With the change of seasons, we have arrived at the Cold Dew, followed by the Frost's Descent. During these seasonal transitions, the leap months in 2023, the leap year in 2024, and the leap sixth month in 2025 have sparked people's curiosity. How are these special years formed? What astronomical principles and folk legends lie behind them?
The Historical Evolution of Calendars
China has had more than 100 different calendars throughout history, such as the "Taichu Calendar" from Emperor Wu of Han, the "Shoushi Calendar" from the Yuan Dynasty, and the "Shixian Calendar" from the Qing Dynasty. Although they differ in astronomical data, their basic design concepts are similar to the current lunar calendar. During the Republic of China period, China began to adopt the Western Gregorian calendar, which is commonly seen on calendars with Arabic numerals. The evolution of these calendars reflects humanity's continuous exploration and precision in measuring time.
The Necessity of Intercalation
A common solar year is about 365 days, while a leap year is about 366 days, whereas a lunar year is approximately 354 days. Since the solar year is about 11 days longer than the lunar year, over time this would lead to seasonal misalignment. To maintain consistency between seasons and the lunar months, ancient calendar scholars used the method of intercalating 7 leap years every 19 years, which was later abandoned by the "Lingde Calendar" of the Tang Dynasty and replaced with the method of intercalating a leap month without a solar term, usually once every 2 to 3 years. The current lunar calendar is a lunisolar calendar, with a common year of about 354 days and a leap year of about 384 days, allowing farmers to plan agricultural activities according to the 24 solar terms.
Determining the Leap Month
In China's lunar calendar, the 24 solar terms are divided into 12 terms and 12 mid-terms. The months without mid-terms are designated as "leap months," called "leap [month]." For example, March 21, 2023, is the mid-term "Spring Equinox," and April 20 is the next mid-term "Grain Rain." The month in between has no mid-term, so it is designated as the leap second month. In 2025, July 25 is the mid-term "Great Heat," and August 23 is the next mid-term "End of Heat." The month in between has no mid-term, so it is designated as the leap sixth month.
Folk Beliefs About Leap Months
Due to the "intercalation" between the solar and lunar calendars, there are folk terms like "double spring year" and "no spring year," accompanied by various agricultural proverbs. While these proverbs enrich folk culture, they cannot be used as a basis for predicting weather or harvests. Moreover, leap sixth months affect the timing of festivals such as the Torch Festival, Qixi Festival, Zhongyuan Festival, and Mid-Autumn Festival, which is determined by the relationship between solar terms and the Gregorian calendar.
Design of the Gregorian Leap Year
The year 2024 is a Gregorian leap year, consisting of 366 days and 53 weeks. The Earth's orbital cycle around the Sun is 365 days, 5 hours, 48 minutes, and 46 seconds, while the Gregorian common year only has 365 days, shortening the tropical year by approximately 0.2422 days. To compensate for this gap, one day is added to February every four years, making the length of that year 366 days, and this year is known as a leap year.
The concept of the leap sixth month involves the coordination between the lunar and Gregorian calendars, as well as the intersection of astronomical principles and folk legends. By understanding how these special years are formed, we can better appreciate the wisdom of ancient Chinese calendars and their impact on modern life. Leap months are not only a necessity for time measurement but also an integral part of cultural heritage, deepening our understanding and respect for natural laws.
