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A year or annum is the orbital period of a planetary body, for example, the Earth, moving in its orbit around the Sun. Due to the Earth's axial tilt, the course of a year sees the passing of the seasons, marked by change in weather, the hours of daylight, and, consequently, vegetation and soil fertility. In temperate and subpolar regions around the planet, four seasons are generally recognized: spring, summer, autumn and winter. In tropical and subtropical regions, several geographical sectors do not present defined seasons; but in the seasonal tropics, the annual wet and dry seasons are recognized and tracked.

An analemma illustrates the changing position of the Sun over the course of a year, as seen at a fixed time of day.

A calendar year is an approximation of the number of days of the Earth's orbital period, as counted in a given calendar. The Gregorian calendar, or modern calendar, presents its calendar year to be either a common year of 365 days or a leap year of 366 days, as do the Julian 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. In English, the abbreviations "y" and "yr" are commonly used ("a" is also used) for the unit of time, though its exact duration may be inconsistent.

In astronomy, the Julian year is a unit of time; it is defined as 365.25 days of exactly 86,400 seconds (SI base unit), totalling exactly 31,557,600 seconds in the Julian astronomical year.

The word year is also used for periods loosely associated with, but not identical to, the calendar or astronomical year, such as the seasonal year, the fiscal year, the academic year, etc. Similarly, year can mean the orbital period of any planet; for example, a Martian year and a Venusian year are examples of the time a planet takes to transit one complete orbit. The term can also be used in reference to any long period or cycle, such as the Great Year.

Contents

English year (via West Saxon ġēar (/jɛar/), Anglian ġēr) continues Proto-Germanic *jǣran (*jē₁ran). Cognates are German Jahr, Old High German jār, Old Norse ár and Gothic jer, from the Proto-Indo-European noun *yeh₁r-om "year, season". Cognates also descended from the same Proto-Indo-European noun (with variation in suffix ablaut) are Avestan yārǝ "year", Greekὥρα (hṓra) "year, season, period of time" (whence "hour"), Old Church Slavonic jarŭ, and Latin hornus "of this year".

Latinannus (a 2nd declension masculine noun;annum is the accusative singular;annī is genitive singular and nominative plural;annō the dative and ablative singular) is from a PIE noun *h₂et-no-, which also yielded Gothic aþn "year" (only the dative plural aþnam is attested).

Although most languages treat the word as thematic *yeh₁r-o-, there is evidence for an original derivation with an *-r/n suffix, *yeh₁-ro-. Both Indo-European words for year, *yeh₁-ro- and *h₂et-no-, would then be derived from verbal roots meaning "to go, move", *h₁ey- and *h₂et-, respectively (compare Vedic Sanskrit éti "goes", atasi "thou goest, wanderest"). A number of English words are derived from Latinannus, such as annual, 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 Latin vetus "old", from the PIE word *wetos- "year", also preserved in this meaning in Sanskrit vat-sa-ras "year" and vat-sa- "yearling (calf)", the latter also reflected in Latin vitulus "bull calf", English wether "ram" (Old English weðer, Gothic wiþrus "lamb").

In some languages, it is common to count years by referencing to one season, as in "summers", or "winters", or "harvests". Examples include Chinese "year", originally , an ideographic compound of a person carrying a bundle of wheat denoting "harvest". Slavic besides godŭ "time period; year" uses lěto "summer; year".

Astronomical years do not have an integer number of days or lunar months. Any calendar that follows an astronomical year must have a system of intercalation such as leap years.

Julian calendar

In the Julian calendar, the average (mean) length of a year is 365.25 days. In a non-leap year, there are 365 days, in a leap year there are 366 days. A leap year occurs every fourth year, or leap year, during which a leap day is intercalated into the month of February. The name "Leap Day" is applied to the added day.

The Revised Julian calendar, proposed in 1923 and used in some Eastern 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 9·10−8). In the year 2800 CE, the Gregorian and Revised Julian calendars will begin to differ by one calendar day.

Gregorian calendar

The Gregorian calendar attempts to cause the northward equinox to fall on or shortly before March 21 and hence it follows the northward equinox year, or tropical year. 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 one ppm (8·10−7) relative to the current length of the mean tropical year (365.24219 days) and even closer to the current March equinox year of365.242374 days that it aims to match. It is estimated that by the year 4000 CE, the northward equinox will fall back by one day in the Gregorian calendar, not because of this difference, but due to the slowing of the Earth's rotation and the associated lengthening of the day.

Other calendars

Further information: Lunisolar calendar

Historically, lunisolar calendars intercalated entire leap months on an observational basis. Lunisolar calendars have mostly fallen out of use except for liturgical reasons (Hebrew calendar, various Hindu calendars).

A modern adaptation of the historical Jalali calendar, known as the Solar Hijri calendar (1925), is a purely solar calendar with an irregular pattern of leap days based on observation (or astronomical computation), aiming to place new year (Nowruz) on the day of vernal equinox (for the time zone of Tehran), as opposed to using an algorithmic system of leap years.

A calendar era assigns a cardinal number to each sequential year, using a reference event in the past (called the epoch) as the beginning of the era.

The Gregorian calendar era is the world's most widely used civil calendar. Its epoch is a 6th century estimate of the date birth of Jesus of Nazareth. Two notations are used to indicate year numbering in the Gregorian calendar: the Christian "Anno Domini" (meaning "in the year of the Lord"), abbreviated AD; and "Common Era", abbreviated CE, preferred by many of other faiths and none. Year numbers are based on inclusive counting, so that there is no "year zero". Years before the epoch are abbreviated BC for Before Christ or BCE for Before the Common Era. In Astronomical year numbering, positive numbers indicate years AD/CE, the number 0 designates 1 BC/BCE, −1 designates 2 BC/BCE, and so on.

Other eras include that of Ancient Rome,Ab Urbe Condita ("from the foundation of the city), abbreviated AUC;Anno Mundi ("year of the world"), used for the Hebrew calendar and abbreviated AM; and the Japanese emperor eras described above. The Islamic Hijri year, (year of the Hijrah,Anno Hegirae abbreviated AH), is a lunar calendar of twelve lunar months and thus is shorter than a solar year.

Financial and scientific calculations often use a 365-day calendar to simplify daily rates.

Fiscal year

A fiscal year or financial year is a 12-month period used for calculating annual financial statements in businesses and other organizations. In many jurisdictions, regulations regarding accounting require such reports once per twelve months, but do not require that the twelve months constitute a calendar year.

For example, in Canada and India the fiscal year runs from April 1; in the United Kingdom it runs from April 1 for purposes of corporation tax and government financial statements, but from April 6 for purposes of personal taxation and payment of state benefits; in Australia it runs from July 1; while in the United States the fiscal year of the federal government runs from October 1.

Academic year

Main article: 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 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 into three roughly equal-length terms (called trimesters or quarters in the United States), roughly coinciding with autumn, winter, and spring. At some, a shortened summer session, sometimes considered part of the regular academic year, is attended by students on a voluntary or elective basis. Other schools break the year into two main semesters, a first (typically August through December) and a second semester (January through May). Each of these main semesters may be split in half by mid-term exams, and each of the halves is referred to as a quarter (or term in some countries). There may also be a voluntary summer session and/or a short January session.

Some other schools, including some in the United States, have four marking periods. Some schools in the United States, notably Boston Latin School, may divide the year into five or more marking periods. Some state in defense of this that there is perhaps a positive correlation between report frequency and academic achievement.

There are typically 180 days of teaching each year in schools in the US, excluding weekends and breaks, while there are 190 days for pupils in state schools in Canada, New Zealand and the United Kingdom, and 200 for pupils in Australia.

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.

Julian year

The 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" used in various scientific contexts. The Julian century of36525 days and the Julian millennium of365250 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, aj, of exactly31557600 seconds.

365.25 d ×86400 s = 1 a = 1 aj =31.5576 Ms

The SI multiplier prefixes may be applied to it to form "ka" (kiloannum), "Ma" (megaannum), etc.

Sidereal, tropical, and anomalistic years

Main articles: Sidereal year and Tropical year
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, Latinsidera, singularsidus). Its average duration is365.256363004 days (365 d 6 h 9 min 9.76 s) (at the epoch J2000.0 = January 1, 2000, 12:00:00 TT).

Today the mean tropical year is defined as the period of time for the mean ecliptic longitude of the Sun to increase by 360 degrees. Since the Sun's ecliptic longitude is measured with respect to the equinox, the tropical year comprises a complete cycle of the seasons and is the basis of solar calendars such as the internationally used Gregorian calendar. The modern definition of mean tropical year differs from the actual time between passages of, e.g., the northward equinox for several reasons explained below. Because of the Earth's axial precession, this year is about 20 minutes shorter than the sidereal year. The mean tropical year is approximately 365 days, 5 hours, 48 minutes, 45 seconds, using the modern definition ( = 365.24219 d × 86 400 s).

The anomalistic year is the time taken for the Earth to complete one revolution with respect to its 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 5, 07:48 UT in 2020), and the aphelion, where the Earth is farthest from the Sun (July 4, 11:35 UT in 2020). 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).

Draconic year

Further information: Draconic period

The draconic year, draconitic year, eclipse year, or ecliptic year is the time taken for the Sun (as seen from the Earth) to complete one revolution with respect to the same lunar node (a point where the Moon's orbit intersects the ecliptic). The year is associated with eclipses: 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 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 of lunar 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

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 synodic 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

The lunar year comprises twelve full cycles of the phases of the Moon, as seen from Earth. It has a duration of approximately 354.37 days. Muslims use this for celebrating their Eids and for marking the start of the fasting month of Ramadan. A Muslim calendar year is based on the lunar cycle. The Jewish calendar is also essentially lunar, except that an intercalary lunar month is added once every two or three years, in order to keep the calendar synchronized with the solar cycle as well. 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 12 schematic months of 30 days each plus 5 epagomenal days. The vague year was used in the calendars of Ethiopia, Ancient Egypt, Iran, Armenia and in Mesoamerica among the Aztecs and Maya. It is still used by many Zoroastrian communities.

Heliacal year

A heliacal year is the interval between the heliacal risings 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 year is the interval between heliacal risings of the star Sirius. It is currently less than the sidereal year and its duration is very close to the Julian year of 365.25 days.

Gaussian year

The Gaussian year is the sidereal year for a planet of negligible mass (relative to the Sun) and unperturbed by other planets that is governed by the Gaussian gravitational constant. Such a planet would be slightly closer to the Sun than Earth's mean distance. Its length is:

365.2568983 days (365 d 6 h 9 min 56 s).

Besselian year

The 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):

B = 1900.0 + (Julian dateTT2415020.31352) /365.242198781

The TT subscript indicates that for this formula, the Julian date should use the Terrestrial Time scale, or its predecessor, ephemeris time.

Variation in the length of the year and the day

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Further information: Length of day (astronometry)

The exact length of an astronomical year changes over time.

  • The positions of the equinox and solstice points with respect to the apsides of Earth's orbit change: the equinoxes and solstices move westward relative to the stars because of precession, and the apsides move in the other direction because of the long-term effects of gravitational pull by the other planets. Since the speed of the Earth varies according to its position in its orbit as measured from its perihelion, Earth's speed when in a solstice or equinox point changes over time: if such a point moves toward perihelion, the interval between two passages decreases a little from year to year; if the point moves towards aphelion, that period increases a little from year to year. So a "tropical year" measured from one passage of the northward ("vernal") equinox to the next, differs from the one measured between passages of the southward ("autumnal") equinox. The average over the full orbit does not change because of this, so the length of the average tropical year does not change because of this second-order effect.
  • Each planet's movement is perturbed by the gravity of every other planet. This leads to short-term fluctuations in its speed, and therefore its period from year to year. Moreover, it causes long-term changes in its orbit, and therefore also long-term changes in these periods.
  • Tidal drag between the Earth and the Moon and Sun increases the length of the day and of the month (by transferring angular momentum from the rotation of the Earth to the revolution of the Moon); since the apparent mean solar day is the unit with which we measure the length of the year in civil life, the length of the year appears to decrease. The rotation rate of the Earth is also changed by factors such as post-glacial rebound and sea level rise.

Numerical value of year variation
Mean year lengths in this section are calculated for 2000, and differences in year lengths, compared to 2000, are given for past and future years. In the tables a day is 86,400 SI seconds long.

Mean year lengths for 2000
Type of year Days Hours Minutes Seconds
Tropical 365 5 48 45
Sidereal 365 6 9 10
Anomalistic 365 6 13 53
Eclipse 346 14 52 55
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

Days Year type
346.62 Draconic, also called eclipse.
354.37 Lunar.
365 Vague, and a common year in many solar calendars.
365.24219 Tropical, also called solar, averaged and then rounded for epoch J2000.0.
365.2425 Gregorian, on average.
365.25 Julian.
365.25636 Sidereal, for epoch J2000.0.
365.259636 Anomalistic, averaged and then rounded for epoch J2011.0.
366 Leap in many solar calendars.

An average Gregorian year is 365.2425 days (52.1775 weeks,8765.82 hours,525949.2 minutes or31556952 seconds). For 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 cycle of the Gregorian calendar has146097 days and hence exactly20871 weeks.

Equinoctial cycle

The Great Year, or equinoctial cycle, corresponds to a complete revolution of the equinoxes around the ecliptic. Its length is about 25,772 years.

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.

Main article: Seasonal year
Further information: Effect of sun angle on climate

A seasonal year is the time between successive recurrences of a seasonal event such as the flooding of a river, the migration of a species of bird, the flowering of a species of plant, the first frost, or the first scheduled game of a certain sport. All of these events can have wide variations of more than a month from year to year.

A common symbol for the year as a unit 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.

In English, the abbreviations "y" or "yr" are more commonly used in non-scientific literature. In some Earth sciences branches (geology and paleontology), "kyr, myr, byr" (thousands, millions, and billions of years, respectively) and similar abbreviations are used to denote intervals of time remote from the present.

The Unified Code for Units of Measure (UCUM) disambiguates the varying symbologies of ISO 1000, ISO 2955 and ANSI X3.50 by using:

at =365.24219 days for the mean tropical year;
aj = 365.25 days for the mean Julian year;
ag =365.2425 days for the mean Gregorian year;

In the UCUM, the symbol "a", without any qualifier, equals 1 aj. The UCUM also minimizes confusion with are, a unit of area, by using the abbreviation "ar".

Since 1989, the International Astronomical Union (IAU) recognizes the symbol "a" rather than "yr" for a year, notes the different kinds of year, and recommends adopting the Julian year of 365.25 days, unless otherwise specified (IAU Style Manual).

Since 1987, the International Union of Pure and Applied Physics (IUPAP) notes "a" as the general symbol for the time unit year (IUPAP Red Book). Since 1993, the International Union of Pure and Applied Chemistry (IUPAC) Green Book also uses the same symbol "a", notes the difference between Gregorian year and Julian year, and adopts the former (a=365.2425 days), also noted in the IUPAC Gold Book.

In 2011, the IUPAC and the International Union of Geological Sciences jointly recommended defining the "annus", with symbol "a", as the length of the tropical year in the year 2000:

a =31556925.445 seconds (approximately365.24219265 ephemeris days)

This differs from the above definition of 365.25 days by about 20 parts 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. (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", and "y" or "yr" for a one-year time period. However, this historical practice does not comply with the NIST Guide, considering the unacceptability of mixing information concerning the physical quantity being measured (in this case, time intervals or points in time) with the units and also the unnaceptability of using abbreviations for units. Furthermore, according to the UK Metric Association (UKMA), language-independent symbols are more universally understood (UKMA Style guide).

SI prefix multipliers

For the following, there are alternative forms that elide the consecutive vowels, such as kilannum, megannum, etc. The exponents and exponential notations are typically used for calculating and in displaying calculations, and for conserving space, as in tables of data.

  • ka (for kiloannum) – a unit of time equal to one thousand, or 103, years, or 1 E3 yr, also known as a millennium in anthropology and calendar uses. The prefix multiplier "ka" is typically used in geology, paleontology, and archaeology for the Holocene and Pleistocene periods, where a non−radiocarbon dating technique such as ice core dating, dendrochronology, uranium-thorium dating or varve analysis is used as the primary method for age determination. If age is determined primarily by radiocarbon dating, then the age should be expressed in either radiocarbon or calendar (calibrated) years Before Present.
  • Ma (for megaannum) – a unit of time equal to one million, or 106, years, or 1 E6 yr. The suffix "Ma" 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 rex was abundant approximately 66 Ma (66 million years) ago. The duration term "ago" may not always be indicated: if the quantity of a duration is specified while not explicitly mentioning a duration term, one can assume that "ago" is implied; the alternative unit "mya" does include "ago" explicitly. It is also written as "million years" (ago) in works for general public use. 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.
  • Ga (for gigaannum) – a unit of time equal to 109 years, or one billion years. "Ga" is commonly used in scientific disciplines such as cosmology and geology to signify extremely long time periods in the past. For example, the formation of the Earth occurred approximately 4.54 Ga (4.54 billion years) ago and the age of the universe is approximately 13.8 Ga.
  • Ta (for teraannum) – a unit of time equal to 1012 years, or one trillion years. "Ta" 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.
  • Pa (for petaannum) – a unit of time equal to 1015 years, or one quadrillion years. The half-life of the nuclide cadmium-113 is about 8 Pa. This symbol coincides with that for the 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) – a unit of time equal to 1018 years, or one quintillion years. The half-life of tungsten-180 is 1.8 Ea.

Abbreviations for "years ago"

Further information: Before Present

In geology and paleontology, a distinction sometimes is made between abbreviation "yr" for years and "ya" for years ago, combined with prefixes for thousand, million, or billion. They are not SI units, using "y" to abbreviate the English "year", but following ambiguous international recommendations, use either the standard English first letters as prefixes (t, m, and b) or metric prefixes (k, M, and G) or variations on metric prefixes (k, m, g). In archaeology, dealing with more recent periods, normally expressed dates, e.g. "22,000 years ago" may be used as a more accessible equivalent of a Before Present ("BP") date.

These abbreviations include:

Non-SI abbreviation Short for... SI-prefixed equivalent Comments and examples
kilo years ka
  • Thousand years
myr
Myr
million years
Mega years
Ma
  • Million years
billion years Ga
kya
kilo years ago time ago in ka
mya
Mya
million years ago
Mega years ago
time ago in Ma
bya
Gya
billion years ago
giga years ago
time ago in Ga

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. This ad hoc distinction between "absolute" time and time intervals is somewhat controversial amongst members of the Geological Society of America.

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.[clarification needed]

Notes

  1. "SI units". IAU. RetrievedFebruary 18, 2010. (See Table 5 and Section 5.15.) Reprinted from: Wilkins, George A. (1989). "The IAU Style Manual"(PDF). IAU Transactions. XXB.
  2. 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)."
  3. Shields, Miriam Nancy (1924). "The new calendar of the eastern churches". Popular Astronomy. 32: 407. Bibcode:1924PA.....32..407S.
  4. 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.
  5. Richards, E.G. (2013). "Calendars". In Urban, S.E.; Seidelmann, P.K. (eds.). Explanatory Supplement to the Astronomical Almanac(PDF) (3rd ed.). Mill Valley, CA: University Science Books. pp. 585, 590. ISBN 978-1-891389-85-6. Archived from the original(PDF) on April 30, 2019. RetrievedMay 9, 2018.
  6. International Earth Rotation and Reference System Service. (2010).IERS EOP PC Useful constants.
  7. 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.
  8. "longitude, ecliptic" and "dynamical equinox". (2018). In "Glossary", The Astronomical Almanac Online. United States Naval Observatory.
  9. Astronomical Almanac for the Year 2011. Washington and Taunton: U.S. Government Printing Office and the U.K. Hydrographic Office. 2009. p. M18 (Glossary).
  10. Astronomical Almanac for the Year 2011. Washington and Taunton: US Government Printing Office and the UK Hydrographic Office. 2009. pp. A1, C2.
  11. Calendar Description and Coordination Maya World Studies Center
  12. Astronomical Almanac for the Year 2010. Washington and Taunton: U.S. Government Printing Office and the U.K. Hydrographic Office. 2008. p. B3.
  13. 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.
  14. 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.
  15. Taff, Lawrence G. (1985). Celestial Mechanics: A Computational Guide for the Practitioner. New York: John Wiley & Sons. p. 103. ISBN 978-0-471-89316-5. 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 the Astronomical Almanac for the Year 2011.
  16. Seidelmann, P. Kenneth (2013). Explanatory Supplement to the Astronomical Almanac. Sean E. Urban (ed.) (3 ed.). Univ Science Books. p. 587. ISBN 978-1-891389-85-6. 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.
  17. "Precession of the Earth's Axis - Wolfram Demonstrations Project". demonstrations.wolfram.com. RetrievedFebruary 10, 2019.
  18. "Science Bowl Questions, Astronomy, Set 2"(PDF). Science Bowl Practice Questions. Oak Ridge Associated Universities. 2009. Archived from the original(PDF) on March 7, 2010. RetrievedDecember 9, 2009.
  19. Thompson, Ambler; Taylor, Barry N. (2008). "Special Publication 811 – Guide for the Use of the International System of Units (SI)"(PDF). National Institute of Standards and Technology (NIST). para 8.1.
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  36. 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 annum: ka, Ma, and Ga for kilo-annum (103 years), Mega-annum (106 years), and Giga-annum (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.{{cite journal}}:Cite journal requires |journal= ()
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Look up year, annus, or ἔτος in Wiktionary, the free dictionary.
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Year Article Talk Language Watch Edit A year or annum is the orbital period of a planetary body for example the Earth moving in its orbit around the Sun Due to the Earth s axial tilt the course of a year sees the passing of the seasons marked by change in weather the hours of daylight and consequently vegetation and soil fertility In temperate and subpolar regions around the planet four seasons are generally recognized spring summer autumn and winter In tropical and subtropical regions several geographical sectors do not present defined seasons but in the seasonal tropics the annual wet and dry seasons are recognized and tracked An analemma illustrates the changing position of the Sun over the course of a year as seen at a fixed time of day A calendar year is an approximation of the number of days of the Earth s orbital period as counted in a given calendar The Gregorian calendar or modern calendar presents its calendar year to be either a common year of 365 days or a leap year of 366 days as do the Julian 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 In English the abbreviations y and yr are commonly used a is also used for the unit of time though its exact duration may be inconsistent In astronomy the Julian year is a unit of time it is defined as 365 25 days of exactly 86 400 seconds SI base unit totalling exactly 31 557 600 seconds in the Julian astronomical year 1 The word year is also used for periods loosely associated with but not identical to the calendar or astronomical year such as the seasonal year the fiscal year the academic year etc Similarly year can mean the orbital period of any planet for example a Martian year and a Venusian year are examples of the time a planet takes to transit one complete orbit The term can also be used in reference to any long period or cycle such as the Great Year 2 Contents 1 Etymology 2 Intercalation 2 1 Julian calendar 2 2 Gregorian calendar 2 3 Other calendars 3 Year numbering 4 Pragmatic divisions 4 1 Fiscal year 4 2 Academic year 5 Astronomical years 5 1 Julian year 5 2 Sidereal tropical and anomalistic years 5 3 Draconic year 5 4 Full moon cycle 5 5 Lunar year 5 6 Vague year 5 7 Heliacal year 5 8 Sothic year 5 9 Gaussian year 5 10 Besselian year 5 11 Variation in the length of the year and the day 5 12 Summary 6 Greater astronomical years 6 1 Equinoctial cycle 6 2 Galactic year 7 Seasonal year 8 Symbols and abbreviations 8 1 SI prefix multipliers 8 2 Abbreviations for years ago 9 See also 10 References 10 1 Notes 11 Further reading 12 External linksEtymologyEnglish year via West Saxon ġear jɛar Anglian ġer continues Proto Germanic jǣran je ran Cognates are German Jahr Old High German jar Old Norse ar and Gothic jer from the Proto Indo European noun yeh r om year season Cognates also descended from the same Proto Indo European noun with variation in suffix ablaut are Avestan yarǝ year Greek ὥra hṓra year season period of time whence hour Old Church Slavonic jarŭ and Latin hornus of this year Latin annus a 2nd declension masculine noun annum is the accusative singular anni is genitive singular and nominative plural annō the dative and ablative singular is from a PIE noun h et no which also yielded Gothic athn year only the dative plural athnam is attested Although most languages treat the word as thematic yeh r o there is evidence for an original derivation with an r n suffix yeh ro Both Indo European words for year yeh ro and h et no would then be derived from verbal roots meaning to go move h ey and h et respectively compare Vedic Sanskrit eti goes atasi thou goest wanderest A number of English words are derived from Latin annus such as annual annuity anniversary etc per annum means each year annō Domini means in the year of the Lord The Greek word for year ἔtos is cognate with Latin vetus old from the PIE word wetos year also preserved in this meaning in Sanskrit vat sa ras year and vat sa yearling calf the latter also reflected in Latin vitulus bull calf English wether ram Old English weder Gothic withrus 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 besides godŭ time period year uses leto summer year IntercalationAstronomical years do not have an integer number of days or lunar months Any calendar that follows an astronomical year must have a system of intercalation such as leap years Julian calendar In the Julian calendar the average mean length of a year is 365 25 days In a non leap year there are 365 days in a leap year there are 366 days A leap year occurs every fourth year or leap year during which a leap day is intercalated into the month of February The name Leap Day is applied to the added day The Revised Julian calendar proposed in 1923 and used in some Eastern Orthodox Churches has 218 leap years every 900 years for the average mean year length of 365 2422222 days close to the length of the mean tropical year 365 24219 days relative error of 9 10 8 In the year 2800 CE the Gregorian and Revised Julian calendars will begin to differ by one calendar day 3 Gregorian calendar The Gregorian calendar attempts to cause the northward equinox to fall on or shortly before March 21 and hence it follows the northward equinox year or tropical year 4 Because 97 out of 400 years are leap years the mean length of the Gregorian calendar year is 365 2425 days with a relative error below one ppm 8 10 7 relative to the current length of the mean tropical year 365 24219 days and even closer to the current March equinox year of 365 242374 days that it aims to match It is estimated that by the year 4000 CE the northward equinox will fall back by one day in the Gregorian calendar not because of this difference but due to the slowing of the Earth s rotation and the associated lengthening of the day Other calendars Further information Lunisolar calendar Historically lunisolar calendars intercalated entire leap months on an observational basis Lunisolar calendars have mostly fallen out of use except for liturgical reasons Hebrew calendar various Hindu calendars A modern adaptation of the historical Jalali calendar known as the Solar Hijri calendar 1925 is a purely solar calendar with an irregular pattern of leap days based on observation or astronomical computation aiming to place new year Nowruz on the day of vernal equinox for the time zone of Tehran as opposed to using an algorithmic system of leap years Year numberingA calendar era assigns a cardinal number to each sequential year using a reference event in the past called the epoch as the beginning of the era The Gregorian calendar era is the world s most widely used civil calendar 5 Its epoch is a 6th century estimate of the date birth of Jesus of Nazareth Two notations are used to indicate year numbering in the Gregorian calendar the Christian Anno Domini meaning in the year of the Lord abbreviated AD and Common Era abbreviated CE preferred by many of other faiths and none Year numbers are based on inclusive counting so that there is no year zero Years before the epoch are abbreviated BC for Before Christ or BCE for Before the Common Era In Astronomical year numbering positive numbers indicate years AD CE the number 0 designates 1 BC BCE 1 designates 2 BC BCE and so on Other eras include that of Ancient Rome Ab Urbe Condita from the foundation of the city abbreviated AUC Anno Mundi year of the world used for the Hebrew calendar and abbreviated AM and the Japanese emperor eras described above The Islamic Hijri year year of the Hijrah Anno Hegirae abbreviated AH is a lunar calendar of twelve lunar months and thus is shorter than a solar year Pragmatic divisionsFinancial and scientific calculations often use a 365 day calendar to simplify daily rates Fiscal year A fiscal year or financial year is a 12 month period used for calculating annual financial statements in businesses and other organizations In many jurisdictions regulations regarding accounting require such reports once per twelve months but do not require that the twelve months constitute a calendar year For example in Canada and India the fiscal year runs from April 1 in the United Kingdom it runs from April 1 for purposes of corporation tax and government financial statements but from April 6 for purposes of personal taxation and payment of state benefits in Australia it runs from July 1 while in the United States the fiscal year of the federal government runs from October 1 Academic year Main article 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 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 into three roughly equal length terms called trimesters or quarters in the United States roughly coinciding with autumn winter and spring At some a shortened summer session sometimes considered part of the regular academic year is attended by students on a voluntary or elective basis Other schools break the year into two main semesters a first typically August through December and a second semester January through May Each of these main semesters may be split in half by mid term exams and each of the halves is referred to as a quarter or term in some countries There may also be a voluntary summer session and or a short January session Some other schools including some in the United States have four marking periods Some schools in the United States notably Boston Latin School may divide the year into five or more marking periods Some state in defense of this that there is perhaps a positive correlation between report frequency and academic achievement There are typically 180 days of teaching each year in schools in the US excluding weekends and breaks while there are 190 days for pupils in state schools in Canada New Zealand and the United Kingdom and 200 for pupils in Australia 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 Astronomical yearsSee also Astronomical month Julian year Main article Julian year astronomy The 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 used in various scientific contexts The Julian century of 36525 days and the Julian millennium of 365250 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 aj of exactly 31557 600 seconds 365 25 d 86400 s 1 a 1 aj 31 5576 Ms The SI multiplier prefixes may be applied to it to form ka kiloannum Ma megaannum etc Sidereal tropical and anomalistic years Main articles Sidereal year and Tropical year 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 365 256363 004 days 365 d 6 h 9 min 9 76 s at the epoch J2000 0 January 1 2000 12 00 00 TT 6 Today the mean tropical year is defined as the period of time for the mean ecliptic longitude of the Sun to increase by 360 degrees 7 Since the Sun s ecliptic longitude is measured with respect to the equinox 8 the tropical year comprises a complete cycle of the seasons and is the basis of solar calendars such as the internationally used Gregorian calendar The modern definition of mean tropical year differs from the actual time between passages of e g the northward equinox for several reasons explained below Because of the Earth s axial precession this year is about 20 minutes shorter than the sidereal year The mean tropical year is approximately 365 days 5 hours 48 minutes 45 seconds using the modern definition 9 365 24219 d 86 400 s The anomalistic year is the time taken for the Earth to complete one revolution with respect to its 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 5 07 48 UT in 2020 and the aphelion where the Earth is farthest from the Sun July 4 11 35 UT in 2020 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 10 Draconic year Further information Draconic period The draconic year draconitic year eclipse year or ecliptic year is the time taken for the Sun as seen from the Earth to complete one revolution with respect to the same lunar node a point where the Moon s orbit intersects the ecliptic The year is associated with eclipses 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 seasons every eclipse year The average duration of the eclipse year is 346 620075 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 18 612815 932 Julian years 6798 331019 days at the epoch J2000 0 Full moon cycle The full moon cycle is the time for the Sun as seen from the Earth to complete one revolution with respect to the perigee of the Moon s orbit This period is associated with the apparent size of the full moon and also with the varying duration of the synodic month The duration of one full moon cycle is 411 784430 29 days 411 days 18 hours 49 minutes 35 seconds at the epoch J2000 0 Lunar year The lunar year comprises twelve full cycles of the phases of the Moon as seen from Earth It has a duration of approximately 354 37 days Muslims use this for celebrating their Eids and for marking the start of the fasting month of Ramadan A Muslim calendar year is based on the lunar cycle The Jewish calendar is also essentially lunar except that an intercalary lunar month is added once every two or three years in order to keep the calendar synchronized with the solar cycle as well Thus a lunar year on the Jewish Hebrew calendar consists of either twelve or thirteen lunar months 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 schematic months of 30 days each plus 5 epagomenal days The vague year was used in the calendars of Ethiopia Ancient Egypt Iran Armenia and in Mesoamerica among the Aztecs and Maya 11 It is still used by many Zoroastrian communities Heliacal year A heliacal year is the interval between the heliacal risings 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 year is the interval between heliacal risings of the star Sirius It is currently less than the sidereal year and its duration is very close to the Julian year of 365 25 days Gaussian year The Gaussian year is the sidereal year for a planet of negligible mass relative to the Sun and unperturbed by other planets that is governed by the Gaussian gravitational constant Such a planet would be slightly closer to the Sun than Earth s mean distance Its length is 365 2568983 days 365 d 6 h 9 min 56 s Besselian year The 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 12 B 1900 0 Julian dateTT 2415 020 31352 365 242198 781 The TT subscript indicates that for this formula the Julian date should use the Terrestrial Time scale or its predecessor ephemeris time Variation in the length of the year and the day This section needs additional citations for verification Please help improve this article by adding citations to reliable sources Unsourced material may be challenged and removed October 2012 Learn how and when to remove this template message Further information Length of day astronometry The exact length of an astronomical year changes over time The positions of the equinox and solstice points with respect to the apsides of Earth s orbit change the equinoxes and solstices move westward relative to the stars because of precession and the apsides move in the other direction because of the long term effects of gravitational pull by the other planets Since the speed of the Earth varies according to its position in its orbit as measured from its perihelion Earth s speed when in a solstice or equinox point changes over time if such a point moves toward perihelion the interval between two passages decreases a little from year to year if the point moves towards aphelion that period increases a little from year to year So a tropical year measured from one passage of the northward vernal equinox to the next differs from the one measured between passages of the southward autumnal equinox The average over the full orbit does not change because of this so the length of the average tropical year does not change because of this second order effect Each planet s movement is perturbed by the gravity of every other planet This leads to short term fluctuations in its speed and therefore its period from year to year Moreover it causes long term changes in its orbit and therefore also long term changes in these periods Tidal drag between the Earth and the Moon and Sun increases the length of the day and of the month by transferring angular momentum from the rotation of the Earth to the revolution of the Moon since the apparent mean solar day is the unit with which we measure the length of the year in civil life the length of the year appears to decrease The rotation rate of the Earth is also changed by factors such as post glacial rebound and sea level rise Numerical value of year variation Mean year lengths in this section are calculated for 2000 and differences in year lengths compared to 2000 are given for past and future years In the tables a day is 86 400 SI seconds long 13 14 15 16 Mean year lengths for 2000 Type of year Days Hours Minutes SecondsTropical 365 5 48 45Sidereal 365 6 9 10Anomalistic 365 6 13 53Eclipse 346 14 52 55Year 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 1160 7 4 5 572000 0 0 0 04000 14 3 5 546000 35 12 10 104Summary Days Year type346 62 Draconic also called eclipse 354 37 Lunar 365 Vague and a common year in many solar calendars 365 24219 Tropical also called solar averaged and then rounded for epoch J2000 0 365 2425 Gregorian on average 365 25 Julian 365 25636 Sidereal for epoch J2000 0 365 259636 Anomalistic averaged and then rounded for epoch J2011 0 366 Leap in many solar calendars An average Gregorian year is 365 2425 days 52 1775 weeks 8765 82 hours 525949 2 minutes or 31556 952 seconds For this calendar a common year is 365 days 8760 hours 525600 minutes or 31536 000 seconds and a leap year is 366 days 8784 hours 527040 minutes or 31622 400 seconds The 400 year cycle of the Gregorian calendar has 146097 days and hence exactly 20871 weeks Greater astronomical yearsEquinoctial cycle The Great Year or equinoctial cycle corresponds to a complete revolution of the equinoxes around the ecliptic Its length is about 25 772 years 17 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 18 Seasonal yearMain article Seasonal year Further information Effect of sun angle on climate A seasonal year is the time between successive recurrences of a seasonal event such as the flooding of a river the migration of a species of bird the flowering of a species of plant the first frost or the first scheduled game of a certain sport All of these events can have wide variations of more than a month from year to year Symbols and abbreviationsA common symbol for the year as a unit of time is a taken from the Latin word annus For example the U S National Institute of Standards and Technology NIST Guide for the Use of the International System of Units SI supports the symbol a as the unit of time for a year 19 In English the abbreviations y or yr are more commonly used in non scientific literature 20 In some Earth sciences branches geology and paleontology kyr myr byr thousands millions and billions of years respectively and similar abbreviations are used to denote intervals of time remote from the present 21 22 The Unified Code for Units of Measure UCUM disambiguates the varying symbologies of ISO 1000 ISO 2955 and ANSI X3 50 by using 23 at 365 24219 days for the mean tropical year aj 365 25 days for the mean Julian year ag 365 2425 days for the mean Gregorian year In the UCUM the symbol a without any qualifier equals 1 aj The UCUM also minimizes confusion with are a unit of area by using the abbreviation ar Since 1989 the International Astronomical Union IAU recognizes the symbol a rather than yr for a year notes the different kinds of year and recommends adopting the Julian year of 365 25 days unless otherwise specified IAU Style Manual 24 25 Since 1987 the International Union of Pure and Applied Physics IUPAP notes a as the general symbol for the time unit year IUPAP Red Book 26 Since 1993 the International Union of Pure and Applied Chemistry IUPAC Green Book also uses the same symbol a notes the difference between Gregorian year and Julian year and adopts the former a 365 2425 days 27 also noted in the IUPAC Gold Book 28 In 2011 the IUPAC and the International Union of Geological Sciences jointly recommended defining the annus with symbol a as the length of the tropical year in the year 2000 29 a 31556 925 445 seconds approximately 365 242192 65 ephemeris days This differs from the above definition of 365 25 days by about 20 parts 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 30 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 and y or yr for a one year time period 30 31 However this historical practice does not comply with the NIST Guide 19 considering the unacceptability of mixing information concerning the physical quantity being measured in this case time intervals or points in time with the units and also the unnaceptability of using abbreviations for units Furthermore according to the UK Metric Association UKMA language independent symbols are more universally understood UKMA Style guide 32 SI prefix multipliers For the following there are alternative forms that elide the consecutive vowels such as kilannum megannum etc The exponents and exponential notations are typically used for calculating and in displaying calculations and for conserving space as in tables of data ka for kiloannum a unit of time equal to one thousand or 103 years or 1 E3 yr also known as a millennium in anthropology and calendar uses The prefix multiplier ka is typically used in geology paleontology and archaeology for the Holocene and Pleistocene periods where a non radiocarbon dating technique such as ice core dating dendrochronology uranium thorium dating or varve analysis is used as the primary method for age determination If age is determined primarily by radiocarbon dating then the age should be expressed in either radiocarbon or calendar calibrated years Before Present Ma for megaannum a unit of time equal to one million or 106 years or 1 E6 yr The suffix Ma 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 rex was abundant approximately 66 Ma 66 million years ago The duration term ago may not always be indicated if the quantity of a duration is specified while not explicitly mentioning a duration term one can assume that ago is implied the alternative unit mya does include ago explicitly It is also written as million years ago in works for general public use 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 Ga for gigaannum a unit of time equal to 109 years or one billion years Ga is commonly used in scientific disciplines such as cosmology and geology to signify extremely long time periods in the past 33 For example the formation of the Earth occurred approximately 4 54 Ga 4 54 billion years ago and the age of the universe is approximately 13 8 Ga Ta for teraannum a unit of time equal to 1012 years or one trillion years Ta 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 Pa for petaannum a unit of time equal to 1015 years or one quadrillion years The half life of the nuclide cadmium 113 is about 8 Pa 34 This symbol coincides with that for the 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 a unit of time equal to 1018 years or one quintillion years The half life of tungsten 180 is 1 8 Ea 35 Abbreviations for years ago Further information Before Present In geology and paleontology a distinction sometimes is made between abbreviation yr for years and ya for years ago combined with prefixes for thousand million or billion 21 36 They are not SI units using y to abbreviate the English year but following ambiguous international recommendations use either the standard English first letters as prefixes t m and b or metric prefixes k M and G or variations on metric prefixes k m g In archaeology dealing with more recent periods normally expressed dates e g 22 000 years ago may be used as a more accessible equivalent of a Before Present BP date These abbreviations include Non SI abbreviation Short for SI prefixed equivalent Comments and exampleskyr kilo years ka Thousand yearsmyr Myr million years Mega years Ma Million yearsbyr billion years Ga Billion years thousand million years or giga year kya kilo years ago time ago in ka Appearance of Homo sapiens circa 200 kya Out of Africa migration circa 60 kya Last Glacial Maximum circa 20 kya Neolithic Revolution circa 10 kyamya Mya million years ago Mega years ago time ago in Ma Pliocene 5 3 to 2 6 mya The last geomagnetic reversal was 0 78 mya 37 The Eemian Stage Last Glacial Period started 0 13 mya The Holocene started 0 01 myabya Gya billion years ago giga years ago time ago in Ga oldest Eukaryotes 2 bya formation of the Earth 4 5 bya Big Bang 13 8 bya Use of mya and bya is deprecated in modern geophysics the recommended usage being Ma and Ga for dates Before Present but m y for the duration of epochs 21 22 This ad hoc distinction between absolute time and time intervals is somewhat controversial amongst members of the Geological Society of America 38 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 clarification needed See also2022 current year Astronomical year numbering Century Decade Epoch reference date ISO 8601 standard for representation of dates and times List of calendars List of years Millennium Orders of magnitude time Unit of timeReferencesNotes SI units IAU Retrieved February 18 2010 See Table 5 and Section 5 15 Reprinted from Wilkins George A 1989 The IAU Style Manual PDF IAU Transactions XXB 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 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 Urban S E Seidelmann P K eds Explanatory Supplement to the Astronomical Almanac PDF 3rd ed Mill Valley CA University Science Books pp 585 590 ISBN 978 1 891389 85 6 Archived from the original PDF on April 30 2019 Retrieved May 9 2018 International Earth Rotation and Reference System Service 2010 IERS EOP PC Useful constants Richards E G 2013 Calendars In S E Urban amp P K Seidelmann Eds Explanatory Supplement to the Astronomical Almanac 3rd ed Mill Valley CA University Science Books p 586 longitude ecliptic and dynamical equinox 2018 In Glossary The Astronomical Almanac Online United States Naval Observatory Astronomical Almanac for the Year 2011 Washington and Taunton U S Government Printing Office and the U K Hydrographic Office 2009 p M18 Glossary Astronomical Almanac for the Year 2011 Washington and Taunton US Government Printing Office and the UK Hydrographic Office 2009 pp A1 C2 Calendar Description and Coordination Maya World Studies Center 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 Touze 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 amp A 282 663S Taff Lawrence G 1985 Celestial Mechanics A Computational Guide for the Practitioner New York John Wiley amp Sons p 103 ISBN 978 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 the Astronomical 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 ISBN 978 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 Precession of the Earth s Axis Wolfram Demonstrations Project demonstrations wolfram com Retrieved February 10 2019 Science Bowl Questions Astronomy Set 2 PDF Science Bowl Practice Questions Oak Ridge Associated Universities 2009 Archived from the original PDF on March 7 2010 Retrieved December 9 2009 a b Thompson Ambler Taylor Barry N 2008 Special Publication 811 Guide for the Use of the International System of Units SI PDF National Institute of Standards and Technology NIST para 8 1 Rowlett Russ Units A How Many A Dictionary of Units of Measurement University of North Carolina Archived from the original on December 20 2008 Retrieved January 9 2009 a b c AGU publications Grammar and Style Guide American Geophysical Union September 1 2017 Archived from the original on September 18 2019 Retrieved January 9 2009 a b North American Commission on Stratigraphic Nomenclature November 2005 North American Stratigraphic Code The American Association of Petroleum Geologists Bulletin Article 13 c ed 89 11 1547 1591 doi 10 1306 07050504129 The Unified Code for Units of Measure UCUM November 21 2017 Retrieved April 23 2022 G A Wilkins Comm 5 IAU Style Manual IAU Transactions XXB 1989 1 SI Units International Astronomical Union Retrieved April 23 2022 IUPAP Red Book Symbols Units Nomenclature and Fundamental Constants in Physics https iupap org wp content uploads 2021 03 A4 pdf E R Cohen T Cvitas J G Frey B Holmstrom K Kuchitsu R Marquardt I Mills F Pavese M Quack J Stohner H L Strauss M Takami and A J Thor Quantities Units and Symbols in Physical Chemistry IUPAC Green Book Third Edition Second Printing IUPAC amp RSC Publishing Cambridge 2008 2 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 Holden Norman E Bonardi Mauro L De Bievre Paul Renne Paul R amp Villa Igor M 2011 IUPAC IUGS common definition and convention on the use of the year as a derived unit of time IUPAC Recommendations 2011 PDF Pure and Applied Chemistry 83 5 1159 1162 doi 10 1351 PAC REC 09 01 22 hdl 10281 21054 S2CID 96753161 a b Biever Celeste April 27 2011 Push to define year sparks time war New Scientist 210 2810 10 Bibcode 2011NewSc 210R 10B doi 10 1016 S0262 4079 11 60955 X Retrieved April 28 2011 Letters About the IUPAC IUGS Common Definition and Convention on the Use of the Year as a Derived Unit of Time Chemistry International Newsmagazine for IUPAC November 19 2011 Retrieved April 23 2022 Style guide UK Metric Association July 12 2017 Retrieved April 23 2022 Arndt Nicholas 2011 Ga in Gargaud Muriel Amils Ricardo Quintanilla Jose Cernicharo Cleaves Henderson James Jim eds Encyclopedia of Astrobiology Berlin Heidelberg Springer p 621 doi 10 1007 978 3 642 11274 4 611 ISBN 978 3 642 11274 4 retrieved December 22 2020 P Belli et al 2007 Investigation of b decay of 113Cd Phys Rev C 76 6 064603 Bibcode 2007PhRvC 76f4603B doi 10 1103 PhysRevC 76 064603 F A Danevich et al 2003 a activity of natural tungsten isotopes Phys Rev C 67 1 014310 arXiv nucl ex 0211013 Bibcode 2003PhRvC 67a4310D doi 10 1103 PhysRevC 67 014310 S2CID 6733875 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 annum ka Ma and Ga for kilo annum 103 years Mega annum 106 years and Giga annum 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 a href wiki Template Cite journal title Template Cite journal cite journal a Cite journal requires journal help Clement Bradford M April 8 2004 Dependence of the duration of geomagnetic polarity reversals on site latitude Nature 428 6983 637 640 Bibcode 2004Natur 428 637C doi 10 1038 nature02459 PMID 15071591 S2CID 4356044 Time Units Geological Society of America Archived from the original on June 16 2016 Retrieved February 17 2010 Further readingFraser Julius Thomas 1987 Time the Familiar Stranger Time The Familiar Stranger illustrated ed Amherst University of Massachusetts Press Bibcode 1988tfs book F ISBN 978 0 87023 576 4 OCLC 15790499 Whitrow Gerald James 2003 What is Time Oxford Oxford University Press ISBN 978 0 19 860781 6 OCLC 265440481 External linksLook up year annus or ἔtos in Wiktionary the free dictionary Wikimedia Commons has media related to Years Images of years Portals Geology Mathematics Astronomy Stars Outer space Science Retrieved from https en wikipedia org w index php title Year amp oldid 1093745496, wikipedia, 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