Years

A year is a unit of time used to measure long intervals and organize civil, agricultural, and astronomical activities. The most common reference is the tropical year, the time for the Sun to return to the same position in the cycle of the seasons, averaging about 365.2422 days. Other astronomical year definitions include the sidereal year (Earth’s orbital return relative to the fixed stars) about 365.25636 days and the anomalistic year about 365.2596 days.

In civil use, a calendar year runs from January 1 to December 31, usually 365 days in a common year and 366 in a leap year. The Gregorian calendar, introduced in 1582, uses a 400-year cycle with leap years on years divisible by 4, except that century years not divisible by 400 are common years. This yields an average year length of 365.2425 days.

Calendars based on lunar or lunisolar cycles diverge more from the tropical year. A lunar year, comprising 12 lunar months, is about 354 days. Lunisolar calendars add leap months to stay in sync with the solar cycle, maintaining roughly a 12-month year over longer cycles. Fiscal years, academic years, and other legal or administrative years are common uses of the term, often not tied to the astronomical year.

Year numbering is a convention for chronology and can be expressed as BCE/CE or BC/AD; astronomical year numbering includes a year zero. In everyday use, a year denotes a single calendar slice within a longer era, providing a frame for events, data, and records.