A year is a unit of time based on how long it takes the Earth to orbit the Sun.OED, s.v. "year". 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 phasessee 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.OED, s.v. "year", entry 2.b.: "transf. Applied to a very long period or cycle (in chronology or mythology, or vaguely in poetic use)."
Calendar year A calendar year is most often an approximation of the number of days of the Earth's orbital period, as counted in a given calendar. The modern world standard calendar (the Gregorian 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 and the Solar Hijri calendar. 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). Lunisolar calendars, traditionally used in Asia, have a year of twelve lunar months but add ("intercalate") a thirteenth leap month every three years or so. The lunar Hijri calendar (known commonly in the West as "the Islamic calendar", although it is not the only one) has a year of twelve lunar months and ignores the solar year.
Abbreviation In English, the unit of time for year is commonly abbreviated as "y" or "yr". The symbol "a" (for , year) is sometimes used in scientific literature, though its exact duration may be inconsistent.
Etymology English year (via West Saxon ġēar (), 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 "year, season". Cognates also descended from the same Proto-Indo-European noun (with variation in suffix ablaut) are Avestan yārǝ "year", Greek () "year, season, period of time" (whence "hour"), Old Church Slavonic jarŭ, and Latin hornus "of this year".
Latin (a 2nd declension masculine noun; is the accusative singular; is genitive singular and nominative plural; the dative and ablative singular) is from a PIE noun , 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 , such as annual, annuity, anniversary, etc.; per annum means "each year", means "in the year of the Lord".
The Greek word for "year", , is cognate with Latin vetus "old", from the PIE word *wetos- "year", also preserved in this meaning in Sanskrit "year" and "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 Astronomical years do not have an integer number of days or lunar months. Any calendar that follows an astronomical year must have a system of intercalation such as leap years.
Julian calendar In the Julian calendar, the average (mean) length of a year is 365.25 days. In a non-leap year, there are 365 days, in a leap year there are 366 days. A leap year occurs every fourth year 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 seconds (SI base unit), totaling exactly 31,557,600 seconds in the Julian astronomical year.
Revised Julian calendar 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 days, close to the length of the mean tropical year, days (relative error of 9·10−8). In the year 2800 CE, the Gregorian and Revised Julian calendars will begin to differ by one calendar day.
Gregorian calendar The Gregorian calendar 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. Because 97 out of 400 years are leap years, the mean length of the Gregorian calendar year is days, with a relative error below one ppm (9·10−7) relative to the current length of the mean tropical year ( days) and even closer to the current March equinox year of days that it aims to match.
Other calendars
Historically, lunisolar calendars intercalated entire leap months on an observational basis. (Examples include the Chinese calendar and similar East and Southeast Asian calendars, the Hebrew calendar, and the various Hindu calendars). Lunisolar calendars have mostly fallen out of use as civil calendar but continue to be used for liturgical reasons.
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, as opposed to using an algorithmic system of leap years.
Year numbering A calendar era assigns a cardinal number to each sequential year, using a reference event in the past (called the epoch) as the beginning of the era.
The Gregorian calendar era is the world's most widely used civil calendar. Its epoch is a 6th century estimate of the date 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 or 0000 designates 1 BC/BCE, −1 or -0001 designates 2 BC/BCE, and so on.
Other eras include that of Ancient Rome, ("from the foundation of the city), abbreviated AUC, and ("year of the world"), used for the Hebrew calendar and abbreviated AM.
Most calendars count years in two eras, one before and one after the epoch. The Japanese imperial eras is the only system still in use that has many eras, adding one with every new tenno.
The Islamic Hijri era, , abbreviated AH, with the Hijrah as its epoch is used primarily with Islamic lunar calendars of twelve lunar months, but also with solar calendars whose year is about 11 days longer and therefore increment the year number less often, resulting in a difference of currently 42 to 43.
Pragmatic divisions Financial and scientific calculations often use a 365-day calendar to simplify daily rates.
Fiscal year
A fiscal year or financial year is a 12-month period used for calculating annual financial statements in businesses and other organizations. In many jurisdictions, regulations regarding accounting require such reports once per twelve months, but do not require that the twelve months constitute a calendar year.
For example, in Canada and India, the fiscal year runs from April 1; in the United Kingdom, it runs from April 1 for purposes of corporation tax and government financial statements, but from April 6 for purposes of personal taxation and payment of state benefits; in Australia, it runs from July 1; while in the United States, the fiscal year of the federal government runs from October 1.
Academic year
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.
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.
Astronomical years
Julian year