A year (from Old English gēr) is the time between two recurrences of an event related to the orbit of the Earth 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.

Etymology

see: annum

West Saxon gear, Anglian gēr 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 lang: 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", lang: ἔτος, is cognate to Latin vetus "old", from PIE *wetus- "year" , also preserved in this meaning in Sanskrit IAST: vat-sa- "yearling (calf)".

The SI prefixes may be applied to the year in English to form kiloyear, megayear, and so on. The form annum is preferred because it is language-neutral (hence kilo-annum, mega-annum, etc.). In such instances, the year in question is always the Julian year of exactly 365.25 days of 86400 seconds.

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 close to 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.

Among solar calendars in wide use today, the Persian calendar is one of the most precise. 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.

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. (This is still used as a convenient time unit in astronomy as shown below.) 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.