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A '''year''' (from [[Old English language|Old English]] ''[[Jēram|gēar]]'') is the [[orbital period]] of the [[Earth]] moving around the [[Sun]]. For an observer on Earth, this corresponds to the period it takes the Sun to complete one course throughout the [[zodiac]] along the [[ecliptic]].
In astronomy, the [[Julian year (astronomy)|Julian year]] is a [[unit of time]], defined as 365.25 [[day]]s of {{gaps|86|400}} [[second|SI second]]s each.<ref>[[International Astronomical Union]] "[http://www.iau.org/science/publications/proceedings_rules/units/ SI units]" accessed February 18, 2010. (See Table 5 and section 5.15.) Reprinted from George A. Wilkins & IAU Commission 5, [http://www.iau.org/static/publications/stylemanual1989.pdf "The IAU Style Manual (1989)"] (PDF file) in ''[[IAU Transactions]]'' Vol. XXB</ref>
There is no universally accepted symbol for the year as a unit of time. The [[International System of Units]] does not propose one. A common abbreviation in international use is '''a''' (for Latin ''[[:wikt:annus|annus]]''), in English also '''y''' or '''yr'''.
Due to the Earth's [[axial tilt]], the course of a year sees the passing of the [[season]]s, marked by changes in [[weather]], hours of [[daylight]], and consequently [[vegetation]] and [[Fertility (soil)|fertility]].
In [[temperate]] and [[Subpolar climate|subpolar]] regions, generally four seasons are recognized: ''[[Spring (season)|spring]]'', ''[[summer]]'', ''[[autumn]]'' and ''[[winter]]'', astronomically marked by the Sun reaching the points of [[equinox]] and [[solstice]], although the climatic seasons [[Seasonal lag|lag behind]] their astronomical markers.
In some [[tropics|tropical]] and [[subtropical]] regions it is more common to speak of the [[wet season|rainy]] (or wet, or [[monsoon]]) season versus the [[dry season]].
A [[calendar year]] is an approximation of the Earth's orbital period in a given [[calendar]].
A calendar year in the [[Gregorian calendar]] (as well as in the [[Julian calendar]]) has either 365 ([[common year]]s) or 366 ([[leap year]]s) days.
The word "year" is also used of periods loosely associated but not strictly identical with either the astronomical or the calendar year, such as the [[seasonal year]], the [[fiscal year]] or the [[academic year]], etc.
By extension, the term ''year'' can mean the orbital period of any [[planet]]: for example, a "Martian year" is the time in which Mars completes its own orbit. The term is also applied more broadly 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>
==Etymology==
{{See|Jēran}}
[[West Saxon]] ''gear'' ({{IPA|jɛar}}), [[Anglian dialects|Anglian]] ''gēr'' continues [[Proto-Germanic]] ''*jǣram'' (''*j[[Eihwaz|ē<sub>2</sub>]]<nowiki></nowiki>ram''). Cognates are [[German language|German]] ''Jahr'', [[Old High German]] ''jar'', [[Old Norse]] ''ár'' and [[Gothic language|Gothic]] ''jer'', all from a [[PIE]] ''*yērom'' "year, season".
Cognates outside of Germanic are [[Avestan]] ''yare'' "year", [[Ancient Greek|Greek]] ''{{lang|grc|ὥρα}}'' "year, season, period of time" (whence "[[hour]]"), [[Old Church Slavonic]] ''jaru'' and [[Latin]] ''hornus'' "of this year".
Latin '''''[[:wikt:annus|Annus]]''''' (a [[2nd declension]] masculine noun; ''annum'' is the [[accusative case|accusative singular]]; ''anni'' is genitive singular and nominative plural; ''anno'' the dative and ablative singular) is from a [[PIE]] noun ''{{PIE|*at-no-}}'', which also yielded [[Gothic language|Gothic]] ''aþnam'' "year".
Both ''*yē-ro-'' and ''*at-no-'' are based on verbal roots expressing movement, ''*at-'' and ''*ey-'' respectively, both meaning "to go" generally.
The Greek word for "year", {{lang|grc|ἔτος}}, is cognate with Latin ''vetus'' "old", from PIE ''*wetus-'' "year", also preserved in this meaning in [[Sanskrit]] ''{{IAST|vat-sa-}}'' "yearling (calf)" and ''{{IAST|vat-sa-ras}}'' "year".
Derived from Latin ''annus'' are a number of English words, such as [[wikt:annual|annual]], [[:wikt:annuity|annuity]], [[:wikt:anniversary|anniversary]], etc.; ''[[:wikt:per annum|per annum]]'' means "each year".
==Seasonal year==
{{See|Effect of sun angle on climate}}
A '''[[seasonal year]]''' is the time between successive recurrences of a [[season]]al 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.
==Calendar year==
A '''[[calendar year]]''' is the time between two dates with the same name in a [[calendar]].
A ''half year'' (one half of a year) may run from January to June, or July to December.
No astronomical year has an integer number of days or lunar months, so any calendar that follows an astronomical year must have a system of [[intercalation]] such as [[leap year]]s. Financial and scientific calculations often use a [[365-day calendar]] to simplify daily rates.
In the [[Julian calendar]], the average 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 4 years.
The [[Gregorian calendar]] attempts to keep the [[vernal equinox]] on or soon before March 21, hence it follows the [[tropical year|vernal equinox year]]. The average length of this calendar's year is {{gaps|365.242|5}} mean solar days (as 97 out of 400 years are leap years); this is within one [[Parts-per notation|ppm]] of the current length of the mean [[tropical year]] ({{gaps|365.242|19}} days). It is estimated that, by the year 4000, the vernal equinox will fall back by one day in the Gregorian calendar, not because of this difference, but because of the slowing down of the Earth's rotation and the associated lengthening of the [[sidereal day]].
The [[Iranian calendar|Persian calendar]], in use in [[Afghanistan]] and [[Iran]], has its year begin on the day of the vernal equinox as determined by astronomical computation (for the time zone of [[Tehran]]), as opposed to using an algorithmic system of leap years.
===Numbering calendar years ===
A [[calendar era]] is used to assign a number to individual years, using a reference point in the past as the beginning of the era. In many countries, the most common era is from the estimated date of the birth of [[Jesus]]; dates in this era are designated ''[[Anno Domini]]'' ("in the year of the Lord", abbreviated ''A.D.'') or ''C.E.'' ([[common era]]). Other eras are also used to enumerate the years in different cultural, religious or scientific contexts.
==Other annual periods==
===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, the federal government of the U.S. has a fiscal year that starts on October 1 instead of January 1. In [[India]] the fiscal year is between April 1 and March 31. In the [[United Kingdom]] and [[Canada]], the financial year runs from April 6 and April 1 respectively, and in [[Australia]] it runs from July 1.
===Academic year===
An '''academic year''' is the annual period during which a student attends an [[educational institution]]. The academic year may be divided into [[academic term]]s, such as semesters or quarters.
Some schools in the UK and USA divide the academic year into ''three'' roughly equal-length terms (called "trimesters" or "quarters" in the USA), 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 an elective summer session and/or a short January session.
Some other schools, including some in the United States, have ''four'' marking periods. The school year in many countries starts in August or September and ends in May, June or July.
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 USA, excluding weekends and breaks, while 190 days for pupils in state schools in the United Kingdom, New Zealand and Canada.
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==<!-- This section is linked from [[Season]] -->
===Julian year===
{{Main|Julian year (astronomy)}}
The '''[[Julian year (astronomy)|Julian year]]''', as used in astronomy and other sciences, is a time unit defined as exactly 365.25 days. This is the normal meaning of the unit "year" (symbol "a" from the [[Latin]] ''annus'') used in various scientific contexts. The Julian century of {{gaps|36|525}} days and the Julian millennium of {{gaps|365|250}} days are used in astronomical calculations. Fundamentally, expressing a time interval in Julian years is a way to precisely specify how many days (not how many "real" years), for long time intervals where stating the number of days would be unwieldy and unintuitive. By convention, the Julian year is used in the computation of the distance covered by a [[light-year]].
In the [[Unified Code for Units of Measure]], the symbol '''a''' (without subscript) always refers to the Julian year '''a<sub>j</sub>''' of exactly {{gaps|31|557|600}} [[second]]s.
365.25 days of {{gaps|86|400}} seconds = 1 a = 1 a<sub>j</sub> = {{gaps|31.557|6}} Ms
The SI multiplier prefixes may be applied to it to form '''ka''' (kiloannum), '''Ma''' (megaannum) etc.
===Sidereal, tropical, and anomalistic years===
:''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 ''sidera'', singular ''sidus''). Its average duration is {{gaps|365.256|363|004}} mean solar days (365 d 6 h 9 min 9.76 s) (at the epoch [[J2000.0]] = <nowiki>January 1, 2000, 12:00:00</nowiki> [[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.]</ref>
The '''[[tropical year]]''' is the period of time for the [[ecliptic longitude]] of the Sun to increase by 360 degrees. Since the Sun's ecliptic longitude is measured with respect to the equinox, the tropical year comprises a complete cycle of the seasons; because of the economic importance of the seasons, the tropical year is the basis of most [[calendar]]s. The tropical year is often defined as the time between [[southern solstice]]s, or between [[northward equinox]]es. 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<ref>
{{cite book
|url=http://asa.usno.navy.mil/SecM/Glossary.html#y
|title=Astronomical Almanac for the Year 2011
|year=2009
|publisher = U.S. Government Printing Office and the U.K. Hydrographic Office
|location = Washington and Taunton
|page = M18 (Glossary)
}}</ref> (= {{gaps|365.242|19}} days).
The '''anomalistic year''' is the time taken for the Earth to complete one revolution with respect to its [[Apsis|apsides]]. The orbit of the Earth is elliptical; the extreme points, called apsides, are the [[perihelion]], where the Earth is closest to the Sun (January 3 in 2011), and the [[aphelion]], where the Earth is farthest from the Sun (July 4 in 2011). The anomalistic year is usually defined as the time between perihelion passages. Its average duration is {{gaps|365.259|636}} 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 = [[U.S. Government Printing Office]] and the [[U.K. Hydrographic Office]]
|location = Washington and Taunton
|pages = A1, C2
}}</ref>
If Earth moved in an ideal [[Kepler orbit]], ''i.e.'' a perfect ellipse with the Sun fixed at one focus, each kind of year would always have the same duration, and the sidereal and anomalistic years would be equal. Because of [[Perturbation (astronomy)|perturbation]]s by the gravity of other planets, Earth's motion varies slightly, causing the sidereal and tropical years to vary in length by about 25 minutes (see table below). Both are affected in the same way, so that the sidereal year is consistently 20 minutes longer than the tropical year, provided that they are measured in the same way.
{|class="wikitable"
! Winter solstice ([[Atomic time]])!! Deviation of the following year's duration from the mean value {{nowrap|{{gaps|365.242|19}} [[SI days]]}}
|-
|align=center|2007-12-22 06:04:04.2
|align=right| +10.51 minutes
|-
|align=center|2008-12-21 12:03:19.7
|align=right| -11.86 minutes
|-
|align=center|2009-12-21 17:40:13.2
|align=right| +15.91 minutes
|-
|align=center|2010-12-21 23:44:53.2
|align=right| -11.94 minutes
|-
|align=center|2011-12-22 05:21:41.8
|align=right| +3.58 minutes
|-
|align=center|2012-12-21 11:14:01.9
|align=right| +2.85 minutes
|-
|align=center|2013-12-21 17:05:38.3
|align=right| +0.86 minutes
|-
|align=center|2014-12-21 22:55:15.2
|align=right| +0.48 minutes
|}
An example of a year that will have a duration exceeding the average value of {{gaps|365.242|19}} [[SI days]] with as much as 24.23 minutes is the one that will begin at winter solstice December 21, 2042 17:47:45.5 (Atomic time).
===Draconic year===
The '''draconic year''', '''draconitic year''', '''eclipse year''', or '''ecliptic year''' is the time taken for the Sun (as seen from the Earth) to complete one revolution with respect to the same [[lunar node]] (a point where the Moon's orbit intersects the ecliptic). This period 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
:{{gaps|346.620|075|883}} 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: {{gaps|18.612|815|932}} Julian years ({{gaps|6|798.331|019}} 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:
:{{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 twelve full cycles of the phases of the Moon, as seen from Earth. It has a duration of approximately 354.37 days.
===Vague year===
The '''vague year''', from ''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 [[Wiktionary:schematic|schematic]] months of 30 days each plus 5 [[Intercalation|epagomenal]] days. The vague year was used in the calendars of [[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] Maya World Studies Center</ref>
===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 interval between heliacal risings of the star [[Sirius]]. It is presently less than the [[#Sidereal, tropical, and anomalistic years|sidereal year]] and its duration is very close to the mean 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:
:{{gaps|365.256|898|3}} days (365 d 6 h 9 min 56 s).
===Besselian year===<!-- This section is linked from [[Universal Time]] -->
The '''[[Besselian epoch|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|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> − {{gaps|2|415|020.313|52}}) / {{gaps|365.242|198|781}}
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===
The exact length of an astronomical year changes over time. The main sources of this change are:
* The precession of the equinoxes changes the position of astronomical events with respect to the apsides of Earth's orbit. An event moving toward [[perihelion]] recurs with a decreasing period from year to year; an event moving toward aphelion recurs with an increasing period from year to year (though this effect does not change the average value of the length of the year).
* Each planet's movement is perturbed by the gravity of every other planet.
*[[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 change. Tidal drag in turn depends on factors such as [[post-glacial rebound]] and [[sea level rise]].
*Changes in the effective mass of the Sun, caused by [[solar wind]] and radiation of energy generated by nuclear fusion and radiated by its surface, will affect the Earth's orbital period over a long time (approximately an extra 1.25 [[microsecond]] per year).<ref>Solar mass is ~2×10<sup>30</sup> kg, decreasing at ~5×10<sup>9</sup> kg/s, or ~8×10<sup>−14</sup> solar mass per year. The period of an orbiting body is proportional to <math>\tfrac{1}{\sqrt{M}}</math>, where M is the mass of the primary.</ref>
*The [[Poynting–Robertson effect]] shortens the year by about 30 [[nanosecond]]s per year.
* [[Gravitational radiation]] shortens the year by about 165 [[attosecond]]s per year.<ref>~300 W of radiation produces ~9.5×10<sup>9</sup> J orbital energy decrease per year; this varies as 1/R, and period varies as R<sup>1.5</sup></ref>
===Summary ===
* 346.62 days: a [[draconitic year]].
* 353, 354 or 355 days: the lengths of common years in some [[lunisolar calendar]]s.
* 354.37 days (12 lunar months): the average length of a year in [[lunar calendar]]s, notably the Muslim calendar.
* 365 days: a vague year and a [[common year]] in many [[solar calendar]]s.
* {{gaps|365.242|19}} days: a mean tropical year (rounded to five decimal places) for the epoch 2000.
* {{gaps|365.242|4}} days: a vernal equinox year (rounded to four decimal places) for the epoch 2000.
* {{gaps|365.242|5}} days: the average length of a year in the Gregorian calendar.
* 365.25 days: the average length of a year in the Julian calendar.
* {{gaps|365.256|4}} days: a sidereal year.
* 366 days: a [[leap year]] in many solar calendars.
* 383, 384 or 385 days: the lengths of leap years in some lunisolar calendars.
* 383.9 days (13 lunar months): a leap 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}} [[hour]]s = {{gaps|525|949.2}} [[minute]]s = {{gaps|31|556|952}} [[second]]s (mean solar, not SI).
A common year is 365 days = {{gaps|8|760}} hours = {{gaps|525|600}} minutes = {{gaps|31|536|000}} seconds.
A leap year is 366 days = {{gaps|8|784}} hours = {{gaps|527|040}} minutes = {{gaps|31|622|400}} seconds.
The 400-year cycle of the Gregorian calendar has {{gaps|146|097}} days and hence exactly {{gaps|20|871}} weeks.
See also [[Gregorian calendar#Leap seconds and other aspects|Leap seconds and other aspects of the Gregorian calendar]].
==Symbol==
There is no universally accepted symbol for the year as a [[unit of time]]. The [[International System of Units]] does not propose one. NIST SP811<ref name=SP811>
{{cite journal
|author=Ambler Thompson, Barry N. Taylor
|url=http://physics.nist.gov/Document/sp811.pdf
|format=PDF
|publisher=[[National Institute of Standards and Technology]] (NIST)
|title=Special Publication 811: Guide for the Use of the International System of Units (SI)
|paragraph=8.1
|year=2008
}}</ref> and ISO 80000-3:2006<ref>
{{cite web
|publisher=[[International Organization for 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 symbol '''[[a (year)|a]]''' is taken from the Latin word ''annus''.<ref name="How">{{cite web
|work=How Many? A Dictionary of Units of Measurement
|author=Russ Rowlett
|publisher=University of North Carolina
|title=Units: A
|url=http://www.unc.edu/~rowlett/units/dictA.html
|accessdate=January 9, 2009
}}</ref>
In English, the abbreviations '''y''' or '''yr''' are sometimes used,
specifically in [[geology]] and [[paleontology]], where ''kyr, myr, byr'' (thousands, millions, and billions of years, respectively) and similar abbreviations are used to denote intervals of time remote from the present.<ref name="How" /><ref name="AGUStyle">
{{cite web
|url=http://www.agu.org/pubs/style_guide_intro.html
|publisher=American Geophysical Union
|title=AGU Editorial Style Guide for Authors
|date=September 21, 2007
|accessdate=2009-01-09
|archiveurl = http://web.archive.org/web/20080714134306/http://www.agu.org/pubs/style_guide_intro.html <!-- Bot retrieved archive --> |archivedate = 2008-07-14}}</ref><ref name="Strat">
{{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 of Petroleum Geologists Bulletin
|volume=89
|issue=11
|month=November
|year=2005
|pages=1547–1591
|doi=
|edition=Article 13 (c)
}}</ref>
===Symbol a===
NIST SP811<ref>{{cite web |author=Ambler Thompson, Barry N. Taylor |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 8.1 |year=2008}}</ref> and ISO 80000-3:2006<ref>{{cite web |publisher=International Organization for Standardization |url=http://www.iso.org/iso/iso_catalogue/catalogue_tc/catalogue_detail.htm?csnumber=31888
|title=ISO 80000-3:2006, Quantities and units
|at=Part 3: Space and time
|location=Geneva
|year=2006}}</ref> suggest the symbol '''a''' (in the [[International System of Units]], although '''a''' is also the symbol for the ''[[Hectare#Are|are]]'', the unit of area used to measure land area, but context is usually enough to disambiguate).
In English, the abbreviations '''y''' and '''yr''' are also used.<ref name="How" />
<ref name="AGUStyle" />
<ref name="Strat" />
The [[Unified Code for Units of Measure]]<ref>{{cite web
|author=Gunther Schadow, Clement J. McDonald
|url=http://aurora.rg.iupui.edu/ucum
|title=Unified Code for Units of Measure
}}</ref> disambiguates the varying symbologies of ISO 1000, ISO 2955 and ANSI X3.50 [http://aurora.regenstrief.org/~ucum/ucum.html#para-31] by using
:''ar'' for [[Hectare#Are|are]] and:
:'''a<sub>t</sub>''' = a_t = {{gaps|365.242|19}} days for the mean [[tropical year]]
:'''a<sub>j</sub>''' = a_j = 365.25 days for the mean Julian year
:'''a<sub>g</sub>''' = a_g = {{gaps|365.242|5}} days for the mean [[Gregorian year]]
:'''a''' = 1 a<sub>j</sub> year (without further qualifier)
A definition jointly adopted by the [[International Union of Pure and Applied Chemistry]] and the [[International Union of Geological Sciences]] is to use ''annus'', with symbol '''a''', for year, defined as the length of the tropical year in the year 2000:<ref>{{cite journal |author=Norman E. Holden, Mauro L. Bonardi1, Paul De Bièvre, Paul R. Renne, and Igor 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 |number=5 |pages=1159–1162 |doi=10.1351/PAC-REC-09-01-22}}</ref><ref name="Biever-war">{{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=April 27, 2011 |accessdate=April 28, 2011}}</ref>
:'''a''' = {{gaps|365.242|192|65}} days = {{gaps|31|556|925.445}} seconds
The notation has proved controversial; it conflicts with an earlier convention among geoscientists to use '''a''' specifically for "years ago", and '''y''' or '''yr''' for a one-year time period.<ref name="Biever-war"/>
==== SI prefix multipliers ====
<!-- This section is linked from [[Cretaceous]], from [[Template:Geologic time scale]] and from [[Theropoda]]. -->
* '''ka''' (for '''kiloannum'''), is a [[millennium|unit of time equal to]] one [[thousand]] (10<sup>3</sup>) years.
* '''Ma''' (for '''megaannum'''), is a unit of [[time]] equal to one [[million]] (10<sup>6</sup>) years. It is commonly used in scientific disciplines such as [[geology]], [[paleontology]], and [[celestial mechanics]] to signify very long time periods into the past or future. For example, the [[dinosaur]] species ''[[Tyrannosaurus|Tyrannosaurus rex]]'' was abundant approximately ''65 Ma'' (65 million years) ago (''ago'' may not always be mentioned; if the quantity is specified while not explicitly discussing a duration, one can assume that "ago" is implied; the alternative but deprecated "[[mya (unit)|mya]]" unit includes "ago" explicitly.). In astronomical applications, the year used is the [[Julian year (astronomy)|Julian year]] of precisely 365.25 days. In geology and paleontology, the year is not so precise and varies depending on the author.
* '''Ga''' (for '''gigaannum'''), is a unit of time equal to 10<sup>9</sup> years (one [[1000000000 (number)|billion]] on the [[long and short scales|short scale]], one [[milliard]] on the [[long and short scales|long scale]]). It is commonly used in scientific disciplines such as [[physical cosmology|cosmology]] and [[geology]] to signify extremely long time periods in the past. For example, the formation of the [[Earth]] occurred approximately ''4.57 Ga'' (4.57 billion years) ago.
* '''Ta''' (for '''teraannum'''), is a unit of time equal to 10<sup>12</sup> years (one [[1000000000000 (number)|trillion]] on the short scale, one billion on the long scale). It is an extremely long unit of time, about 70 times as long as the [[age of the universe]]. It is the same order of magnitude as the expected life span of a small [[red dwarf star]].
* '''Pa''' (for '''petaannum'''), is a unit of time equal to 10<sup>15</sup> years (one [[Orders of magnitude (numbers)#1015|quadrillion]] on the short scale, one billiard on the long scale). The half-life of the nuclide [[cadmium]]-113 is about 8 Pa. <!--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, though both are infrequently used and context will normally be sufficient to distinguish time from pressure values.
* '''Ea''' (for '''exaannum'''), is a unit of time equal to 10<sup>18</sup> years (one [[quintillion]] on the short scale, one trillion on the long scale). The half-life of [[tungsten]]-180 is 1.8 Ea.<ref>{{cite journal| author = F. A. Danevich ''et 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}}</ref>
===Symbols y and yr===
In [[astronomy]], [[geology]], and [[paleontology]], the abbreviation ''yr'' for "years" and ''ya'' for "years ago" are sometimes used, combined with prefixes for "thousand", "million", or "billion".<ref
name="AGUStyle" /><ref
name="NASC">{{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 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 annum: ka, Ma, and Ga for kilo-annum (10<sup>3</sup> years), Mega-annum (10<sup>6</sup> years), and Giga-annum (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 1950 AD, 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.
}}</ref>
They are not SI units, using ''y'' to abbreviate English [[:wikt:year|year]], but following ambiguous international recommendations, use either the standard English first letters as prefixes (t,m,and b) and/or the familiar metric multiplier prefixes (k, m, and g). These abbreviations include:
{|class="wikitable"
|-
| || SI-prefixed equivalent || order of magnitude
|-
|'''kyr'''|| "ka" ||* Thousands forms
|-
|'''myr''' || "Ma" ||* Millions forms
|-
|'''byr''' ||"Ga" ||* Billions forms
|-
|'''tya''' or '''kya''' || "ka ago" ||{{Main|1 E10 s|1 E11 s|1 E12 s}}
*Appearance of ''[[Homo sapiens]], ca. 200 tya
*[[Out-of-Africa migration]], ca. 60 tya
*[[Last Glacial Maximum]], ca. 20 tya
*[[Neolithic Revolution]], ca. 10 tya
|-
|'''mya''' || "Ma ago" || {{Main|1 E13 s|1 E14 s|1 E15 s}}
*[[Pliocene]] 5.3 to 2.6 mya
**The [[Brunhes–Matuyama reversal|last geomagnetic reversal]] was 0.78 mya<ref>{{cite journal|author=Bradford M. 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=637–40}}</ref>
**The ([[Eemian Stage]]) [[Ice Age]] started 0.13 mya
*[[Holocene]] started 0.01 mya
|-
|'''bya''' or '''gya''' || "Ga ago" || {{Main|1 E16 s|1 E17 s|1 E18 s}}
*oldest [[Eukaryotes]], 2 bya
*age of the [[Earth]], 4.5 bya
*[[Big Bang]], 13.7 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 duration 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=http://www.geosociety.org/TimeUnits/|accessdate=February 17, 2010}}</ref>
Note that on graphs using "ya" units on the horizontal axis time flows from right to left, which may seem counter-intuitive. If the "ya" units are on the vertical axis, time flows from top to bottom which is probably easier to understand than conventional notation.
=="Great 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, and cannot be determined precisely as the precession speed is variable.
===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
| accessdate=December 9, 2009}}</ref>
==See also==
{{Portal|Time}}
{{div col|2}}
* [[{{CURRENTYEAR}}]]
* [[Astronomical year numbering]]
* [[ISO 8601]]: standard for representation of dates and times
* [[Jēran]]
* [[List of calendars]]
* [[List of years]]
* [[Man-hour]]
* [[Orders of magnitude (time)]]
* [[Unit of time]]
{{div col end}}
==References==
===Notes===
{{Reflist}}
==Further reading==
* {{Cite book
| title=Time, the Familiar Stranger
| last=Fraser
| first=Julius Thomas
| edition=illustrated
| location=Amherst
| publisher=[[University of Massachusetts Press]]
| year=1987
| isbn=0870235761
| oclc=15790499
}}
* {{Cite book
| title=What is Time?
| last=Whitrow
| first=Gerald James
| location=Oxford
| publisher=[[Oxford University Press]]
| year=2003
| isbn=0198607814
| oclc=265440481
}}
{{Time Topics}}
{{Time measurement and standards}}
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{{পোখালি}}
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