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A year (from Old English ȝēr; symbol a or sometimes y) is the amount of time it takes the Earth to make one revolution around the Sun. By extension, this can be applied to any planet: for example, a "Martian year" is the time in which Mars completes its own orbit.

Although there is no universally accepted symbol for the year, NIST SP811^[1] and ISO 80000-3:2006^[2] suggest the symbol a in the International System of Units. (Although a is also the symbol for the are unit of area, context is usually enough to disambiguate.) In English, the deprecated abbreviations kyr, Myr, Gyr (for kiloyear, megayear and gigayear) are still used informally when denoting intervals of time remote from the present.^[3]^[4]^[5]

Etymology

Further information: Jēran and annum

West Saxon ȝear, Anglian ȝēr continues Proto-Germanic *jǣram (*jē₂ram). Cognates are 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 annum is from a base *at-no-. Both *yē-ro- and *at-no- are based on verbal roots expressing movement, *at- and *ey- respectively, both meaning "to go" generally. Latin annum has a cognate in Gothic aþnam "year". 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)".

Calendar year

A calendar year is the time between two dates with the same name in a calendar.

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 (which can be thought of as 97 out of 400 years being leap years) whereas the vernal equinox year is 365.2424 days.

The Persian calendar is one of the most precise, among solar calendars in wide use today. Rather than being based on numerical rules, the Persian year begins on the day (for the time zone of Tehran) on which the vernal equinox actually falls, as determined by precise astronomical computations. The Persian Calendar is in use in Afghanistan and Iran.

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.

In the Julian calendar, the average length of a year was 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.

A half year (one half of a year) may run from January to June or July to December.

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.

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, the federal government of the U.S. has a fiscal year that starts on October 1st instead of January 1. 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.

Academic year

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.

Many schools in the UK and USA divide the academic year into three roughly equal-length trimesters (called terms in the UK), more or less coinciding with autumn, winter, and spring. At some, a shortened summer session, not usually 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 (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 an elective summer session, and/or a short January session.
Some other schools, including some in the United States, have four marking periods. The school year in many countries starts in August or September and ends in May, June or July.
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 180 days of teaching each year in schools in the USA, excluding weekends and breaks, and 190 days for pupils in state schools in the United Kingdom, New Zealand and Canada.

Astronomical years

Julian year

Main article: Julian year (astronomy)

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.

In the Unified Code for Units of Measure, the symbol a (without subscript) always refers to the Julian year a_j of exactly 31557600 seconds.

365.25 days of 86400 seconds = 1 a = 1 a_j = 31.5576 Ms

The SI multiplier prefixes may be applied to it to form ka (kiloannum), Ma (megaannum) etc.

Sidereal, tropical, and anomalistic years

The relations among these are considered more fully in Precession (astronomy).

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:

365.256 363 051 days (365 d 6 h 9 min 9.7676 s) (at the epoch J2000.0 = 2000 January 1 12:00:00 TT).

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:

365.242 189 67 days (365 d 5 h 48 min 45 s) (at the epoch J2000.0).

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:

365.259 635 864 days (365 d 6 h 13 min 52 s) (at the epoch J2000.0).

The anomalistic year is slightly longer than the sidereal year because of the precession of the apsides (or anomalistic precession).

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 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:

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

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).

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:

411.784 430 29 days (411 d 18 h 49 min 34 s) (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.

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 (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.)

Sothic year

The Sothic year is the interval between heliacal risings of the star Sirius. Its duration is very close to the mean Julian year of 365.25 days.

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 Earth's mean distance. Its length is:

365.256 898 3 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 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:

B = 2,000 + (JD - 2,451,544.53) /365.242189

Great year

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.

Galactic year

The Galactic year is the time it takes Earth's solar system to revolve once around the galactic center. It comprises roughly 226 million Earth years.

Variation in the length of the year and the day

The exact length of an astronomical year changes over time. The main sources of this change are:

The precession of the equinoxes changes the position of astronomical events with respect to the apsides of Earth's orbit.
An event moving toward perihelion recurs with a decreasing period from year to year; an event moving toward aphelion recurs with an increasing period from year to year (though this effect does not change the average value of the length of the year).

Each planet's movement is perturbed by the gravity of every other planet.

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 change. Tidal drag in turn depends on factors such as post-glacial rebound and sea level rise.

Changes in the effective mass of the Sun, caused by solar wind and radiation of energy generated by nuclear fusion and radiated by its surface, will affect the Earth's orbital period over a long time (approximately an extra 1.25 microsecond per year^[6]).

The Poynting-Robertson effect shortens the year by about 30 nanoseconds per year.

Gravitational radiation shortens the year by about 165 attoseconds per year.^[7]

Summary of various kinds of year

346.62 days: a draconitic year in some septenary calendars.
353, 354 or 355 days: the lengths of common years in some lunisolar calendars.
354.37 days (12 lunar months): the average length of a year in lunar calendars.
365 days: a common year in many solar calendars.
365.24219 days: a mean tropical year near the year 2000.
365.2424 days: a vernal equinox year.
365.2425 days: the average length of a year in the Gregorian calendar.
365.25 days: the average length of a year in the Julian calendar.
365.2564 days: a sidereal year.
366 days: a leap year in many solar calendars.
383, 384 or 385 days: the lengths of leap years in some lunisolar calendars.
383.9 days (13 lunar months): a leap year in some lunisolar calendars.

An average Gregorian year is 365.2425 days = 52.1775 weeks, 8,765.82 hours = 525,949.2 minutes = 31,556,952 seconds (mean solar, not SI).

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.

Numeration or designation of year numbers

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.

Notes

^ Ambler Thompson, Barry N. Taylor (2008). "Special Publication 811: Guide for the Use of the International System of Units (SI)" (PDF). National Institute of Standards and Technology (NIST). http://physics.nist.gov/Document/sp811.pdf.
^ "ISO 80000-3:2006, Quantities and units - Part 3: Space and time". Geneva, Switzerland: International Organization for Standardization. 2006. http://www.iso.org/iso/iso_catalogue/catalogue_tc/catalogue_detail.htm?csnumber=31888.
^ "AGU Editorial Style Guide for Authors". American Geophysical Union. 21 September 2007. http://www.agu.org/pubs/style_guide_intro.html. Retrieved 2009-01-09. "Annum (a) is used to denote year as a unit of measure; use a, ka, Ma, or Ga rather than year (or yr) kyr, Myr (or m.y.), or Gyr."
^ North American Commission on Stratigraphic Nomenclature (November 2005). "North American Stratigraphic Code". The American Association of Petroleum Geologists Bulletin 89 (11): 1547-1591. http://ngmdb.usgs.gov/Info/NACSN/Code2/code2.html#Article13.
^ Rowlett, Russ. "a". How Many? A Dictionary of Units of Measurement. University of North Carolina. http://www.unc.edu/~rowlett/units/dictA.html. Retrieved 2009-01-09.
^ Solar mass is ~2×10³⁰ kg, decreasing at ~5×10⁹ kg/s, or ~8×10⁻¹⁴ solar mass per year. The period of an orbiting body is proportional to $\frac{1}{\sqrt{M}}$ , where M is the mass of the primary.
^ ~300 W of radiation produces ~9.5×10⁹ J orbital energy decrease per year; this varies as 1/R, and period varies as R^1.5

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