A year (from Old English ȝēr) is the orbital period of the Earth moving around the Sun. For an observer on Earth, this corresponds to the period it takes the Sun to complete one course throughout the zodiac along the ecliptic.
There is no universally accepted symbol for the year as a unit of time. The International System of Units does not propose one. A common abbreviation in international use is a (for Latin annus), in English also y or yr.
Due the Earth's axial tilt, the course of a year sees the passing of the seasons, marked by changes in weather, and hours of daylight, and consequently vegetation and fertility. In temperate and subpolar regions, generally four seasons are recognized: spring, summer, autumn and winter, astronomically marked by the Sun reaching the points of equinox and solstice, although the climatic seasons lags behind ther astronomical markers. In some tropical and subtropical regions it is more common to speak of the rainy (or wet, or monsoon) season versus the dry season.
A calendar year is an approximation of the Earth's orbital period in a given calendar. A calendar year in the Gregorian calendar (as well as in the Julian calendar) has either 365 (common years) or 366 (leap years) days.
The word "year" is also used of periods loosely associated but not strictly identical with either the astronomical or the calendar year, such as the seasonal year, the fiscal year or the academic year, etc. By extension, the term year can mean the orbital period of any planet: for example, a "Martian year" is the time in which Mars completes its own orbit. The term is also applied more broadly to any long period or cycle, such as the Platonic "Great Year".
West Saxon ȝear, Anglian ȝēr continues Proto-Germanic *jǣram (*jē2ram). Cognates are German Jahr, Old High German jar, Old Norse ár and Gothic jer, all from a PIE *yērom "year, season". Cognates outside of Germanic are Avestan yare "year", Greek ὥρα "year, season, period of time" (whence "hour"), Old Church Slavonic jaru and Latin hornus "of this year".
Latin Annus (a 2nd declension masculine noun; annum is the accusative singular; anni is genitive singular and nominative plural; anno the locative singular) is from a PIE noun *at-no-, which also yielded Gothic aþnam "year".
Both *yē-ro- and *at-no- are based on verbal roots expressing movement, *at- and *ey- respectively, both meaning "to go" generally.
The Greek word for "year", ἔτος, is cognate to Latin vetus "old", from PIE *wetus- "year" , also preserved in this meaning in Sanskrit vat-sa- "yearling (calf)".
A seasonal year is the time between successive recurrences of a seasonal event such as the flooding of a river, the migration of a species of bird, the flowering of a species of plant, the first frost, or the first scheduled game of a certain sport. All of these events can have wide variations of more than a month from year to year.
A half year (one half of a year) may run from January to June, or July to December.
No astronomical year has an integer number of days or lunar months, so any calendar that follows an astronomical year must have a system of intercalation such as leap years. Financial and scientific calculations often use a 365-day calendar to simplify daily rates.
In the Julian calendar, the average 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 4 years.
The Gregorian calendar attempts to keep the vernal equinox on or soon before March 21; hence it follows the vernal equinox year. The average length of this calendar's year is 365.2425 mean solar days (as 97 out of 400 years are leap years); this is within one ppm of the current length of the mean tropical year (365.24219 days). The vernal equinox is estimated to fall back in the Gregorian calendar by one day by the year 4000 not because of this difference but because of the slowing down of the Earth's rotation and the associated lengthening of the sidereal day.
The Persian calendar, in use in Afghanistan and Iran, has its year begin on the day of the vernal equinox as determined by astronomical computation (for the time zone of Tehran), as opposed to using an algorithmic system of leap years.
A calendar era is used to assign a number to individual years, using a reference point in the past as the beginning of the era. In many countries, the most common era is from the estimated date of the birth of Jesus Christ; dates in this era are designated anno Domini ("in the year of the Lord", abbreviated A.D.) or, more neutrally, C.E. (common era). Other eras are also used to enumerate the years in different cultural, religious or scientific contexts.
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, the federal government of the U.S. has a fiscal year that starts on October 1 instead of January 1. In India the fiscal year is between April 1 and March 31. In the United Kingdom and Canada, the financial year runs from April 6 and April 1 respectively, and in Australia it runs from July 1.
An academic year refers to the annual period during which a student attends school, college or university.
The school year can be divided up in various ways, two of which are most common in North American educational systems.
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 36525 days and the Julian millennium of 365250 days are used in astronomical calculations. Fundamentally, expressing a time interval in Julian years is a way to precisely specify how many days (not how many "real" years), for long time intervals where stating the number of days would be unwieldy and unintuitive. By convention, the Julian year is used in the computation of the distance covered by a light-year.
365.25 days of 86400 seconds = 1 a = 1 aj = 31.5576 Ms
The SI multiplier prefixes may be applied to it to form ka (kiloannum), Ma (megaannum) etc.
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 duration in SI days of 86,400 SI seconds each is on average:
The tropical year is the time taken for the Earth to complete one revolution with respect to the framework provided by the intersection of the ecliptic (the plane of the orbit of the Earth) and the plane of the equator (the plane perpendicular to the rotation axis of the Earth). The exact length of a tropical year slightly depends on the chosen starting point: for example the vernal equinox year is the time between successive vernal equinoxes. The mean tropical year (averaged over all ecliptic points) is:
The tropical year is shorter than the sidereal year because of the precession of the equinoxes.
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 3 in 2010), and the aphelion, where the Earth is farthest from the Sun (July 6 in 2010). The anomalistic year is usually defined as the time between two successive perihelion passages. Its average duration is:
The anomalistic year is slightly longer than the sidereal year because of the precession of the apsides (also known as anomalistic precession, orbital precession, and, perihelion precession.)
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). This period 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:
This term is sometimes erroneously used to designate the draconic or nodal period of lunar precession, that is the time it takes for a complete revolution of the Moon's ascending node around the ecliptic: 18.612 815 932 Julian years (6798.331 019 days; at the epoch J2000.0).
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:
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.
The vague year is an integral approximation to the seasonal year equaling 365 days. 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 Ancient Egypt, Iran, Armenia and in Mesoamerica among the Aztecs and Maya.
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. (To visualise: the constellation Crux, which rose and set as seen from the Mediterranean in ancient Greek times, is never above the horizon in current times.)
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:
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 1 January. It is named after the 19th century German astronomer and mathematician Friedrich Bessel. An approximate formula to compute the current time in Besselian years from the Julian day is:
The exact length of an astronomical year changes over time. The main sources of this change are:
the Muslim calendar.
A common year is 365 days = 8,760 hours = 525,600 minutes = 31,536,000 seconds.
A leap year is 366 days = 8,784 hours = 527,040 minutes = 31,622,400 seconds.
The 400-year cycle of the Gregorian calendar has 146,097 days and hence exactly 20,871 weeks.
There is no universally accepted symbol for the year as a unit of time. The International System of Units does not propose one. NIST SP811 and ISO 80000-3:2006 suggest the symbol a, generally read as the Latin annus or annum. In English, the abbreviations y or yr are sometimes used, specifically in geology and paleontology, where kyr, myr, byr (thousands, millions and billions of years, respectively) and similar abbreviations remain in use to denote intervals of time remote from the present .
NIST SP811 and ISO 80000-3:2006 suggest the symbol a (in the International System of Units, a is also the symbol for the unit of area called the "are", but context is usually enough to disambiguate). In English, the abbreviations y and yr are also used.  
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 English year. These abbreviations include:
|SI-prefixed equivalent||order of magnitude|
|tya or kya||"ka ago"|
|bya or gya||"Ga ago"|
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.
The Great year, Platonic year, or Equinoctial cycle corresponds to a complete revolution of the equinoxes around the ecliptic. Its length is about 25,700 years, and cannot be determined precisely as the precession speed is variable.
|From Poems of Cheer (1910)|
What can be said in New Year rhymes,
That's not been said a thousand times?
The new years come, the old years go,
We know we dream, we dream we know.
We rise up laughing with the light,
We lie down weeping with the night.
We hug the world until it stings,
We curse it then and sigh for wings.
We live, we love, we woo, we wed,
We wreathe our brides, we sheet our dead.
We laugh, we weep, we hope, we fear,
And that's the burden of the year.
|This work is in the public domain in
the United States because it was published before
January 1, 1923.
The author died in 1919, so this work is also in the public domain in countries and areas where the copyright term is the author's life plus 80 years or less. This work may also be in the public domain in countries and areas with longer native copyright terms that apply the rule of the shorter term to foreign works.
Among the ancient Egyptians the year consisted of twelve months of thirty days each, with five days added to make it a complete revolution of the earth round the sun. The Jews reckoned the year in two ways, (1) according to a sacred calendar, in which the year began about the time of the vernal equinox, with the month Abib; and (2) according to a civil calendar, in which the year began about the time of the autumnal equinox, with the month Nisan. The month Tisri is now the beginning of the Jewish year.
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One year is about 365 days long (except in a Leap year). It is the time it takes the Earth to go completely around (Orbit) the sun once. A year is actually 365¼ days long, but a calendar has 365 days, except in a leap year.
There are several ways used to measure the length of a year.
Solar and lunar years are used by different societies in daily life. The other measurements are used by astronomers.
|Months of the Year|