Chinese calendar

The Chinese calendar is a lunisolar calendar created by or commonly used by the Chinese people. While this description is generally accurate, it does not provide a definitive or complete answer. A total of 102 calendars have been officially recorded in classical historical texts.^[1] In addition, many more calendars were created privately, with others being built by people who adapted Chinese cultural practices, such as the Koreans, Japanese, Vietnamese, and many others, over the course of a long history.

A Chinese calendar consists of twelve months, each aligned with the phases of the moon, along with an intercalary month inserted as needed to keep the calendar in sync with the seasons. It also features twenty-four solar terms, which track the position of the sun and are closely related to climate patterns. Among these, the winter solstice is the most significant reference point and must occur in the eleventh month of the year. Each month contains either twenty-nine or thirty days. The sexagenary cycle for each day runs continuously over thousands of years and serves as a determining factor to pinpoint a specific day amidst the many variations in the calendar. In addition, there are many other cycles attached to the calendar that determine the appropriateness of particular days, guiding decisions on what is considered auspicious or inauspicious for different types of activities.

The variety of calendars arises from deviations in algorithms and assumptions about inputs. The Chinese calendar is location-sensitive, meaning that calculations based on different locations, such as Beijing and Nanjing, can yield different results. This has even led to occasions where the Mid-Autumn Festival was celebrated on different days between mainland China and Hong Kong in 1978,^[2] as some almanacs based on old imperial rule. The sun and moon do not move at a constant speed across the sky. While ancient Chinese astronomers were aware of this fact, it was simpler to create a calendar using average values. There was a series of struggles over this issue, and as measurement techniques improved over time, so did the precision of the algorithms. The driving force behind all these variations has been the pursuit of a more accurate description and prediction of natural phenomena.

The calendar during imperial times was regarded as sacred and mysterious. Rulers, with their mandate from Heaven, worked tirelessly to create an accurate calendar capable of predicting climate patterns and astronomical phenomena, which were crucial to all aspects of life, especially agriculture, fishing, and hunting. This, in turn, helped maintain their authority and secure an advantage over rivals. In imperial times, only the rulers had the authority to announce a calendar. An illegal calendar could be considered a serious offence, often punishable by capital punishment.

Early calendars were also lunisolar, but they were less stable due to their reliance on direct observation. Over time, increasingly refined methods for predicting lunar and solar cycles were developed, eventually reaching maturity around 104 BC, when the Taichu Calendar (太初曆), namely the genesis calendar, was introduced during the Han dynasty. This calendar laid the foundation for subsequent calendars, with its principles being followed by calendar experts for over two thousand years. Over centuries, the calendar was refined through advancements in astronomy and horology, with dynasties introducing variations to improve accuracy and meet cultural or political needs.

Improving accuracy has its downsides. The solar terms, namely solar positions, calculated based on the predicted location of the sun, make them far more irregular than a simple average model. In practice, solar terms do not need to be maximally precise because climate does not change overnight. The introduction of the leap second to the Chinese calendar is somewhat excessive, as it makes future predictions more challenging. This is particularly true since the leap second is typically announced six months in advance, which can complicate the determination of which day the new moon or solar terms fall on, especially when they occur close to midnight.

While modern China primarily adopts the Gregorian calendar for official purposes, the traditional calendar remains culturally significant, influencing festivals and cultural practices, determining the timing of Chinese New Year with traditions like the twelve animals of the Chinese zodiac still widely observed. The winter solstice serves as another New Year, a tradition inherited from ancient China. Beyond China, it has shaped other East Asian calendars, including the Korean, Vietnamese, and Japanese lunisolar systems, each adapting the same lunisolar principles while integrating local customs and terminology.

The sexagenary cycle, a repeating system of Heavenly Stems and Earthly Branches, is used to mark years, months, and days. Before adopting their current names, the Heavenly Stems were known as the "Ten Suns" (十日),^[3][4] having research that it is a remnant of an ancient solar calendar.^[5]

Epochs, or fixed starting points for year counting, have played an essential role in the Chinese calendar's structure. Some epochs are based on historical figures, such as the inauguration of the Yellow Emperor (Huangdi), while others marked the rise of dynasties or significant political shifts. This system allowed for the numbering of years based on regnal eras, with the start of a ruler's reign often resetting the count.

The Chinese calendar also tracks time in smaller units, including months, days, double-hour, hour and quarter periods. These timekeeping methods have influenced broader fields of horology, with some principles, such as precise time subdivisions, still evident in modern scientific timekeeping. The continued use of the calendar today highlights its enduring cultural, historical, and scientific significance.