Lunar Calendar - Lunisolar Calendars

Lunisolar Calendars

Most lunar calendars are in fact lunisolar calendars. That is, months reflect the lunar cycle, but then intercalary months (e.g. "second Adar" in the Hebrew calendar) are added to bring the calendar year into synchronisation with the solar year. Some examples are the Chinese and Hindu calendars, and most calendar systems used in antiquity.

All these calendars have a variable number of months in a year. The reason for this is that a year is not evenly divisible by an exact number of lunations, so without the addition of intercalary months the seasons would drift each year. This results in a thirteen-month year every two or three years.

Some lunar calendars are calibrated by annual natural events which are affected by lunar cycles as well as the solar cycle. An example of this is the lunar calendar of the Banks Islands which includes three months in which the edible palolo worm mass on the beaches. These events occur at the last quarter of the lunar month, as the reproductive cycle of the palolos is synchronised with the moon.

Even though the Gregorian calendar is in common and legal use, lunar calendars serve to determine traditional holidays in parts of the world such as India, China, Korea, Japan, Vietnam and Nepal. Some examples include Diwali, Chinese New Year/Tết (Vietnamese New Year), Mid-Autumn Festival/Chuseok and Nepal Sambat.