of a day, {{val|8765.82}} [[hour]]s, {{val|525949.2}} [[minute]]s or {{val|31556952}} [[second]]s). Note however that in absolute time the average Gregorian year is not adequately defined unless the period of the averaging (start and end dates) is stated, because each period of 400 years is longer (by more than 1000 seconds) than the preceding one as the rotation of the Earth slows. In this calendar, a common year is 365 days ({{val|8760}} hours, {{val|525600}} minutes or {{val|31536000}} seconds), and a leap year is 366 days ({{val|8784}} hours, {{val|527040}} minutes or {{val|31622400}} seconds). The 400-year civil cycle of the Gregorian calendar has {{val|146097}} days and hence exactly {{val|20871}} 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.<ref>{{Cite journal |doi = 10.1051/0004-6361:20041335|title = A long-term numerical solution for the insolation quantities of the Earth|journal = Astronomy & Astrophysics|volume = 428|pages = 261–285|year = 2004|last1 = Laskar|first1 = J.|last2 = Robutel|first2 = P.|last3 = Joutel|first3 = F.|last4 = Gastineau|first4 = M.|last5 = Correia|first5 = A. C. M.|last6 = Levrard|first6 = B.|bibcode = 2004A&A...428..261L|doi-access = free}}</ref><ref>{{Cite web|url=http://demonstrations.wolfram.com/PrecessionOfTheEarthsAxis/|title=Precession of the Earth's Axis - Wolfram Demonstrations Project|website=demonstrations.wolfram.com|language=en|access-date=February 10, 2019}}</ref>

=== 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.<ref>
{{cite web
 |url         = http://www.orau.gov/SCIENCEBOWL/teams/files/astrset2.pdf
 |title       = Science Bowl Questions, Astronomy, Set 2
 |work        = Science Bowl Practice Questions
 |year        = 2009
 |publisher   = Oak Ridge Associated Universities
 |access-date  = December 9, 2009
|archive-url  = https://web.archive.org/web/20100307191635/http://www.orau.gov/sciencebowl/teams/files/astrset2.pdf
 |archive-date = March 7, 2010
}}</ref>

== IUPAC–IUGS proposal ==

In 2011, a task group of the IUPAC and the [[International Union of Geological Sciences]] (IUGS) jointly recommended defining the annus for geological purposes as
<blockquote>1 a&nbsp;=&nbsp;{{val|31556925.445}} seconds<ref name="IUPAC–IUGS">{{cite journal |first1=Norman E. |last1=Holden |first2=Mauro L. |last2=Bonardi |first3=Paul |last3=De Bièvre |first4=Paul R. |last4=Renne |first5=Igor M. |last5=Villa  |name-list-style=amp |title=IUPAC-IUGS common definition and convention on the use of the year as a derived unit of time (IUPAC Recommendations 2011) |journal=[[Pure and Applied Chemistry]] |year=2011 |volume=83 |pages=1159–1162 |doi=10.1351/PAC-REC-09-01-22 |issue=5|s2cid=96753161 |url=http://doc.rero.ch/record/303694/files/pac-rec-09-01-22.pdf |hdl=10281/21054 |hdl-access=free }}</ref> (approximately {{val|365.24219265}} [[ephemeris day]]s)</blockquote>

They chose a value close to the length of tropical year for the [[Epoch (astronomy)|epoch]] 2000.0 (which is roughly the length of the tropical year 2000; the length of the tropical year is slowly decreasing).<ref name="IUPAC–IUGS"/> However, the definition is as a multiple of the second, the SI base unit of time, and independent of astronomical definitions, since "[d]efinitions of the annus that are based on an intermediate relationship via the day, such as the Julian and Gregorian year, bear an inherent, pre-programmed obsolescence because of the variability of Earth's orbital movement".<ref name="IUPAC–IUGS"/> It differs from the Julian year of 365.25 days (3.1557600 × 10<sup>7</sup> s) by about 21 [[parts per million]].

As of April 2025, the IUPAC Green Book (4th edition) provides a definition of the year as a = {{val|31556925.9747}} seconds.<ref>{{Cite web |title=Green Book |url=https://iupac.org/what-we-do/books/greenbook/ |access-date=2025-04-11 |website=IUPAC {{!}} International Union of Pure and Applied Chemistry |language=en-US}}</ref>

== Seasonal year ==
{{main|Seasonal year}}
{{Further|Effect of Sun angle on climate}}

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 and abbreviations ==
A common symbol for the year as a [[unit of time]] is "a", taken from the Latin word {{Lang|la|annus}}.
For example, the U.S. [[National Institute of Standards and Technology]] (NIST) ''Guide for the Use of the International System of Units (SI)'' supports the symbol "a" as the unit of time for a year.<ref name=NIST>
{{cite web
 |first1=Ambler |last1=Thompson |first2=Barry N. |last2=Taylor 
 |url=https://physics.nist.gov/cuu/pdf/sp811.pdf
 |publisher=National Institute of Standards and Technology (NIST)
 |title=Special Publication 811 – Guide for the Use of the International System of Units (SI)
 |at=para 8.1
 |year=2008
}}</ref>

In English, the abbreviations "y" or "yr" are more commonly used in non-scientific literature.<ref name="How">
{{cite web
 |last=Rowlett |first=Russ
 |title=Units: A
 |url=http://www.unc.edu/~rowlett/units/dictA.html
 |access-date=January 9, 2009
 |work=How Many? A Dictionary of Units of Measurement
 |publisher=University of North Carolina
 |archive-date=December 20, 2008
 |archive-url=https://web.archive.org/web/20081220111445/http://www.unc.edu/~rowlett/units/dictA.html
 }}</ref> In some [[Earth sciences]] branches ([[geology]] and [[paleontology]]), "[[kyr]], [[myr]], [[byr]]" (thousands, millions, and billions of years, respectively) and similar abbreviations are used to denote intervals of time remote from the present.<ref name="AGUStyle">
{{cite web
 |url=https://www.agu.org/Publish-with-AGU/Publish/Author-Resources/Grammar-Style-Guide#datetime
 |publisher=American Geophysical Union
 |title=AGU publications: Grammar and Style Guide
 |date= September 1, 2017
 |access-date= January 9, 2009
 |archive-url = https://web.archive.org/web/20190918210347/https://www.agu.org/Publish-with-AGU/Publish/Author-Resources/Grammar-Style-Guide#datetime
 |archive-date = September 18, 2019
}}</ref><ref name="Strat">{{cite journal
 |author=North American Commission on Stratigraphic Nomenclature
 |url=https://ngmdb.usgs.gov/Info/NACSN/Code2/code2.html#Article13
 |title=North American Stratigraphic Code
 |journal=The American Association of Petroleum Geologists Bulletin
 |volume=89
 |issue=11
 |date=November 2005
 |pages=1547–1591
 |doi=10.1306/07050504129
 |edition=Article 13 (c)
 |bibcode=2005BAAPG..89.1547.
 |url-access=subscription
 |archive-date=February 2, 2014
 |access-date=January 6, 2009
 |archive-url=https://web.archive.org/web/20140202155725/http://ngmdb.usgs.gov/Info/NACSN/Code2/code2.html#Article13
 }}</ref> In [[astronomy]] the abbreviations kyr, Myr and Gyr are in common use for kiloyears, megayears and gigayears.<ref name="MNRAS Style">
{{cite web
 |url=https://academic.oup.com/mnras/pages/general_instructions
 |publisher=Oxford University Press
 |title=General Instructions - Monthly Notices of the Royal Astronomical Society - Oxford Academic
 |date=November 3, 2022
}}</ref><ref name="AAS Style">
{{cite web
 |url=https://journals.aas.org/aas-style-guide/
 |publisher=The American Astronomical Society
 |title=AAS Style Guide - AAS Journals
 |date=November 3, 2022
}}</ref>

The [[Unified Code for Units of Measure]] (UCUM) disambiguates the varying symbologies of ISO 1000, ISO 2955 and ANSI X3.50 by using:<ref name="UCUM">{{cite web | title=The Unified Code for Units of Measure | website= UCUM | date=November 21, 2017 | url=https://ucum.org/ucum.html#para-31 | access-date=July 27, 2022}}</ref>
: a<sub>t</sub> = {{val|365.24219}} days for the mean tropical year;
: a<sub>j</sub> = 365.25 days for the mean Julian year;
: a<sub>g</sub> = {{val|365.2425}} days for the mean [[Gregorian year]];
In the UCUM, the symbol "a", without any qualifier, equals 1 a<sub>j</sub>.
The UCUM also minimizes confusion with ''[[Hectare#Are|are]]'', a unit of area, by using the abbreviation "ar".

Since 1989, the [[International Astronomical Union]] (IAU) recognizes the symbol "a" rather than "yr" for a year, notes the different kinds of year, and recommends adopting the Julian year of 365.25 days, unless otherwise specified (IAU ''Style Manual'').<ref>G.A. Wilkins, Comm. 5, "IAU Style Manual", IAU Transactions XXB (1989), [https://www.iau.org/static/publications/stylemanual1989.pdf] {{Webarchive|url=https://web.archive.org/web/20190411205339/https://www.iau.org/static/publications/stylemanual1989.pdf|date=April 11, 2019}}.</ref><ref name="IAU">{{cite web | title=SI Units | website=International Astronomical Union | url=https://www.iau.org/publications/proceedings_rules/units/ | access-date=April 23, 2022}}</ref>

Since 1987, the [[International Union of Pure and Applied Physics]] (IUPAP) notes "a" as the general symbol for the time unit year ([[IUPAP Red Book|IUPAP ''Red Book'']]).<ref>IUPAP Red Book: Symbols, Units, Nomenclature and Fundamental Constants in Physics. https://iupap.org/wp-content/uploads/2021/03/A4.pdf {{Webarchive|url=https://web.archive.org/web/20230101212618/https://iupap.org/wp-content/uploads/2021/03/A4.pdf |date=January 1, 2023 }}</ref>
Since 1993, the [[International Union of Pure and Applied Chemistry]] (IUPAC) [[IUPAC Green Book|''Green Book'']] also uses the same symbol "a", notes the difference between Gregorian year and Julian year, and adopts the former (a = {{val|365.2425}} days),<ref>E.R. Cohen, T. Cvitas, J.G. Frey, B. Holmström, K. Kuchitsu, R. Marquardt, I. Mills, F. Pavese, M. Quack, J. Stohner, H.L. Strauss, M. Takami, and A.J. Thor, ''Quantities, Units and Symbols in Physical Chemistry'', IUPAC ''Green Book'', Third Edition, Second Printing, IUPAC & RSC Publishing, Cambridge (2008) [http://iupac.org/wp-content/uploads/2015/07/Green-Book-PDF-Version-2011.pdf] {{Webarchive|url=https://web.archive.org/web/20190417213638/https://iupac.org/wp-content/uploads/2015/07/Green-Book-PDF-Version-2011.pdf|date=April 17, 2019}}</ref> also noted in the [[IUPAC Gold Book|IUPAC ''Gold Book'']].<ref name="Gold">{{cite book | title=The IUPAC Compendium of Chemical Terminology | chapter=year | publisher=International Union of Pure and Applied Chemistry (IUPAC) | publication-place=Research Triangle Park, NC | date=February 24, 2014 | doi=10.1351/goldbook.y06723 | page=}}</ref>

In 2011, a task group of IUPAC and IUGS recommended the use of a as the symbol for the annus (along with multiples such as Ma) for both time intervals and absolute ages.<ref name="IUPAC–IUGS"/> This proved controversial as it conflicts with an earlier convention among geoscientists to use "a" specifically for absolute age before the present (e.g. 1&nbsp;Ma for 1 million years ago), and "y" or "yr" (and My, Myr etc) for a time interval or period of time.<ref name="Biever-war">{{cite journal |first=Celeste |last=Biever |title=Push to define year sparks time war |journal=[[New Scientist]] |volume=210 |issue=2810 |page=10 |url=https://www.newscientist.com/article/dn20423-push-to-define-year-sparks-time-war.html |date=April 27, 2011 |access-date=April 28, 2011|bibcode=2011NewSc.210R..10B |doi=10.1016/S0262-4079(11)60955-X |url-access=subscription }}</ref><ref name="iupac2011">{{cite web | title=Letters About the IUPAC-IUGS Common Definition and Convention on the Use of the Year as a Derived Unit of Time
 | website=Chemistry International -- Newsmagazine for IUPAC | date=November 19, 2011 | url=http://publications.iupac.org/ci/2011/3306/pac_letters-sup.html | access-date=April 23, 2022}}</ref>

=== SI prefix multipliers ===
{{see also|Orders of magnitude (time)}}
<!-- This section is linked from [[Cretaceous]], from [[Template:Geologic time scale]] and from [[Theropoda]]. -->
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.
{|class="wikitable"
|+Units of time with SI prefixes
!width=120pt|Symbol!!Definition!!Common scientific uses and notes
|-
|{{Anchor|Kiloannus|Kiloannus}} ka (for ''kiloannus'')|| One [[thousand]] or 10<sup>3</sup> years, also known as a [[millennium]] in [[anthropology]] and calendar uses.||Geology, paleontology, and [[archaeology]] for the [[Holocene]] and [[Pleistocene]] [[period (geology)|periods]], where a non−[[radiocarbon dating]] technique such as [[ice core]] dating, [[dendrochronology]], [[uranium-thorium dating]] or [[varve]] analysis is used as the primary 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]].
|-
|{{Anchor|Megaannus|Megaannus}} Ma (for ''megaannus'')||One [[million]] or 10<sup>6</sup> years.||Geology, paleontology, and [[celestial mechanics]]. 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.
|-
|{{Anchor|Gigaannus|Gigaannus}} Ga (for ''gigaannus'')||One [[1000000000 (number)|billion]] or 10<sup>9</sup> years.||[[physical cosmology|Cosmology]] and geology.<ref>{{Citation|last=Arndt|first=Nicholas|title=Ga|date=2011|encyclopedia=Encyclopedia of Astrobiology|page=621|editor-last=Gargaud|editor-first=Muriel|place=Berlin, Heidelberg|publisher=Springer|language=en|doi=10.1007/978-3-642-11274-4_611|isbn=978-3-642-11274-4|editor2-last=Amils|editor2-first=Ricardo|editor3-last=Quintanilla|editor3-first=José Cernicharo|editor4-last=Cleaves|editor4-first=Henderson James (Jim)}}</ref> For example, [[history of 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.
|-
|{{Anchor|Teraannus|Teraannus}} Ta (for ''teraannus'')||One [[1000000000000 (number)|trillion]] or 10<sup>12</sup> years.||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]].
|-
| {{Anchor|Petaannus|Petaannus}} Pa (for ''petaannus'')||One [[10^15|quadrillion]] or 10<sup>15</sup> years.||The [[half-life]] of the [[nuclide]] [[cadmium-113]] is about 8&nbsp;Pa.<ref>{{cite journal| author = P. Belli| display-authors = etal| title = Investigation of β decay of <sup>113</sup>Cd| journal = [[Phys. Rev. C]] |volume = 76|article-number = 064603|year = 2007 |doi = 10.1103/PhysRevC.76.064603|bibcode = 2007PhRvC..76f4603B| issue = 6 }}</ref><!--The half-life of the nuclear isomer [[tantalum]]-180m is about 1 Pa.<ref>[http://www.eurekalert.org/features/doe/2005-08/drnl-ttp082205.php Testing the physics of nuclear isomers] ''Eurekalert'' (August 2005)</ref> (error in source -- it in not half-life, it is the lower limit on half-life, that means that the half-life can be, for example, 1e21 yr or 1e29 yr or any other value > 1e15 yr. The decay of Ta-180m had never been observed).--> This symbol coincides with that for the [[pascal (unit)|pascal]] without a multiplier prefix, but context will normally be sufficient to distinguish long time periods from pressure values.
|-
|{{Anchor|Exaannus|Exaannus}} Ea (for ''exaannus'')||One [[quintillion]] or 10<sup>18</sup> years.||The half-life of [[tungsten-180]] is 1.8 Ea.<ref>{{cite journal| author = F.A. Danevich| display-authors = etal| title = α activity of natural tungsten isotopes| journal = [[Phys. Rev. C]] |volume = 67| issue = 1|article-number = 014310|year = 2003| arxiv = nucl-ex/0211013|doi = 10.1103/PhysRevC.67.014310|bibcode = 2003PhRvC..67a4310D | s2cid = 6733875}}</ref>
|}

=== Abbreviations for "years ago" ===
{{anchor|Abbreviations yr and ya}}{{anchor|years ago}}{{anchor|kya}}{{anchor|Mya}}{{anchor|Bya}}
{{further|Before Present}}

In geology and paleontology, a distinction sometimes is made between abbreviation "yr" for ''years'' and "ya" for ''years ago'', combined with prefixes for thousand, million, or billion.<ref name="AGUStyle" /><ref name="NASC">{{cite web
 |author=North American Commission on Stratigraphic Nomenclature
 |url=https://ngmdb.usgs.gov/Info/NACSN/Code2/code2.html#Article13
 |title=North American Stratigraphic Code (Article 13 (c))
 |quote=(c) Convention and abbreviations. – The age of a stratigraphic unit or the time of a geologic event, as commonly determined by numerical dating or by reference to a calibrated time-scale, may be expressed in years before the present. The unit of time is the modern year as presently recognized worldwide.  Recommended (but not mandatory) abbreviations for such ages are SI (International System of Units) multipliers coupled with "a" for annus: ka, Ma, and Ga for kilo-annus (10<sup>3</sup> years), Mega-annus (10<sup>6</sup> years), and Giga-annus (10<sup>9</sup> years), respectively. Use of these terms after the age value follows the convention established in the field of C-14 dating. The "present" refers to AD 1950, and such qualifiers as "ago" or "before the present" are omitted after the value because measurement of the duration from the present to the past is implicit in the designation. In contrast, the duration of a remote interval of geologic time, as a number of years, should not be expressed by the same symbols. Abbreviations for numbers of years, without reference to the present, are informal (e.g., y or yr for years; my, m.y., or m.yr. for millions of years; and so forth, as preference dictates). For example, boundaries of the Late Cretaceous Epoch currently are calibrated at 63 Ma and 96 Ma, but the interval of time represented by this epoch is 33 m.y.
 |access-date=January 6, 2009
 |archive-date=February 2, 2014
 |archive-url=https://web.archive.org/web/20140202155725/http://ngmdb.usgs.gov/Info/NACSN/Code2/code2.html#Article13
 }}</ref> In archaeology, dealing with more recent periods, normally expressed dates, e.g. "10,000 BC", may be used as a more traditional form than [[Before Present]] ("BP").

These abbreviations include:
{|class="wikitable"
! rowspan=2|{{Anchor|mya}}Non-SI<br />abbreviations
! rowspan=2|Short for
! rowspan=2|SI-prefixed<br />equivalent
! rowspan=2|Definition
! colspan=2|Examples
|-
!Event
!Time
|-
| {{center|[[kyr]]}} || kilo years || ka ||Thousand years||colspan=2|
|-
| {{center|[[myr]]<br />Myr}} || million years<br />Mega years || Ma ||Million years||colspan=2|
|-
| {{center|[[billion years|byr]]<br />Gyr}} || billion years<br />Giga years || Ga ||Billion years<br />([[Long and short scales|thousand million]] years)||colspan=2|
|-
| {{center|kya}} || kilo years ago || || Time ago in ka ||
* Appearance of ''[[Homo sapiens]]''
* [[Out-of-Africa migration]]
* [[Last Glacial Maximum]]
* [[Neolithic Revolution]]
||
Around 200 kya<br />Around 60 kya<br />Around 20 kya<br />Around 10 kya
|-
| {{center|mya<br />Mya}}|| million years ago<br />Mega years ago || ||Time ago in Ma ||
* [[Cretaceous–Paleogene extinction event]]
* [[Pliocene]]
* [[Brunhes–Matuyama reversal|Last geomagnetic reversal]]<ref>
{{cite journal
 |first=Bradford M.
 |last=Clement
 |title=Dependence of the duration of geomagnetic polarity reversals on site latitude
 |date=April 8, 2004
 |journal=Nature
 |volume=428
 |pmid=15071591
 |doi=10.1038/nature02459
 |issue=6983
 |pages=637–640
 |bibcode = 2004Natur.428..637C
 |s2cid=4356044
}}</ref>
* The ([[Eemian Stage]]) [[Last Glacial Period]] started
* The [[Holocene]] started
| Approximately 66 mya<br />5.3 to 2.6 mya<br />0.78 mya<br />0.13 mya<br />0.01 mya
|-
| {{center|bya<br />Gya}} || billion years ago<br/>giga years ago|| ||Time ago in Ga ||
* Oldest [[Eukaryote]]s
* Formation of the [[Earth]]
* [[Big Bang]]
|2 bya<br />4.5 bya<br />13.8 bya
|}

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 durations of epochs.<ref name="AGUStyle" /><ref name="Strat" /> This ''ad hoc'' distinction between "absolute" time and time intervals is somewhat controversial amongst members of the Geological Society of America.<ref>{{cite web|publisher=Geological Society of America|title=Time Units|url=https://www.geosociety.org/TimeUnits/|access-date=February 17, 2010| archive-date=June 16, 2016|archive-url=https://web.archive.org/web/20160616100504/https://www.geosociety.org/TimeUnits/}}</ref>

== See also ==
{{div col|colwidth=30em}}
* [[Astronomical year numbering]]
* [[Century]]
* [[Decade]]
* [[Epoch]]
* [[ISO 8601]]
* [[List of calendars]]
* [[List of years]]
* [[Millennium]]
* [[Orders of magnitude (time)]]
* [[Unit of time]]
{{div col end}}

== References ==
=== Notes ===
{{reflist}}

== Further reading ==
* {{cite book
 | title=Time, the Familiar Stranger
 | url=https://archive.org/details/timefamiliarstra0000fras
 | url-access=registration
 | last=Fraser
 | first=Julius Thomas
 | edition=illustrated
 | location=Amherst
 | publisher=University of Massachusetts Press
 | year=1987
 | isbn=978-0-87023-576-4
 | oclc=15790499
 | bibcode=1988tfs..book.....F
}}
* {{cite book
 | title=What is Time?
 | last=Whitrow
 | first=Gerald James
 | location=Oxford
 | publisher=Oxford University Press
 | year=2003
 | isbn=978-0-19-860781-6
 | oclc=265440481
}}

{{Time Topics}}
{{Time measurement and standards}}
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[[Category:Types of year|*]]
[[Category:Years| ]]