ⓘ Year


The light-year is a unit of length used to express astronomical distances and measures about 9.46 trillion kilometres or 5.88 trillion miles. As defined by the International Astronomical Union, a light-year is the distance that light travels in vacuum in one Julian year. Because it includes the word "year", the term light-year may be misinterpreted as a unit of time. The light-year is most often used when expressing distances to stars and other distances on a galactic scale, especially in non-specialist and popular science publications. The unit most commonly used in professional astrometr ...

New Year

New Year is the time or day at which a new calendar year begins and the calendars year count increments by one. Many cultures celebrate the event in some manner and the 1st day of January is often marked as a national holiday. In the Gregorian calendar, the most widely used calendar system today, New Year occurs on January 1 New Years Day. This was also the first day of the year in the original Julian calendar and of the Roman calendar after 153 BC. During the Middle Ages in western Europe, while the Julian calendar was still in use, authorities moved New Years Day, depending upon locale, ...

Chinese New Year

Chinese New Year is the Chinese festival that celebrates the beginning of a new year on the traditional Chinese calendar. The festival is commonly referred to as the Spring Festival in China as the spring season in the lunisolar calendar traditionally starts with lichun, the first of the twenty-four solar terms which the festival celebrates around the time of. Marking the end of winter and the beginning of the spring season, observances traditionally take place from New Year’s Eve, the evening preceding the first day of the year to the Lantern Festival, held on the 15th day of the year. Th ...

International Geophysical Year

The International Geophysical Year was an international scientific project that lasted from 1 July 1957 to 31 December 1958. It marked the end of a long period during the Cold War when scientific interchange between East and West had been seriously interrupted. Sixty-seven countries participated in IGY projects, although one notable exception was the mainland Peoples Republic of China, which was protesting against the participation of the Republic of China. East and West agreed to nominate the Belgian Marcel Nicolet as secretary general of the associated international organization. The IGY ...

Islamic New Year

The Islamic New Year, also called the Hijri New Year or Arabic New Year, is the day that marks the beginning of a new Hijri year, and is the day on which the year count is incremented. The first day of the Islamic year is observed by Muslims on the first day of the month of Muharram. The epoch of the Islamic era was set as 622 Common Era, the year of the emigration of Muhammad and his followers from Mecca to Medina, known as the Hijra. All religious duties, such as prayer, fasting in the month of Ramadan, and pilgrimage, and the dates of significant events, such as celebration of holy nigh ...

Old New Year

The Old New Year or the Orthodox New Year is an informal traditional holiday, celebrated as the start of the New Year by the Julian calendar. In the 20th and 21st centuries, the Old New Year falls on January 14 in the Gregorian calendar. The same day is celebrated in India as the sun ends its southward journey and starts moving northward: Thai Pongal. This traditional dating of the New Year is sometimes commonly called "Orthodox" because it harks back to a time when governments in Russia and Eastern Europe used the Julian Calendar, which is still used by some jurisdictions of the Orthodox ...


ⓘ Year

A year is the orbital period of the Earth moving in its orbit around the Sun. Due to the Earths 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.

A calendar year is an approximation of the number of days of the Earths 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; see below. 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. The ISO standard ISO 80000-3, Annex C, supports the symbol a for Latin annus to represent a year of either 365 or 366 days. In English, the abbreviations y and yr are commonly used.

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.


1. Etymology

English year via West Saxon gēar /jɛar, Anglian gēr) continues Proto-Germanic *jǣran *jē₁ran. Cognates are German Jahr, Old High German jār, Old Norse ar 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".

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. A number of English words are derived from Latin annus, such as annual, annuity, anniversary, etc.; per annum means "each year", anno Domini 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 weder, 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 leto "summer; year".

In the International System of Quantities ISO 80000-3, the year symbol, a is defined as either 365 days or 366 days.


2.1. Intercalation 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 365.242 2222 days, close to the length of the mean tropical year, 365.242 19 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.


2.2. Intercalation 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 is 365.242 5 days; with a relative error below one ppm 8 10 −7 relative to the current length of the mean tropical year 365.242 19 days and even closer to the current March equinox year of 365.242 374 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 Earths rotation and the associated lengthening of the day.


2.3. Intercalation Other calendars

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.


3. Year numbering

A calendar era assigns a cardinal number to each sequential year, using a reference point in the past as the beginning of the era.

The worldwide standard is the Anno Domini, although some prefer the term Common Era because it has no explicit reference to Christianity. It was introduced in the 6th century and was intended to count years from the nativity of Jesus.

The Anno Domini era is given the Latin abbreviation AD for Anno Domini "in the year of the Lord", or alternatively CE for "Common Era". Years before AD 1 are abbreviated BC for Before Christ or alternatively BCE for Before the Common Era. Year numbers are based on inclusive counting, so that there is no "year zero". In the modern alternative reckoning of Astronomical year numbering, positive numbers indicate years AD, the number 0 designates 1 BC, −1 designates 2 BC, and so on.


4.1. Pragmatic divisions 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.


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

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.


5. Astronomical years

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.


5.1. Astronomical years 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" symbol "a" from the Latin annus used in various scientific contexts. The Julian century of 36 525 days and the Julian millennium of 365 250 days are used in astronomical calculations. Fundamentally, expressing a time interval in Julian years is a way to precisely specify how many days not how many "real" years, for long time intervals where stating the number of days would be unwieldy and unintuitive. By convention, the Julian year is used in the computation of the distance covered by a light-year.

In the Unified Code for Units of Measure, the symbol, a without subscript, always refers to the Julian year, a j, of exactly 31 557 600 seconds.

365.25 days of 86 400 seconds = 1 a = 1 a j = 31.5576 Ms

The SI multiplier prefixes may be applied to it to form ka kiloannus, Ma megaannus, etc.


5.2. Astronomical years Sidereal, tropical, and anomalistic years

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. Its average duration is 365.256 363 004 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 Suns ecliptic longitude is measured with respect to the equinox, the tropical year comprises a complete cycle of the seasons; because of the biological and socio-economic importance of the seasons, the tropical year is the basis of most calendars. 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 Earths 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 days of 86400 SI seconds

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.


5.3. Astronomical years Draconic year

The draconic year, draconitic year, eclipse year, or ecliptic year is the time taken for the Sun as seen from the Earth to complete one revolution with respect to the same lunar node a point where the Moons 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

346.620 075 883 days 346 d 14 h 52 min 54 s at the epoch J2000.0.

This term is sometimes erroneously used for the draconic or nodal period of lunar precession, that is the period of a complete revolution of the Moons ascending node around the ecliptic: 18.612 815 932 Julian years 6 798.331 019 days; at the epoch J2000.0.


5.4. Astronomical years 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 Moons 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:

411.784 430 29 days 411 days 18 hours 49 minutes 35 seconds at the epoch J2000.0.


5.5. Astronomical years 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. It is still used by many Zoroastrian communities.


5.6. Astronomical years 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.


5.7. Astronomical years 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.


5.8. Astronomical years 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 Earths mean distance. Its length is:

365.256 8983 days 365 d 6 h 9 min 56 s.

5.9. Astronomical years 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 date TT − 2 415 020.313 52 / 365.242 198 781

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


5.10. Astronomical years Variation in the length of the year and the day

The exact length of an astronomical year changes over time.

  • Each planets 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.
  • The positions of the equinox and solstice points with respect to the apsides of Earths 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, Earths 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.
  • 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.

5.11. Astronomical years 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.


5.12. Astronomical years Summary

An average Gregorian year is 365.2425 days. For this calendar, a common year is 365 days 8760 hours, 525 600 minutes or 31 536 000 seconds, and a leap year is 366 days 8784 hours, 527 040 minutes or 31 622 400 seconds. The 400-year cycle of the Gregorian calendar has 146 097 days and hence exactly 20 871 weeks.


6. "Greater" astronomical years

Equinoctial cycle

The Great Year, or equinoctial cycle, corresponds to a complete revolution of the equinoxes around the ecliptic. Its length is about 25.700 years.

Galactic year

The Galactic year is the time it takes Earths Solar System to revolve once around the galactic center. It comprises roughly 230 million Earth years.


7. Seasonal year

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.


8. Symbols

In the International System of Quantities the symbol for the year as a unit of time is a, taken from the Latin word annus.

In English, the abbreviations "y" or "yr" are more commonly used in non-scientific literature, but also specifically in geology and paleontology, where "kyr, myr, byr" and similar abbreviations are used to denote intervals of time remote from the present.


8.1. Symbols Symbol

NIST SP811 and ISO 80000-3:2006 support the symbol a as the unit of time for a year. In English, the abbreviations y and yr are also used.

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

a t = 365.242 19 days for the mean tropical year; a j = 365.25 days for the mean Julian year; a g = 365.2425 days for the mean Gregorian year;


a, without a qualifier = 1 a j ; and, ar for are, is a unit of area.

The International Union of Pure and Applied Chemistry IUPAC and the International Union of Geological Sciences have jointly recommended defining the annus, with symbol a, as the length of the tropical year in the year 2000:

a = 31 556 925.445 seconds approximately 365.242 192 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. 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.


8.2. Symbols SI prefix multipliers

For the following, there are alternative forms which 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.

  • Ea for exaannum – a unit of time equal to 10 18 years, or one quintillion years. The half-life of tungsten-180 is 1.8 Ea.
  • Ga for gigaannum – a unit of time equal to 10 9 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 universe is approximately 13.8 Ga.
  • Pa for petaannum – a unit of time equal to 10 15 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.
  • Ma for megaannum – a unit of time equal to one million, or 10 6, 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.
  • ka for kiloannum – a unit of time equal to one thousand, or 10 3, 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: e.g. ice core dating, dendrochronology, uranium-thorium dating, or varve analysis; is used as the primary dating 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.
  • Ta for teraannum – a unit of time equal to 10 12 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.

8.3. Symbols Abbreviations yr and ya

In astronomy, geology, and paleontology, the abbreviation yr for years and ya for years ago are sometimes used, combined with prefixes for thousand, million, or billion. 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:

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.