See soorce fer Year

{{fixscots|date=15 May 2013}}

A '''yeir''' (frae [[Auld Inglis leid|Auld Inglis]] ''[[:wikt:gēar|gēar]]''; [[Gothic]] ''[[:wikt:jēr|jēr]]''; [[Runic]] ''[[:wikt:jēran|jēran]]'') is a [[unit]] o [[time]] based on tha [[orbital period]] o tha [[The Yird|Yird]] movin aroond tha [[Sun]]. Fur an observer at is oan tha Yird, thes corresponds tae tha period it taes tha sun tae complete a bodie coorse throoghit tha [[Zodiac]] alang tha [[ecliptic]]. In [[astronomy]], tha [[Julian yeir (astronomy)|Julian year]] is a unit o time at is defined as 365.25 [[day]]s o {{gaps|86|400}} [[Seicont|SI seconds]] each (nae ''[[lowp second]]s'').[[Internaitional Astronomical Union]]"[http://www.iau.org/science/publications/proceedings_rules/units/ SI units]" accessed Februar 18, 2010. (see table fife an section fife.15.) reprintit frae [[George A. Wilkins]] & IAU commission fife, [http://www.iau.org/static/publications/stylemanual1989.pdf "The IAU Style Manual (1989)"] (PDF file) in ''[[IAU Transactions]]'' vol. xxb Thaur is nae universally acceptit seembol fur the year as a unit o time. The [[Internaitional Seestem o Units]] disnae propose a bodie. A common abbreviation in internaitional uisage is '''a''' (fur the [[Laitin laid|Laitin]] wird ''[[:wikt:annus|annus]]''), in [[Scots leid|Scots]] an [[Inglis leid|Inglis]] an aw '''y''' or '''yr'''. Due tae the Yird's [[axial kelter]], the coorse o a year sees the passin o the [[saison]]s, merked bi varioos changes in [[weaither]], hoors o [[daylicht]], an consequently [[vegetation]] an [[Fertility (soil)|fertility]]. In [[temperate]] an [[Subpolar climate|subpolar]] regions, generally fower [[saison]]s ur recognised: ''[[Ware (saison)|ware]]'', ''[[simmer]]'', ''[[hairst]]'' an ''[[winter]]'', each wi different [[temperatur]] an [[wather]], astronomically merked bi the sun reachin the pynts o [[equinox]] an [[solstice]]&mdash;althoogh the climatic saisons [[Saisonal lag|lag]] behind thair astronomical markers. In some [[tropics|tropical]] an [[subtropical]] regions it is mair common tae spick o the [[Wit saison|rainy]] (ur dreich, or [[monsuin]]) saison versus the [[dry saison]]. A [[calendar year]] is an approximation o the Yird's orbital period in a gien [[calendar]]. A calendar year in the [[Gregorian calendar]] (as weel as in the [[Julian calendar]]) haes aither 365 ([[common year]]s), or 366 ([[lowp year]]s) days. The wird "year" is an aw uised o periods loosely associatit but nae strictly identical wi aither the astronomical or the calendar year, sic as the [[saisonal year]], the [[fiscal year]] or the [[academic year]], etc. Bi extension, the term ''year'' can pure techt the orbital period o onie [[planit]]: fur example, a "Maurtian year" is the time in which [[Maurs]] completes its ain orbit. The term is an aw applied mair broadly tae onie lang period or cycle, sic as the "[[great year]]".[[oed]], s.v. "year", entry tae.b.: "''transf.'' Applied tae a huir uv a lang period or cycle (in [[chronology]] an [[mythology]], an vaguely in [[Poetry|poetic]] uise)."

The year is aft dividit intae twal consecutive periods o time cried [[month]]s: [[Januar]], [[Februar]], [[Mairch]], [[Apryle]], [[Mey]], [[Juin]], [[Julie]], [[Augist]], [[September]], [[October]], [[November]], an [[Dizember]].

== Etymology ==
{{further|jēran}}
[[West Saxon dialect|West saxon]] ''gear'' ({{IPA|jɛar}}), [[Anglian dialects|Anglian]] ''gēr'' continues [[Proto-Germanic]] ''*jǣram'' (''*j[[eihwaz|ē2]]ram''). [[Cognate]]s ur [[German language|German]] ''jahr'', [[auld high german]] ''jar'', [[auld norse]] ''ár'' an [[Gothic language|Gothic]] ''jer'', aw frae a [[pie]] ''*yērom'' ("year" or "saison"). Cognates ootwith o Germanic or [[Avestan]] ''yare'' ("year"), [[Ancient Greek leid|ancient Greek]] ''{{lang|grc|ὥρα}}'' ("year", "saison", or "period o time"), whence "[[hoor]]", [[auld kirk slavonic]] ''jaru'' an [[Laitin]] ''hornus'' ("o thes year"). Laitin ''[[:wikt:annus|annus]]'' (a [[2nt declension]] masculine [[noon]]); ''annum'' is the [[Accusatife case|accusatife singular]]; ''anni'' is [[genitife]] [[singular]] an [[nominatife]] [[plural]]; ''anno'' is the datife an ablatife singular) is frae a [[pie]] noin ''{{pie|*at-no-}}'', which yielded [[Gothic language|Gothic]] ''aþnam'' ("year") an aw. Baith ''*yē-ro-'' an ''*at-no-'' ur based oan verbal ruits fur expressin movement&mdash;''*at-'' an ''*ey-'', respectively&mdash;baith meanin "tae go" fur ordinar. The Greek wird fur "year", {{lang|grc|ἔτος}}, is cognate wi Laitin ''vetus'' ("old"), frae peh ''*wetus-'' ("year"), preserved in thes meanin in [[sanskrit]] ''{{iast|vat-sa-}}'' ("yearlin cauf") an ''{{iast|vat-sa-ras}}'' ("year") an aw. Derived frae the Laitin ''annus'' ur a nummer o Scots wirds, sic as [[wikt:annual|annual]], [[:wikt:annuity|annuity]], [[:wikt:anniversary|anniversary]], etc.; ''[[:wikt:per annum|per annum]]'' means "each year".

== Saisonal yeir ==
A [[saisonal yeir]] is the time atween successife recurrences o a [[saison]]al event, sic as the [[fluid (naitural disaster)|fluid]]in o a river, the [[migration]] o a [[species]] o [[spyug]], the [[flouer]]in o a species o [[plaunt]], the first [[frost]] o the yeir, or the first scheduled gam o a certaint [[sport]]. Aw thir events can hae wide variations o mair nor a [[month]] frae yeir tae yeir.

== Calendar yeir ==
A [[calendar yeir]] is the time atween tois dates wi the sam nam in a [[calendar]]. A half-year (ain half o a year) main rin frae Januar tae [[Juin]], or [[Julie]] tae [[Dizember]]. No astronomical yeir haes an [[integer]] [[nummer]] o days or lunar months, sae onie calendar at follows an astronomical yeir main hae a system o [[Intercalation (timekeepin)|intercalation]] sic as [[lowp year]]s. Financial an scienteefic calculations aft uise a 365-day calendar tae simplify daily rates. In the [[Julian calendar]], the average lenth o a yeir is 365.25 days. In a non-lowp yeir, thaur ur 365 days; in a [[lowp yeir]], thaur ur 366 days. A lowp yeir occurs aboot every fower yeirs. The [[Gregorian calendar]] attempts tae keep the [[northward equinox]] oan or shortly afair Mairch 21, hence it follows the [[tropical yeir|northward equinox year]]. The pure techt lenth o the yeir is {{gaps|365.2425}} days (as 97 it o 400 yeirs or lowp yeirs); thes is within a bodie [[pairts per million|ppm]] o the current lenth o the pure techt [[tropical yeir]] ({{gaps|365.242|19}} days). It is estimatit at bi the yeir 4000, the northward equinox will faa back bi a bodie day in the Gregorian calendar, nae coz o thes difference, but coz o the slowin doon o the Yird's rotation an the associatit lengthenin o the [[Sidereal day]]. The [[Revised Julian calendar]], as uised in some [[Eastern Orthodox church]]es, an aw tries tae synchronize wi the tropical yeir. The average lenth o thes calendar's yeir is {{gaps|365.242|222}} days (as 218 it o 900 year or lowp yeirs). Gregorian an revised Julian calendars will stairt tae differ in 2800. The [[iranian calendar|persian calendar]], in uise in [[Afghanistan]] an [[Iran]], haes its yeir begin oan the day o the northward equinox as determined bi astronomical computation (fur the time zain o [[tehran]]), as opposed tae uisin an algorithmic system o lowp yeirs.

=== Nummerin calendar yeirs ===
A [[calendar era]] is uised tae assign a [[nummer]] tae each individual yeir, uisin a reference pynt in the pest as the beginnin o the era. In mony coontries, the maist common era is frae the tradeetional (thoogh noo believed incorrect) yeir o the birth o [[Jesus]]. Dates in thes era ur designatit as ''[[anno domini]]'' (Laitin fur "in the yeir o the Lord"), abbreviatit AD (fur ''Anno Domini'') or CE (fur "[[common era]]"). The yeir afair yin AD or CE is designatit yin "[[befair Christ]]" (BC) or "befair the common era" (BCE); the yeir afair at tae BC; etc. Hence thaur wisnae [[0 (year)|year 0]] AD/CE. When the computations involvin yeirs ur dain involvin baith yeirs AD an years BC, it is common tae uise [[astronomical yeir nummerin]], in which yin BC is designatit "0", tae BC is designatit "−1", an sae oan. Ither eras ur an aw uised tae enumerate the yeirs in different cultural, religioos, an scienteefic contexts.

== Ither annual periods==

=== Fiscal year ===
A [[fiscal year]] or financial year is a 12-month period uised fur calculatin annual financial statements in businesses an ither organisations. In mony jurisdictions, regulations regardin accoontin require sic reports ance per twal months, but dinnae require at the twal months constitute a calendar year. Fur example, in [[Canada]] an [[Indie]] the fiscal year runs frae [[Apryle]] 1; in the [[Unitit Kinrick]] it runs frae Apryle yin fur purposes o corporation tax an government financial statements, but frae Apryle sex fur purposes o personal taxation an payment o state benefits; in [[Australia]] it runs frae Julie 1, while in the [[Unitit States]], the fiscal year o the [[Federal government o the Unitit States|federal government]] runs frae [[October]] yin.

=== Academic year===
An academic year is the annual period durin which a student attends an [[eddication]]al [[Eddicational institution|institution]]. The academic year main be dividit intae [[academic term]]s, sic as semesters an quarters. Some [[schuil]]s in the UK an USA divide the academic year intae ''three'' rooghly equal-lenth terms (cried "trimesters" or "quarters" in the US), rooghly coincidin wi hairst, winter, an ware. At some, a shortened [[simmer]] session, whiles considered a pairt o the regular academic year, is attended bi students oan a voluntary or electife basis. Ither schuils break the year intae ''twa'' main semesters, a first (teepically [[Augist]] ben Dizember) an a second semester (Januar ben [[Mey]]). Each o thir main semesters main be split in half bi mid-term examinaitions, an each o the halves is referred tae as a "[[quarter]]" (or "term" in some coontries). Thaur main an aw be an electife simmer session and/ur a cuttie Januar session. Some ither schuils, includin some in the Unitit States, hae ''foor'' eddicational periods o markin. The skale year in mony coontries starts in Augist or [[September]] an ends in Mey, Juin, or Julie. Some schuils in the Unitit States, notably [[Boston Laitin Schuil]], main divide the year intae ''fife or mair'' markin periods. Some state in defence o thes at thaur is aiblins a [[positife correlation]] atween report frequency an academic achievement. Thaur ur teepically 180 days o teachin each year in schuils in the usa, excludin weekends an breaks, while 190 days fur pupils in state schuils in the Unititd Kinrick, [[New Zealain]] an Canada an aw. Tho, in [[Indie]], the academic year normally starts frae Juin yin an ends oan Mey 31. Thocht schuils stairt closin in the end o [[Ware (saison)|ware]], the actual academic closure is oan Mey 31, an in [[Nepal]] it starts frae Julie 15.{{citation needed|date=May 2013}} schuils an varsities in australia teepically hae academic years at rooghly align wi the calendar year (i.e. startin in [[Februar]] or [[Mairch]] an endin in October tae Dizember), as the [[soothern hemisphaur]] experiences simmer frae Dizember tae Februar. In [[Israel]], the academic year begins aroond October or [[November]], aligned wi the second month o the [[Hebrew calendar]].

== Astronomical year ==

=== Julian year ===
{{main|Julian year (astronomy)}}
The [[Julian year (astronomy)|Julian year]], as uised in [[astronomy]] an ither sciences, is a unit o time defined as exactly 365.25 days. Thes is the normal meanin o the unit "year" (seembol "a" frae the [[Laitin]] ''annus'') uised in varioos scienteefic contexts. The Julian century o {{gaps|36|525}} days an the Julian millennium o {{gaps|365|250}} days ur uised in astronomical calculations. Fundamentally, expressin a time interval in Julian years is a wey tae precisely specify hoo mony days (nae hoo mony "real" years), fur lang time intervals whaur statin the nummer o days woods be unwieldy an unintuitife. Bi convention, the Julian year is uised in the computation o the distance covered bi a [[lichtyear]]. In the [[unified code fur units o measure]], the seembol '''a''' (withit [[subscript]]) aye refers tae the Julian year '''aj''' o exactly {{gaps|31|557|600}} [[seicont]]s. 365.25 days o {{gaps|86|400}} seconds = yin a = yin aj = {{gaps|31.557|6}} ms the SI multiplier prefixes main be applied tae it tae f'rm '''ka''' (kiloannum), '''ma''' (megaannum) etc.

=== Sidereal, tropical, an anomalistic years ===
Each o thir three year can be luisely cried an "astronomical year". The [[sidereal year]] is the time taen fur the Yird tae complete a bodie revolution o its [[orbit]], as measured against a fixed fram o reference (sic as the fixed stars, Laitin ''sidera'', singular ''sidus''). Its average duration is {{gaps|365.256|363|004}} pure techt solar days (365 d sex h 9 min 9.76 s) (at the epoch [[j2000.0]] = Januar yin, 2000, 12:00:00 [[terrestrial time|tt]]). Internaitional Yird rotation and reference system service. (2010).[http://hpiers.obspm.fr/eop-pc/models/constants.html iers eop pc uiseful constants.] the [[tropical year]] is the period o time fur the [[ecliptic longitude]] o the sun tae increase bi 360 degrees. Syne the sun's ecliptic longitude is measured wi respect tae the equinox, the tropical year comprises a complete cycle o the saisons; coz o the biological an socio-economic importance o the saisons, the tropical year is the basis o maist [[calendar]]s. The tropical year is commonly defined<ref>{{cite beuk |url=http://books.google.com/?id=uj4jhgjanb4c&pg=pa80&lpg=pa80&dq=newcomb+tropical+year#v=onepage&q=newcomb%20tropical%20year&f=false |title=explanatory supplement to the astronomical almanac |author=P.K.Seidelman (ed.) |year=1992 |page=80 |isbn=978-1-891389-45-0 }}{{Dead link|date=August 2021 |bot=InternetArchiveBot |fix-attempted=yes }}</ref> fur the pure techt motion o the sun alang the ecliptic. Thes differs frae the actual time atween passages o, e.g., the [[northward equinox]] fur several reasons explained ablo. Coz o the Yird's [[axial precession (astronomy)|axial precession]], thes year is abit 20 [[meenit]]s shorter than the Sidereal year. The pure techt tropical year is approximately 365 days, fife hoors, 48 meenits, 45 seconds<ref>{{cite beuk |url=http://asa.usno.navy.mil/secm/glossary.html#y |title=astronomical almanac fur the year 2011 |year=2009 |publisher=U.S. Government Printin Office an the U.K. Hydrographic Office |location=Washington an Taunton |page=m18 (glossary) |access-date=2013-05-14 |archive-date=2017-08-19 |archive-url=https://web.archive.org/web/20170819204825/http://asa.usno.navy.mil/SecM/Glossary.html#y |url-status=dead }}</ref> (= {{gaps|365.242|19}} days). The anomalistic year is the time taen fur the Yird tae complete a bodie revolution wi respect tae its [[apsis|apsides]]. The orbit o the Yird is [[Ellipse|elliptical]]; the extreme pynts, cried "[[apsides]]", or the [[perihelion]]—whaur the Yird is closest tae the sun (aboot 3 Januar in 2011)—an the [[aphelion]], whaur the Yird is farthest frae the sun (Julie fuir in 2011). The anomalistic year is uisually defined as the time atween perihelion passages. Its average duration is {{gaps|365.259|636}} days (365 d sex h 13 min 52.6 s) (at the epoch j2011.0).<ref>{{cite beuk |title=astronomical almanac fur the year 2011 |year=2009 |publisher = [[U.S. Government Printin Office]] an the [[U.K. Hydrographic Office]] |location = Washington an Taunton |pages = a1, c2 }}</ref> if Yird moved in an ideal [[kepler orbit]], ''i.e.'' a perfect ellipse wi the sun fixed at a bodie focus, each kin o year woods aye hae the sam duration, an the sidereal an anomalistic years woods be equal. Coz o [[perturbation (astronomy)|perturbations]] bi the [[gravity]] o ither planits, Yird's motion varies slichtly, causin the sidereal an tropical years tae vary in lenth bi abit 25 meenits (see the table ablo). Baith ur affectit in the sam wey, sae at the sidereal year is consistently 20 meenits langer than the tropical year, providit at thay ur measured in huir uv a sam wey.

=== Draconic year ===
The draconic year, draconitic year, eclipse year, or ecliptic year is the time taen fur the sun (as seen frae the Yird) tae complete a bodie revolution wi respect tae the sam [[lunar node]] (a pynt whaur the [[muin]]'s orbit intersects the [[ecliptic]]). Thes period is associatit wi [[eclipse]]s: thir occur anly when baith the sun an the muin ur near thir nodes; sae eclipses occur within abit a month o every half o an eclipse year. Hence, thaur ur tois [[eclipse saison]]s every eclipse year. The average duration o the eclipse year is:{{gaps|346.620|075|883}} days (346 d 14 h 52 min 54 s) (at the epoch j2000.0). This term is whiles erroneoosly uised fur the draconic or nodal period o [[lunar precession]]; at is, the period o a complete revolution o the muin's ascendin node aroond the ecliptic: {{gaps|18.612|815|932}} Julian years ({{gaps|6|798.331|019}} days, at the epoch j2000.0).

=== Full muin cycle ===
The [[full muin cycle]] is the time fur the sun (as seen frae the Yird) tae complete a bodie revolution wi respect tae the [[perigee]] o the orbit o the muin. Thes period is associatit wi the apparent size o the [[full muin]], an wi the varyin duration o the [[month|synodic month]] an aw. The duration o a bodie foo muin cycle is: :{{gaps|411.784|430|29}} days (411 d 18 h 49 min 34 s) (at the epoch j2000.0).

=== Lunar year ===
The [[lunar year]] comprises twal foo cycles o the phases o the muin, as seen frae the Yird. It haes a duration o approximately 354.37 days. [[Muslims]] uise thes fur celebratin thair [[Musleem holidays|eids]] an fur markin the stairt o the fastin month o ''[[Ramadan]]''. A Musleem calender year is based oan the [[lunar cycle]].

=== Vague year ===
The "vague year", frae ''annus vagus'' ("wanderin year"), is an integral approximation tae the year equalin 365 days, which wanders in relation tae mair exact years. teepically, the vague year is dividit intae 12 [[wiktionary:schematic|schematic]] months o 30 days each plus fife [[Intercalation (timekeepin)|epagomenal]] days. Thes "vague year" wis uised in the calendars o [[Egyptian calendar|ancient Egyp]], [[Iranian calendar|Iran]], [[Armenian calendar|Armenie]] an in [[mesoamerican calendars|mesoamerica]] amang the [[Aztec calendar|Aztecs]] an [[Haab|Maya]].[http://www.mayacalendar.com/descripcion.html calendar description an coordinaition] Maya Warld Studies Center.

=== Heliacal year ===
A [[heliacal year]] is the interval atween the [[heliacal risin]]s o a staurn. It differs frae the [[sidereal year]] fur stars awa frae the [[ecliptic]] due mainly tae the [[precession o the equinoxes]].

=== Sothic year ===
The [[Sothic cycle|Sothic year]] is the interval atween heliacal risins o the staurn [[sirius]]. It is presently less nor the [[#sidereal, tropical, an anomalistic years|sidereal year]] an its duration is huir uv a close tae the pure techt Julian year o 365.25 days.
=== Gaussian year ===
The [[Gaussian year]] is the sidereal year fur a [[planit]] o negligible mass (relatife tae the sun) an unperturbed bi ither planits at is governed bi the [[Gaussian gravitational constant]]. Sic a planit woods be slichtly closer tae the sun than the Yird's pure techt distance. Its lenth is: :{{gaps|365.256|898|3}} days (365 d sex h 9 min 56 s).

=== Besselian year ===
The [[Besselian epoch|Besselian year]] is a tropical year at starts when the (fictitioos) pure techt sun reaches an ecliptic longitude o 280°. Thes is currently oan or close tae Januar yin. It wis named efter the [[19th century]] German astronomer an mathematician [[Friedrich Bessel]]. The followin [[equation]] can be uised in irder tae compute the current besselian epoch (in years):<ref>{{cite beuk |title=[[astronomical almanac]] fur the year 2010 |year=2008 |publisher = U.S. Government Printing Office and the U.K. Hydrographic Office |location = Washington an Taunton |page = b3 }}</ref> b = 1900.0 + (Julian datett − {{gaps|2|415|020.313|52}}) / {{gaps|365.242|198|781}} the "tt" [[subscript]] indicates at fur thes [[formula]], the Julian date shoods uise the [[terrestrial time]] scale or its predecessur, [[ephemeris time]].

=== Variation in the lengths o the year an the day ===
{{refimprove section|date=May 2013}}
The exact lenth o an astronomical year changes ower time.[http://asa.usno.navy.mil/secm/glossary.html#dynamical-equinox the astronomical almanac online] {{Webarchive|url=https://web.archive.org/web/20170819204825/http://asa.usno.navy.mil/SecM/Glossary.html#dynamical-equinox |date=2017-08-19 }}{{failed verification|date=May 2013}} the main soorces o thes chynge are:
* The poseetions o the equinox an solstice pynts wi respect tae the apsides o Yird's orbit chynge: the first coz o [[precession]], an the apsides coz o the long-term effects o gravitational pull bi the ither planits. Syne the speed o the Yird varies accordin tae its poseetion in its orbit as measured frae its [[perihelion]], Yird's speed when in a solstice or equinox pynt changes ower time: if sic a pynt moves toward [[perihelion]], the interval atween tois passages decreases a bit frae year tae year; if the pynt moves towards [[aphelion]], en at period inreases a bit frae year tae year. Sae a "tropical year" measured frae a bodie passage o the northward ("vernal") equinox tae the neist, it differs frae the a bodie measured atween passages o the soothward (hairst) equinox. The average ower the foo orbit disnae chynge coz o thes, sae the lenth o the average tropical year disnae chynge coz o thes second-order effect.
* Each planit's movement is perturbed bi the gravity o every ither planit. Thes leids tae short-term fluctuations in its speed, an therefair its period frae year tae year. Mairowre, it haes caused long-term changes in its orbit, an therefair long-term changes in thir periods an aw.
*[[Tidal drag]] atween the Yird an the muin (an sun) increases the lenth o the day an o the month (bi transferrin angular momentum frae the rotation o Yird tae the revolution o the muin); syne the apparent pure techt solar day is the unit wi which we measure the lenth o the year in civil life, the lenth o the year appears tae decrease. The rotation rate o the Yird is changed an aw bi factors sic as [[post-glacial reboond]] an sea level rise.

==== Numerical value o year variation ====
Mean year lenths in thes section ur calculatit fur [[2000]], an differences in year lenths compared tae 2000 ur gien fur pest an future years. In the tables a day is 86,400 SI seconds lang.<ref>{{cite beuk
|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 beuk
|last1=Taff
|first1=Lawrence G.
|title=Celestial Mechanics: A Computational Guide for the Practitioner
|year=1985
|isbn=0-471-89316-1
|place=New York
|publisher=John Wiley & Sons
|page=103
}} Values in tables agree closely fur 2000, an depairt bi as much as 44 seconds fur the years furthest in the pest or future; the expressions ur simpler than those recommended in the ''Astronomical Almanac fur the year 2011''.</ref><ref>{{cite book |edition=3 |publisher=Univ Science Books |isbn=1-891389-85-8 |last=Seidelmann |first=P. Kenneth |others=Sean E. Urban (ed.) |title=Explanatory Supplement to the Astronomical Almanac |year=2013 |page=587}} Tabulates length o tropical year frae −500 tae 2000 at 500-year intervals usin a [[formula]] bi laskar (1986); agrees closely wi values in thes section near 2000, depairts bi sex seconds in −500.</ref>
{| class="wikitable"
|+Mean year lenths fur 2000
|-
! Teep o year !! days !! hoors !! meenits || seconds
|-
| tropical || 365 || 5 || 48 || 45
|-
| sidereal || 365 || 6 || 9 || 10
|-
| anomalistic || 365 || 6 || 13 || 53
|-
| eclipse || 346 || 14 || 52 || 55
|}

{| class="wikitable"
|+Lenth o year: difference frae 2000 in seconds<br />(anly [[positive]] when lenth fur tabulatit year is greater than the lenth fur 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 ===
* 346.62 days: a [[dracontic year|draconitic year]].
* 353, 354, or 355 days: the lenths o common years in some [[lunisolar calendar]]s.
* 354.37 days (12 lunar months): the average lenth o the years in [[lunar calendar]]s, notably the musleem calendar.
* 365 days: a vague year ane a [[common year]] in mony [[solar calendar]]s.
* {{gaps|365.242|19}} days: a pure techt tropical year (roonded tae fife decimal places) fur the [[epoch]] [[2000]].
* {{gaps|365.242|4}} days: a vernal equinox year (roonded tae fower decimal places) fur the epoch 2000.
* {{gaps|365.242|5}} days: the average lenth o a year in the Gregorian calendar.
* 365.25 days: the average lenth o a year in the Julian calendar.
* {{gaps|365.256|4}} days: a sidereal year.
* 366 days: a [[lowp year]] in mony solar calendars.
* 383, 384, or 385 days: the lenths o lowp years in some lunisolar calendars.
* 383.9 days (13 lunar months): a lowp year in some lunisolar calendars. An average Gregorian year is {{gaps|365.242|5}} [[day]]s = {{gaps|52.177|5}} [[week]]s = {{gaps|8|765.82}} [[hoor]]s = {{gaps|525|949.2}} [[meenit]]s = {{gaps|31|556|952}} [[seicont]]s (mean solar, nae si). A common year is 365 days = {{gaps|8|760}} hoors = {{gaps|525|600}} meenits = {{gaps|31|536|000}} seconds. A lowp year is 366 days = {{gaps|8|784}} hoors = {{gaps|527|040}} meenits = {{gaps|31|622|400}} seconds. The 400-year cycle o the Gregorian calendar haes {{gaps|146|097}} days, an hence exactly {{gaps|20|871}} [[week]]s.

== Symbol ==
Thaur is nae universally acceptit seembol fur the year as a unit o time. The [[Internaitional Seestem o Units]] disnae propose a bodie. Nist sp811 <ref>{{cite journal |author=Ambler Thompson, Barry N. Taylor |url=http://physics.nist.gov/document/sp811.pdf |format=pdf |publisher=[[Naitional Institute o Standards an Technology]] (nist) |title=special publication 811: Guide for the use of the International System of Units (SI) |at=paragraph 8.1 |year=2008 |journal= |access-date=2013-05-14 |archive-date=2008-05-11 |archive-url=https://web.archive.org/web/20080511154858/http://physics.nist.gov/Document/sp811.pdf |url-status=dead }}</ref> an ISO 80000-3:2006 <ref>{{cite web |publisher=[[internaitional organization fur standardization]] |url=http://www.iso.org/iso/iso_catalogue/catalogue_tc/catalogue_detail.htm?csnumber=31888 |title=ISO 80000-3:2006, quantities and units – part 3: space and time |location=geneva, switzerland |year=2006 }}</ref> suggest the seembol ''[[a (year)|a]]'', taen frae the Laitin wird ''annus''.<ref>{{cite web |work=haw mony? a dictionary o units o measurement |author=Russ Rowlett |publisher=university o north carolina |title=units: a |url=http://www.unc.edu/~rowlett/units/dicta.html |accessdate=2009-01-09 |archive-date=2008-12-20 |archive-url=https://web.archive.org/web/20081220111445/http://www.unc.edu/~rowlett/units/dictA.html |url-status=dead }}</ref> in Scots, the abbreviations "y" an "yr" ur whiles uised, specifically in [[geology]] an [[paleontology]], whaur "kyr, myr, byr" (thoosands, millions, an even billions o years, respectively) an seemilar abbreviations ur uised tae denote intervals o time remote frae the present.<ref>{{cite web |url=http://www.agu.org/pubs/style_guide_intro.html |publisher=american geophysical union |title=agu editorial style guide fur authors |date=September 21, 2007 |accessdate=2009-01-09 |archiveurl=https://web.archive.org/web/20080714134306/http://www.agu.org/pubs/style_guide_intro.html |archivedate=2008-07-14 |url-status=dead }} {{cite journal |author=North American Commission on Stratigraphic Nomenclature |url=http://ngmdb.usgs.gov/info/nacsn/code2/code2.html#article13 |title=north american stratigraphic code |journal=The American Association o Petroleum Geologists Bulletin |volume=89 |issue=11 |date=November 2005 |pages=1547–1591 |doi= |edition=article 13 (c) |access-date=2013-05-14 |archive-date=2014-02-02 |archive-url=https://web.archive.org/web/20140202155725/http://ngmdb.usgs.gov/Info/NACSN/Code2/code2.html#article13 |url-status=dead }}</ref>

=== Symbol "A" ===
Nist sp811 <ref>{{cite web |author=Ambler Thompson, Barry N. Taylur |url=http://physics.nist.gov/document/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 eeecht.1 |year=2008 |access-date=2013-05-14 |archive-date=2008-05-11 |archive-url=https://web.archive.org/web/20080511154858/http://physics.nist.gov/Document/sp811.pdf |url-status=dead }}</ref> an ISO 80000-3:2006 <ref>{{cite web |publisher=[[Internaitional Organization fur Standardization]] (ISO) |url=http://www.iso.org/iso/iso_catalogue/catalogue_tc/catalogue_detail.htm?csnumber=31888 |title=ISO 80000-3:2006, quantities an units |at=part 3: space and time |location=[[Geneva]] |year=2006}}</ref> suggest the seembol '''a''' (in the [[Internaitional Seestem o Units]], althoogh '''a''' is the seembol an aw fur the ''[[hectare#are|are]]'', the unit o aurie uised tae measure lain aurie, but context is uisually enaw tae disambiguate). The abbreviations '''y''' an '''yr''' ur uised an aw. The [[unified code fur units o measure]]<ref>{{cite web |author=Gunther Schadaw, Clement J. Mcdonald |url=http://aurora.rg.iupui.edu/ucum |title=unified code fur units o measure |access-date=2013-05-14 |archive-date=2008-05-20 |archive-url=https://web.archive.org/web/20080520112256/http://aurora.rg.iupui.edu/ucum |url-status=dead }}</ref> disambiguates the varyin symbologies of ISO 1000, ISO 2955 an ansi x3.50<ref>http://aurora.regenstrief.org/~ucum/ucum.html#para-31 {{Webarchive|url=https://web.archive.org/web/20100304195312/http://aurora.regenstrief.org/~ucum/ucum.html#para-31 |date=2010-03-04 }}</ref> bi usin :''ar'' fur [[hectare#are|are]] and: :'''at''' = a_t = {{gaps|365.242|19}} days fur the pure techt [[tropical year]] :'''aj''' = a_j = 365.25 days fur the pure techt Julian year :'''ag''' = a_g = {{gaps|365.242|5}} days fur the pure techt [[Gregorian year]] :'''a''' = yin aj year (withit further qualifier) a definition jointly adoptit bi the [[Internaitional Union o Pure an Applee'd Chemistry]] (IUPAC) an the [[Internaitional Union o Geological Sciences]] (IUGS) is tae uise ''annus'', wi seembol '''a''', fur year, defined as the lenth o the tropical year in the year 2000:<ref>{{cite journal |author=Norman E. Holden, Mauro L. Bonardi, Paul de Bièvre, Paul R. Renne, an Igur M Villa |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}}</ref><ref>{{cite journal |author=Celeste Biever |title=Push to Define Year Sparks Time War |journal=[[New Scientist]] |url=http://www.newscientist.com/article/dn20423-push-to-define-year-sparks-time-war.html |date=Apryle 27, 2011 |accessdate=Apryle 28, 2011}}</ref> :'''a''' = {{gaps|365.242|192|65}} days = {{gaps|31|556|925.445}} seconds the notation haes proved controversial; it conflicts wi an earlier convention amang geoscientists tae uise '''a''' specifically fur "years ago", an '''y''' or '''yr''' fur a period o ane year.

==== SI prefix multipliers ====
Fur the followin, thaur ur alternatife forms which elide the consecutife vowels, sic as ''kilannum'', ''megannum'', etc.
* '''ka''' (fur kiloannum), is a [[millennium|unit o time equal tae]] a bodie [[thoosand]] (103) years. Thes is teepically uised in [[geology]], [[paleontology]], an [[archaeology]] fur [[holocene]] an [[pleistocene]] periods whaur a non-[[radiocarbon datin]] technique (i.e., [[ice core]] datin, [[dendrochronology]], [[uranium-thorium datin]], or [[varve]] analysis) is uised as the primary datin method fur age determinaition. If age is primarily determined bi [[radiocarbon datin]], en the age shoods be expressed in aither radiocarbon or calendar (calibratit) years [[befair present]].
* '''ma''' (fur megaannum), is a unit o [[time]] equal tae a bodie [[million]] (106) years. It is commonly uised in scienteefic disciplines sic as [[geology]], [[paleontology]], an [[celestial mechanics]] tae signify huir uv a lang time periods intae the pest or future. Fur example, the [[dinosaur]] species ''[[tyrannosaurus|tyrannosaurus rex]]'' wis abundant approximately ''65 ma'' (65 million years) ago (''ago'' mauna aye be mentioned; if the quantity is specified while nae explicitly discussin a duration, a bodie can assume at "ago" is implied; the alternatife "[[mya (unit)|mya]]" unit includes "ago" explicitly). In astronomical applications, the year uised is the [[Julian year (astronomy)|Julian year]] o precisely 365.25 days. In [[geology]] an [[paleontology]], the year isnae sae precise an varies—dependin oan the authur.
* '''ga''' (fur gigaannum), is a unit o time equal tae 109 year (ain [[1000000000 (nummer)|billion]] oan the [[lang an cuttie scales|short scale]], a bodie [[milliard]] oan the [[lang an cuttie scales|lang scale]]). It is commonly uised in scienteefic disciplines sic as [[physical cosmology|cosmology]] an [[geology]] tae signify extremely lang time periods in the pest. Fur example, the formation o the [[Yird]] occurred approximately ''4.57 ga'' (4.57 billion years) ago.
* '''ta''' (fur teraannum), is a unit o time equal tae 1012 year (ain [[1000000000000 (nummer)|trillion]] oan the cuttie scale, a bodie billion oan the lang scale). It is an extremely lang unit o time, abit 70 times as lang as the [[age o the universe]]. It's the sam order o magnitude as the expectit life span o a wee [[red dwarf star]].
* '''pa''' (fur petaannum), is a unit o time equal tae 1015 year (ain [[10^15|quadrillion]] oan the cuttie scale, a bodie billiard oan the lang scale). The [[half-life]] o the [[nuclide]] o [[cadmium]]-113 is abit eeecht pa.<ref>{{cite journal| author = p. belli ''it al.''| title = Investigation of β decay of 113cd| journal = [[Phys. Rev. C]] |volume = 76|page = 064603|year = 2007 |doi = 10.1103/physrevc.76.064603|bibcode = 2007phrvc..76f4603b| issue = sex }}</ref> thes seembol coincides wi at fur the [[pascal (unit)|pascal]] withit a multiplier prefix, thocht baith ur infrequently uised an context will normally be sufficient tae distinguish time frae pressure values.
* '''ea''' (fur exaannum), is a unit o time equal tae 1018 year (ain [[quintillion]] oan the cuttie scale, a bodie trillion oan the lang scale). The half-life o [[tungsten]]-180 is yin.8 ea.<ref>{{cite journal| author = f. a. danevich ''it al.''| title = α activity of natural tungsten isotopes| journal = [[phys. rev. c]] |volume = 67|page = 014310|year = 2003| arxiv = nucl-ex/0211013|doi = 10.1103/physrevc.67.014310|bibcode = 2003phrvc..67a4310d }}</ref>

=== Symbols "y" an "yr" ===
In [[astronomy]], [[geology]], an [[paleontology]], the abbreviations '''yr''' fur "year" an '''ya''' fur "year ago" ur whiles uised, combined wi prefixes fur "thoosand", "million", or "billion".<ref>{{cite journal |author=North American Commission on Stratigraphic Nomenclature |url=http://ngmdb.usgs.gov/info/nacsn/code2/code2.html#article13 |title=North American Stratigraphic Code |chapter=article 13 (c) |quote=(c) convention an abbreviations. |journal= |access-date=2013-05-14 |archive-date=2014-02-02 |archive-url=https://web.archive.org/web/20140202155725/http://ngmdb.usgs.gov/Info/NACSN/Code2/code2.html#article13 |url-status=dead }}</ref> The age o a stratigraphic unit or the time o a geologic event, as commonly determined bi numerical datin or bi reference tae a calibratit time-scale, main be expressed in years afair the present. The unit o time is the modren year as presently recognised warldwide. Recommended but nae mandatory abbreviations fur sic ages ur SI (Internaitional Seestem o Units) multipliers coopled wi "a" fur annum: ka, ma, an ga fur kilo-annum (103 year), mega-annum (106 year), an giga-annum (109 year), respectively. The uisin o thir terms efter the age value follows the convention established in the field o [[carbon-14 datin]]. The "present" refers tae 1950 ad, an sic qualifiers as "ago" or "befair the present" ur omittit efter the value coz measurement o the duration frae the present tae the pest is implicit in the designaition. In contraest, the duration o ane remote interval o geologic time, as a nummer o years, shoods nae be expressed bi the sam seembols. Abbreviations fur nummers o years, withit reference tae the present, ur cried informal (e.g., '''y''' or '''yr''' fur "years"; '''mah''', ''m.y.'', or '''m.yr.''' fur millions o years; an sae forth, as preference dictates). Fur example, boondaries o the late cretaceoos epoch currently ur calibratit at 63 ma an 96 ma, but the interval o time representit bi thes epoch is 33 m.y. thay arenae SI units, uisin '''y''' tae abbreviate Scots [[:wikt:year|year]], but followin ambiguoos internaitional recommendations, uise aither the standard Scots first letters as prefixes ("t", "m", an "b") or [[metric prefix]]es ("k", "m", an "g"). Thir abbreviations include:

{| class="wikitable"
|-
| Non-SI abbreviation || SI-prefixed equivalent || Order o magnitude
|-
| '''kyr'''|| ka ||
* Thoosand years
|-
| '''myr''' || Ma ||
* Million years
|-
| '''byr'''  || Ga ||
* Billion years ([[Long an short scales|thoosand million]] years)
|-
| '''tya''' or '''kya''' || "ka ago" || {{Main|1 E10 s|1 E11 s|1 E12 s}}
* Appearance o ''[[Homo sapiens]]'', ca. 200 tya
* [[Oot o Africa migration]], ca. 60 tya
* [[Lest Glacial Maximum]], ca. 20 tya
* [[Neolithic Revolution]], ca. 10 tya
|-
| {{Anchor|mya}}'''mya''' || "Ma ago" || {{Main|1 E13 s|1 E14 s|1 E15 s}}
* [[Pliocene]], 5.3 tae 2.6 mya
** The [[Brunhes–Matuyama reversal|lest geomagnetic reversal]] wis 0.78 mya<ref>{{cite journal|author=Bradford M. Clement|title=Dependence of the duration of geomagnetic polarity reversals on site latitude|url=https://archive.org/details/sim_nature-uk_2004-04-08_428_6983/page/637|date=Apryle 8, 2004|journal=Nature|volume=428|pmid=15071591|doi=10.1038/nature02459 |issue=6983 |pages=637–40|bibcode = 2004Natur.428..637C }}</ref>
** The ([[Eemian Stage]]) [[Ice Age]] startit 0.13 mya
* The [[Holocene]] startit 0.01 mya
|-
| '''bya''' or '''gya''' || "Ga ago" || {{Main|1 E16 s|1 E17 s|1 E18 s}}
* Oldest [[Eukaryotes]], 2 bya
* Age o the [[Yird]], 4.5 bya
* [[Big Bang]], 13.8 bya
|}

Uisage o "mya" an "bya" is deprecatit in modren geophysics, the recommended usage til "ma" an "ga" fur dates [[befair present]], but "m.y." fur the duration o epochs. Thes ''ad hoc'' distinction atween "absolute" time an time intervals is somewhat controversial amongst members o the [[Geological Society o America]].<ref>{{cite web|publisher=geological society o america|title=time units|url=http://www.geosociety.org/timeunits/|accessdate=2010-2-17|archive-date=2016-06-16|archive-url=https://web.archive.org/web/20160616100504/http://www.geosociety.org/TimeUnits/|url-status=dead}}</ref> note at oan graphs uisin "ya" units oan the horizontal axis time flows frae reit tae left, which main seem coonter-intuitife. If the "ya" units ur oan the vertical axis, time flows frae tap tae bottom which is probably easier tae kin than conventional notation.

== Great years ==

=== Equinoctial cycle ===
The "[[great year]]", or ''equinoctial cycle'', corresponds tae a complete revolution o the equinoxes aroond the ecliptic. Its lenth is abit 25,700 year, an cannae be determined precisely as the [[astronomical precession|precession]] speed is variable.

=== Galactic year ===
The [[galactic year]] is the time it taes the Yird's [[solar system]] tae revolve ance aroond the [[galactic center]]. It comprises rooghly 230 million Yird years.<ref>{{cite web | url=http://www.orau.gov/sciencebowl/teams/files/astrset2.pdf | title=science bowl questions, astronomy, sit tae | work=science bowl practice questions | year=2009 | publisher=Oak Ridge Associated Universities | accessdate=2009-12-9 | archive-date=2010-03-07 | archive-url=https://web.archive.org/web/20100307191635/http://www.orau.gov/sciencebowl/teams/files/astrset2.pdf | url-status=dead }}</ref>

== See an aw ==
{{portal|time}}
{{div col|2}}
* [[{{CURRENTYEAR}}]]
* [[Astronomical year nummerin]]
* [[Befair present]]
* [[ISO 8601]]: standard fur representation o dates an times
* [[List o calendars]]
* [[List o years]]
* [[Man-hoor]]
* [[Orders o magnitude (time)]]
* [[Unit o time]]
{{div col end}}

== References ==

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

== Further readin ==
* {{cite beuk | title=time, the familiar stranger | last=Fraser | first=Julius Thomas | edition=illustratit | location=Amherst | publisher=[[University of Massachusetts Press]] | year=1987 | isbn=0-87023-576-1 | oclc=15790499 }}
* {{cite beuk | title=what is time? | last=Whitrow | first=Gerald Jimmy | location=Oxford | publisher=[[Oxford University Press]] | year=2003 | isbn=0-19-860781-4 | oclc=265440481 }}
{{time topics}}