Why Is There a Leap Month?

A leap month is an extra month added to the lunar calendar to reconcile the cycle of the moon's phases with the changes of the four seasons. A lunar month is calculated according to the waxing and waning of the moon, averaging about 29.5 days, so twelve months together total only around 354 days, roughly 11 days shorter than the time it takes the Earth to orbit the sun. Without adjustment, within just a few years the lunar months would fall noticeably out of step with the real seasons.

The method of placing leap months is closely tied to the 24 solar terms. The solar terms are defined by the sun's position, and half of them are called jieqi (nodal terms) while the other half are called zhongqi (medial terms). A normal lunar month generally contains one zhongqi. When a certain lunar month contains no zhongqi, that month is designated a leap month, keeping the name of the previous month with the word "leap" added, for example leap fourth month. Traditionally there is a saying of "seven leap months in nineteen years," which shows that the appearance of leap months generally follows a discernible pattern.

For agriculture, leap months have practical value. The 24 solar terms that guide farming follow the sun, so as long as the lunar months line up with the seasons, the solar terms can steadily fall in their corresponding periods, and farming arrangements such as sowing, raising seedlings, irrigation and harvesting have a reliable basis. The leap month is precisely the adjustment that keeps the lunar calendar from drifting further and further off, ensuring that the Spring Festival always falls near the end of winter and the beginning of spring, and allowing the farming proverbs and solar-term experience of past generations to be passed down.

As for common questions, a leap month is not a fixed month that is always repeated; which month is leap is calculated year by year according to the rules for placing leap months, and its frequency of occurrence is also uneven. The solar terms are determined by the sun's position and fall on roughly similar dates in the Gregorian calendar each year, so a leap month will not disrupt the solar terms themselves. The various customs that have formed around leap months in folk tradition are local practices and are a separate matter from the principles of the calendar. Once you understand the logic of placing leap months, reading the lunar calendar and planning farm work both become clearer.