A year or annus is the
orbital period of a planetary body, for example, the
Earth, moving in
its orbit around the
Sun. Due to the Earth's
axial tilt, the course of a year sees the passing of the
seasons, marked by change in
weather, the hours of
daylight
Daylight is the combination of all direct and indirect sunlight during the daytime. This includes direct sunlight, diffuse sky radiation, and (often) both of these reflected by Earth and terrestrial objects, like landforms and buildings. Sunligh ...
, and, consequently,
vegetation and
soil fertility
Soil fertility refers to the ability of soil to sustain agricultural plant growth, i.e. to provide plant habitat and result in sustained and consistent yields of high quality. . In
temperate and
subpolar regions around the planet, four seasons are generally recognized:
spring
Spring(s) may refer to:
Common uses
* Spring (season), a season of the year
* Spring (device), a mechanical device that stores energy
* Spring (hydrology), a natural source of water
* Spring (mathematics), a geometric surface in the shape of a ...
,
summer
Summer is the hottest of the four temperate seasons, occurring after spring and before autumn. At or centred on the summer solstice, the earliest sunrise and latest sunset occurs, daylight hours are longest and dark hours are shortest, wit ...
,
autumn
Autumn, also known as fall in American English and Canadian English, is one of the four temperate seasons on Earth. Outside the tropics, autumn marks the transition from summer to winter, in September ( Northern Hemisphere) or March ( Sou ...
and
winter. In
tropical and
subtropical
The subtropical zones or subtropics are geographical zone, geographical and Köppen climate classification, climate zones to the Northern Hemisphere, north and Southern Hemisphere, south of the tropics. Geographically part of the Geographical z ...
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
Generally speaking, a calendar year begins on the New Year's Day of the given calendar system and ends on the day before the following New Year's Day, and thus consists of a whole number of days. A year can also be measured by starting on any o ...
is an approximation of the number of days of the Earth's orbital period, as counted in a given
calendar
A calendar is a system of organizing days. This is done by giving names to periods of time, typically days, weeks, months and years. A date is the designation of a single and specific day within such a system. A calendar is also a physi ...
. The
Gregorian calendar, or modern calendar, presents its calendar year to be either a
common year
A common year is a calendar year with 365 days, as distinguished from a leap year, which has 366. More generally, a common year is one without intercalation. The Gregorian calendar (like the earlier Julian calendar) employs both common years ...
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).
In English, the
unit of time for year is commonly abbreviated as "y" or "yr". The symbol "a" is more common in scientific literature, though its exact duration may be inconsistent.
In astronomy, the
Julian year is a unit of time defined as 365.25
day
A day is the time period of a full rotation of the Earth with respect to the Sun. On average, this is 24 hours, 1440 minutes, or 86,400 seconds. In everyday life, the word "day" often refers to a solar day, which is the length between two so ...
s of exactly 86,400
second
The second (symbol: s) is the unit of time in the International System of Units (SI), historically defined as of a day – this factor derived from the division of the day first into 24 hours, then to 60 minutes and finally to 60 seconds ...
s (
SI base unit
The SI base units are the standard units of measurement defined by the International System of Units (SI) for the seven base quantities of what is now known as the International System of Quantities: they are notably a basic set from which all ...
), 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 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, or the flowering of a species of plant.
The need for farmers to predict seasonal events led to th ...
, the
fiscal year, the
academic year
An academic year or school year is a period of time which schools, colleges and universities use to measure a quantity of study.
School holiday
School holidays (also referred to as vacations, breaks, and recess) are the periods during which sch ...
, etc. Similarly, ''year'' can mean the orbital period of any
planet; for example, a
Martian year
Though no standard exists, numerous calendars and other timekeeping approaches have been proposed for the planet Mars. The most commonly seen in the scientific literature denotes the time of year as the number of degrees from the northern vernal ...
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.
Etymology
English ''year'' (via
West Saxon ''ġēar'' (),
Anglian ''ġēr'') continues
Proto-Germanic ''*jǣran'' (''*j
ē₁ran''). Cognates are
German
German(s) may refer to:
* Germany (of or related to)
**Germania (historical use)
* Germans, citizens of Germany, people of German ancestry, or native speakers of the German language
** For citizens of Germany, see also German nationality law
**Ger ...
''Jahr'',
Old High German ''jār'',
Old Norse ''ár'' and
Gothic
Gothic or Gothics may refer to:
People and languages
*Goths or Gothic people, the ethnonym of a group of East Germanic tribes
**Gothic language, an extinct East Germanic language spoken by the Goths
**Crimean Gothic, the Gothic language spoken b ...
''
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
Avestan (), or historically Zend, is an umbrella term for two Old Iranian languages: Old Avestan (spoken in the 2nd millennium BCE) and Younger Avestan (spoken in the 1st millennium BCE). They are known only from their conjoined use as the scrip ...
''yārǝ'' "year",
Greek () "year, season, period of time" (whence "
hour
An hour (symbol: h; also abbreviated hr) is a unit of time conventionally reckoned as of a day and scientifically reckoned between 3,599 and 3,601 seconds, depending on the speed of Earth's rotation. There are 60 minutes in an hour, and 24 ho ...
"),
Old Church Slavonic
Old Church Slavonic or Old Slavonic () was the first Slavic languages, Slavic literary language.
Historians credit the 9th-century Byzantine Empire, Byzantine missionaries Saints Cyril and Methodius with Standard language, standardizing the lan ...
''jarŭ'', and
Latin ''hornus'' "of this year".
Latin (a
2nd declension The second declension is a category of nouns in Latin and Greek with similar case formation. In particular, these nouns are thematic, with an original ''o'' in most of their forms. In Classical Latin, the short ''o'' of the nominative and accusativ ...
masculine noun; is the
accusative singular; is genitive singular and nominative plural; the dative and ablative singular) is from a
PIE
A pie is a baked dish which is usually made of a pastry dough casing that contains a filling of various sweet or savoury ingredients. Sweet pies may be filled with fruit (as in an apple pie), nuts ( pecan pie), brown sugar ( sugar pie), swe ...
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
An anniversary is the date on which an event took place or an institution was founded in a previous year, and may also refer to the commemoration or celebration of that event. The word was first used for Catholic feasts to commemorate saints ...
, 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
Intercalation may refer to:
* Intercalation (chemistry), insertion of a molecule (or ion) into layered solids such as graphite
*Intercalation (timekeeping), insertion of a leap day, week or month into some calendar years to make the calendar foll ...
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, 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 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 attempts to cause the
northward equinox
The March equinox or northward equinox is the equinox on the Earth when the subsolar point appears to leave the Southern Hemisphere and cross the celestial equator, heading northward as seen from Earth. The March equinox is known as the vern ...
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 days; with a relative error below one
ppm (8·10
−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. 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 Earth's rotation and the associated lengthening of the day.
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 calendar
The Hindu calendar, Panchanga () or Panjika is one of various lunisolar calendars that are traditionally used in the Indian subcontinent and Southeast Asia, with further regional variations for social and Hindu religious purposes. They adopt a s ...
s).
A modern adaptation of the historical
Jalali calendar, known as the
Solar Hijri calendar (1925), is a purely
solar calendar
A solar calendar is a calendar whose dates indicate the season or almost equivalently the apparent position of the Sun relative to the stars. The Gregorian calendar, widely accepted as a standard in the world, is an example of a solar calendar.
T ...
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 Spring equinox or vernal equinox or variations may refer to:
* March equinox, the spring equinox in the Northern Hemisphere
* September equinox, the spring equinox in the Southern Hemisphere
Other uses
* Nowruz, Persian/Iranian new year which be ...
(for the time zone of
Tehran), 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
The civil calendar is the calendar, or possibly one of several calendars, used within a country for civil, official, or administrative purposes. The civil calendar is almost always used for general purposes by people and private organizations.
Th ...
. Its epoch is a
6th century estimate of the date of birth of
Jesus of Nazareth
Jesus, likely from he, יֵשׁוּעַ, translit=Yēšūaʿ, label=Hebrew/Aramaic ( AD 30 or 33), also referred to as Jesus Christ or Jesus of Nazareth (among other names and titles), was a first-century Jewish preacher and religious ...
. 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, ("from the foundation of
the city), abbreviated AUC; ("year of the world"), used for the
Hebrew calendar and abbreviated AM; and the Japanese emperor eras described above. The Islamic
Hijri year, (year of the
Hijrah, abbreviated AH), is a
lunar calendar of twelve
lunar months and thus is shorter than a solar year.
Pragmatic divisions
Financial and scientific calculations often use a
365-day calendar A 365-day calendar consists of exactly 365 days per year (in common years), and is primarily used in computer models and as an assumption in every-day calculations. For example, a calculation of a daily rate may use an annual total divided by exactl ...
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
Australia, officially the Commonwealth of Australia, is a Sovereign state, sovereign country comprising the mainland of the Australia (continent), Australian continent, the island of Tasmania, and numerous List of islands of Australia, sma ...
it runs from July 1; while in the
United States the fiscal year of the
federal government
A federation (also known as a federal state) is a political entity characterized by a union of partially self-governing provinces, states, or other regions under a central federal government (federalism). In a federation, the self-governin ...
runs from October 1.
Academic year
An academic year is the annual period during which a student attends an
educational institution
An educational institution is a place where people of different ages gain an education, including preschools, childcare, primary-elementary schools, secondary-high schools, and universities. They provide a large variety of learning environments an ...
. The academic year may be divided into
academic term
An academic term (or simply term) is a portion of an academic year, the time during which an educational institution holds classes. The schedules adopted vary widely.
In most countries, the academic year begins in late summer or early autumn and ...
s, 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, 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 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
The Boston Latin School is a public exam school in Boston, Massachusetts. It was established on April 23, 1635, making it both the oldest public school in the British America and the oldest existing school in the United States. Its curriculum f ...
, 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
The Julian year, as used in astronomy and other sciences, is a time unit defined as exactly 365.25 days of 86,400
SI seconds each ("
ephemeris days"). This is the normal meaning of the unit "year" used in various scientific contexts. The Julian century of ephemeris days and the Julian millennium of 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
A light-year, alternatively spelled light year, is a large unit of length used to express astronomical distances and is equivalent to about 9.46 trillion kilometers (), or 5.88 trillion miles ().One trillion here is taken to be 1012 ...
.
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
The International Union of Geological Sciences (IUGS) is an international non-governmental organization devoted to international cooperation in the field of geology.
About
The IUGS was founded in 1961 and is a Scientific Union member of the Inte ...
, see below), the symbol ''a'' (without subscript) always refers to the Julian year, ''a
j'', of exactly
second
The second (symbol: s) is the unit of time in the International System of Units (SI), historically defined as of a day – this factor derived from the division of the day first into 24 hours, then to 60 minutes and finally to 60 seconds ...
s.
:365.25 d × s = 1 a = 1 a
j =
Ms
The
SI multiplier prefixes may be applied to it to form "ka", "Ma", etc.
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 (such as the fixed stars, Latin , singular ). Its average duration is days (365 d 6 h 9 min 9.76 s) (at the epoch
J2000.0
In astronomy, an epoch or reference epoch is a moment in time used as a reference point for some time-varying astronomical quantity. It is useful for the celestial coordinates or orbital elements of a celestial body, as they are subject to pertu ...
= 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 Sun's ecliptic longitude is measured with respect to the equinox, the tropical year comprises a complete cycle of the seasons and is the basis of
solar calendar
A solar calendar is a calendar whose dates indicate the season or almost equivalently the apparent position of the Sun relative to the stars. The Gregorian calendar, widely accepted as a standard in the world, is an example of a solar calendar.
T ...
s 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 ( = 365.24219 d × 86 400 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
An apsis (; ) is the farthest or nearest point in the orbit of a planetary body about its primary body. For example, the apsides of the Earth are called the aphelion and perihelion.
General description
There are two apsides in any ellip ...
. 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).
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 lunar node is either of the two orbital nodes of the Moon, that is, the two points at which the orbit of the Moon intersects the ecliptic. The ''ascending'' (or ''north'') node is where the Moon moves into the northern ecliptic hemisphere, w ...
(a point where the Moon's orbit intersects the ecliptic). The year is associated with
eclipse
An eclipse is an astronomical event that occurs when an astronomical object or spacecraft is temporarily obscured, by passing into the shadow of another body or by having another body pass between it and the viewer. This alignment of three ce ...
s: 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
: 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
Lunar precession is a term used for three different precession motions related to the Moon.
First, it can refer to change in orientation of the lunar rotational axis with respect to a reference plane, following the normal rules of precession foll ...
, that is the period of a complete revolution of the Moon's ascending node around the ecliptic: Julian years ( days; at the epoch J2000.0).
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 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:
: days (411 days 18 hours 49 minutes 35 seconds) (at the epoch J2000.0).
Lunar year
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 celebrating their
Eids and for marking the start of the fasting month of
Ramadan
, type = islam
, longtype = Religious
, image = Ramadan montage.jpg
, caption=From top, left to right: A crescent moon over Sarıçam, Turkey, marking the beginning of the Islamic month of Ramadan. Ramadan Quran reading in Bandar Torkaman, Iran. ...
. A Muslim calendar year is based on the lunar cycle. The
Jewish calendar is also essentially lunar, except that an intercalary lunar month is added once every two or three years, in order to keep the calendar synchronized with the solar cycle as well. Thus, a lunar year on the Jewish (Hebrew) calendar consists of either twelve or thirteen lunar months.
Vague year
The vague year, from 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
The intercalary month or epagomenal days. of the ancient Egyptian, Coptic, and Ethiopian calendars are a period of five days in common years and six days in leap years in addition to those calendars' 12 standard months, sometimes reckoned as thei ...
days. The vague year was used in the calendars of
Ethiopia,
Ancient Egypt,
Iran,
Armenia and in
Mesoamerica among the
Aztecs
The Aztecs () were a Mesoamerican culture that flourished in central Mexico in the post-classic period from 1300 to 1521. The Aztec people included different Indigenous peoples of Mexico, ethnic groups of central Mexico, particularly those g ...
and
Maya. It is still used by many Zoroastrian communities.
Heliacal year
A heliacal year is the interval between the
heliacal rising
The heliacal rising ( ) or star rise of a star occurs annually, or the similar phenomenon of a planet, when it first becomes visible above the eastern horizon at dawn just before sunrise (thus becoming "the morning star") after a complete orbit of ...
s of a star. It differs from the
sidereal year
A sidereal year (, ; ), also called a sidereal orbital period, is the time that Earth or another planetary body takes to orbit the Sun once with respect to the fixed stars.
Hence, for Earth, it is also the time taken for the Sun to return to the ...
for stars away from the
ecliptic due mainly to the
precession of the equinoxes.
Sothic year
The
Sothic year
The Sothic cycle or Canicular period is a period of 1,461 Egyptian civil years of 365 days each or 1,460 Julian years averaging 365¼ days each. During a Sothic cycle, the 365-day year loses enough time that the start of its ...
is the interval between heliacal risings of the star
Sirius. It is currently less than the
sidereal year
A sidereal year (, ; ), also called a sidereal orbital period, is the time that Earth or another planetary body takes to orbit the Sun once with respect to the fixed stars.
Hence, for Earth, it is also the time taken for the Sun to return to the ...
and its duration is very close to the Julian year of 365.25 days.
Gaussian year
The
Gaussian year
A Gaussian year is defined as 365.2568983 days. It was adopted by Carl Friedrich Gauss as the length of the sidereal year in his studies of the dynamics of the solar system.
A slightly different value is now accepted as the length of the sidereal y ...
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:
: days (365 d 6 h 9 min 56 s).
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 − ) /
The TT subscript indicates that for this formula, the Julian date should use the
Terrestrial Time
Terrestrial Time (TT) is a modern astronomical time standard defined by the International Astronomical Union, primarily for time-measurements of astronomical observations made from the surface of Earth.
For example, the Astronomical Almanac uses T ...
scale, or its predecessor,
ephemeris time.
Variation in the length of the year and the day
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
Tidal acceleration is an effect of the tidal forces between an orbiting natural satellite (e.g. the Moon) and the primary planet that it orbits (e.g. Earth). The acceleration causes a gradual recession of a satellite in a prograde orbit away from ...
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 86,400 SI seconds long.
Summary
Some of the year lengths in this table are in average
solar days, which are slowly getting longer and are now around 86,400.002
SI seconds.
An average Gregorian year may be said to be 365.2425
day
A day is the time period of a full rotation of the Earth with respect to the Sun. On average, this is 24 hours, 1440 minutes, or 86,400 seconds. In everyday life, the word "day" often refers to a solar day, which is the length between two so ...
s (52.1775
weeks, and if an hour is defined as one twenty-fourth of a day,
hour
An hour (symbol: h; also abbreviated hr) is a unit of time conventionally reckoned as of a day and scientifically reckoned between 3,599 and 3,601 seconds, depending on the speed of Earth's rotation. There are 60 minutes in an hour, and 24 ho ...
s,
minute
The minute is a unit of time usually equal to (the first sexagesimal fraction) of an hour, or 60 seconds. In the UTC time standard, a minute on rare occasions has 61 seconds, a consequence of leap seconds (there is a provision to insert a nega ...
s or
second
The second (symbol: s) is the unit of time in the International System of Units (SI), historically defined as of a day – this factor derived from the division of the day first into 24 hours, then to 60 minutes and finally to 60 seconds ...
s). Note however that in absolute time the average Gregorian year does not exist, because each period of 400 years is longer (by more than 1000 seconds) than the preceding one as the rotation of the earth slows. For this calendar, a common year is 365 days ( hours, minutes or seconds), and a leap year is 366 days ( hours, minutes or seconds). The 400-year civil cycle of the Gregorian calendar has days and hence exactly weeks.
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 Earth's
Solar System to revolve once around the
Galactic Center. It comprises roughly 230 million Earth years.
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.
Symbols and abbreviations
A common symbol for the year as a
unit of time is "a", taken from the Latin word .
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.
[
]
In English, the abbreviations "y" or "yr" are more commonly used in non-scientific literature.
In some
Earth sciences branches (
geology and
paleontology), "
kyr
The abbreviation kyr means "thousand years".
kyr was formerly common in some English language works, especially in geology and astronomy, for the unit of 1,000 years or millennium. The "k" is the unit prefix for kilo- or thousand with the suffix ...
,
myr
The abbreviation Myr, "million years", is a unit of a quantity of (i.e. ) years, or 31.556926 teraseconds.
Usage
Myr (million years) is in common use in fields such as Earth science and cosmology. Myr is also used with Mya (million years ago). ...
,
byr" (thousands, millions, and billions of years, respectively) and similar abbreviations are used to denote intervals of time remote from the present.
[
][
] In
astronomy the abbreviations kyr, Myr and Gyr are in common use for kiloyears, megayears and gigayears.
[
][
]
The
Unified Code for Units of Measure (UCUM) disambiguates the varying symbologies of ISO 1000, ISO 2955 and ANSI X3.50 by using:
:a
t = days for the mean tropical year;
:a
j = 365.25 days for the mean Julian year;
:a
g = days for the mean
Gregorian year
Gregorian may refer to:
*The thought or ideology of Pope Gregory I or Pope Gregory VII (also called ''Gregorianism'')
*Things named for Pope Gregory I:
**Gregorian chant, the central tradition of Western plainchant, a form of monophonic, unaccompa ...
;
In the UCUM, the symbol "a", without any qualifier, equals 1 a
j.
The UCUM also minimizes confusion with ''
are
Are commonly refers to:
* Are (unit), a unit of area equal to 100 m2
Are, ARE or Åre may also refer to:
Places
* Åre, a locality in Sweden
* Åre Municipality, a municipality in Sweden
**Åre ski resort in Sweden
* Are Parish, a municipa ...
'', 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'').
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'').
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), also noted in the
IUPAC ''Gold Book''.
In 2011, the IUPAC and the
International Union of Geological Sciences
The International Union of Geological Sciences (IUGS) is an international non-governmental organization devoted to international cooperation in the field of geology.
About
The IUGS was founded in 1961 and is a Scientific Union member of the Inte ...
jointly recommended defining the "annus", with symbol "a", as the length of the tropical year in the year 2000:
:a = seconds (approximately
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" (e.g. 1 Ma for 1 million years ago), and "y" or "yr" for a one-year time period.
However, this historical practice does not comply with the NIST ''Guide'',
considering the unacceptability of mixing information concerning the
physical quantity
A physical quantity is a physical property of a material or system that can be quantified by measurement. A physical quantity can be expressed as a ''value'', which is the algebraic multiplication of a ' Numerical value ' and a ' Unit '. For examp ...
being measured (in this case, time intervals or points in time) with the units and also the unnaceptability of using abbreviations for units.
Furthermore, according to the
UK Metric Association (UKMA), language-independent symbols are more universally understood (UKMA ''Style guide'').
SI prefix multipliers
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.
* ka (for ''kiloannus'') – a
unit of time equal to one
thousand or 10
3 years, 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
Radiocarbon dating (also referred to as carbon dating or carbon-14 dating) is a method for determining the age of an object containing organic material by using the properties of radiocarbon, a radioactive isotope of carbon.
The method was dev ...
technique such as
ice core
An ice core is a core sample that is typically removed from an ice sheet or a high mountain glacier. Since the ice forms from the incremental buildup of annual layers of snow, lower layers are older than upper ones, and an ice core contains ic ...
dating,
dendrochronology
Dendrochronology (or tree-ring dating) is the scientific method of dating tree rings (also called growth rings) to the exact year they were formed. As well as dating them, this can give data for dendroclimatology, the study of climate and atmos ...
,
uranium-thorium dating or
varve analysis is used as the primary method for age determination. If age is determined primarily by
radiocarbon dating
Radiocarbon dating (also referred to as carbon dating or carbon-14 dating) is a method for determining the age of an object containing organic material by using the properties of radiocarbon, a radioactive isotope of carbon.
The method was dev ...
, then the age should be expressed in either radiocarbon or calendar (calibrated) years
Before Present.
* Ma (for ''megaannus'') – a
unit of time equal to one
million
One million (1,000,000), or one thousand thousand, is the natural number following 999,999 and preceding 1,000,001. The word is derived from the early Italian ''millione'' (''milione'' in modern Italian), from ''mille'', "thousand", plus the au ...
or 10
6 years. The suffix "Ma" is commonly used in scientific disciplines such as 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'') – a unit of time equal to one
billion or 10
9 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 the universe is approximately 13.8 Ga.
* Ta (for ''teraannus'') – a unit of time equal to one
trillion
''Trillion'' is a number with two distinct definitions:
* 1,000,000,000,000, i.e. one million million, or (ten to the twelfth power), as defined on the short scale. This is now the meaning in both American and British English.
* 1,000,000,000,0 ...
or 10
12 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
''Red Dwarf'' is a British science fiction comedy franchise created by Rob Grant and Doug Naylor, which primarily consists of a television sitcom that aired on BBC Two between 1988 and 1999, and on Dave since 2009, gaining a cult following. T ...
.
* Pa (for ''petaannus'') – a unit of time equal to one
quadrillion or 10
15 years. The
half-life of the
nuclide
A nuclide (or nucleide, from nucleus, also known as nuclear species) is a class of atoms characterized by their number of protons, ''Z'', their number of neutrons, ''N'', and their nuclear energy state.
The word ''nuclide'' was coined by Truman ...
cadmium-113
Naturally occurring cadmium (48Cd) is composed of 8 isotopes. For two of them, natural radioactivity was observed, and three others are predicted to be radioactive but their decays have not been observed, due to extremely long half-lives. The tw ...
is about 8 Pa. This symbol coincides with that for the
pascal
Pascal, Pascal's or PASCAL may refer to:
People and fictional characters
* Pascal (given name), including a list of people with the name
* Pascal (surname), including a list of people and fictional characters with the name
** Blaise Pascal, Fren ...
without a multiplier prefix, though both are infrequently used and context will normally be sufficient to distinguish time from pressure values.
* Ea (for ''exaannus'') – a unit of time equal to one
quintillion or 10
18 years. The half-life of
tungsten-180
Naturally occurring tungsten (74W) consists of five isotopes. Four are considered stable (182W, 183W, 184W, and 186W) and one is slightly radioactive, 180W, with an extremely long half-life of 1.8 ± 0.2 exayears (1018 years). On average, tw ...
is 1.8 Ea.
Abbreviations for "years ago"
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.
[
] 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:
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.
See also
*
: current year
*
Astronomical year numbering
*
Century
A century is a period of 100 years. Centuries are numbered ordinally in English and many other languages. The word ''century'' comes from the Latin ''centum'', meaning ''one hundred''. ''Century'' is sometimes abbreviated as c.
A centennial or ...
*
Decade
*
Epoch (reference date)
*
ISO 8601: standard for representation of dates and times
*
List of calendars
*
List of years
*
Millennium
*
Orders of magnitude (time)
*
Unit of time
*
Annual
References
Notes
Further reading
*
*
External links
*
Images of years
{{Authority control
*