Lunar year

A lunar calendar is a calendar based upon cycles of the Moon's phases (synodic months), in contrast to solar calendars based solely upon the solar year. A purely lunar calendar is also distinguished from lunisolar calendars whose lunar months are brought into alignment with the solar year through some process of intercalation. The details of when months begin varies from calendar to calendar, with some using new, full, or crescent moons and others employing detailed calculations.

Because each lunation is a little less than 29 days, 12 hours, 44 minutes, and 3 seconds (29.530588 days), it is common for the months of lunar calendars to vary between 29 and 30 days. Because the period of twelve such lunations—a lunar year—is a little less than 354 days, 8 hours, 48 minutes, and 34 seconds (354.367056 days), purely lunar calendars lose around 11 days per year relative to the Gregorian calendar. In purely lunar calendars such as the Islamic calendar, the lack of intercalation causes the lunar months to cycle through all the seasons of the Gregorian year over the course of a 33-lunar-year cycle.

Although the Gregorian calendar is in common and legal use in most countries, traditional lunar and lunisolar calendars continue to be used throughout the Old World to determine religious festivals and national holidays. Such holidays include Ramadan (Islamic calendar); the Chinese, Japanese, Korean, Vietnamese, and Mongolian New Year (Chinese, Japanese, Korean, Vietnamese, and Mongolian calendars); the Nepali New Year (Nepali calendar); the Mid-Autumn Festival and Chuseok (Chinese and Korean calendars); Loi Krathong (Thai calendar); and Diwali (Hindu calendars).

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