Monthlength

Monthlength is a concept describing the duration of a month, typically measured in days. It has two primary meanings: civil monthlength, which refers to calendar months in a given system, and lunar monthlength, which refers to the length of a lunar cycle in astronomy.

In the Gregorian civil calendar, monthlengths vary by month: January, March, May, July, August, October, and December have 31 days; April, June, September, and November have 30 days; February has 28 days in common years and 29 days in leap years.

In lunar and lunisolar calendars, monthlength is tied to the motion of the Moon. The mean synodic month is about 29.53059 days. Many lunar calendars use months of 29 or 30 days, often alternating or determined by lunar sightings. The Islamic calendar is strictly lunar, with months of 29 or 30 days and an average year of 354 or 355 days. Lunisolar calendars keep months roughly aligned with the Moon while adding leap months to stay in phase with the solar year; examples include the Hebrew and some East Asian calendars.

Because the solar year is about 365.24 days, while a lunar year is shorter, most calendars employ intercalation to prevent drift. In the Gregorian system, a leap day is added to February every four years, with century-year rules refining the cycle. Lunisolar systems insert extra months on irregular schedules to maintain seasonal alignment.

See also calendar month, lunar month, synodic month, intercalation.