The term 'year' is also used to indicate other periods of roughly similar duration, such as the roughly 354-day cycle of twelve of the Moon's phases (seelunar calendar), as well as periods loosely associated with the calendar or astronomical year, such as theseasonal year, thefiscal year, theacademic year, etc.
By extension, the term "year" can also be applied to the time taken for anyastronomical object to revolve around itsprimary, as for example theMartian year of roughly 1.88 Earth years.
The term can also be used in reference to any long period or cycle, such as theGreat Year.[2]
Calendar year
Acalendar year is an approximation of the number of days of the Earth's orbital period, as counted in a givencalendar. TheGregorian calendar, or modern calendar, presents its calendar year to be either acommon year of 365 days or aleap year of 366 days, as do theJulian calendars. For the Gregorian calendar, the average length of the calendar year (the mean year) across the complete leap cycle of 400 years is 365.2425 days (97 out of 400 years are leap years).[3]
Abbreviation
In English, theunit of time for year is commonly abbreviated as "y" or "yr". The symbol "a" (forLatin:annus, year) is sometimes used in scientific literature, though its exact duration may be inconsistent.[citation needed]
Although most languages treat the word as thematic*yeh₁r-o-, there is evidence for an original derivation with an*-r/n suffix,*yeh₁-ro-. Both Indo-European words for year,*yeh₁-ro- and*h₂et-no-, would then be derived from verbal roots meaning "to go, move",*h₁ey- and*h₂et-, respectively (compareVedic Sanskritéti "goes",atasi "thou goest, wanderest"). A number of English words are derived from Latinannus, such asannual,annuity,anniversary, etc.;per annum means "each year",annō Dominī means "in the year of the Lord".
The Greek word for "year",ἔτος, is cognate with Latinvetus "old", from the PIE word*wetos- "year", also preserved in this meaning inSanskritvat-sa-ras "year" andvat-sa- "yearling (calf)", the latter also reflected in Latinvitulus "bull calf", Englishwether "ram" (Old Englishweðer, Gothicwiþrus "lamb").
In some languages, it is common to count years by referencing to one season, as in "summers", or "winters", or "harvests". Examples include Chinese年 "year", originally秂, an ideographic compound of a person carrying a bundle of wheat denoting "harvest". Slavic besidesgodŭ "time period; year" useslěto "summer; year".
Intercalation
Astronomical years do not have aninteger number of days or lunar months. Any calendar that follows an astronomical year must have a system ofintercalation such as leap years.
Julian calendar
In theJulian calendar, the average (mean) length of a year is 365.25 days. In a non-leap year, there are 365 days, in a leap year there are 366 days. A leap year occurs every fourth year during which a leap day isintercalated into the month of February. The name "Leap Day" is applied to the added day.
In astronomy, theJulian year is a unit of time defined as 365.25 days, each of exactly86400seconds (SI base unit), totaling exactly 31,557,600 seconds in the Julian astronomical year.[4][5]
Revised Julian calendar
TheRevised Julian calendar, proposed in 1923 and used in someEastern Orthodox Churches, has 218 leap years every 900 years, for the average (mean) year length of365.2422222 days, close to the length of the mean tropical year,365.24219 days (relative error of 9·10). In the year 2800 CE, the Gregorian and Revised Julian calendars will begin to differ by one calendar day.[6]
Gregorian calendar
TheGregorian calendar aims to ensure that thenorthward equinox falls on or shortly before March 21 and hence it follows thenorthward equinox year, ortropical year.[7] Because 97 out of 400 years are leap years, the mean length of the Gregorian calendar year is365.2425 days; with a relative error below oneppm (8·10) relative to the current length of the meantropical year (365.242189 days) and even closer to the currentMarch equinox year of365.242374 days that it aims to match.
Historically, lunisolar calendars intercalated entireleap months on an observational basis. Lunisolar calendars have mostly fallen out of use except for liturgical reasons (Hebrew calendar, variousHindu calendars).
A modern adaptation of the historicalJalali calendar, known as theSolar Hijri calendar (1925), is a purelysolar calendar with an irregular pattern of leap days based on observation (or astronomical computation), aiming to place new year (Nowruz) on the day ofvernal equinox (for the time zone ofTehran), as opposed to using an algorithmic system of leap years.
Year numbering
Acalendar era assigns acardinal number to each sequential year, using a reference event in the past (called theepoch) as the beginning of the era.
The Gregorian calendar era is the world's most widely usedcivil calendar.[8] Its epoch is a6th century estimate of the date of birth ofJesus of Nazareth. Two notations are used to indicate year numbering in the Gregorian calendar: the Christian "Anno Domini" (meaning "in the year of the Lord"), abbreviated AD; and "Common Era", abbreviated CE, preferred by many of other faiths and none. Year numbers are based oninclusive counting, so that there is no "year zero". Years before the epoch are abbreviated BC forBefore Christ or BCE forBefore the Common Era. InAstronomical year numbering, positive numbers indicate years AD/CE, the number0 designates 1 BC/BCE, −1 designates 2 BC/BCE, and so on.
Afiscal 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, inCanada andIndia the fiscal year runs from April 1; in theUnited Kingdom it runs from April 1 for purposes of corporation tax and government financial statements, but from April 6 for purposes of personal taxation and payment of state benefits; inAustralia it runs from July 1; while in theUnited States the fiscal year of thefederal government runs from October 1.
An academic year is the annual period during which a student attends aneducational institution. The academic year may be divided intoacademic terms, such as semesters or quarters. The school year in many countries starts in August or September and ends in May, June or July. In Israel the academic year begins around October or November, aligned with the second month of the Hebrew calendar.
Some schools in the UK, Canada and the United States divide the academic year intothree roughly equal-length terms (calledtrimesters orquarters in the United States), roughly coinciding with autumn, winter, and spring. At some, a shortened summer session, sometimes considered part of the regular academic year, is attended by students on a voluntary or elective basis. Other schools break the year intotwo 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 aquarter (orterm in some countries). There may also be a voluntary summer session or a short January session.
Some other schools, including some in the United States, havefour marking periods. Some schools in the United States, notablyBoston Latin School, may divide the year intofive or more marking periods. Some state in defense of this that there is perhaps apositive correlation between report frequency and academic achievement.
There are typically 180 days of teaching each year in schools in the US, excluding weekends and breaks, while there are 190 days for pupils in state schools in Canada, New Zealand and the United Kingdom, and 200 for pupils in Australia.
In India the academic year normally starts from June 1 and ends on May 31. Though schools start closing from mid-March, the actual academic closure is on May 31 and in Nepal it starts from July 15.[citation needed]
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.
The Julian year, as used in astronomy and other sciences, is a time unit defined as exactly 365.25 days of86400SI seconds each ("ephemeris days"). This is the normal meaning of the unit "year" used in various scientific contexts. The Julian century of36525 ephemeris days and the Julian millennium of365250 ephemeris days are used in astronomical calculations. Fundamentally, expressing a time interval in Julian years is a way to precisely specify an amount of time (not how many "real" years), for long time intervals where stating the number of ephemeris days would be unwieldy and unintuitive. By convention, the Julian year is used in the computation of the distance covered by alight-year.
Each of these three years can be loosely called anastronomical year.
The sidereal year is the time taken for the Earth to complete one revolution of itsorbit, as measured against a fixed frame of reference (such as the fixed stars, Latinsidera, singularsidus). Its average duration is365.256363004 days (365 d 6 h 9 min 9.76 s) (at the epochJ2000.0 = January 1, 2000, 12:00:00TT).[10]
Today the mean tropical year is defined as the period of time for the meanecliptic longitude of the Sun to increase by 360 degrees.[11] Since the Sun's ecliptic longitude is measured with respect to the equinox,[12] the tropical year comprises a complete cycle of the seasons and is the basis ofsolar calendars such as the internationally usedGregorian calendar. The modern definition of mean tropical year differs from the actual time between passages of, e.g., the northward equinox, by a minute or two, for several reasons explained below. Because of the Earth'saxial precession, this year is about 20 minutes shorter than the sidereal year. The mean tropical year is approximately 365 days, 5 hours, 48 minutes, 45 seconds, using the modern definition[13] (=365.24219 d ×86400 s). The length of the tropical year varies a bit over thousands of years because the rate of axial precession is not constant.
The anomalistic year is the time taken for the Earth to complete one revolution with respect to itsapsides. The orbit of the Earth is elliptical; the extreme points, called apsides, are theperihelion, where the Earth is closest to the Sun, and theaphelion, where the Earth is farthest from the Sun. The anomalistic year is usually defined as the time between perihelion passages. Its average duration is 365.259636 days (365 d 6 h 13 min 52.6 s) (at the epoch J2011.0).[14]
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 samelunar node (a point where the Moon's orbit intersects the ecliptic). The year is associated witheclipses: 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 twoeclipse seasons every eclipse year. The average duration of the eclipse year is
346.620075883 days (346 d 14 h 52 min 54 s) (at the epoch J2000.0).
This term is sometimes erroneously used for the draconic or nodal period oflunar precession, that is the period of a complete revolution of the Moon's ascending node around the ecliptic:18.612815932 Julian years (6798.331019 days; at the epoch J2000.0).
Full moon cycle
Thefull moon cycle is the time for the Sun (as seen from the Earth) to complete one revolution with respect to theperigee of the Moon's orbit. This period is associated with the apparent size of thefull moon, and also with the varying duration of thesynodic month. The duration of one full moon cycle is:
411.78443029 days (411 days 18 hours 49 minutes 35 seconds) (at the epoch J2000.0).
Lunar year
Thelunar year comprises twelve full cycles of the phases of the Moon, as seen from Earth. It has a duration of approximately 354.37 days.Muslims use this for religious purposes, including calculating the date of theHajj and the fasting month ofRamadan, and thus also theEids. TheJewish calendar is also mainly lunar, but with the addition of an intercalary lunar month once every two or three years, designed to keep the calendar broadly synchronous with the solar cycle. Thus, a lunar year on the Jewish (Hebrew) calendar consists of either twelve or thirteen lunar months.
Vague year
The vague year, fromannus 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 12schematic months of 30 days each plus 5epagomenal days. The vague year was used in the calendars ofEthiopia,Ancient Egypt,Iran,Armenia and inMesoamerica among theAztecs andMaya.[15] It is still used by many Zoroastrian communities.
TheSothic year is the heliacal year, the interval between heliacal risings, of the starSirius. It is currently less than thesidereal year and its duration is very close to the Julian year of 365.25 days.
Gaussian year
TheGaussian year is the sidereal year for a planet of negligible mass (relative to the Sun) and unperturbed by other planets that is governed by theGaussian gravitational constant. Such a planet would be slightly closer to the Sun than Earth's mean distance. Its length is:
365.2568983 days (365 d 6 h 9 min 56 s).
Besselian year
TheBesselian 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 mathematicianFriedrich Bessel. The following equation can be used to compute the current Besselian epoch (in years):[16]
B = 1900.0 + (Julian dateTT −2415020.31352) /365.242198781
The TT subscript indicates that for this formula, the Julian date should use theTerrestrial Time scale, or its predecessor,ephemeris time.
The exact length of an astronomical year changes over time.
The positions of the equinox and solstice points with respect to the apsides of Earth's orbit change: the equinoxes and solstices move westward relative to the stars because ofprecession, and the apsides move in the other direction because of the long-term effects of gravitational pull by the other planets. Since the speed of the Earth varies according to its position in its orbit as measured from its perihelion, Earth's speed when in a solstice or equinox point changes over time: if such a point moves toward perihelion, the interval between two passages decreases a little from year to year; if the point moves towards aphelion, that period increases a little from year to year. So a "tropical year" measured from one passage of the northward ("vernal") equinox to the next, differs from the one measured between passages of the southward ("autumnal") equinox. The average over the full orbit does not change because of this, so the length of the average tropical year does not change because of this second-order effect.
Each planet's movement is perturbed by the gravity of every other planet. This leads to short-term fluctuations in its speed, and therefore its period from year to year. Moreover, it causes long-term changes in its orbit, and therefore also long-term changes in these periods.
Tidal drag between the Earth and the Moon and Sun increases the length of the day and of the month (by transferring angular momentum from the rotation of the Earth to the revolution of the Moon); since the apparent mean solar day is the unit with which we measure the length of the year in civil life, the length of the year appears to decrease. The rotation rate of the Earth is also changed by factors such aspost-glacial rebound andsea level rise.
Numerical value of year variation Mean year lengths in this section are calculated for 2000, and differences in year lengths, compared to 2000, are given for past and future years. In the tables a day is86400 SI seconds long.[17][18][19][20]
Year length difference from 2000 (seconds; positive when length for tabulated year is greater than length in 2000)
Year
Tropical
Sidereal
Anomalistic
Eclipse
−4000
−8
−45
−15
−174
−2000
4
−19
−11
−116
0
7
−4
−5
−57
2000
0
0
0
0
4000
−14
−3
5
54
6000
−35
−12
10
104
Summary
Some of the year lengths in this table are in averagesolar days, which are slowly getting longer (at a rate that cannot be exactly predicted in advance) and are now around86400.002SI seconds.
An average Gregorian year may be said to be 365.2425days (52.1775weeks, and if an hour is defined as one twenty-fourth of a day,8765.82hours,525949.2minutes or31556952seconds). Note however that in absolute time the average Gregorian year is not adequately defined unless the period of the averaging (start and end dates) is stated, because each period of 400 years is longer (by more than 1000 seconds) than the preceding one as the rotation of the Earth slows. In this calendar, a common year is 365 days (8760 hours,525600 minutes or31536000 seconds), and a leap year is 366 days (8784 hours,527040 minutes or31622400 seconds). The 400-year civil cycle of the Gregorian calendar has146097 days and hence exactly20871 weeks.
Greater astronomical years
Equinoctial cycle
TheGreat Year, or equinoctial cycle, corresponds to a complete revolution of the equinoxes around the ecliptic. Its length is about 25,700 years.[21][22]
A seasonal year is the time between successive recurrences of a seasonal event such as the flooding of a river, the migration of a species of bird, the flowering of a species of plant, the first frost, or the first scheduled game of a certain sport. All of these events can have wide variations of more than amonth from year to year.
Symbols and abbreviations
A common symbol for the year as aunit of time is "a", taken from the Latin wordannus.For example, the U.S.National Institute of Standards and Technology (NIST)Guide for the Use of the International System of Units (SI) supports the symbol "a" as the unit of time for a year.[24]
In English, the abbreviations "y" or "yr" are more commonly used in non-scientific literature.[25] In someEarth sciences branches (geology andpaleontology), "kyr,myr,byr" (thousands, millions, and billions of years, respectively) and similar abbreviations are used to denote intervals of time remote from the present.[26][27] Inastronomy the abbreviations kyr, Myr and Gyr are in common use for kiloyears, megayears and gigayears.[28][29]
In the UCUM, the symbol "a", without any qualifier, equals 1 aj.The UCUM also minimizes confusion withare, a unit of area, by using the abbreviation "ar".
Since 1989, theInternational Astronomical Union (IAU) recognizes the symbol "a" rather than "yr" for a year, notes the different kinds of year, and recommends adopting the Julian year of 365.25 days, unless otherwise specified (IAUStyle Manual).[30][31]
In 2011, the IUPAC and theInternational Union of Geological Sciences jointly recommended defining the "annus", with symbol "a", as the length of the tropical year in the year 2000:[35]
a =31556925.445 seconds (approximately365.24219265ephemeris days)
This differs from the above definition of 365.25 days by about 20parts per million. The joint document says that definitions such as the Julian year "bear an inherent, pre-programmed obsolescence because of the variability of Earth's orbital movement", but then proposes using the length of the tropical year as of 2000 AD (specified down to the millisecond), which suffers from the same problem.[36] (The tropical year oscillates with time by more than a minute.)
The notation has proved controversial as it conflicts with an earlier convention among geoscientists to use "a" specifically for "years ago" (e.g. 1 Ma for 1 million years ago), and "y" or "yr" for a one-year time period.[36][37]However, this historical practice does not comply with the NISTGuide,[24] considering the unacceptability of mixing information concerning thephysical quantity being measured (in this case, time intervals or points in time) with the units and also the unacceptability of using abbreviations for units.Furthermore, according to theUK Metric Association (UKMA), language-independent symbols are more universally understood (UKMAStyle guide).[38]
For the following, there are alternative forms that elide the consecutive vowels, such askilannus,megannus, etc. The exponents and exponential notations are typically used for calculating and in displaying calculations, and for conserving space, as in tables of data.
Geology, paleontology, andcelestial mechanics. In astronomical applications, the year used is the Julian year of precisely 365.25 days. In geology and paleontology, the year is not so precise and varies depending on the author.
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 smallred dwarf.
Thehalf-life of thenuclidecadmium-113 is about 8 Pa.[40] This symbol coincides with that for thepascal without a multiplier prefix, but context will normally be sufficient to distinguish long time periods from pressure values.
In geology and paleontology, a distinction sometimes is made between abbreviation "yr" foryears and "ya" foryears ago, combined with prefixes for thousand, million, or billion.[26][42] In archaeology, dealing with more recent periods, normally expressed dates, e.g. "10,000 BC", may be used as a more traditional form thanBefore Present ("BP").
Use of "mya" and "bya" is deprecated in modern geophysics, the recommended usage being "Ma" and "Ga" for datesBefore Present, but "m.y." for the durations of epochs.[26][27] Thisad hoc distinction between "absolute" time and time intervals is somewhat controversial amongst members of the Geological Society of America.[44]
^Shields, Miriam Nancy (1924). "The new calendar of the eastern churches".Popular Astronomy.32: 407.Bibcode:1924PA.....32..407S.
^Ziggelaar, A. (1983)."The Papal Bull of 1582 Promulgating a Reform of the Calendar". In G. V. Coyne; M. A. Hoskin; O. Pedersen (eds.).Gregorian Reform of the Calendar: Proceedings of the Vatican Conference to Commemorate its 400th Anniversary. Vatican City: Pontifical Academy of Sciences. p. 223.
^Richards, E.G. (2013). Calendars. In S.E. Urban & P.K. Seidelmann (Eds.),Explanatory Supplement to the Astronomical Almanac (3rd ed.). Mill Valley, CA: University Science Books. p. 586.
^"longitude, ecliptic".Archived from the original on September 8, 2023, and"dynamical equinox".Archived from the original on September 8, 2023, (c. 2022). In "Glossary",The Astronomical Almanac Online. United States Naval Observatory.
^"Glossary".Astronomical Applications Department. United States Naval Observatory. c. 2022. s.v. year, tropical.Archived from the original on September 8, 2023. RetrievedNovember 6, 2023.
^Astronomical Almanac for the Year 2010. Washington and Taunton: U.S. Government Printing Office and the U.K. Hydrographic Office. 2008. p. B3.
^U.S. Naval Observatory Nautical Almanac Office and Her Majesty's Nautical Almanac Office (2010).Astronomical Almanac for the year 2011. Washington: U.S. Government Printing Office. pp. C2, L8.
^Simon, J.L.; Bretagnon, P.; Chapront, J.; Chapront-Touzé, M.; Francou, G.; Laskar, J. (February 1994). "Numerical expressions for precession formulae and mean elements for the Moon and planets".Astronomy and Astrophysics.282 (2):663–683.Bibcode:1994A&A...282..663S.
^Taff, Lawrence G. (1985).Celestial Mechanics: A Computational Guide for the Practitioner. New York: John Wiley & Sons. p. 103.ISBN978-0-471-89316-5. Values in tables agree closely for 2000, and depart by as much as 44 seconds for the years furthest in the past or future; the expressions are simpler than those recommended in theAstronomical Almanac for the Year 2011.
^Seidelmann, P. Kenneth (2013).Explanatory Supplement to the Astronomical Almanac. Sean E. Urban (ed.) (3 ed.). Univ Science Books. p. 587.ISBN978-1-891389-85-6. Tabulates length of tropical year from −500 to 2000 at 500 year intervals using a formula by Laskar (1986); agrees closely with values in this section near 2000, departs by 6 seconds in −500.
^Rowlett, Russ."Units: A".How Many? A Dictionary of Units of Measurement. University of North Carolina. Archived fromthe original on December 20, 2008. RetrievedJanuary 9, 2009.
^E.R. Cohen, T. Cvitas, J.G. Frey, B. Holmström, K. Kuchitsu, R. Marquardt, I. Mills, F. Pavese, M. Quack, J. Stohner, H.L. Strauss, M. Takami, and A.J. Thor,Quantities, Units and Symbols in Physical Chemistry, IUPACGreen Book, Third Edition, Second Printing, IUPAC & RSC Publishing, Cambridge (2008)[2]Archived April 17, 2019, at theWayback Machine
^"year".The IUPAC Compendium of Chemical Terminology. Research Triangle Park, NC: International Union of Pure and Applied Chemistry (IUPAC). February 24, 2014.doi:10.1351/goldbook.y06723.
^"Style guide".UK Metric Association. July 12, 2017. RetrievedApril 23, 2022.
^Arndt, Nicholas (2011),"Ga", in Gargaud, Muriel; Amils, Ricardo; Quintanilla, José Cernicharo; Cleaves, Henderson James (Jim) (eds.),Encyclopedia of Astrobiology, Berlin, Heidelberg: Springer, p. 621,doi:10.1007/978-3-642-11274-4_611,ISBN978-3-642-11274-4, retrievedDecember 22, 2020
^North American Commission on Stratigraphic Nomenclature."North American Stratigraphic Code (Article 13 (c))".(c) Convention and abbreviations. – The age of a stratigraphic unit or the time of a geologic event, as commonly determined by numerical dating or by reference to a calibrated time-scale, may be expressed in years before the present. The unit of time is the modern year as presently recognized worldwide. Recommended (but not mandatory) abbreviations for such ages are SI (International System of Units) multipliers coupled with "a" for annus: ka, Ma, and Ga for kilo-annus (103 years), Mega-annus (106 years), and Giga-annus (109 years), respectively. Use of these terms after the age value follows the convention established in the field of C-14 dating. The "present" refers to AD 1950, and such qualifiers as "ago" or "before the present" are omitted after the value because measurement of the duration from the present to the past is implicit in the designation. In contrast, the duration of a remote interval of geologic time, as a number of years, should not be expressed by the same symbols. Abbreviations for numbers of years, without reference to the present, are informal (e.g., y or yr for years; my, m.y., or m.yr. for millions of years; and so forth, as preference dictates). For example, boundaries of the Late Cretaceous Epoch currently are calibrated at 63 Ma and 96 Ma, but the interval of time represented by this epoch is 33 m.y.
