Year
The year is a unit of time based on the roughly 3651/4 days taken by the Earth to revolve around the Sun.
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.
The symbol "a" (for Latin: annus, year) is sometimes used in scientific literature, though its exact duration may be inconsistent.
[citation needed] English year (via West Saxon ġēar (/jɛar/), Anglian ġēr) continues Proto-Germanic *jǣran (*jē₁ran).
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.
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".
Historically, lunisolar calendars intercalated entire leap months on an observational basis.
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.
At some, a shortened summer session, sometimes considered part of the regular academic year, is attended by students on a voluntary or elective basis.
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).
[citation needed] Schools and universities in Australia typically have academic years that roughly align with the calendar year (i.e., starting in February or March and ending in October to December), as the southern hemisphere experiences summer from December to February.
The 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.
The mean tropical year is approximately 365 days, 5 hours, 48 minutes, 45 seconds, using the modern definition[13] (= 365.24219 d × 86400 s).
The average duration of the eclipse year is 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).
This period is associated with the apparent size of the full moon, and also with the varying duration of the synodic month.
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 vague year was used in the calendars of Ethiopia, Ancient Egypt, Iran, Armenia and in Mesoamerica among the Aztecs and Maya.
It differs from the sidereal year for stars away from the ecliptic due mainly to the precession of the equinoxes.
[17][18][19][20] 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.
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.
The Great Year, or equinoctial cycle, corresponds to a complete revolution of the equinoxes around the ecliptic.
[25] 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.
[26][27] In astronomy the abbreviations kyr, Myr and Gyr are in common use for kiloyears, megayears and gigayears.
[32] 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),[33] also noted in the IUPAC Gold Book.
[26][42] In archaeology, dealing with more recent periods, normally expressed dates, e.g. "10,000 BC", may be used as a more traditional form than Before Present ("BP").
