Year

A year is a unit of time based on the orbital period of a celestial body, most commonly the Earth around the Sun. In everyday use, the civil year or calendar year consists of 12 months and about 365 days; a common year has 365 days, while a leap year has 366 days to keep the calendar in alignment with the solar year.

Several distinct definitions of a year are used in astronomy and calendrics. The tropical or solar year, which governs the cycle of the seasons, averages about 365.2422 days. The sidereal year, the time it takes Earth to complete one orbit relative to the fixed stars, lasts about 365.25636 days. The anomalistic year, the interval between successive perihelion passages, is around 365.25964 days. These measures differ slightly due to orbital dynamics and precession.

Most modern civil calendars are Gregorian, introduced in 1582 to correct errors of the Julian calendar. The Gregorian rule assigns leap years to years divisible by 4, except centuries not divisible by 400. This yields 366 days in leap years and 365 in common years; the reform involved skipping 10 days during the transition in October 1582 in many regions, with adoption spreading over subsequent centuries.

Beyond the Gregorian system, many cultures use lunisolar or lunar calendars. Some, like the Hebrew calendar, insert leap months in a 19-year cycle to align with the solar year, while purely lunar calendars, such as the Islamic calendar, alternate about 354 and 355 days. The year is a fundamental structural element in timekeeping, calendars, and scheduling for civil, religious, and cultural purposes.