yearlength

Yearlength refers to the duration of a year, the period associated with the Earth's orbit around the Sun, or the length of a calendar year used in timekeeping. In astronomy and calendrics, several distinct year definitions are used, depending on what is measured (seasonal cycle, orbital mechanics, or calendar conventions).

The tropical year, about 365.24219 days, measures the interval between successive vernal equinoxes and is the basis for most solar calendars. The sidereal year, about 365.25636 days, is the time for Earth to complete one orbit relative to the fixed stars. The anomalistic year, about 365.25964 days, is the interval between successive perihelion passages.

Lunar years, defined as 12 synodic lunar months, average about 354.36705 days. A lunisolar year, used by some calendars (such as the Hebrew calendar in fixed cycles), averages about 365.2468 days and introduces leap months to stay aligned with the solar year.

In Gregorian calendars, a common year has 365 days, and a leap year has 366 days. The system is designed so that the average year length over a 400-year cycle is 365.2425 days, approximating the tropical year. The 400-year cycle contains 97 leap years.

The concept of yearlength reflects the mismatch between orbital motion and civil timekeeping. Over long times, calendar reform addresses drift between calendar dates and seasons caused by the difference between the tropical year and the assumed year length of the calendar.