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An animation of the inner Solar System planets' orbit around the Sun. The duration of the year is the time taken to go around the Sun.

A year is a unit of time based on how long it takes the Earth to orbit the Sun.[1] In scientific use, the tropical year (approximately 365 solar days, 5 hours, 48 minutes, 45 seconds) and the sidereal year (about 20 minutes longer) are more exact. The modern calendar year, as reckoned according to the Gregorian calendar, approximates the tropical year by using a system of leap years.

The term 'year' is also used to indicate other periods of roughly similar duration, such as the lunar year (a roughly 354-day cycle of twelve of the Moon's phases – see lunar calendar), as well as periods loosely associated with the calendar or astronomical year, such as the seasonal year, the fiscal year, the academic year, etc.

Due to the Earth's axial tilt, the course of a year sees the passing of the seasons, marked by changes 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.

By extension, the term 'year' can also be applied to the time taken for the orbit of any astronomical object around its primary – for example the Martian year of roughly 1.88 Earth years.

The term can also be used in reference to any long period or cycle, such as the Great Year.[2]

Calendar year

[edit]

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 (97 out of 400 years are leap years).[3]

Abbreviation

[edit]

In English, the unit of time for year is commonly abbreviated as "y" or "yr". The symbol "a" (for Latin: annus, year) is sometimes used in scientific literature, though its exact duration may be inconsistent.[citation needed]

Etymology

[edit]

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", Greek ὥρα (hṓra) "year, season, period of time" (whence "hour"), Old Church Slavonic jarŭ, and Latin hornus "of this year".[citation needed]

Latin annus (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 Latin annus, 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 年 "year", originally 秂, an ideographic compound of a person carrying a bundle of wheat denoting "harvest". Slavic besides godŭ "time period; year" uses lěto "summer; year".

Intercalation

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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

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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 during which a leap day is intercalated into the month of February. The name "Leap Day" is applied to the added day.

In astronomy, the Julian year is a unit of time defined as 365.25 days, each of exactly 86400 seconds (SI base unit), totaling exactly 31,557,600 seconds in the Julian astronomical year.[4][5]

Revised Julian calendar

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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). In the year 2800 CE, the Gregorian and Revised Julian calendars will begin to differ by one calendar day.[6]

Gregorian calendar

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The Gregorian calendar aims to ensure that the northward equinox falls on or shortly before March 21 and hence it follows the northward equinox year, or tropical year.[7] 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) relative to the current length of the mean tropical year (365.242189 days) and even closer to the current March equinox year of 365.242374 days that it aims to match.

Other calendars

[edit]
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 numbering

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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.[8] Its epoch is a 6th century estimate of the date of 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 imperial eras. 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 divisions

[edit]

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

Fiscal year

[edit]
Main article: 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

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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 in the Northern Hemisphere starts in August or September and ends in May, June or July, providing a summer break from study between academic years. 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 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.[citation needed]

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 years

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See also: Astronomical month

Julian year

[edit]
Main article: Julian year (astronomy)

The Julian year, as used in astronomy and other sciences, is a time unit now defined as exactly 365.25 days of 86400 SI seconds each[9] ("ephemeris days"). This is one meaning of the unit "year" used in various scientific contexts. The Julian century of 36525 ephemeris days and the Julian millennium of 365250 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 a light-year.

In the Unified Code for Units of Measure (but not according to the International Union of Pure and Applied Physics or the International Union of Geological Sciences, see below), the symbol 'a' (without subscript) always refers to the Julian year, 'aj', of exactly 31557600 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", "Ma", etc.[10]

The scientific Julian year is not to be confused with a year in the Jullian calendar. The scientific Julian year is a multiple of the SI second; it is today "astronomical" only in the sense "used in astronomy", whilst true astronomical years are based on the movements of celestial bodies.

Sidereal, tropical, and anomalistic years

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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 365.256363004 days (365 d 6 h 9 min 9.76 s) (at the epoch J2000.0 = January 1, 2000, 12:00:00 TT).[11]

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.[12] Since the Sun's ecliptic longitude is measured with respect to the equinox,[13] 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, by a minute or two, 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[14] (= 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 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, and the aphelion, 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).[15]

Draconic year

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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

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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

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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 religious purposes, including calculating the date of the Hajj and the fasting month of Ramadan, and thus also the Eids. The Jewish 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

[edit]

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.[16] It is still used by many Zoroastrian communities.

Heliacal year

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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

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The Sothic year is the heliacal year, 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

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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

[edit]

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):[17]

B = 1900.0 + (Julian dateTT − 2415020.31352) / 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

[edit]
Further information: ΔT (timekeeping)

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 86400 SI seconds long.[18][19][20][21]

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

[edit]

Some of the year lengths in this table are in average solar days, which are slowly getting longer (at a rate that cannot be exactly predicted in advance) and are now around 86400.002 SI seconds.

Days Year type
346.62 Draconic, also called eclipse
354.37 Lunar
365 Solar days: 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, solar days averaged over the 400-year cycle
365.25 Julian, solar days averaged over the four-year cycle
365.25636 Sidereal, for epoch J2000.0
365.259636 Anomalistic, averaged and then rounded for epoch J2011.0
366 Leap year in many solar calendars

An average Gregorian year may be said to be 365.2425 days (52.1775 weeks, and if an hour is defined as one twenty-fourth of a day, 8765.82 hours, 525949.2 minutes or 31556952 seconds). 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 or 31536000 seconds), and a leap year is 366 days (8784 hours, 527040 minutes or 31622400 seconds). The 400-year civil cycle of the Gregorian calendar has 146097 days and hence exactly 20871 weeks.

Greater astronomical years

[edit]

Equinoctial cycle

[edit]

The Great Year, or equinoctial cycle, corresponds to a complete revolution of the equinoxes around the ecliptic. Its length is about 25,700 years.[22][23]

Galactic year

[edit]

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.[24]

IUPAC–IUGS proposal

[edit]

In 2011, a task group of the IUPAC and the International Union of Geological Sciences (IUGS) jointly recommended defining the annus for geological purposes as

1 a = 31556925.445 seconds[25] (approximately 365.24219265 ephemeris days)

They chose a value close to the length of tropical year for the epoch 2000.0 (which is roughly the length of the tropical year 2000; the length of the tropical year is slowly decreasing).[25] However, the definition is as a multiple of the second, the SI base unit of time, and independent of astronomical definitions, since "[d]efinitions of the annus that are based on an intermediate relationship via the day, such as the Julian and Gregorian year, bear an inherent, pre-programmed obsolescence because of the variability of Earth's orbital movement".[25] It differs from the Julian year of 365.25 days (3.1557600 × 107 s) by about 21 parts per million.

As of April 2025, the IUPAC Green Book (4th edition) provides a definition of the year as a = 31556925.9747 seconds.[26]

Seasonal year

[edit]
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.

Symbols and abbreviations

[edit]

A 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.[27]

In English, the abbreviations "y" or "yr" are more commonly used in non-scientific literature.[28] 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.[29][30] In astronomy the abbreviations kyr, Myr and Gyr are in common use for kiloyears, megayears and gigayears.[31][32]

The Unified Code for Units of Measure (UCUM) disambiguates the varying symbologies of ISO 1000, ISO 2955 and ANSI X3.50 by using:[10]

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).[33][34]

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).[35] 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),[36] also noted in the IUPAC Gold Book.[37]

In 2011, a task group of IUPAC and IUGS recommended the use of a as the symbol for the annus (along with multiples such as Ma) for both time intervals and absolute ages.[25] This proved controversial as it conflicts with an earlier convention among geoscientists to use "a" specifically for absolute age before the present (e.g. 1 Ma for 1 million years ago), and "y" or "yr" (and My, Myr etc) for a time interval or period of time.[38][39]

SI prefix multipliers

[edit]
See also: Orders of magnitude (time)

For the following, there are alternative forms that elide the consecutive vowels, such as kilannus, 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.

Units of time with SI prefixes
Symbol Definition Common scientific uses and notes
ka (for kiloannus) One thousand or 103 years, also known as a millennium in anthropology and calendar uses. 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 megaannus) One million or 106 years. Geology, paleontology, and celestial 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.
Ga (for gigaannus) One billion or 109 years. Cosmology and geology.[40] 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 teraannus) One trillion or 1012 years. 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 petaannus) One quadrillion or 1015 years. The half-life of the nuclide cadmium-113 is about 8 Pa.[41] This symbol coincides with that for the pascal without a multiplier prefix, but context will normally be sufficient to distinguish long time periods from pressure values.
Ea (for exaannus) One quintillion or 1018 years. The half-life of tungsten-180 is 1.8 Ea.[42]

Abbreviations for "years ago"

[edit]

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.[29][43] In archaeology, dealing with more recent periods, normally expressed dates, e.g. "10,000 BC", may be used as a more traditional form than Before Present ("BP").

These abbreviations include:

Non-SI
abbreviations 		Short for SI-prefixed
equivalent 		Definition Examples
Event Time
kyr
 kilo years ka Thousand years
myr
Myr
 million years
Mega years		 Ma Million years
byr
Gyr
 billion years
Giga years		 Ga Billion years
(thousand million years)
kya
 kilo years ago Time ago in ka

Around 200 kya
Around 60 kya
Around 20 kya
Around 10 kya

mya
Mya
 million years ago
Mega years ago		 Time ago in Ma Approximately 66 mya
5.3 to 2.6 mya
0.78 mya
0.13 mya
0.01 mya
bya
Gya
 billion years ago
giga years ago		 Time ago in Ga 2 bya
4.5 bya
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 durations of epochs.[29][30] This ad hoc distinction between "absolute" time and time intervals is somewhat controversial amongst members of the Geological Society of America.[45]

See also

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References

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Further reading

[edit]
Revisions and contributorsEdit on WikipediaRead on Wikipedia

Year

View on Grokipedia
from Grokipedia
A year is a unit of time based on 's around the Sun, approximately 365.25 days in length, serving as a fundamental measure for calendars, seasons, and astronomical cycles. In astronomy, the —the time between successive vernal equinoxes—measures 365 days, 5 hours, 48 minutes, and 46 seconds (or 365.2422 mean solar days) and aligns with seasonal changes due to 's and . By contrast, the , defined as the period for to complete one full relative to distant , lasts about 365.256363 mean solar days, roughly 20 minutes longer than the tropical year owing to the of 's equinoxes. Calendars adapt these astronomical years into practical systems: solar calendars like the Gregorian approximate the tropical year at 365.2425 days by omitting leap days in most century years, ensuring alignment with seasons over millennia. Lunar calendars, such as the Islamic, base the year on 12 synodic months (about 354.367 days), drifting relative to seasons without intercalation, while lunisolar systems like the Hebrew or Chinese add extra months periodically to synchronize lunar cycles with the solar year. Beyond these, years appear in fiscal, academic, or cultural contexts, but all derive from the core astronomical concept of 's annual revolution.

Basics and Definition

Calendar Year

A calendar year serves as the primary in civil calendars, representing an approximation of the period required for to complete one full around the Sun, lasting roughly 365 days. This division facilitates the organization of daily life, administrative functions, and long-term planning in societies worldwide. Unlike more precise astronomical measurements, the calendar year prioritizes practicality over exact orbital duration to align with human needs. In the predominant , adopted globally for civil purposes, the year commences on and spans 365 days in common years, with an additional day—February 29—inserted in to account for the fractional excess in the . This structure ensures that the calendar remains synchronized with seasonal changes over extended periods. The system, introduced in 1582 by , refined earlier Julian reforms to better approximate the while maintaining a fixed starting date. Historically, the year as a solar-based temporal unit originated in ancient agricultural societies, where tracking the Sun's annual cycle was essential for timing planting, harvesting, and related communal activities. Early civilizations developed years to correlate human endeavors with predictable environmental rhythms. This foundational role underscores the calendar year's evolution from a tool of to a of modern and . While derived from the astronomical year, the year emphasizes accessibility and consistency for non-scientific applications.

Etymology

The English word "year" derives from the *yēr-, meaning "year" or "," likely referring to a complete cycle of time associated with natural rhythms. This root passed into Proto-Germanic as *jēran, evolving into gēar (in ) or ger (in Anglian), where it denoted a full revolution of seasons; by , it had shifted to yer, standardizing as "year" to signify approximately 365 days or 12 months. Cognates in related languages, such as ár, jār, and Gothic jer, demonstrate the term's deep embedding in the Indo-European family, emphasizing seasonal recurrence across Germanic tongues. In contrast, the Latin annus, also meaning "year," originates from Proto-Indo-European *at-no-, a form of the root *at- ("to go"), interpreted as "that which goes around," symbolizing the orbital path of celestial bodies. This etymology highlights a conceptual focus on rather than seasons, influencing like French an and Italian anno, and underscoring the year's astronomical basis in . Biblical and ancient Near Eastern traditions further shaped the term's historical usage, particularly through . In Biblical Hebrew, šānāh ("year") comes from the root š-n-h, connoting "to repeat" or "to change," which captures the cyclical repetition of time and seasons in agricultural societies. This root connects to Akkadian šattu ("year"), used in Mesopotamian texts to mark periods of or lunar cycles, both deriving from Proto-Semitic *šanat-. In biblical contexts, such as genealogies or prophetic timelines in Genesis and Daniel, šānāh denotes eras of divine intervention or human lifespan.

Abbreviation

In English, the standard abbreviation for "year" is "yr.", which serves for both singular and plural forms, as in "1 yr." or "5 yrs.". This is commonly employed in informal writing, tables, charts, and technical contexts where brevity is essential, while the full word "year" or "years" is preferred in formal . Similarly, "y." appears occasionally in scientific or engineering notations, though "yr." remains more prevalent in general usage. Style guides provide specific recommendations on its application. generally discourages abbreviating "year" in running text to maintain clarity but permits "yr." in informal settings, footnotes, or visual aids like graphs. advises spelling out "year" in body text except in user interfaces or space-constrained elements, aligning with broader editorial practices that prioritize over contraction. A historical variation derives from Latin, where "ann." abbreviates "annus," meaning year, and is still used in academic, legal, or classical contexts to denote annual periods. Internationally, abbreviations vary; for instance, the Australian Government Style Manual accepts either "y" or "yr." for consistency within documents. In standardized data exchange, the format represents years as four-digit numerals (e.g., 2025) in dates like YYYY-MM-DD, and uses "Y" in duration strings such as "P1Y" for , ensuring unambiguous global .

Calendar Systems and Adjustments

Intercalation in the Julian Calendar

The was introduced by in 45 BCE as a reform of the earlier to better align civil timekeeping with the solar year. This system established a standard year of 365 days divided into 12 months, with an intercalary day added every fourth year to compensate for the extra time in the solar cycle. The additional day was inserted at the end of February, creating in . Under the Julian rule, any year divisible by 4 qualifies as a , resulting in three common years of 365 days followed by one of 366 days. This mechanism yields an average calendar year length of 365.25 days, calculated as (3 × 365 + 366) / 4. The approach was a significant improvement over prior systems but still imperfect, as the true length of the —the time for to complete one orbit around the Sun relative to the equinoxes—is approximately 365.2422 days. The overestimate of about 0.0078 days (or 11 minutes and 14 seconds) per year caused a gradual drift, known as the quarter-day error. Over centuries, this accumulation shifted the calendar relative to the seasons; by the , the vernal had drifted by 10 days earlier than intended. This misalignment affected agricultural and religious timing, highlighting the need for further adjustments.

Intercalation in the Gregorian Calendar

The Gregorian calendar was introduced by Pope Gregory XIII through the papal bull Inter Gravissimas issued on February 24, 1582, as a reform to address the accumulating errors in the preceding Julian calendar by adjusting the timing of leap years and immediately correcting the date drift. To implement the change, Catholic countries in Europe skipped 10 days in October 1582, with Thursday, October 4, being followed directly by Friday, October 15, thereby realigning the calendar with the vernal equinox for accurate computation of Easter. The revised leap year rules established that a year is a if it is divisible by 4, except for years divisible by 100, which are not unless also divisible by 400; this results in 97 every 400 years, yielding an average length of 365.2425 days. Compared to the Julian calendar's average of 365.25 days, this refinement reduces the annual drift to approximately 0.0003 days, or about 1 day every 3,300 years, ensuring greater long-term alignment with the . Adoption of the varied widely due to religious and political divisions, with immediate uptake in Catholic nations like , , , , and in 1582, while Protestant countries delayed implementation owing to opposition to papal authority. and its colonies, for instance, retained the until 1752, when an skipped 11 days (September 2 followed by September 14) to synchronize civil dating. Eastern Orthodox countries experienced even longer delays, with many rejecting the reform initially due to concerns over calculations; while some, such as and , adopted it in the early 20th century ( in 1923), others like switched only in 1918, and certain Orthodox churches continue using the for liturgical purposes into the present day.

Intercalation in Other Calendars

The Hebrew calendar is a lunisolar system that intercalates an extra month, Adar II, in seven years of a 19-year cycle—specifically the 3rd, 6th, 8th, 11th, 14th, 17th, and 19th years—to align its 12 lunar months (typically 354 days) with the solar year of approximately 365.25 days. This Metonic cycle ensures that holidays like Passover remain in spring, preventing seasonal drift. In contrast, the is purely lunar, consisting of 12 months totaling 354 or 355 days, with no intercalation to adjust for the solar year. This results in the calendar drifting backward by about 11 days annually relative to the seasons, causing festivals such as to shift through the year—for example, occurring in different seasons every few years, which can affect fasting conditions from summer heat to winter cold. The , also lunisolar, inserts seven leap months over a 19-year cycle to synchronize its lunar months with the solar year, similar to the Hebrew approach. Additionally, it includes rules for occasional adjustments, such as ensuring the falls within the 11th month, to maintain alignment with seasonal agricultural cycles.

Year Numbering Systems

Year numbering systems provide a sequential method for labeling years in historical, religious, and cultural contexts, the of across time. These systems typically dates to a significant , such as a foundational event or religious , and vary widely depending on the tradition. While intercalation helps align years with astronomical cycles to maintain sequential accuracy, numbering systems focus on the assignment of ordinal values to those years. The most widely used year numbering system in the modern era is the Anno Domini (AD), or Common Era (CE), which counts years from the estimated birth of Jesus Christ. This system was introduced in 525 CE by the Scythian monk Dionysius Exiguus, who devised it as part of his Easter tables to replace the earlier Diocletian era, basing year 1 on what he calculated as the incarnation year. Dionysius's numbering omitted a year zero, transitioning directly from 1 BC to AD 1, a convention that persists in the Gregorian calendar. The AD/CE notation became standard in Christian Europe by the 8th century through the works of scholars like Bede and was later globalized via European colonialism and international standards. Other historical systems employ different epochs tied to foundational events. The Roman Ab Urbe Condita (AUC) system, meaning "from the founding of the city," numbered years from the legendary establishment of in 753 BCE, as standardized by the scholar in the 1st century BCE; for example, the year 1 BCE corresponded to AUC 753. This chronology was used in Roman historiography, such as Livy's , though it fell out of common use after the adoption of the Christian era. In the Islamic tradition, the Hijri (AH) calendar numbers years from the Hijra, the migration of Prophet Muhammad from to in 622 CE, formalized as the epoch by Caliph Umar ibn al-Khattab in the to standardize Muslim dating. The , or (AM), counts years from the biblical creation of the world, traditionally dated to 3761 BCE in the rabbinic chronology established by the 4th century CE, making the current year approximately 5786 AM as of 2025 CE. Transitioning between these systems presents challenges, particularly for eras before the AD/CE epoch. The Before Christ (BC) or Before Common Era (BCE) notation, introduced in the 17th century by Denis Pétau for BC and popularized in the 19th-20th centuries for BCE to promote secular usage, counts backwards from year 1 (e.g., 753 BCE is 753 years before AD 1). The absence of a in the —where the Julian/Gregorian system is extended backward—means there is no year 0, so calculations between BC and AD eras require adjusting by one year (e.g., to AD 1 spans one year, not two). This discontinuity can complicate astronomical and historical computations, often addressed by using that assigns 0 to .

Practical and Cultural Divisions

Fiscal Year

A is a 12-month period adopted by governments, es, and organizations for financial reporting, budgeting, taxation, and auditing purposes, which may or may not align with the standard running from to 31. This structure provides flexibility to better synchronize financial cycles with operational realities, such as seasonal business activities or administrative needs, rather than adhering strictly to the . While many entities opt for the for simplicity, fiscal years are commonly offset to optimize planning and reporting timelines. Variations in fiscal year dates arise from historical, economic, and practical considerations tailored to a country's business cycles, agricultural seasons, or retail patterns. In the United States, the federal government's fiscal year spans October 1 to September 30, a shift implemented in 1976 to allow Congress more time to review and pass budgets without overlapping the holiday season's disruptions. The United Kingdom's fiscal year runs from April 6 to April 5, rooted in medieval agricultural tax traditions that aligned collections with post-harvest periods. Japan's fiscal year, from April 1 to March 31, coordinates with the academic calendar and corporate reporting cycles to facilitate smoother economic data aggregation. These offsets help avoid peak spending or revenue periods, such as year-end holidays or spring planting seasons in agrarian economies. The choice of fiscal year significantly influences budgeting processes, tax filing deadlines, audit schedules, and the timing of economic indicators like GDP reports. For instance, misalignments can complicate international comparisons of financial data, prompting organizations like the International Monetary Fund to adjust for fiscal versus calendar reporting in their analyses. In countries adhering to the calendar year, such as Canada or Germany, financial reporting integrates seamlessly with global standards, but offset fiscal years in others demand careful reconciliation for cross-border trade and investment. Overall, this system supports more accurate reflections of an entity's financial health by capturing complete operational cycles.

Academic Year

The academic year serves as the foundational period for organizing educational activities in schools and universities, typically encompassing a structured sequence of terms or semesters that deliver the bulk of instruction. In the , it commonly spans from late August or September to May or , divided into two semesters—fall and spring—or three terms, with breaks for winter holidays and spring. This configuration provides approximately 180 instructional days annually, allowing alignment with seasonal weather patterns that favor indoor learning during colder months and outdoor activities during summer vacations. Although often attributed to agricultural needs in pre-industrial societies, the of the longer summer break traces its historical roots primarily to 19th-century urban reforms addressing concerns from summer and in cities, with rural agrarian influences playing a secondary role in shorter years overall. In recent decades, some regions have piloted year-round models to redistribute these days more evenly, reducing summer learning loss while maintaining total instructional time; for instance, programs in and test balanced calendars with shorter, interspersed breaks. Variations exist by hemisphere and region to accommodate local climates and holidays. In the , such as , the academic year runs from late to mid-, structured into four terms with holidays between them, culminating in a summer break during the hottest months of and . This typically yields 180–200 instructional days, similar to northern counterparts, but shifted to leverage milder winter weather for extended school sessions.

Other Pragmatic Divisions

In , the year refers to the annual period encompassing one complete cycle of cultivation and for a specific product, with start and end dates varying by type and regional practices. For instance, in the , USDA marketing years for grains differ: corn and typically run from September 1 to August 31, while , oats, and run from June 1 to May 31, aligning with planting and harvesting cycles. This division facilitates tracking production, trade, and economic factors like yield variability influenced by weather and pests. In the , the designates the annual production cycle for vehicle models, often spanning from September of one calendar year to August of the next, to synchronize with sales cycles and the introduction of new features in the fall. This pragmatic division allows manufacturers to update designs, comply with regulatory standards, and align releases with consumer buying patterns, such as post-summer demand. For example, 2026 model year vehicles began appearing on dealer lots in August 2025, marking the transition from prior models. Beyond standard fiscal and academic years, tax year variations for self-employed individuals often involve quarterly divisions to manage estimated payments throughout the calendar year. In the United States, self-employed taxpayers must generally pay estimated income and self-employment taxes four times annually, with deadlines on April 15, June 15, September 15, and January 15 of the following year, based on projected annual liability. In the European Union, value-added tax (VAT) reporting periods similarly include quarterly options, tailored by member state; for instance, in Germany, businesses with turnover below certain thresholds file quarterly VAT returns due by the 10th of the month following the quarter's end, while higher-turnover entities file monthly. Belgium requires quarterly filers to submit by the 25th day after the quarter ends. These divisions ensure timely revenue collection while accommodating business cash flows.

Astronomical Years

Julian Year

The Julian year is a standardized unit of time in astronomy, defined as exactly 365.25 mean solar days, equivalent to 31,557,600 seconds. This fixed length, denoted by the symbol a or aJ, provides a consistent interval for precise calculations, independent of variations in or orbital irregularities. The concept originated in the as an adaptation of the average year length in the , which inserts a leap day every four years to approximate the . Astronomer formalized its use in his 1898 Tables of the Sun, where he employed the Julian year (and its multiple, the Julian century of 36,525 days) alongside a more precise "solar century" for computations, promoting it for uniformity in astronomical tables. This adoption helped standardize time units across international ephemerides following a 1896 conference of national almanac directors. In modern applications, the Julian year serves as the default "year" in the (IAU) standards for measurements like the and rates, unless a specific type such as the is indicated. It is particularly valuable in for defining parameters like the and computing trajectories, as seen in 's astrodynamic models. Additionally, it facilitates historical astronomy by providing a timescale that bypasses civil calendar discrepancies in analyzing ancient observations.

Sidereal, Tropical, and Anomalistic Years

The is the duration required for to complete one full revolution around the Sun relative to the distant fixed stars, representing the true unaffected by 's . This period is determined observationally and aligns with Kepler's third law, which relates the TT of a to its semi-major axis aa via T2∝a3T^2 \propto a^3, where the constant of proportionality depends on the . For , the mean length of the is 365.256363 mean solar days. In contrast, the measures the time between successive vernal es, serving as the fundamental basis for defining seasons and aligning calendars like the Gregorian system with solar cycles. It is shorter than the due to the of Earth's rotational axis, which causes the equinox points to shift westward against the stellar background at a rate of approximately 50.3 arcseconds per year, effectively reducing the interval by about 20 minutes over one . The mean tropical year length is 365.24219 mean solar days, derived from precise calculations accounting for this precessional effect. The anomalistic year is the interval between successive passages of through perihelion, the point of closest approach to the Sun, influenced by the slow of the orbital apsides arising from planetary gravitational perturbations and Earth's of about 0.0167. This apsidal motion advances the perihelion by roughly 11.6 arcseconds per year, making the anomalistic year slightly longer than the . Its mean length is 365.259636 mean solar days, also rooted in Keplerian adjusted for these perturbations. These year lengths, while close to the Julian year's fixed 365.25 days, vary slightly due to long-term dynamical effects like and eccentricity changes. The following table summarizes their mean durations in mean solar days for comparison:
Year TypeMean Length (days)Reference Point
Sidereal365.256363
Tropical365.24219Vernal equinoxes
Anomalistic365.259636Perihelion passages

Draconic Year

The draconic year, also known as the eclipse year, is the time required for the Sun, as observed from , to complete one full cycle relative to the lunar nodes—the points where the Moon's orbit intersects the ecliptic plane. This period specifically measures the interval for the Moon's ascending node to regress by 360 degrees relative to the vernal equinox. Its length is 346.62 days, which is shorter than the due to the westward regression of the nodes. The draconic year plays a key role in eclipse prediction because solar and lunar can only occur when the Sun is positioned near one of the lunar nodes, aligning the , , and Sun along the . Eclipse seasons happen twice within each draconic year, roughly 173 days apart, as the Sun passes through the nodal line. Multiples of the draconic year help synchronize these alignments over longer periods; for instance, one saros cycle—comprising 242 draconic months—is approximately equivalent to 19 draconic years, repeating similar eclipse geometries after about 6,585 days. This duration arises from the interplay of Earth's orbital and the dynamics of the Moon's , where solar gravitational perturbations cause the lunar nodes to regress westward at a rate of about 19.35 degrees per . The relative motion between the Sun's annual progression along the and this nodal regression shortens the effective cycle for the Sun to realign with a node, yielding the 346.62-day period. This regression stems from the exerted by the Sun on the slightly inclined and Moon's , a fundamental aspect of three-body .

Metonic Cycle and Full Moon Cycle

The is an approximately 19-year period in which the phases of the , including full moons, recur on the same dates of the year. This alignment occurs because 19 tropical years closely match the duration of 235 synodic months, totaling 6,939.69 days. The cycle was discovered by the Athenian astronomer Meton around 432 BCE, who observed this near-equivalence and introduced it into the of ancient . The cycle is synonymous with the in this context, referring to the repetition of full moons—and equivalently, new moons—on nearly identical calendar dates after 19 years, with a discrepancy of just a few hours. This periodicity arises from the synodic month, the time between consecutive full moons, which serves as a foundational component of the lunar year. The underpins the structure of various lunisolar calendars by providing a framework for intercalation, where an additional month is added seven times within the 19-year span to synchronize lunar phases with seasonal solar progression. In the , for instance, this results in occurring in years 3, 6, 8, 11, 14, 17, and 19 of each Metonic cycle, ensuring that festivals tied to lunar dates remain aligned with agricultural seasons.

Lunar Year

The lunar year is defined as a period consisting of 12 synodic months, representing the time for the Moon to complete 12 cycles of phases as observed from . Each synodic month averages 29.53059 days in length, yielding a total of approximately 354.367 days for the lunar year. This measure contrasts with solar years, which are based on around the Sun and average about 365.242 days. Purely lunar calendars, such as the , adhere strictly to this 12-month structure without intercalary adjustments to align with solar cycles, resulting in years of 354 or 355 days depending on the observation of new moons. The shorter duration compared to the tropical solar year leads to a seasonal drift of roughly 11 days per year, causing dates in lunar calendars to shift progressively through the seasons over time. Historically, the lunar year served as the foundational unit in ancient Mesopotamian calendars, where it guided timekeeping for administrative, agricultural, and religious purposes based on observed lunar phases. In modern contexts, it remains central to religious observances in , determining the timing of events like and the independently of seasonal changes.

Vague Year

The vague year refers to a fixed of exactly 365 days, serving as a simple approximation to the while disregarding the additional fractional day of approximately 0.25 days. This structure overlooks the need for intercalation, resulting in a gradual misalignment with the actual over time. In ancient astronomical and calendrical systems, the vague year provided a straightforward framework for timekeeping without the complexity of leap adjustments. A prominent example is the ancient Egyptian civil calendar, which divided the year into 12 months of 30 days each, plus 5 epagomenal days at the end, totaling 365 days. This configuration caused the calendar to drift backward by roughly one day every four years relative to the seasons, accumulating a full year's shift over about 1,460 years. The vague year proved practical for administrative and ritual purposes in Egyptian society, where seasonal alignment was monitored separately through astronomical observations. In theoretical and early computational contexts, the vague year's fixed length facilitated simplified models of celestial periods and time reckoning, particularly before more precise measurements necessitated refinements like the Julian year of 365.25 days.

Heliacal and Sothic Years

The heliacal year in ancient was defined by the interval between consecutive heliacal risings of prominent stars, particularly Sirius (known as Sothis), marking its first visibility just before dawn after a period of due to proximity to the Sun. This event, observed from locations like Memphis around 3000 BCE, typically spanned approximately 365.25 days, aligning closely with the and serving as a natural marker. The Sothic year, by contrast, was the fixed civil calendar year of exactly 365 days, consisting of 12 months of 30 days each plus 5 epagomenal days, without intercalation for the fractional solar day. This shorter year caused the calendar to drift relative to the seasons and stellar events by one day every four years, leading to the —a period of 1,461 Sothic years (or approximately 1,460 astronomical years of 365.25 days each)—over which the of Sirius realigned with the civil on I Akhet 1. Culturally, the heliacal rising of Sirius held profound significance in ancient Egypt, as it reliably predicted the annual inundation of the River, which deposited nutrient-rich essential for and societal sustenance. Priests and astronomers monitored this phenomenon to synchronize the with seasonal cycles, associating Sirius with the Sothis and using it to guide planting and harvest timings, thereby underpinning the economic and religious fabric of dynastic eras.

Gaussian and Besselian Years

The Gaussian year is a fundamental unit in , defined as the sidereal of a of negligible mass in an unperturbed elliptical with a semi-major axis of 1 (AU), lasting exactly 365.2568983 days. Introduced by in his studies of solar system dynamics, this duration derives from Kepler's third law and the k=0.01720209895k = 0.01720209895, through the relation P=2π/kP = 2\pi / k, where PP is the . It can be expressed as 365.25+0.0068983365.25 + 0.0068983 days, providing a precise baseline that incorporates gravitational influences on planetary motion without perturbations from other bodies. In astronomical epoch calculations, the Gaussian year serves as a reference for adjusting positions from the standard J2000.0 epoch, which uses a mean Julian year of 365.25 days, to account for subtle gravitational effects in the solar system's barycentric framework. This adjustment ensures accuracy in deriving the AU as 149,597,870.66 km, fixed by international agreement to support ephemeris computations and spacecraft navigation. Its application in celestial coordinate systems facilitates the transformation of positions perturbed by interplanetary gravity, maintaining consistency in long-term orbital predictions. The Besselian year, developed by Friedrich Wilhelm Bessel, is a variable time unit starting from the instant when the fictitious mean Sun reaches a mean of 280° (equivalent to 18ʰ 40ᵐ), typically near January 1 in the . Unlike fixed years, its length is slightly shorter than the , given by the tropical year minus 0.148T0.148 T seconds, where TT is the number of tropical centuries elapsed since 1900.0. This definition aligns the year with the dynamical , making it suitable for historical ephemerides that require synchronization with Earth's orbital motion. Historically employed in pre-1984 astronomical tables, the Besselian year was essential for computing —the small, periodic wobble in Earth's axis due to lunar and solar torques—and in celestial coordinates. It enabled precise designations like B1950.0 for star catalogs and planetary positions, where parameters were tabulated relative to the vernal . Although superseded by Julian epochs in modern systems like J2000.0, the Besselian year remains relevant for interpreting legacy data in , particularly in applications involving -based reference frames. The Besselian year relates closely to the , differing mainly in its alignment to the vernal for enhanced accuracy in seasonal astronomical computations.

Variations in Year and Day Lengths

The length of a day on varies depending on the reference point. The sidereal day, defined as the time required for to complete one relative to distant stars, measures approximately 23 hours, 56 minutes, and 4 seconds. In contrast, the mean solar day, the average interval between successive transits of the Sun across the meridian, lasts 24 hours. This roughly 4-minute difference results from 's orbital motion around the Sun, which causes the direction to the Sun to shift eastward by about 1 degree per day relative to the stars. Secular changes in day length are primarily driven by tidal friction, where gravitational interactions between , dissipate as heat in Earth's oceans and solid body. This process slows , lengthening the day at a theoretical rate of 2.3 milliseconds per century based on models of tidal . Observed rates from ancient records and modern measurements, however, average about 1.7 milliseconds per century over the past 2,500 years, with variations attributed to additional geophysical factors like and core-mantle interactions. These changes in day length have compounding effects on measurements of the year, as the —serving as the baseline for seasonal cycles—comprises a fixed physical but an evolving number of mean solar days. The slowing reduces the number of days per tropical year by approximately 9.7 microseconds per century, maintaining the year's absolute duration in uniform time (such as atomic seconds) nearly constant while altering alignments over millennia. This indirect lengthening in year measures, tied to the 1.7 milliseconds per century observed day increase, necessitates periodic adjustments in timekeeping systems to synchronize with astronomical events. The implications of these variations are significant for both historical and future applications. Ancient eclipse records, analyzed through models incorporating tidal slowing, allow reconstruction of past rotation rates, revealing that days were shorter by several hours hundreds of millions of years ago. Looking ahead, the cumulative effect could require refinements to calendars like the Gregorian system within thousands of years to prevent seasonal drift, as the decreasing number of days per year accumulates to shifts of seconds over centuries. Such adjustments highlight the dynamic nature of Earth's rotational and orbital systems under tidal influences.

Longer Astronomical Cycles

Equinoctial Cycle

The equinoctial cycle refers to the approximately 25,772-year period over which Earth's rotational axis completes one full precession, causing the vernal equinox to shift westward along the ecliptic by about 360 degrees relative to the fixed stars. This motion arises from the gravitational torque exerted by the Sun and Moon on Earth's equatorial bulge, resulting in a slow wobble of the axis that traces a cone in space. The cycle shifts the positions of the equinoxes and solstices through the zodiac constellations, moving the vernal equinox from Aries into Pisces around 100 BCE and toward Aquarius in the coming millennia. The primary effects of the equinoctial cycle include long-term changes in the alignment of seasons with stellar backgrounds, as the points of gradually drift relative to constellations, altering the celestial scenery during key astronomical events. This also influences seasonal contrasts by varying the timing of Earth's perihelion relative to solstices over the cycle, contributing to climatic variations such as intensified winters in one hemisphere. Superimposed on this smooth are variations—small, periodic oscillations in the axis orientation with periods of 18.6 years and shorter, primarily driven by the Moon's orbital perturbations around the Earth-Moon barycenter. The equinoctial cycle was discovered around 130 BCE by the Greek astronomer of , who identified the phenomenon by comparing his observations of solstice positions with those from earlier Babylonian records spanning 169 years, revealing a cumulative shift of approximately 2 degrees. Hipparchus quantified the annual rate as about 1 degree per century, laying the foundation for later astronomical models. This discovery highlighted the dynamic nature of Earth's orientation, distinguishing from the fixed stellar sphere assumed in prior Greek astronomy. The is shortened by this precessional effect compared to the .

Galactic Year

The galactic year, also referred to as the cosmic year, represents the of the Solar System around the of the galaxy. This duration measures the time required for and the Sun to complete one full along their nearly circular path in the galactic disk. The length of the galactic year is estimated at approximately 225–250 million years. This value is derived from the Solar System's orbital velocity of about 220–230 km/s relative to the , combined with its average distance of roughly 8 kiloparsecs (26,000 light-years) from that center. The period can be calculated using the formula for orbital motion, where the of the orbit (approximately 2π times the ) is divided by the velocity, yielding the timeframe for one . Estimates of the galactic year rely on observations of the Milky Way's rotation curve, which plots orbital speeds versus distance from and reveals a nearly flat velocity profile indicative of significant influence. Key data come from , particularly 21 cm wavelength emissions from neutral gas, allowing astronomers to map velocities of interstellar clouds via the . The distance to the is further refined through analysis of dynamics, where proper motions and radial velocities of these ancient star systems provide constraints on the galaxy's mass distribution and scale. On cosmic timescales, the contextualizes biological and geological evolution, as the Solar System's position within the shifts gradually over such intervals. For example, the of non-avian dinosaurs around 66 million years ago occurred roughly one-quarter to one-third of a galactic year in the past, highlighting how evolutionary milestones unfold against the backdrop of galactic motion.

Other Extended Cycles

The ecliptic plane, defined by Earth's orbit around the Sun, experiences gradual variations due to gravitational perturbations from other planets, causing its inclination relative to the solar system's invariable plane to oscillate with an amplitude of about 3° over multimillion-year timescales. Such changes contribute to long-term variations in the orientation of the , though the primary remains highly stable. Complementing is the obliquity cycle, part of the Milankovitch orbital variations, where Earth's oscillates between 22.1° and 24.5° with a period of about 41,000 years. These variations in tilt affect the distribution of solar insolation across latitudes, with higher obliquity enhancing seasonal contrasts and lower values diminishing them, thereby linking orbital dynamics to climatic cycles spanning tens of thousands of years. Other Milankovitch cycles include the eccentricity cycle, where Earth's varies between 0.005 and 0.058 over approximately 100,000 years, influencing the overall shape of the orbit and the intensity of seasonal differences. Additionally, the apsidal precession cycle, with a period of about 21,000–23,000 years, causes the point of perihelion to shift relative to the equinoxes, modulating the timing of closest solar approach and contributing to hemispheric asymmetries in insolation. These combined cycles drive long-term fluctuations, including ice ages, over periods of 10,000 to 100,000 years.

Specialized Concepts

Seasonal Year

The seasonal year refers to the time interval between the recurrence of corresponding points in the seasonal cycle, such as from one vernal equinox to the next or one summer solstice to the next, which varies slightly due to Earth's orbital characteristics. Unlike the fixed-length tropical year, which averages these intervals at approximately 365.2422 days, the seasonal year manifests as unequal durations for the four seasons in each hemisphere, primarily because of Earth's axial tilt of 23.4 degrees combined with the elliptical shape of its orbit (eccentricity of about 0.0167). The tilt is responsible for the existence of seasons by varying the amount of sunlight received at different latitudes throughout the year, while the eccentricity causes the planet to move faster in its orbit when closer to the Sun (perihelion) and slower when farther away (aphelion), leading to shorter seasons near perihelion and longer ones near aphelion. In the , this results in a shorter winter of about 89 days and a longer summer of about 93 days, with spring and autumn intermediate at roughly 92 and 89 days, respectively; the configuration is reversed in the , where summer is shorter and winter longer. These lengths are calculated by measuring the intervals between the astronomical events defining the seasons: the vernal equinox marks the start of spring, the begins summer, the autumnal equinox initiates fall, and the starts winter, with precise timings determined from Earth's position relative to the Sun using . The current positioning of perihelion in early —near the 's —accelerates orbital motion during that season, shortening it, while aphelion in early extends Northern summer; this dynamic more significantly affects the 's summer due to the timing alignment. These disparities in seasonal durations have shaped human activities, particularly and cultural festivals, by influencing growing periods and resource availability in each hemisphere. In the Northern Hemisphere, the extended summer facilitates longer crop maturation times, supporting diverse harvests that underpin festivals like , which celebrate abundance at season's end. Conversely, the shorter Southern Hemisphere summer can constrain agricultural yields in regions like and , prompting adapted farming practices.

IUPAC–IUGS Proposal for Year Definition

In 2011, the International Union of Pure and Applied Chemistry (IUPAC) and the (IUGS) jointly recommended a standardized definition of the year as a derived for use in scientific , particularly in physics, chemistry, and geosciences. The proposal designates the year as the Julian year, defined precisely as 365.25 days, with each day consisting of exactly 86,400 SI seconds, yielding a total of 31,557,600 seconds. This fixed duration serves as an invariant reference, independent of . The rationale for this definition stems from the need to eliminate inconsistencies arising from the variable length of the astronomical year, which fluctuates due to factors such as Earth's orbital and tidal interactions. Prior conventions, established in by IUPAC and in 1968 by IUGS, relied on the of 1900 (approximately 31,556,925.9747 seconds), but these differed slightly in application and tied the unit to a specific historical , complicating cross-disciplinary comparisons. By adopting the Julian year, the proposal aligns the unit with the stable SI second, facilitating precise calculations in fields like , isotopic decay, and thermodynamic modeling without requiring adjustments for temporal variations. This recommendation builds on the historical Julian year, originally an average calendar length from the Julian reform, but redefines it strictly in SI terms for modern use. As of 2025, it is the endorsed standard for , with widespread in peer-reviewed publications and international data standards to promote uniformity, though some astronomical contexts retain traditional variants for specific observations.

Notation and Usage

Symbols and Standard Abbreviations

In , the year as a is commonly symbolized by "a", derived from the Latin annum, particularly in astronomy and physics contexts where precision is required. This symbol aligns with recommendations from the (IAU) and is explicitly endorsed in ISO/IEC 80000-3 for denoting calendar cycles equivalent to one . The Unicode representation for "a" is the standard Latin lowercase letter U+0061, ensuring compatibility in digital typesetting without special characters. An alternative symbol frequently used in astronomy is "yr", reflecting common practice in observational data and catalogs to distinguish it from other uses of "a", such as . In physics nomenclature, the International Union of Pure and (IUPAP) also adopts "a" as the general for the year, emphasizing its role in fundamental constants and experimental reporting. Standard abbreviations for the year include "y" in and texts for brevity, though it is informal and requires contextual clarity to avoid confusion with other units like the yard (yd). The "yr" is widely accepted in English-language , often without a period in technical prose. In formal Latin-derived contexts, such as historical or legal documents, "annum" appears fully or abbreviated as part of phrases like per annum. Contextual usage varies: in tables and equations, compact symbols like "a" or "yr" are preferred for space efficiency and alignment with ISO 80000 guidelines, while prose typically spells out "year" to enhance readability and prevent ambiguity, especially in multidisciplinary publications. These notations can be combined with SI prefixes for larger scales, such as "Ga" for giga-annum in geological or cosmological discussions.

SI Prefix Multipliers

SI prefixes, as defined by the International Bureau of Weights and Measures (BIPM), are applied to the year—symbolized as "a" for annum—to express very long or short timescales in scientific disciplines such as , , and . These multipliers follow the standard decimal system for powers of ten, enabling concise notation for durations far beyond or below a single year. For instance, the prefix "kilo-" (k, 10³) forms "ka" for kiloannum, representing 1,000 years, while "mega-" (M, 10⁶) yields "Ma" for megaannum, denoting one million years. This system ensures uniformity in reporting vast temporal scales without relying on cumbersome numerical expansions. The following table summarizes common SI prefixes used with the year unit in :
PrefixSymbolPower of TenNameEquivalent Years
kilo-k10³Kiloannum1,000
mega-M10⁶Megaannum1,000,000
giga-G10⁹Gigaannum1,000,000,000
tera-T10¹²Teraannum1,000,000,000,000
These prefixes are particularly standardized in Earth sciences by organizations like the (USGS), which endorses "ka," "Ma," and "Ga" for stratigraphic dating to avoid ambiguity in historical records. In , such notations pinpoint evolutionary milestones; for example, the mass , which eliminated non-avian dinosaurs, is dated to 66 Ma based on radiometric and stratigraphic evidence from the Chicxulub impact site. In cosmology, gigaannum (Ga) scales the universe's timeline, with the current age estimated at 13.8 Ga from cosmic microwave background measurements by the Planck satellite, providing a benchmark for the hot Big Bang epoch. Smaller prefixes like "milli-" (m, 10⁻³) for milli-annum (0.001 years) appear in precise astrophysical modeling of stellar cycles, though they are less common than larger multipliers. This prefix application enhances interdisciplinary communication by aligning temporal units with the SI framework, prioritizing clarity in data presentation across scales from millennia to eons.

Abbreviations for Historical Time Periods

In historical dating, particularly within archaeology and geology, the abbreviation "BP" stands for "Before Present," where the present is conventionally defined as the year 1950 CE to standardize radiocarbon measurements from the era when the method was developed. This fixed reference point ensures consistency across studies, as it avoids variations from ongoing atomic testing that began altering atmospheric carbon-14 levels post-1950. For example, an event dated to 10,000 BP corresponds approximately to 8050 BCE, calculated by subtracting the BP value from 1950 CE. BP dates are typically derived from radiocarbon (¹⁴C) analysis and require to account for fluctuations in atmospheric over time, converting raw radiocarbon years to years. The International Calibration (IntCal) working group provides standardized curves, such as IntCal20, which extend back to about 55,000 years BP using data from tree rings, corals, and other precisely dated materials to achieve high precision. Calibrated BP dates are often denoted as "cal BP" to distinguish them from uncalibrated ones, ensuring accuracy in chronological reconstructions. For longer timescales, "ka BP" denotes thousands of years before present (with "ka" from the SI prefix "kilo-"), commonly applied in ice core and sediment core dating to describe events spanning millennia. This notation facilitates analysis of paleoclimatic records, such as timelines or sediment deposition rates, where multiple radiocarbon dates from core layers confirm stratigraphic sequences. Uncalibrated versions are specified as "¹⁴C ka BP," while calibrated forms use "cal ka BP" for alignment with calendar timescales. These abbreviations offer a neutral, scientifically oriented framework that simplifies comparisons across global datasets and sidesteps the cultural or religious implications associated with BCE/CE or BC/AD systems. International conventions, including those from IntCal, emphasize calibrated dates for precision, updating older uncalibrated examples to reflect current standards.

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