Year

The year is a unit of time based on the roughly 365¼ days taken by the Earth to revolve around the Sun.^[1] The contemporary calendar year, based on the Gregorian solar calendar, approximates this cycle.

The term "year" is also used to indicate other periods of roughly similar duration, such as the roughly 354-day cycle of the Moon's phases (see 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

By extension, the term "year" can also be applied to the time taken for any astronomical object to revolve around its primary, as 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.^[2]

A calendar year is an approximation of the number of days of the Earth's orbital period, as counted in a given calendar. The Gregorian calendar, or modern 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. 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).^[3]

In English, the

Financial and scientific calculations often use a 365-day calendar to simplify daily rates.

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.

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 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 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

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.^{[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 Julian year, as used in astronomy and other sciences, is a time unit defined as exactly 365.25 days of 86400

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, see below), the symbol 'a' (without subscript) always refers to the Julian year, 'a_j', of exactly 31557600 seconds.

365.25 d × 86400 s = 1 a = 1 a_j = 31.5576 Ms

The SI multiplier prefixes may be applied to it to form "ka", "Ma", etc.^[9]

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

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). The year 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

346.620075883 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, 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).

The

The Sothic year is the heliacal year, the interval between heliacal risings, of the star Sirius. It is currently less than the sidereal year and its duration is very close to the Julian year of 365.25 days.

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:

365.2568983 days (365 d 6 h 9 min 56 s).

The

The TT subscript indicates that for this formula, the Julian date should use the Terrestrial Time scale, or its predecessor, ephemeris time.

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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 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 86400 SI seconds long.[17]^[18][19][20]

Some of the year lengths in this table are in average

An average Gregorian year may be said to be 365.2425 days (52.1775 weeks, and if an hour is defined as one twenty-fourth of a day, 8765.82 hours, 525949.2 minutes or 31556952 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. In this calendar, a common year is 365 days (8760 hours, 525600 minutes or 31536000 seconds), and a leap year is 366 days (8784 hours, 527040 minutes or 31622400 seconds). The 400-year civil cycle of the Gregorian calendar has 146097 days and hence exactly 20871 weeks.

The Great Year, or equinoctial cycle, corresponds to a complete revolution of the equinoxes around the ecliptic. Its length is about 25,700 years.^[21][22]

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.^[23]

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 common symbol for the year as a unit of time is "a", taken from the Latin word annus. 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.^[24]

In English, the abbreviations "y" or "yr" are more commonly used in non-scientific literature.

The Unified Code for Units of Measure (UCUM) disambiguates the varying symbologies of ISO 1000, ISO 2955 and ANSI X3.50 by using:^[9]

a_t = 365.24219 days for the mean tropical year;

a_j = 365.25 days for the mean Julian year;

a_g = 365.2425 days for the mean

Gregorian year

;

In the UCUM, the symbol "a", without any qualifier, equals 1 a_j. The UCUM also minimizes confusion with are, 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).^[30][31]

Since 1987, the

In 2011, the IUPAC and the International Union of Geological Sciences jointly recommended defining the "annus", with symbol "a", as the length of the tropical year in the year 2000:^[35]

a = 31556925.445 seconds (approximately 365.24219265

ephemeris days

)

This differs from the above definition of 365.25 days by about 20

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.[36]^[37] However, this historical practice does not comply with the NIST Guide,^[24] considering the unacceptability of mixing information concerning the physical quantity being measured (in this case, time intervals or points in time) with the units and also the unacceptability of using abbreviations for units. Furthermore, according to the UK Metric Association (UKMA), language-independent symbols are more universally understood (UKMA Style guide).^[38]

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.

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.^[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").

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 durations of epochs.^[26][27] This ad hoc distinction between "absolute" time and time intervals is somewhat controversial amongst members of the Geological Society of America.^[44]