In this blog post, we will take a closer look at the true nature of the lunar calendar, including the lunisolar calendar, leap months, and the method of calculating the length of the year.
The calendar traditionally used in East Asia is commonly referred to as the “lunar calendar,” but it is actually a “lunisolar calendar” that reflects both the movements of the sun and the moon. In other words, it is not a calendar based solely on the shape of the moon, but a complex system that also takes into account seasonal changes based on the movement of the sun. In the lunar calendar, the cycle of the moon, or sakumangwol, which is approximately 29.53 days, is used as the basic unit of a month.
The lunar month refers to the cycle from the new moon, when the moon is not visible, to the full moon, when the moon is full, and then until it disappears again. Accordingly, the length of a month was divided into 30 days for a long month (daewol) and 29 days for a short month (sowol), and these two types of months alternated in no particular order.
The first day of each month was designated as the day when the sun and moon were aligned with the earth and the moon was completely invisible, i.e., the new moon. However, there was a limitation in that it was difficult to reflect seasonal changes when dates were determined based solely on the movement of the moon. To compensate for this, East Asia established 24 solar terms based on the movement of the sun and marked them on the calendar.
The 24 solar terms divide the seasonal changes caused by the sun’s movement along the ecliptic into 15-day intervals and are named after the beginning of spring, the beginning of summer, and the beginning of autumn, among others. They became an important reference point for farming and daily life.
The problem was that the cycles of the sun and moon did not match perfectly. A year based on the movement of the sun is approximately 365.25 days, but a year based on the movement of the moon is approximately 354.36 days (29.53 days × 12 months), resulting in a difference of approximately 11 days. Without resolving this difference, the calendar would be out of sync with the actual seasons every year, so a mechanism was needed to correct this, and that is the role of the intercalary month.
The method devised to determine when and in which month to insert a leap month is called the “no-middle leap month method.” In this method, the ecliptic, which is the orbit of the sun, is divided into 12 equal parts, and two of the 24 solar terms are assigned to each part in order. These solar terms were further divided into seasonal terms and middle terms. For example, the beginning of spring, the beginning of summer, and the beginning of autumn were assigned to the first half of each month and called “seasonal terms,” while the end of winter, the beginning of spring, and the beginning of summer were assigned to the second half of each month and called “middle terms.” This division went beyond simply conveying information on the calendar and became an important standard for coordinating the rhythms of society as a whole, including agriculture and rituals.
Meanwhile, the 12 divisions of the ecliptic correspond to the 12 months based on the solar calendar, in which case the average length of each month is approximately 30.44 days (365.25 days ÷ 12 months). This differs by about one day from the length of a month in the lunar calendar (approximately 29.53 days), and this slight difference accumulates over the years, causing discrepancies in the calendar. As a result, there are cases where a particular month does not include Junggi, which is called a “month without Junggi.” In the lunisolar calendar, an intercalary month is inserted into the month without a middle day to correct the difference between the solar and lunar cycles. As a result of this adjustment, intercalary months were generally inserted seven times in 19 years.
The creation of a calendar, which required such complex calculations and astronomical knowledge, went beyond simply counting days and belonged to the field of calendar science, which involved observing and calculating the movements of celestial bodies. In traditional East Asian societies, calendar science was more than just a simple skill; it had political and philosophical significance and was deeply connected to the concept of the will of heaven.
The will of heaven refers to the legitimacy of a monarch’s rule, which is granted by heaven. Monarchs must not go against the will of heaven and must listen to the voices of the people. One way to achieve this was to establish and announce an accurate calendar. By creating an accurate and sophisticated calendar and providing it to the people, monarchs sought to establish their authority as beings who understood the will of heaven and put it into practice. Thus, the calendar became a symbol of rule and a basis for legitimacy.
Furthermore, the sophistication of the calendar system did not depend solely on the movements of the sun and moon. In East Asia, even the movements of the planets were included in calendar calculations. Strictly speaking, the cycles of the sun and moon alone are sufficient for creating a calendar. In the solar calendar, only the movement of the sun needs to be considered, and in the lunar calendar, only the movement of the moon needs to be considered. Even the lunisolar calendar is basically composed of only the movements of the sun and moon. Therefore, the movements of the planets do not directly affect the creation of calendars. Nevertheless, East Asians paid attention to the movements of the planets, especially the five planets—Mercury, Venus, Mars, Jupiter, and Saturn.
This tendency became clear at least from the Han Dynasty, and the precise observation and recording of the orbits and cycles of the five planets became an important part of the calendar system. This can be seen as a reflection of the East Asian worldview, which sought to understand the will of heaven through the changes in the celestial bodies, and the ruling system, which sought to maintain order within that world.
As a result, calendars in East Asia were not merely tools for recording and announcing dates. They were the crystallization of the calendar system and a symbolic means of connecting the celestial world with the human world, as well as a complex system that encompassed both political and cultural meanings. For example, while the solar calendar only shows the position of the sun on the ecliptic, the traditional calendars of East Asia were able to contain a wider range of information.
In particular, rather than simply indicating the date as the month and day, it became possible to indicate the year, month, and day, enabling a more specific and sophisticated concept of time. Closely related to this concept of “year” was the planet Jupiter, known as “Seisei” in Japanese.
East Asians had known since ancient times that Jupiter’s orbital period was approximately 12 years, and based on this knowledge, they developed a unique method of calculating years called the “year of the planet Jupiter.” The year of the planet Jupiter was a method of estimating the year based on the position of Jupiter in the sky, similar to reading the time on a clock divided into 12 equal parts.
As such, the calendars of traditional East Asian societies were not mere records of time, but rather comprehensive cultural products that connected the universe and humans, the heavens and rulers, and nature and society. They contained a complex mix of meanings and roles, including precise astronomy, deep philosophical thinking, symbols of political authority, and practical functions that supported the lives of the people.
