Year

A year measures the time Earth takes to complete one orbit around the Sun. More precise scientific definitions include the tropical year (roughly 365 days, 5 hours, 48 minutes, and 45 seconds) and the sidereal year (about 20 minutes longer). The Gregorian calendar approximates the tropical year by adding leap years. The word "year" also describes other cycles of similar length, like the lunar year (about 354 days based on twelve lunar phases), and periods tied to the calendar or astronomy, such as the seasonal, fiscal, or academic year.

Earth's axial tilt causes the seasons over the course of a year, marked by changes in weather, daylight hours, and effects on vegetation and soil. Temperate and subpolar regions generally recognize four seasons: spring, summer, autumn, and winter.

In tropical and subtropical areas, some regions lack defined seasons, but seasonal tropics track annual wet and dry seasons. The term "year" can also apply to the orbital period of any astronomical object around its primary, like a Martian year of roughly 1.88 Earth years. It may refer to any long cycle, such as the Great Year.

Calendar year

A calendar year typically approximates the number of days in Earth's orbit, as counted in a given calendar. The Gregorian calendar, the modern world standard, has either a common year of 365 days or a leap year of 366 days, similar to the Julian and Solar Hijri calendars.

Over its 400-year leap cycle, the Gregorian calendar's average year length is 365.2425 days (97 leap years). Lunisolar calendars, traditional in Asia, have twelve lunar months but add a thirteenth leap month every three years or so. The lunar Hijri calendar (often called the Islamic calendar in the West) has twelve lunar months and disregards the solar year.

Abbreviation

In English, "year" is commonly abbreviated as "y" or "yr." The symbol "a" (from Latin annus) appears in scientific literature, though its exact duration can vary.

Etymology

The English word "year" (from West Saxon ġēar, Anglian ġēr) continues Proto-Germanic jǣran. Cognates include German Jahr, Old High German jār, Old Norse ár, and Gothic jer, from the Proto-Indo-European noun yeh₁r-om meaning "year, season." Other cognates include Avestan yārǝ, Greek hṓra (source of "hour"), Old Church Slavonic jarŭ, and Latin hornus ("of this year").

Latin annus comes from PIE h₂et-no-, also yielding Gothic aþn. Many English words derive from Latin annus, such as annual, annuity, and anniversary; per annum means "each year," and annō Dominī means "in the year of the Lord." The Greek word ἔτος ("year") is cognate with Latin vetus ("old"), from PIE wetos-, also seen in Sanskrit vat-sa-ras and vat-sa- ("yearling"), and Latin vitulus ("bull calf"), English wether ("ram"). Some languages count years by referencing seasons, like "summers" or "winters." Chinese 年 ("year") originally depicted a person carrying wheat, meaning "harvest." Slavic uses godŭ ("time period; year") and lěto ("summer; year").

Julian calendar

Astronomical years do not have an integer number of days or lunar months, so any calendar following an astronomical year needs intercalation, such as leap years. In the Julian calendar, the average year length is 365.25 days, with a leap year every fourth year adding a leap day to February. In astronomy, the Julian year is defined as exactly 365.25 days of 86,400 seconds each, totaling 31,557,600 seconds. The Revised Julian calendar, proposed in 1923 and used by some Eastern Orthodox Churches, has 218 leap years every 900 years, giving an average year of 365.2422222 days—close to the mean tropical year of 365.24219 days.

In the year 2800 CE, the Gregorian and Revised Julian calendars will differ by one day. The Gregorian calendar aims to keep the northward equinox on or before March 21, following the tropical year. With 97 leap years in 400 years, its mean year is 365.2425 days, with a relative error below one part per million.