Year

A year is the orbital period of the Earth moving in its orbit around the Sun. Due to the Earth's axial tilt, the course of a year sees the passing of the seasons, marked by changes in weather, the hours of daylight, and consequently vegetation and fertility. In temperate and subpolar regions, generally four seasons are recognized: spring, summer, autumn and winter. In seasonal tropical and subtropical regions, the wet (rainy or monsoon) season and the dry season are generally recognized.

A calendar year is an approximation of the Earth's orbital period in a given calendar. The Gregorian, or modern, calendar, presents the calendar year to be either a common year of 365 days or a leap year of 366 days, as does the Julian calendar; see below. The average year length across the complete leap cycle (400 years) of the Gregorian calendar is 365.2425 days. The ISO standard ISO 80000-3, Annex C, supports the symbol "a" (for Latin annus) to represent a year of either 365 or 366 days. In English, the abbreviations "y" and "yr" are used.