Year

Cartel with the year change sample between 2018 and 2019, both belonging to the Gregorian calendar.

A year, Earth year, sidereal, sidereal year or Julian year (symbol: a) refers to the orbital time of planet Earth, that is, the time it takes that planet to go around the Sun completely. The term year is also used to refer to the orbital period of any planet and, by extension, it is applied to others orbital cycles.

A calendar year is an approximation of the number of days in the Earth's orbital period, as counted in a given calendar. The Gregorian calendar, or modern calendar, presents its calendar year as a 365-day common year or as a 366-day leap year, just like the Julian calendars; "see" bottom. For the Gregorian calendar, the average length of the calendar year (the mean year) over the full 400-year leap cycle is 365.2425 days. ISO 80000-3, Annex C, supports the symbol a (from Latin [:wikt:annus|annus]]) to represent a year of 365 or 366 days. In English, the abbreviations y and yr are often used.

In astronomy, the Julian year is a unit of time; is defined as 365.25 days of exactly 86,400 seconds (SI base unit), which add up to 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, fiscal year, school year, etc. Similarly, year can mean the orbital period of any planet; for example, a Martian year and a Venusian year are examples of the time it takes for a planet 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 (/ jɛar/), Anglo-Saxon ġēr) continues Proto-Germanic *jǣran (*jē₁ran). Cognates are German Jahr, Old High German jār, Old Norse ár, and Gothic [:wikt:𐌾𐌴𐍂|jer]], from the Proto-Indo-European noun Template:PIE "year, season". Cognates also descending from the same Proto-Indo-European noun (with variation in suffix ablaut) are Avestan yārǝ "year", Greek ὥρα (hṓra) "year, season, period of time" (whence "hora"), Old Church Slavonic jarŭ, and Latin hornus "of this year".

Latin annus (a 2nd declension masculine noun; annum is the accusative singular; annī is genitive singular and nominative plural; annō the dative and ablative singular) comes from a noun in Proto-Indo-European-PIE Template:PIE, which also gave rise to Gothic aþn & #34;year" (only the dative plural aþnam is attested).

Although most languages treat the word as stem *yeh₁r-o-, there is evidence of an original derivation with a suffix *-r/n, *yeh₁-ro-. Both Indo-European words for year, *yeh₁-ro- and *h₂et-no-, would then derive from verb roots meaning "to go, move", *h₁ey- and *h₂et-, respectively (compare Vedic Sanskrit éti "va", atasi "vagas, lazy"). Several English words derive from the Latin annus, such as annual, annuity, anniversary, etc.; per annum means "every year", annō Dominī means "in the year of the Lord".

The Greek word for "year", ἔτος, is a cognate of the Latin vetus "old", from the PIE word *wetos- "year", also preserved in this meaning in Sanskrit. Template:IAST "year" and Template:IAST "yearling (calf)", the latter also reflected in the Latin [:wikt:vitulus|vitulus]] "calf", English wether "ram" (Old English weðer, Gothic wiþrus "lamb").

In some languages, it is common to count years by referring to a season, as in "summers", or "winters", or "harvests". Some examples are Chinese 年 "year", originally 秂, an ideographic compound of a person carrying a bundle of wheat denoting "harvest". Slavic plus [:wikt:Reconstruction:Proto-Slavic/godъ|godŭ] "time period; year" uses [:wikt:Reconstruction:Proto-Slavic/lěto|lěto] "summer; year".

Collation

Astronomical years do not have an integer number of lunar days or months. Any calendar that follows an astronomical year must have a system of intercalation like leap years.

Julian calendar

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 four years, or leap year, during which a leap day is inserted in the month of February. The name "Leap Day" applies to the added day.

The Revised Julian Calendar, proposed in 1923 and used in some Eastern Orthodox Churches It has 218 leap years every 900 years, with a mean year length of 365,242,222 2 days, close to the length of the mean tropical year, 365,242,19 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 intends for the northerly equinox to fall on or shortly before March 21, so it follows the year of the northerly equinox, or tropical year. Since 97 of the 400 years are leap years, the average duration of the year of the Gregorian calendar is 365,242 days; with a relative error of less than one ppm (8-10^-7) with respect to the current duration of the mean tropical year 365.242 days and even closer to the current year of the March equinox of 365.242 days that he intends equalize. It is estimated that by the year 4000 AD, the northward equinox will be one day behind in the Gregorian calendar, not because of this difference, but because of the slowing down of the Earth's rotation and the consequent lengthening of the day.

Other calendars

Historically, lunisolar calendars intercalated entire leap months observationally. Lunisolar calendars have fallen out of use, except for liturgical reasons (Hebrew calendar, various Hindu calendars).

A modern adaptation of the historic Jalali calendar, known as the Solar Hijri calendar (1925), is a purely solar calendar with an irregular pattern of leap days based on observation (or astronomical calculation), the purpose of which is to locate the year new (Nouruz) on the day of the quinxovernal (for the time zone of Tehran), instead of using an algorithmic system of leap years.

Computation

The simplicity of the definition hides the complexity of its computation.

Several years are considered according to the reference chosen to determine its duration:

The year or sidereal year when:

• The time between two consecutive steps of the Earth for the same point of its orbit. Usually used by astronomers, it is the most accurate measure of a year.
• Reference: the stars.
• Duration: 366.255 936 steel days, or 365.256 363 004 average solar days, 365 days 6 hours 9 minutes 9.76 seconds, i.e. less half (1/2) solar day.

Year

The tropical year, solar year or tropical year when:

• The time between two successive steps of the Sun by the average equinox. In other words, time from spring to spring for example. Reference: equinox vernal. Mesopotamian and Egyptian civilizations achieved values very close to the real for the tropic year, especially considering the shortage of media of the time.
• Duration: 365.242 190 402 average solar days, 365 d 5 h 48 m 45.22 s.

Calendar year when:

• The number of full days considered as one year for civil or religious purposes. In order for it to fit the astronomical cycles, which matter day fractions, this time compute varies every year.

Other definitions

In astronomy and astrophysics, in addition to the sidereal year, the following are of interest:

• The anomalistic year: time elapsed between two successive steps of the Earth by the perihelium of its orbit (365.2596 days)
• Cosmic or galactic year: time spent in an orbit of the Sun around the center of the Milky Way (about 220 million years)
• Platonic year: time in which the Earth axis describes a complete circle in the celestial sphere due to the precession (about 25 800 years)
• The year of eclipse: time between two successive steps of the Sun by a node of the orbit of the Moon, when an eclipse can happen.

Variations in the length of the year and the day

The exact length of an astronomical year changes over time. The main causes of these changes are:

• The precession of equinoxes changes the position of astronomical events regarding the axes of the Earth's orbit. An event that moves towards the perihellio is repeated with a shorter period of one year to another; an event that moves towards the perihellio will be repeated with a longer period of one year to another (although this effect does not change the medium value of the duration of the year).
• The gravitational influence of the moon and planets changes the movement of the Earth in a constant orbit around the Sun. Earth's orbit varies in a chaotic way, but at a much more limited interval than the orbits of the nearest planets.
• The tide resistance between the Earth and the Moon and the Sun increases the duration of the day and the month (transferring the angular moment of the rotation of the Earth to the revolution of the Moon); as the apparent medium solar day is the unity with which the length of the day is measured in civil life, it makes the effect that changes the duration of the year. In turn, tide resistance depends on factors such as postglaciar bounce and sea level rise.
• Changes in the actual mass of the Sun, caused by the solar wind and the energy radiation generated by nuclear fusion and radiated by its surface, will affect the orbital period of the Earth over a long period (approximately 1.25 microseconds per year).
• Other effects tend to shorten the Earth orbital period, such as the Poynting-Robertson effect (approximately 30 nanoseconds per year) or gravitational wave (approximately 165 attoseconds per year).

Example of solar analem, showing the curve in the form of "eight" lying down, for an observer in the northern hemisphere, where the position that the sun is taking at the same time every day for a year is reflected.

Historical evolution of the solar year

Egyptian Year

I thought a year was 365 days. The Egyptian calendar suffered very remarkable defamations of time and was aimed at reforming during the Canopo Congress (see Canopo Decree), where it was concluded that the year lasted 365.25 days.

Julian solar year

Based on the Canopo Decree, it was estimated that a year lasted 365.25 solar days, i.e. 365 days and 6 hours. It also considered that 366 solar days were counted every four years. That fourth year he was called a bisister because the ancient Romans counted on February 24 twice.

Gregorian solar year

It's the calendar year. It is a correction in the conteum of the Julian solar year, because in 1582 some astronomers discovered a gradual desphase of time in the Julian calendar. After a long investigation, they concluded that a year lasted approximately 365,2425 solar days, i.e. 365 days, 5 hours, 49 minutes and 12 seconds. It was also determined that secular years could be bisy, only if they were divisible between 400.

Symbol

The symbol established for the year in ISO 80000 is a (from the Latin annus). Ma (megayears, millions of years) and ka (kiloyears, thousands of years) can be derived from it, using multiples.

Leap Year

Every four years a day must be added to adjust the calendar year to the calendar year.

The Romans placed it between the twenty-third and twenty-fourth of February, and for superstitious reasons it had no name or number: if the twenty-third was the day sextum ante calendas martii, of this day of more they said day bisextum, the other sixth.

This year was believed to bring bad luck. During the added day (February 29) demons and evil spirits ruled. Those born on this day are "strange" beings, at least because they only have a birthday every four years. There is a belief that in the leap year all the olive leaves are turned downwards and those born in a leap year do not catch smallpox.

Astronomical Years

Julian Year

The Julian year, as used in astronomy and other sciences, is a unit of time defined as exactly 365.25 days. This is the normal meaning of the unit "year" used in various scientific contexts. The Julian century of 36,525 days and the Julian millennium of 365,250 days are used in astronomical calculations. Fundamentally, expressing a time interval in Julian years is a way of specifying precisely how many days (not how many "actual" years), for long time intervals where stating the number of days would be unwieldy and unintuitive. By convention, the Julian year is used in calculating the distance traveled in a light-year.

In the Unified Code of Measurement Units, the symbol a (without subscript) always refers to the Julian year, a_j, of exactly 31,557,600 seconds.

365.25 d × 86 400 s = 1 a = 1 to_j = 31.5576 ms

Sidereal, tropical and anomalous years

The relationships between them are studied in more detail in Precession of the Equinoxes.

Each of these three years can be called, in a general way, an astronomical year.

A sidereal year is the time it takes the Earth to complete one revolution of its orbit, measured with respect to a fixed frame of reference (such as fixed stars, Latin sidera, singular sidus). Its average duration is 365,256,363,004 days (365 d 6 h 9 min 9.76 s) (in epoch J2000.0 = 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 ecliptic longitude of the Sun is measured with respect to the equinox, the tropical year comprises a complete cycle of seasons; Due to the biological and socioeconomic importance of the seasons, the tropical year is the basis of most calendars. The modern definition of the mean tropical year differs from the actual time between the passages of, say, the northward equinox for several reasons explained below. Due to 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 (= 365,242 19 d × 86,400 s).

The anomalous year is the time it takes for the Earth to complete one revolution with respect to its apse. Earth's orbit is elliptical; the extreme points, called apses, are perihelion, where Earth is closest to the Sun (January 5, 07:48 UT in 2020), and aphelion, where Earth is farthest from the Sun (July 4, 11:35 UT in 2020). The anomalous year is usually defined as the time between perihelion passes. Its average duration is 365,259,636 days (365 d 6 h 13 min 52.6 s) (in epoch J2011.0).

Draconic Year

The draconic year, draconitic year, eclipse year or ecliptic year is the time it takes for the Sun (as seen from Earth) to complete one revolution with respect to the same lunar node. (point at which the Moon's orbit intersects the ecliptic). The year is associated with eclipses: these occur only when both the Sun and the Moon are close to these nodes; so eclipses occur within about a month of every half-year of eclipse. Therefore, 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) (in epoch J2000.0).

This term is sometimes erroneously used for the draconic or nodal period of lunar precession, i.e. the period of one complete revolution of the Moon's ascending node around the ecliptic: 18.612815932 Julian years, i.e. 6798,331019 days; in the J2000.0 epoch).

Full Moon Cycle

The full moon cycle is the time it takes for the Sun (as viewed from 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 variable duration of the synodic month. The duration of a full moon cycle is: 411.78443029 days (411 days 18 hours 49 minutes 35 seconds) (in epoch J2000.0).

Moon Year

The lunar year comprises twelve complete cycles of the phases of the Moon, as seen from Earth. It has a duration of approximately 354.37 days. Muslims use it to celebrate their Eids and to mark the start of the fasting month of Ramadan. The Muslim calendar year is based on the lunar cycle. The Jewish calendar is also essentially lunar, except that a lunar intercalary month is added once every two or three years, to keep the calendar in sync with the solar cycle as well. Thus, a lunar year in the Jewish (Hebrew) calendar consists of twelve or thirteen lunar months.

Lazy year

The vague year, from annus vagus or wandering year, is an integral approximation of the year equal to 365 days, which wanders relative to with more exact years. Normally 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 Ethiopia, Ancient Egypt, Iran, Armenia, and in Mesoamerica among the Aztecs and Mayas. It is still used by many Zoroastrian communities.

Heliac Year

A heliacal year is the interval between the heliacal risings of a star. It differs from the sidereal year for stars far from the ecliptic mainly due to the precession of the equinoxes.

Sotic Year

The Sothic year is the interval between the heliacal rises 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.

Gaussian year

The Gaussian year is the sidereal year for a planet of negligible mass (with respect to the Sun) and not disturbed by other planets that is governed by the Gaussian gravitational constant. Such a planet would be slightly closer to the Sun than the average distance from Earth. Its length is: 365.2568983 days (365 d 6 h 9 min 56 s).

Besselian Year

This section is linked from Universal Time

The Besselian Year is a tropical year that begins when the (fictional) mean Sun reaches an ecliptic longitude of 280°. Currently, this occurs on or about January 1. It is named after the 19th-century German astronomer and mathematician Friedrich Bessel. The following equation can be used to calculate the current Besselian epoch (in years):

B = 1900.0 + (Juliandate)_TT -2 415 020,31352 / 365,242 198 781

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

Other years

Fiscal Year

A fiscal year, or financial year, is a 12-month period used to calculate annual financial reports in businesses and other organizations. In most jurisdictions there are accounting laws that require these reports once every twelve months. This year does not necessarily correspond to the calendar year.

Judicial Year

Period, generally from September 1 of one year to July 31 of the following year, during which the normal tasks of the courts and justice administration bodies are carried out.

School Year

The school year, academic year or academic year, is the period of the year in which students go to their educational centers. It generally lasts 9 months and can be divided into semesters, trimesters or four-month periods depending on the country and institution.

In different countries the school year or academic year begins and ends on different dates. In most South American countries, for example, the year starts in March and ends in December. In Europe, on the other hand, it generally starts in September and ends in June.

Farm year

It is the period of agricultural work from the beginning of the preparation work to the harvest. These jobs do not coincide with the calendar year. For legal purposes, each cycle counts as one agricultural year. See also agricultural year and agricultural calendar.

Forest Year

A forest year is a 12-month period, generally from October 1 of one year to September 30 of the following year, during which forest harvesting takes place.

Liturgical year

This is a year organized in such a way that the main events of the life of Christ and the most important mysteries of Christianity are celebrated successively. It consists of two parts: that of the temporal, or cycle of mobile festivals, and centered on the celebration of Easter, and the part of the santoral, or calendar of fixed festivals.

Jubilee or Holy Year

The jubilee or holy year is a celebration that takes place in different historical Christian Churches, particularly the Catholic Church and the Orthodox Church, and that commemorates a sabbatical year with particular meanings.

Gap Year

Initially according to Jewish law, the sabbatical year is the last year of every seven years during which the land could not be cultivated and each master had to free a Hebrew slave, if he wanted it. On the other hand, in countries with an Anglo-Saxon cultural tradition, the sabbatical year referred to the course or semester that is granted every six or seven years to a professor to dedicate it to his studies or research, exempting him from teaching.

Particular years

• Olympic Year: Year counted from the Olympics. It was introduced in Greece by the Timeo historian of Taumenius, who calculated the years, from 776 to C., according to the list of winners of the Olympic Games. The system was implemented from 260 to C. (130.a olympiad), and was abolished in time of Theodosius the Great, in 395.
• Zodiac Year: The year of the Egyptian calendar in which the zodiac period ended and therefore coincided approximately the calendar year and the tropic year.
• Year of the foundation of Rome: Year counted from the supposed date of the foundation of Rome, 753 BC.
• Year of consulate: Year from the appointment of the consuls. The origin of this computation, which the Roman law commanded to use for the dating of public documents, dates back to the beginning of the republic (509 BC).
• Year of Empire: Year counted from the proclamation of the emperor. This formula was attached to that of the year of the consulate, in the lower Roman empire, and finally replaced it.
• Year of confusion or Annus confusionis: The year 46 BC, in which the Julian reform took place. It lasted 445 days.
• Year of the Kingdom: Year counted from the king's elevation to the throne. This system of dating was used by the Christian kingdoms of the Middle Ages, by analogy with the year of the empire or the pontificate, until, gradually, it was replaced by that of the Christian era.

Length of years of the different planets

The duration is indicated in Earth days. Rounded to two decimal places.

Anecdotes caused by the adoption of the Gregorian calendar

• Saint Teresa of Avila died on Thursday, October 4, 1582 and was buried the following day Friday, October 15. Santa Teresa's party is held on October 15.
• Shakespeare and Cervantes died on the same date, April 23, 1616, but not the same day. Cervantes died on April 23, 1616 according to the Gregorian calendar in force in Spain since 1582. Shakespeare died on May 3, 1616 according to the Julian calendar in force in England until 1752.
• Russia did not accept the reform of the Gregorian calendar until the soviets arrived. The adoption of the new calendar was made so that on 1 February 1918 it became on 14 February. The curious case is that the so-called “Oct Revolution”, in the former USSR, was held in November. This was due to the fact that the revolution had taken place on October 25, 26, 1917, according to the Julian calendar in force in the tsarist Russia, a date that, when the new regime adopted the Gregorian calendar, was to fall on November 7-8. The birthday of the revolution was always held in November and sometimes even referred to the “Nov Revolution.”