Chinese Lunar Calendar

Chinese lunar calendar, as the traditional calendar of China, is also known as rural calendar, Hua calendar, Xia calendar, Han calendar, Chinese calendar and so on. Originated in the Yellow Emperor Xuanyuan Period (2717 - 2599 BC), the Chinese calendar is not a pure lunar calendar but a lunisolar calendar.

From the stem-branch system in ancient times to the Gregorian calendar used in the late Qing Dynasty (1644 - 1911 AD), the Chinese calendar was revised for many times and up to 102 calendars were used in the history of China. These calendars had significant impacts on Chinese culture and civilization, such as stem-branch calendar, Xia calendar, Shang calendar, Zhou calendar, Taichu calendar in the Western Han Dynasty (206 BC - 24 AD), Huangji calendar in the Sui Dynasty (581 - 618 AD), and Dayan calendar in the Tang Dynasty (618 - 907 AD). Some calendars had never been officially used but contributed to health preserving, medicine, ideology, science, astronomy, mathematics, and other fields. When the People's Republic of China was founded in 1949, the Gregorian calendar was adopted to number the years with the Common Era system; at the same time, the traditional Chinese calendar - lunar calendar was retained and the national standard Calculation and Promulgation of the Chinese Calendar was issued in 2017. The current lunar calendar is calculated by the Purple Hills Observatory, Chinese Academy of Sciences (CAS) and it is part of the official almanac - Chinese Astronomical Almanac.

The Chinese calendar is a lunisolar calendar rather than a lunar one, which takes the cycle of the moon phases, i.e. the synodic month, as the length of a month, incorporates the elements of 24 solar terms of the stem-branch calendar, refers to the solar year as the length of a year and adjusts the average calendar year to the tropical year by adding a leap month. Due to the absorption of stem-branch calendar and 24 solar terms, the Chinese lunar calendar can reflect the change of seasons, guide farming and so on. Therefore, it contains both lunar and solar calendar elements and is a lunisolar calendar.

The Chinese calendar takes into account the relationship between the sun, the moon and the earth. The lunar calendar is regardless of the earth's orbit around the sun and cannot reflect the seasons since there is no fixed time for the changes of seasons. Different from the solar year which is 365 or 366-day, the lunar year sometimes differs with the solar year by one month in terms of days. In order to coordinate the number of days between the solar year and the lunar year, the lunisolar calendar, i.e. the Chinese lunar calendar, is introduced and it includes common years and leap years. A common year, a.k.a. nonleap year, has 12 months while a leap year consists of 13 months. A month can be a 30-day long month or 29-day short month; the average calendar month is equal to a synodic month. The long or short months are decided by calculation. This is done by adding 24 solar terms to the calendar and by using "7 leap years in 19 lunar years" (i.e. adding 7 leap years in each 19 lunar years. There is one extra leap month in a leap year, thus the year has 383-385 days while a common year has 353-355 days) to make the average number of days in a lunar year in line with that in a tropical year. As a result, the Chinese lunar calendar corresponds both to the phases of the moon and to the periodic motion of the earth around the sun.

In the Chinese lunar calendar, the first month is traditionally called Zheng Yue (the first month, not January) while the 11th month and the 12th month are called Dong Yue (eleventh month) and La Yue (end-of-year month) respectively; however, Dong Yue is rarely used at present. The Chinese idiom of Han Dong La Yue (literally chilly winter) used to indicate the cold climate indicate that the 10th, 11th and the 12th months in the Chinese lunar calendar are the coldest of a year.

In a lunar month, the first 10 days are usually called ''Chu+Date'' in sequence. For example, the 2nd day of the 1st month is called Zheng Yue Chu Er (Er means second in Chinese), i.e. 1st Month 2nd Day. The 11th to the 29th or 30th day are directly called with the number of the date. For example, the 15th day of the 1st month is called Zheng Yue Shi Wu (Shi Wu means fifteenth in Chinese), i.e. 1st Month 15th Day. Traditionally, the 21st to 29th days are called ''Nian+Date'' (Nian means twenty in Chinese). For example, the 22nd day of the 1st month is called Zheng Yue Nian Er, i.e. 1st Month 22nd Day.

The 24 solar terms, an integral part of the Chinese lunar calendar, reflect the annual apparent motion of the sun. As the 24 seasonal points of the stem-branch calendar, the 24 solar terms are distributed in 12 months, and each month contains two solar terms. The 24 solar terms adopts the cycle of the earth's revolution around the sun, which basically summarizes the different positions of the sun on the ecliptic at different times of the year, the exact time of four seasons, the occurrence rules of natural phenomena like rainfall and snowfall, and the time of some phenological phenomena in nature. The 24 solar terms are determined according to the positions of the earth in the ecliptic (i.e. the earth's orbit around the sun) and each corresponds to a certain position of the sun every time it moves 15 degrees on the ecliptic. The 24 solar terms were established in ancient times and incorporated into the Taichu calendar in the Western Han Dynasty (206 BC - 24 AD) as a supplementary calendar to guide farming. Through the 24 solar terms, the Chinese lunar calendar accurately reflects the changes at all seasons to facilitate people's production, living and agricultural cultivation. Following the rules of the Chinese lunar calendar, the 24 solar terms are calculated by calendarists with the astronomical algorithms thru actual observations. The lunar months generally correspond to the 12 minor solar terms (each falls upon the middle of a month) while the 12 major solar terms can fall on the second half of the last month or the first half of the current month in the Chinese lunar calendar.

The major solar terms and the minor appear alternately and each lasts about 15 days. The two are collectively known as the solar terms.

List of 24 Solar Terms: Start of Spring, Rain Water, Insects Awaken, Vernal Equinox, Clear and Bright, Grain Rain, Summer Begins, Grain Buds, Grain Ear, Summer Solstice, Slight Heat, Great Heat, Autumn Begins, Heat Withdraws, White Dews, Autumn Equinox, Cold Dews, Frost Falls, Winter Begins, Light Snow, Heavy Snow, Winter Solstice, Slight Cold, Great Cold