# Year

A **yeir** (frae Auld Inglis *gēar*; Gothic *jēr*; Runic *jēran*) is a unit o time based on tha orbital period o tha 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 year is a unit o time at is defined as 365.25 days o 86400 SI seconds each (nae *lowp seconds*).Internaitional Astronomical Union"SI units Archived 2013-07-16 at the Wayback Machine" accessed Februar 18, 2010. (see table fife an section fife.15.) reprintit frae George A. Wilkins & IAU commission fife, "The IAU Style Manual (1989)" Archived 2011-07-26 at the Wayback Machine (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 wird *annus*), in Scots an Inglis an aw **y** or **yr**. Due tae the Yird's axial kelter, the coorse o a year sees the passin o the saisons, merked bi varioos changes in weaither, hoors o daylicht, an consequently vegetation an fertility. In temperate an subpolar regions, generally fower saisons ur recognised: *ware*, *simmer*, *hairst* an *winter*, each wi different temperatur an wather, astronomically merked bi the sun reachin the pynts o equinox an solstice—althoogh the climatic saisons lag behind thair astronomical markers. In some tropical an subtropical regions it is mair common tae spick o the 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 years), or 366 (lowp years) 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 poetic uise)."

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

## Etymology

West saxon *gear* (jɛar), Anglian *gēr* continues Proto-Germanic *\*jǣram* (*\*jē2ram*). Cognates ur German *jahr*, auld high german *jar*, auld norse *ár* an Gothic *jer*, aw frae a pie *\*yērom* ("year" or "saison"). Cognates ootwith o Germanic or Avestan *yare* ("year"), ancient Greek *ὥρα* ("year", "saison", or "period o time"), whence "hoor", auld kirk slavonic *jaru* an Laitin *hornus* ("o thes year"). Laitin *annus* (a 2nt declension masculine noon); *annum* is the accusatife singular; *anni* is genitife singular an nominatife plural; *anno* is the datife an ablatife singular) is frae a pie noin *\*at-no-*, which yielded Gothic *aþnam* ("year") an aw. Baith *\*yē-ro-* an *\*at-no-* ur based oan verbal ruits fur expressin movement—*\*at-* an *\*ey-*, respectively—baith meanin "tae go" fur ordinar. The Greek wird fur "year", ἔτος, is cognate wi Laitin *vetus* ("old"), frae peh *\*wetus-* ("year"), preserved in thes meanin in sanskrit **vat-sa-** ("yearlin cauf") an **vat-sa-ras** ("year") an aw. Derived frae the Laitin *annus* ur a nummer o Scots wirds, sic as annual, annuity, anniversary, etc.; *per annum* means "each year".

## Saisonal yeir

A saisonal yeir is the time atween successife recurrences o a saisonal event, sic as the fluidin o a river, the migration o a species o spyug, the flouerin 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 sic as lowp years. 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 northward equinox year. The pure techt lenth o the yeir is 365.2425 days (as 97 it o 400 yeirs or lowp yeirs); thes is within a bodie ppm o the current lenth o the pure techt tropical yeir (365.24219 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 churches, an aw tries tae synchronize wi the tropical yeir. The average lenth o thes calendar's yeir is 365.242222 days (as 218 it o 900 year or lowp yeirs). Gregorian an revised Julian calendars will stairt tae differ in 2800. The 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 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 runs frae October yin.

### Academic year

An academic year is the annual period durin which a student attends an eddicational institution. The academic year main be dividit intae academic terms, sic as semesters an quarters. Some schuils 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, the actual academic closure is oan Mey 31, an in Nepal it starts frae Julie 15.[*citation needit*] 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

The 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 36525 days an the Julian millennium o 365250 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 31557600 seiconts. 365.25 days o 86400 seconds = yin a = yin aj = 31.5576 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 365.256363004 pure techt solar days (365 d sex h 9 min 9.76 s) (at the epoch j2000.0 = Januar yin, 2000, 12:00:00 tt). Internaitional Yird rotation and reference system service. (2010).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 calendars. The tropical year is commonly defined 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, thes year is abit 20 meenits shorter than the Sidereal year. The pure techt tropical year is approximately 365 days, fife hoors, 48 meenits, 45 seconds (= 365.24219 days). The anomalistic year is the time taen fur the Yird tae complete a bodie revolution wi respect tae its apsides. The orbit o the Yird is 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 365.259636 days (365 d sex h 13 min 52.6 s) (at the epoch j2011.0). 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 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 eclipses: 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 saisons every eclipse year. The average duration o the eclipse year is:346.620075883 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: 18.612815932 Julian years (6798.331019 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 synodic month an aw. The duration o a bodie foo muin cycle is: :411.78443029 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 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 schematic months o 30 days each plus fife epagomenal days. Thes "vague year" wis uised in the calendars o ancient Egyp, Iran, Armenie an in mesoamerica amang the Aztecs an Maya.calendar description an coordinaition Maya Warld Studies Center.

### Heliacal year

A heliacal year is the interval atween the heliacal risins 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 year is the interval atween heliacal risins o the staurn sirius. It is presently less nor the 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: :365.2568983 days (365 d sex h 9 min 56 s).

### Besselian year

The 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): b = 1900.0 + (Julian datett − 2415020.31352) / 365.242198781 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

Template:Refimprove section
The exact lenth o an astronomical year changes ower time.the astronomical almanac online Archived 2017-08-19 at the Wayback Machine 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.

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 |

Lenth o year: difference frae 2000 in seconds
(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 draconitic year.
* 353, 354, or 355 days: the lenths o common years in some lunisolar calendars.
* 354.37 days (12 lunar months): the average lenth o the years in lunar calendars, notably the musleem calendar.
* 365 days: a vague year ane a common year in mony solar calendars.
* 365.24219 days: a pure techt tropical year (roonded tae fife decimal places) fur the epoch 2000.
* 365.2424 days: a vernal equinox year (roonded tae fower decimal places) fur the epoch 2000.
* 365.2425 days: the average lenth o a year in the Gregorian calendar.
* 365.25 days: the average lenth o a year in the Julian calendar.
* 365.2564 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 365.2425 days = 52.1775 weeks = 8765.82 hoors = 525949.2 meenits = 31556952 seiconts (mean solar, nae si). A common year is 365 days = 8760 hoors = 525600 meenits = 31536000 seconds. A lowp year is 366 days = 8784 hoors = 527040 meenits = 31622400 seconds. The 400-year cycle o the Gregorian calendar haes 146097 days, an hence exactly 20871 weeks.

## 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 an ISO 80000-3:2006 suggest the seembol *a*, taen frae the Laitin wird *annus*. 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.

### Symbol "A"

Nist sp811 an ISO 80000-3:2006 suggest the seembol **a** (in the Internaitional Seestem o Units, althoogh **a** is the seembol an aw fur the *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 disambiguates the varyin symbologies of ISO 1000, ISO 2955 an ansi x3.50 bi usin :*ar* fur are and: :**at** = a\_t = 365.24219 days fur the pure techt tropical year :**aj** = a\_j = 365.25 days fur the pure techt Julian year :**ag** = a\_g = 365.2425 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: :**a** = 365.24219265 days = 31556925.445 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 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 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 includes "ago" explicitly). In astronomical applications, the year uised is the 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 billion oan the short scale, a bodie milliard oan the lang scale). It is commonly uised in scienteefic disciplines sic as 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 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 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. thes seembol coincides wi at fur the 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.

### 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". 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 year, but followin ambiguoos internaitional recommendations, uise aither the standard Scots first letters as prefixes ("t", "m", an "b") or metric prefixes ("k", "m", an "g"). Thir abbreviations include:

|  |  |  |
| --- | --- | --- |
| Non-SI abbreviation | SI-prefixed equivalent | Order o magnitude |
| **kyr** | ka | * Thoosand years
 |
| **myr** | Ma | * Million years
 |
| **byr** | Ga | * Billion years (thoosand million years)
 |
| **tya** or **kya** | "ka ago" | Main airticles: 1 E10 s, 1 E11 s, an 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
 |
| **mya** | "Ma ago" | Main airticles: 1 E13 s, 1 E14 s, an 1 E15 s
* Pliocene, 5.3 tae 2.6 mya
	+ The lest geomagnetic reversal wis 0.78 mya
	+ The (Eemian Stage) Ice Age startit 0.13 mya
* The Holocene startit 0.01 mya
 |
| **bya** or **gya** | "Ga ago" | Main airticles: 1 E16 s, 1 E17 s, an 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. 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 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.

## See an aw

* 2026
* 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

## Further readin

* Fraser, Julius Thomas (1987). *time, the familiar stranger* (illustratit ed.). Amherst: University of Massachusetts Press. ISBN 0-87023-576-1. OCLC 15790499.
* Whitrow, Gerald Jimmy (2003). *what is time?*. Oxford: Oxford University Press. ISBN 0-19-860781-4. OCLC 265440481.

Template:Time topics