Chinese Calendar - Purpose of The Intercalary Months

Purpose of The Intercalary Months

Most people, upon using or studying the Chinese calendar, are perplexed by the intercalary month because of its seemingly unpredictable nature. As mentioned above, the intercalary month refers to additional months added to the calendar in some years to correct for its deviation from the astronomical year, a function similar to that of the extra day in February in leap years.

However, because of the complex astronomical knowledge required to calculate if and when an intercalary month needs to be inserted, to most people, it is simply a mystery. This has led to a superstition that intercalary months in certain times of the year bring bad luck.

The main purpose of the intercalary month is to correct for deviations of the calendrical year from the astronomical year. Because the Chinese calendar is mainly a lunar calendar, its standard year is 354 days, whereas the astronomical year is approximately 365¼ days. Without the intercalary month, this deviation would build up over time, and the Spring festival, for example, would no longer fall in Spring. Thus, the intercalary month serves a valuable purpose in ensuring that the year in the Chinese calendar remains approximately in line with the astronomical year.

The intercalary month is inserted whenever the Chinese calendar moves too far from the stage of progression of the earth in its orbit. Thus, for example, if the beginning of a certain month in the Chinese calendar deviates by a certain number of days from its equivalent in a solar calendar, an intercalary month needs to be inserted.

The practical benefit of this system is that the calendar is able to approximately keep in pace with the solar cycle, while at the same time retaining months that roughly correspond with lunar cycles. Hence the term lunisolar calendar. The latter is important because many traditional festivals correspond to significant events in the moon's cycle. For example, the mid-autumn festival is always on a day of the full moon.